1. Principles of building wireless telecommunication systems
1.1 Architecture of cellular communication systems.
1.2 Service of the subscriber by the network.
1.3 Methods for separating subscribers in cellular communications
1.4 DECT standard for communication.
1.5 Standards Bluetooth, Wi-Fi (802.11, 802.16).
2. Systems of complex signals for telecommunication systems.
2.1 Signal spectra
2.2 Correlation properties of signals
2.3 Types of complex signals
2.4 Derived signal systems
3. Modulation of complex signals
3.1 Geometric representation of signals
3.2 Methods of phase keying of signals (FM2, FM4, OFM).
3.3 Modulation with minimal frequency shift.
3.4 Quadrature modulation and its characteristics (QPSK, QAM).
3.5 Implementation of quadrature modems.
4. Characteristics of signal reception in telecommunication systems.
4.1 Probability of discrimination errors M of known signals
4.2 Probability of errors in distinguishing M fluctuating signals.
4.3 Calculation of discrimination errors of M signals with unknowns
non-energy settings.
4.4 Comparison of synchronous and asynchronous communication systems.
5. Conclusion.
6. References
1. Principles of building wireless telecommunication systems
1.1 Architecture of cellular communication systems
A cellular communication system is a complex and flexible technical system that allows for a wide variety, both in terms of configuration options and the set of functions performed. An example of the complexity and flexibility of the system is that it can transmit both speech and other types of information, in particular text messages and computer data. In terms of voice transmission, in turn, ordinary two-way telephone communication, multilateral telephone communication (the so-called conference call - with more than two subscribers participating in a conversation at the same time), voice mail can be implemented. When organizing a normal two-way telephone conversation, starting with a call, the modes of auto redial, call waiting, call forwarding are possible.
A cellular communication system is built as a collection of cells, or cells, covering a service area, such as a metropolitan area. Cells are usually schematically depicted in the form of regular equal-sized hexagons (Fig. 1.1.), Which, by resemblance to bee honeycombs, was the reason to call the system cellular. Cellular, or cellular, structure of the system is directly related to the principle of frequency reuse - the basic principle of the cellular system, which determines the efficient use of the allocated frequency range and high system capacity.
Rice. 1.1. Cells (cells) of the system covering the entire service area.
In the center of each cell there is a base station serving all mobile stations (subscriber radiotelephone sets) within its cell (Fig. 1.2.). When a subscriber moves from one cell to another, his service is transferred from one base station to another. All base stations of the system, in turn, are connected to the switching center, from which there is access to the Interconnected Communications Network (ICN) of Russia, in particular, if it happens in a city, access to a regular urban wired telephone network.
Rice. 1.2. One cell with a base station in the center serving all mobile stations in the cell.
On fig. 1.3. a functional diagram corresponding to the described structure is given.
Rice. 1.3. Simplified functional diagram of a cellular communication system: BS - base station; PS - mobile station (subscriber radiotelephone).
In reality, cells are never strictly geometric. Real cell boundaries have the form of irregular curves depending on the propagation and attenuation conditions of radio waves, i.e. on the terrain, the nature and density of vegetation and buildings, and similar factors. Moreover, cell boundaries are generally not well-defined, since the handoff boundary of a mobile station from one cell to another may shift to some extent with changes in radio wave propagation conditions and depending on the direction of movement of the mobile station. Similarly, the position of the base station only approximately coincides with the center of the cell, which, moreover, is not so easy to determine unambiguously if the cell has an irregular shape. If the base stations use directional (not isotropic in the horizontal plane) antennas, then the base stations actually end up at the cell boundaries. Further, a cellular communication system may include more than one switching center, which may be due to the evolution of the development of the system or the limited capacity of the switch. It is possible, for example, the structure of the system of the type shown in Fig. 1.4. - with several switching centers, one of which can be conditionally called "head" or "leading".
Rice. 1.4. Cellular communication system with two switching centers.
Consider a mobile station, which is the simplest element of a cellular communication system in terms of functionality and design, and besides, it is the only element of the system that is actually available to the user.
The block diagram of the mobile station is shown in fig. 1.5. It consists of:
Control block;
Transceiver unit;
Antenna block.
Rice. 1.5. Block diagram of a mobile station (subscriber radiotelephone).
The transceiver unit, in turn, includes a transmitter, a receiver, a frequency synthesizer and a logic unit.
The antenna unit is the simplest in composition: it includes the antenna itself and the receive-transmit switch. The latter for a digital station can be an electronic switch that connects the antenna either to the output of the transmitter or to the input of the receiver, since the mobile station of a digital system never receives and transmits simultaneously.
The control unit includes a handset - microphone and speaker, keyboard and display. The keyboard (a dialing field with numeric and function keys) is used to dial the phone number of the called subscriber, as well as commands that determine the operating mode of the mobile station. The display serves to display various information provided by the device and the operating mode of the station.
The transceiver unit is much more complicated.
The transmitter includes:
Analog-to-digital converter (ADC) - converts the signal from the microphone output into digital form, and all subsequent processing and transmission of the speech signal is carried out in digital form, up to the reverse digital-to-analog conversion;
The speech encoder encodes the speech signal, i.e., converts a digital signal according to certain laws in order to reduce its redundancy, i.e. in order to reduce the amount of information transmitted over a communication channel;
Channel encoder - adds additional (redundant) information to the digital signal received from the output of the speech encoder, designed to protect against errors during signal transmission over the communication line; for the same purpose, information is subjected to a certain repackaging (multiplication); in addition, the channel encoder introduces control information from the logic block into the transmitted signal;
Modulator - carries out the transfer of information of the encoded video signal to the carrier frequency.
The composition of the receiver basically corresponds to the transmitter, but with the inverse functions of its constituent blocks:
The demodulator extracts from the modulated radio signal an encoded video signal carrying information;
The channel decoder extracts control information from the input stream and sends it to the logical block; the received information is checked for errors, and the selected errors are corrected if possible; before further processing, the received information is subjected to reverse (with respect to the encoder) repackaging;
The speech decoder restores the speech signal coming to it from the channel decoder, converting it into a natural form, with its inherent redundancy, but in digital form;
A digital-to-analog converter (DAC) converts the received speech signal into analog form and feeds it to the speaker output;
The equalizer serves to partially compensate for signal distortion due to multipath propagation; in essence, it is an adaptive filter that is tuned according to the training sequence of symbols that is part of the transmitted information; the equalizer block is not, generally speaking, functionally necessary and in some cases may be absent.
For a combination of an encoder and a decoder, the name codec is sometimes used.
In addition to the transmitter and receiver, the transceiver unit includes a logic unit and a frequency synthesizer. The logical unit is, in fact, a microcomputer with its own operational and permanent memory, which controls the operation of the mobile station. The synthesizer is a source of carrier frequency oscillations used to transmit information over a radio channel. The presence of a local oscillator and a frequency converter is due to the fact that different parts of the spectrum are used for transmission and reception.
The block diagram of the base station is shown in fig. 1.6.
Rice. 1.6. Base station block diagram.
The presence of several receivers and the same number of transmitters allows simultaneous operation on several channels with different frequencies.
Receivers and transmitters of the same name have common tunable reference oscillators, which ensure their coordinated tuning when switching from one channel to another. To ensure simultaneous operation of N receivers for one receiving and N transmitters for one transmitting antenna, a power divider for N outputs is installed between the receiving antenna and receivers, and a power divider for N inputs is installed between the transmitters and the transmitting antenna.
The receiver and transmitter have the same structure as in the mobile station, except that there is no DAC and ADC here, since both the transmitter input signal and the receiver output signal are digital.
The communication line interface unit packs the information transmitted over the communication line to the switching center and unpacks the information received from it.
The base station controller, which is a sufficiently powerful and perfect computer, provides control over the operation of the station, as well as monitoring the performance of all its units and nodes.
The switching center is the brain center and at the same time the dispatching center of the cellular communication system, to which information flows from all base stations are closed and through which access to other communication networks is provided - a fixed telephone network, long-distance communication networks, satellite communications, and other cellular networks.
The block diagram of the switching center is shown in fig. 1.7. The switch performs the switching of information flows between the corresponding communication lines. It can, in particular, direct the flow of information from one base station to another, or from a base station to a fixed communication network, or vice versa.
The switch is connected to the communication lines through the appropriate communication controllers that perform intermediate processing (packing/unpacking, buffer storage) of information flows. The general management of the operation of the switching center and the system as a whole is carried out from the central controller, which has powerful software. The work of the switching center involves the active participation of operators, therefore, the center includes the appropriate terminals, as well as means of displaying and recording (documenting) information. The operator enters data on subscribers and their service conditions, initial data on the operating modes of the system.
Rice. 1.7. Block diagram of the switching center.
Important elements of the system are databases - home registry, guest registry, authentication center, hardware registry. The home register contains information about all subscribers registered in this system and about the types of services that can be provided to them. The location of the subscriber is also fixed here to organize his call, and the actually rendered services are recorded. The guest register contains approximately the same information about subscribers - guests (roamers), i.e. about subscribers registered in another system, but currently using cellular communication services in this system. The Authentication Center provides procedures for authenticating subscribers and encrypting messages. The hardware register, if it exists, contains information about the mobile stations in operation for their serviceability and authorized use.
1.2 Subscriber service by the network
Interface - a system of signals through which the devices of a cellular communication system are connected to each other. Each cellular standard uses several interfaces (different in different standards).
Of all the interfaces used in cellular communication, one occupies a special place - this is the exchange interface between the mobile and base stations. It is called the air interface. The terrestrial interface is necessarily used in any cellular communication system, with any of its configurations and in the only possible variant for its cellular communication standard.
The on-air interface of the D-AMPS system of the IS-54 standard is relatively simple (Fig. 1.8.).
A traffic channel is a voice or data transmission channel. The transmission of information in the traffic channel is organized by successive frames with a duration of 40 ms. Each frame consists of six time intervals - slots; the slot duration (6.67 ms) corresponds to 324 bits. With full-rate coding, two slots are allocated for one speech channel in each frame, i.e. A 20-millisecond segment of speech is packed into one slot, which is one-third as long. With half-rate coding, one speech channel is assigned one slot in the frame, i.e. the packing of the speech signal is twice as dense as in full-rate coding.
Fig.1.8. Frame and slot structure of the D-AMPS system (traffic channel; IS-54 standard): Data – speech information; Sync(Sc) – synchronizing (training) sequence; SACCH – control channel slow alignment information; CDVCC(CC) - coded digital color confirmation code; G - protective blank; R is the transmitter pulse front interval; V,W,X,Y - hexadecimal zeros; Res - reserve.
The slot has a slightly different structure in the forward traffic channel - from the base station to the mobile station and in the reverse traffic channel - from the mobile station to the base station. In both cases, 260 bits are allocated for speech transmission. Another 52 bits are occupied by control and auxiliary information. It includes: a 28-bit training sequence used for slot identification within a frame, slot timing, and equalizer tuning; 12-bit SACCH signaling (monitoring and control) message; 12-bit coded digital coloring code (CDVCC) field used to identify the mobile station when its signal is received by the base station (the code is assigned by the base station individually for each channel, i.e. for each mobile station and is relayed by the latter back to the base station).
The remaining 12 bits are not used in the forward channel (reserve), and in the reverse channel they serve as a guard interval, during which no useful information is transmitted.
At the initial stage of establishing a connection, a shortened slot is used, in which the synchronization sequence and the CDVCC code are repeated many times, separated by zero numbers of different lengths. There is an additional protective blank at the end of the shortened slot. The mobile station transmits short slots until the base station selects the required time delay, determined by the distance between the mobile station and the base station.
There are several communication channels: frequency, physical and logical.
A frequency channel is a frequency band allocated for the transmission of information of one communication channel. Several physical channels can be placed in one frequency channel, for example, in the TDMA method.
A physical channel in a time division multiple access (TDMA) system is a time slot with a specific number in the air interface frame sequence.
Logical channels are divided according to the type of information transmitted in the physical channel into a traffic channel and a control channel. The control channel carries signaling information including control information and equipment status monitoring information, and the traffic channel carries voice and data.
(Traffic is a collection of messages transmitted over a communication line).
Consider the operation of a mobile station within one cell of its ("home") system, without handover. In this case, the operation of the mobile station can be divided into four stages, which correspond to four modes of operation:
Power on and initialization;
Standby mode;
Communication (call) establishment mode;
Mode of communication (telephone conversation).
After turning on the mobile station, initialization is performed - the initial start. During this stage, the mobile station is configured to work as part of the system - according to the signals regularly transmitted by the base stations on the appropriate control channels, after which the mobile station goes into standby mode.
While in standby mode, the mobile station monitors:
Changes in system information - these changes can be associated both with changes in the operating mode of the system, and with the movements of the mobile station itself;
System commands - for example, a command to confirm its performance;
Receiving a call from the system;
Call initialization by own subscriber.
In addition, the mobile station can periodically, for example, once every 10...15 minutes, confirm its operability by transmitting appropriate signals to the base station. In the switching center for each of the included mobile stations, a cell is fixed in which it is “registered”, which facilitates the organization of the procedure for calling a mobile subscriber.
If a call comes from the system to the number of a mobile subscriber, the switching center directs this call to the base station of the cell in which the mobile station is “registered”, or to several base stations in the vicinity of this cell, taking into account the possible movement of the subscriber over the time elapsed from the moment the last "registration", and the base stations transmit it on the appropriate call channels. The mobile station, which is in the idle mode, receives the call and answers it through its base station, simultaneously transmitting the data necessary to carry out the authentication procedure. If the authentication is successful, a traffic channel is assigned and the number of the corresponding frequency channel is reported to the mobile station. The mobile station tunes in to a dedicated channel and, together with the base station, performs the necessary steps to prepare a communication session. At this stage, the mobile station tunes in to the specified slot number in the frame, adjusts the time delay, adjusts the level of radiated power, and so on. The choice of time delay is made for the purpose of temporal coordination of slots in the frame when organizing communication with mobile stations located at different distances from the base station. In this case, the time delay of the packet transmitted by the mobile station is regulated by the commands of the base station.
The base station then issues a ringing (ringing) message, which is acknowledged by the mobile station, and the caller is able to hear the ringing signal. When the called subscriber answers the call, the mobile station issues a call termination request. When the connection is terminated, the communication session begins.
During the conversation, the mobile station processes the transmitted and received speech signals, as well as control signals transmitted simultaneously with speech. At the end of the conversation, service messages are exchanged between the mobile and base station, after which the transmitter of the mobile station turns off and the station goes into standby mode.
If the call originates from the mobile station, i.e. the subscriber dials the number of the called subscriber and presses the "call" button on the control panel, then the mobile station transmits a message through its base station indicating the called number and data for authenticating the mobile subscriber. After authentication, the base station assigns a traffic channel, and the next steps to prepare a communication session are the same as when a call is received from the system.
The base station then informs the switching center about the readiness of the mobile station, the switching center transfers the call to the network, and the mobile station subscriber gets the opportunity to hear "call" or "busy" signals. The connection is terminated on the network side.
Each time a connection is established, authentication and identification procedures are performed.
Authentication is a procedure for confirming the authenticity (validity, legality, availability of rights to use cellular services) of a subscriber of a mobile communication system. The need to introduce this procedure is caused by the inevitable temptation to gain unauthorized access to cellular services.
Identification is the procedure for establishing that a mobile station belongs to one of the groups with certain properties or characteristics. This procedure is used to identify lost, stolen or malfunctioning devices.
The idea of the authentication procedure in a digital cellular communication system is to encrypt some identifier passwords using quasi-random numbers periodically transmitted to the mobile station from the switching center, and an encryption algorithm individual for each mobile station. Such encryption, using the same initial data and algorithms, is performed both at the mobile station and at the switching center, and authentication is considered successful if both results match.
The identification procedure consists in comparing the identifier of the subscriber set with the numbers contained in the corresponding “black lists” of the equipment register in order to withdraw stolen and technically faulty devices from circulation. The device identifier is made such that it is difficult and uneconomical to change or counterfeit it.
When moving a mobile station from one cell to another, its service is transferred from the base station of the first cell to the base station of the second (Fig. 1.9.). This process is called handover. It occurs only when the mobile station crosses a cell boundary during a communication session and the communication is not interrupted. If the mobile station is in standby mode, it simply tracks these movements according to the information of the system transmitted over the control channel, and at the right time changes to a stronger signal from another base station.
Rice. 1.9. Handover from cell A to cell B when the mobile station crosses a cell boundary.
The need for a handover occurs when the quality of the communication channel, as measured by signal strength and/or bit error rate, falls below an acceptable limit. In the D-AMPS standard, a mobile station measures these characteristics only for a working cell, but when the communication quality deteriorates, it reports this through the base station to the switching center, and at the command of the latter, similar measurements are performed by mobile stations in neighboring cells. Based on the results of these measurements, the switching center selects the cell to which the service should be handed over.
The service is transferred from the cell with the worst quality of the communication channel to the cell with the best quality, and the specified difference must be at least some predetermined value. If this condition is not required, then, for example, when the mobile station moves approximately along the cell boundary, multiple handovers from the first cell to the second and back are possible, leading to loading the system with meaningless work and reducing the quality of communication.
Having made a decision to handover and selecting a new cell, the switching center informs the base station of the new cell about this, and the mobile station issues the necessary commands through the base station of the old cell indicating the new frequency channel, working slot number, etc. The mobile station is reconfigured to a new channel and set up to work with the new base station, following approximately the same steps as in the preparation of the communication session, after which communication continues through the base station of the new cell. At the same time, a break in a telephone conversation does not exceed a fraction of a second and remains invisible to the subscriber.
A cellular communication system can provide a roaming function - this is a procedure for providing cellular communication services to a subscriber of one operator in the system of another operator.
An idealized and simplified roaming organization scheme is as follows: a cellular subscriber who finds himself on the territory of a "foreign" system that allows roaming, initiates a call as if he were on the territory of his "own" system. The switching center, making sure that this subscriber does not appear in its home register, perceives it as a roamer and enters it into the guest register. At the same time, it queries the home register of the roamer's "native" system for information related to it, necessary for organizing service, and reports in which system the roamer is currently located; the latter information is recorded in the home register of the "native" system of the roamer. After that, the roamer uses cellular communications as at home.
1.3 Methods for separating subscribers in cellular communications
A link resource represents the time and bandwidth available for signal transmission on a particular system. To create an efficient communication system, it is necessary to plan the distribution of the resource among the users of the system so that time/frequency is used as efficiently as possible. The result of such planning should be equal access of users to the resource. There are three main methods for separating subscribers in a communication system.
1. Frequency division. Certain sub-bands of the usable frequency band are allocated.
2. Temporal separation. Subscribers are allocated periodic time intervals. Some systems allow users a limited amount of time to communicate. In other cases, the time that users access a resource is determined dynamically.
3. Code division. Certain elements of a set of orthogonally (or almost orthogonally) distributed spectral codes are distinguished, each of which uses the entire frequency range.
With frequency division (FDMA), the communication resource is distributed according to Fig. 1.10. Here, the distribution of signals or users over a frequency range is long-term or permanent. A communication resource can simultaneously contain several signals spaced apart in the spectrum.
The primary frequency range contains signals that use the frequency range between f 0 and f 1 , the second one between f 2 and f 3 , and so on. The regions of the spectrum that lie between the usable bands are called guard bands. The guard bands act as a buffer to reduce interference between adjacent (in frequency) channels.
Rice. 1.10. Frequency division seal.
In order for an unmodulated signal to use a higher frequency range, it is converted by superimposing or mixing (modulating) this signal and a sinusoidal signal of a fixed frequency.
With time division (TDMA), a communication resource is distributed by providing each of the M signals (users) of the entire spectrum for a small period of time, called a time interval (Fig. 1.11.). The time intervals that separate the intervals used are called guard intervals.
The guard interval creates some temporal uncertainty between adjacent signals and acts as a buffer, thereby reducing interference. Time is usually broken down into intervals called frames. Each frame is divided into time slots that can be distributed among users. The general structure of frames repeats periodically, such that a TDMA data transmission is one or more time slots that repeat periodically throughout each frame.
Rice. 1.11. Seal with temporary separation.
Code division multiple access (CDMA) is a practical application of spread spectrum techniques that can be divided into two main categories: direct sequence spread spectrum and frequency hopping spread spectrum.
Let us consider the spectrum extension by the direct sequence method. Spread spectrum gets its name from the fact that the bandwidth used for signal transmission is much larger than the minimum required for data transmission. So, N users receive an individual code g i (t), where i = 1,2,…,N. The codes are approximately orthogonal.
A block diagram of a typical CDMA system is shown in fig. 1.12.
Rice. 1.12. Multiple access code division.
The first block of the circuit corresponds to the data modulation of the carrier wave Acosω 0 t. The output of the modulator belonging to the user from group 1 can be written as follows: s 1 (t)=A 1 (t)cos(ω 0 t+φ 1 (t)).
The type of the received signal can be arbitrary. The modulated signal is multiplied by the spreading signal g 1 (t) assigned to group 1; the result g 1 (t)s 1 (t) is transmitted over the channel. Similarly, for users of groups from 2 to N, the product of the code function and the signal is taken. Quite often, access to the code is limited to a well-defined group of users. The resulting channel signal is a linear combination of all transmitted signals. Neglecting delays in signal transmission, the specified linear combination can be written as follows: g 1 (t)s 1 (t)+ g 2 (t)s 2 (t)+…+ g N (t)s N (t).
Multiplication of s 1 (t) and g 1 (t) results in a function whose spectrum is the convolution of the spectra of s 1 (t) and g 1 (t). Since the signal s 1 (t) can be considered narrowband (compared to g 1 (t)), the bands g 1 (t)s 1 (t) and g 1 (t) can be considered approximately equal. Consider a receiver configured to receive messages from user group 1. Let us assume that the received signal and the code g 1 (t) generated by the receiver are fully synchronized with each other. The first step of the receiver is to multiply the received signal by g 1 (t). As a result, a function g 1 2 (t)s 1 (t) and a set of side signals g 1 (t)g 2 (t)s 2 (t)+ g 1 (t)g 3 (t)s 3 (t )+…+ g 1 (t)g N (t)s N (t). If the code functions g i (t) are mutually orthogonal, the received signal can be perfectly extracted in the absence of noise, because
.
Side signals are easily eliminated by the system, since
.
The main advantages of CDMA are privacy and noise immunity.
1. Privacy. If the code of a user group is known only to authorized members of that group, CDMA ensures communication confidentiality, since unauthorized persons who do not have the code cannot access the transmitted information.
2. Noise immunity. Modulating a signal with a sequence on transmission requires it to be re-modulated with the same sequence on reception (which is equivalent to demodulating the signal), resulting in the restoration of the original narrowband signal. If the interference is narrowband, then the demodulating direct sequence, when received, acts on it as a modulating one, i.e. “smeares” its spectrum over a wide band W ss, as a result of which only 1/G of the interference power falls into the narrow band of the signal W s, so that the narrow-band interference will be attenuated G times, where G=W ss /W s (W ss is the extended spectrum band, W s is the original spectrum). If the interference is broadband - with a band of the order of W ss or wider, then demodulation will not change the width of its spectrum, and the interference will fall into the signal band weakened as many times as its band is wider than the band W s of the original signal.
1.4 Standard DECT for communication
DECT systems and devices are distributed in more than 30 countries on all continents of the planet. In fact, DECT is a set of specifications that define radio interfaces for various types of communication networks and equipment. DECT combines the requirements, protocols and messages that ensure the interaction of communication networks and terminal equipment. The organization of the networks themselves and the arrangement of equipment are not included in the standard. The most important task of DECT is to ensure the compatibility of equipment from different manufacturers.
Initially, DECT was focused on telephony - radio extenders, wireless office PBXs, providing radio access to public telephone networks. But the standard turned out to be so successful that it began to be used in data transmission systems, wireless subscriber access to public communication networks. DECT has found use in multimedia applications and home radio networks, for Internet access and facsimile.
What is a DECT radio interface? In the 20 MHz band (1880 - 1900 MHz), 10 carrier frequencies are allocated with an interval of 1.728 MHz. DECT uses time division access technology - TDMA. The time spectrum is divided into separate frames of 10ms (Fig. 1.13.). Each frame is divided into 24 time slots: 12 slots for receiving (from the point of view of a wearable terminal) and 12 for transmitting. Thus, on each of the 10 carrier frequencies, 12 duplex channels are formed - a total of 120. Duplex is provided by time division (with an interval of 5 ms) of reception / transmission. The 32-bit sequence “101010…” is used for synchronization. DECT provides speech compression in accordance with adaptive differential PCM technology at a rate of 32 Kbps. Therefore, the information part of each slot is 320 bits. When transmitting data, it is possible to combine time slots. The radio path uses Gaussian frequency modulation.
Base stations (BS) and subscriber terminals (AT) DECT constantly scan all available channels (up to 120). In this case, the signal strength on each of the channels is measured, which is entered into the RSSI list. If the link is busy or noisy, the RSSI is high. The BS selects the channel with the lowest RSSI value for continuous transmission of service information about subscriber calls, station ID, system capabilities, etc. This information plays the role of reference signals for the AT - according to them, subscriber devices determine whether there is a right to access a particular BS, whether it provides the services required by the subscriber, whether there is free capacity in the system and choose the BS with the highest quality signal.
In DECT, the communication channel always defines AT. When a connection is requested from the BS (incoming connection), the AT receives a notification and selects a radio channel. The service information is transmitted by the base station and analyzed by the user terminal constantly, therefore, the AT is always synchronized with the closest available BS. When establishing a new connection, the AT selects the channel with the lowest RSSI value - this ensures that the new connection occurs on the cleanest channel available. This procedure of dynamic channel allocation allows you to get rid of frequency planning - the most important feature of DECT.
Rice. 1.13. DECT spectrum.
Since the AT constantly, even when a connection is established, analyzes the available channels, they can be dynamically switched during a communication session. Such switching is possible both to another channel of the same BS, and to another BS. This procedure is called "handover". In handover, the AT establishes a new connection, and for some time communication is maintained on both channels. Then the best one is chosen. Automatic switching between channels of different BS occurs almost imperceptibly for the user and is completely initiated by the AT.
It is essential that the signal power in the radio path of the DECT equipment is very low - from 10 to 250 mW. Moreover, 10 mW is practically the nominal power for microcellular systems with a cell radius of 30-50 m inside the building and up to 300-400 m in open space. Transmitters with a power of up to 250 mW are used for radio coverage of large areas (up to 5 km).
With a power of 10 mW, it is possible to locate base stations at a distance of 25 m. As a result, a record density of simultaneous connections (about 100 thousand subscribers) is achieved, provided that the BS is located according to the hexagon scheme in the same plane (on the same floor).
To protect against unauthorized access in DECT systems, the BS and AT authentication procedure is used. AT is registered in the system or at individual base stations to which it has access. Authentication occurs with each connection: the BS sends a "request" to the AT - a random number (64 bits). Based on this number and the authentication key, the AT and the BS calculate an authentication response (32 bits) using a given algorithm, which the AT sends to the BS. The BS compares the calculated response with the received one and, if they match, allows the AT to connect. DECT has a standard DSAA authentication algorithm.
As a rule, the authentication key is calculated based on the subscriber's authentication key UAK with a length of 128 bits or the authentication code AC (16 - 32 bits). UAK is stored in AT ROM or in a DAM card, which is similar to a SIM card. AC can also be manually written to the AT ROM or entered during authentication. Together with UAK, a personal user identifier UPI is also used, 16-32 bits long, entered only manually. In addition, unauthorized retrieval of information in systems with TDMA is extremely complex and is available only to specialists.
1.5 Standards Bluetooth , Wi - fi (802.11, 802.16)
The Bluetooth specification describes a packet method for transmitting information with time multiplexing. Radio traffic occurs in the frequency band 2400-2483.5 MHz. The radio path uses the method of spreading the spectrum by means of frequency hops and two-level Gaussian frequency modulation.
The frequency hopping method implies that the entire frequency band allocated for transmission is divided into a certain number of sub-channels with a width of 1 MHz each. The channel is a pseudo-random sequence of hops over 79 or 23 RF subchannels. Each channel is divided into 625 µs time segments, with each segment corresponding to a specific sub-channel. The transmitter uses only one subchannel at a time. Jumps occur synchronously in the transmitter and receiver in a pre-fixed pseudo-random sequence. Up to 1600 frequency jumps can occur per second. This method provides confidentiality and some noise immunity of transmissions. Noise immunity is ensured by the fact that if the transmitted packet could not be received on any subchannel, then the receiver reports this and the packet transmission is repeated on one of the following subchannels, already at a different frequency.
The Bluetooth protocol supports both point-to-point and point-to-multipoint connections. Two or more devices using the same channel form a piconet. One of the devices works as a master, and the rest as slaves. There can be up to seven active slaves in one piconet, with the remaining slaves in the "parked" state, remaining synchronized with the master. Interacting piconets form a "distributed network".
Each piconet has only one master device, but slave devices can be part of different piconets. In addition, the main device of one piconet can be a slave in another (Fig. 1.14.). Piconets are not synchronized with each other in time and frequency - each of them uses its own sequence of frequency hops. In the same piconet, all devices are synchronized in time and frequency. The hop sequence is unique to each piconet and is determined by the address of its primary device. The cycle length of the pseudo-random sequence is 2 27 elements.
Rice. 1. 14. A piconet with one slave device a), several b) and a distributed network c).
The Bluetooth standard provides for duplex transmission based on time division. The master device transmits packets in odd time segments, and the slave device in even ones (Fig. 1.15.). Packets, depending on their length, can take up to five time segments. In this case, the channel frequency does not change until the end of the packet transmission (Fig. 1.16.).
Rice. 1. 15. Timing diagram of the channel.
The Bluetooth protocol can support an asynchronous data channel, up to three synchronous (constant rate) voice channels, or a simultaneous asynchronous data and synchronous voice channel.
With a synchronous connection, the master device reserves time segments following so-called synchronous intervals. Even if a packet is received with an error, it is not retransmitted during a synchronous connection. Asynchronous communication uses time segments that are not reserved for a synchronous connection. If no address is specified in the address field of an asynchronous packet, the packet is considered "broadcast" - it can be read by all devices. An asynchronous connection allows retransmission of packets received with errors.
Rice. 1. 16. Transmission of packets of various lengths.
A standard Bluetooth packet contains a 72-bit access code, a 54-bit header, and an information field of no more than 2745 bits. The access code identifies packets belonging to the same piconet and is also used for synchronization and query procedures. It includes a preamble (4 bits), a sync word (64 bits) and a trailer - 4 bits of the checksum.
The header contains communication control information and consists of six fields: AM_ADDR – 3-bit address of the active element; TYPE - 4-bit data type code; FLOW - 1 bit of data flow control, indicating the readiness of the device to receive; ARQN - 1 bit of confirmation of correct reception; SEQN - 1 bit used to determine the sequence of packets; HEC - 8-bit checksum.
The information field, depending on the type of packets, may contain either voice fields or data fields, or both types of fields at the same time.
Consider the IEEE 802.11 standard used in local data networks - i.e. in Ethernet-like wireless networks, which are fundamentally asynchronous in nature.
IEEE 802.11 considers two lower levels of the open systems interaction model - physical (the method of working with the transmission medium, speed and modulation methods are determined) and the data link level, and at the last level the lower sublevel is considered - MAC, i.e. channel access control (transmission medium). IEEE 802.11 uses the 2.400 - 2.4835 GHz band with a bandwidth of 83.5 MHz and provides for packet transmission with 48-bit address packets.
The standard provides for two main ways to organize a local network - according to the principle "each with each" (communication is established directly between two stations, all devices must be in the radio visibility zone, no administration occurs) and in the form of a structured network (an additional device appears - an access point, as a rule, stationary and operating on a fixed channel; communication between devices occurs only through access points, through which access to external wired networks is also possible).
As a rule, control functions are distributed among all devices of the IEEE 802.11 network - DCF mode. However, for structured networks, PCF mode is possible, when control is transferred to one specific access point. The need for PCF arises when transmitting delay-sensitive information. After all, IEEE 802.11 networks operate on the principle of competitive access to the channel - there are no priorities. In order to set them if necessary, the PCF mode was introduced. However, operation in this mode can occur only at certain periodically repeating intervals.
For the security of data transmission at the MAC level, station authentication and encryption of transmitted data are provided.
IEEE 802.11 implements Carrier Sense Multiple Access with Collision Detection. The station can start transmission only if the channel is free. If stations detect that multiple stations are trying to operate on the same channel, they all stop transmitting and try to resume transmission after a random amount of time. Thus, even when transmitting, the device must monitor the channel, i.e. work for the reception.
Before the first attempt to access the channel, the device loads the duration of a random waiting interval into a special counter. Its value is decremented at the specified frequency while the channel is idle. As soon as the counter is reset, the device can occupy the channel. If the channel is occupied by another device before the counter is reset, the counting stops, keeping the reached value. On the next attempt, the count starts from the saved value. As a result, those who did not have time last time get more chances to occupy the channel next time. This is not the case with wired Ethernet networks.
The packets through which the transmission takes place are actually formed at the MAC layer; at the physical layer, a physical layer header (PLCP) is added to them, consisting of a preamble and the PLCP header itself. There are three types of MAC layer packets - data packets, control packets and control packets. Their structure is the same. Each packet includes a MAC header, an information field, and a checksum.
Fixed access broadband urban wireless data networks use the IEEE 802.16 standard.
The IEEE 802.16 standard describes the operation in the range of 10 - 66 GHz systems with a point-to-multipoint architecture (from the center to many). This is a bi-directional system, i.e. downstream (from the base station to subscribers) and upstream (to the base station) flows are provided. In this case, channels are assumed to be broadband (about 25 MHz), and transmission rates are high (for example, 120 Mbps).
The IEEE 802.16 standard provides for a single carrier modulation scheme (in each frequency channel) and allows three types of quadrature amplitude modulation: four-position QPSK and 16-position 16-QAM (mandatory for all devices), as well as 64-QAM (optional).
Data at the physical layer is transmitted as a continuous sequence of frames. Each frame has a fixed duration - 0.5; 1 and 2 ms. The frame consists of a preamble (32 QPSK-character sync sequence), a control section, and a sequence of data packets. Since the system defined by the IEEE 802.16 standard is bidirectional, a duplex mechanism is required. It provides for both frequency and time separation of the uplink and downlink. In temporal channel duplexing, a frame is divided into downlink and uplink subframes separated by a special interval. In frequency duplexing, the uplink and downlink are each broadcast on their own carrier.
The IEEE 802.16 MAC layer is subdivided into three sub-layers - a service translation sub-layer (services are different applications), a main sub-layer, and a security sub-layer. At the protection sublayer, authentication mechanisms and data encryption are implemented. At the service transformation sublayer, the data streams of the upper layer protocols are transformed for data transmission over IEEE 802.16 networks. For each type of upper-level application, the standard provides a different transformation mechanism. At the main MAC sublayer, data packets are formed, which are then transmitted to the physical layer and broadcast through the communication channel. A MAC packet includes a header and a data field, which may be followed by a checksum.
A key point in the IEEE 802.16 standard is the concept of a service flow and the related concepts of "connection" and "connection identifier" (CID). A service flow in the IEEE 802.16 standard is a data flow associated with a specific application. In this context, a connection is the establishment of a logical connection at the MAC layers on the transmitting and receiving side for the transmission of a service flow. Each connection is assigned a 16-bit CID, which is uniquely associated with the type and characteristics of the connection. The service flow is characterized by a set of requirements for the information transmission channel (to the symbol delay time, the level of delay fluctuations and the guaranteed bandwidth). Each service flow is assigned an SFID identifier, based on which the BS determines the necessary parameters of a specific connection associated with this service flow.
The basic principle of providing channel access in the IEEE 802.16 standard is access on demand. No AS (subscriber station) can transmit anything, except for requests for registration and provision of a channel, until the BS allows it to do so, i.e. will allocate a time slot in the uplink and indicate its location. The SS can either request a certain bandwidth size in the channel or request a change in the channel resource already provided to it. The IEEE 802.16 standard provides for two modes of granting access - for each individual connection and for all connections of a particular AS. It is obvious that the first mechanism provides more flexibility, but the second one significantly reduces the volume of overhead messages and requires less hardware performance.
2. Systems of complex signals for telecommunication systems
2.1 Signal spectra
The signal spectrum s(t) is determined by the Fourier transform
In general, the spectrum is a complex function of the frequency ω. The spectrum can be represented as
,
where |S(ω)| is the amplitude, and φ(ω) is the phase spectrum of the signal s(t).
The signal spectrum has the following properties:
1. Linearity: if there is a set of signals s 1 (t), s 2 (t), ..., and s 1 (t) S 1 (ω), s 2 (t) S 2 (ω), ..., then the sum of the signals Fourier transform as follows:
where a i are arbitrary numerical coefficients.
2. If the signal s(t) corresponds to the spectrum S(ω), then the same signal shifted by t 0 corresponds to the spectrum S(ω) multiplied by e - jωt 0 s(t-t 0)S(ω)e - jωt 0 .
3. If s(t)S(ω), then
4. If s(t)S(ω) and f(t)=ds/dt, then f(t)F(ω)=jωS(ω).
5. If s(t)S(ω) and g(t)=∫s(t)dt, then g(t)G(ω)=S(ω)/jω.
6. If u(t)U(ω), v(t)V(ω) and s(t)=u(t)v(t), then
.
The signal is found over the spectrum using the inverse Fourier transform
.
Consider the spectra of some signals.
1. Rectangular pulse.
Fig.2.1. Spectrum of a rectangular pulse.
2. Gaussian impulse.
s(t)=Uexp(-βt 2)
Fig.2.2. Gaussian pulse spectrum.
3. Smoothed impulse
Using numerical integration, we find the spectrum S(ω).
S(0)=2.052 S(6)=-0.056
S(1)=1.66 S(7)=0.057
S(2)=0.803 S(8)=0.072
S(3)=0.06 S(9)=0.033
S(4)=-0.259 S(10)=-0.0072
S(5)=-0.221 S(ω)=S(-ω)
Rice. 2.3. The spectrum of the smoothed pulse.
2.2 Correlation properties of signals
To compare signals shifted in time, the autocorrelation function (ACF) of the signal is introduced. It quantitatively determines the degree of difference between the signal u(t) and its time-shifted copy u(t - τ) and is equal to the scalar product of the signal and the copy:
It is directly seen that at τ=0 the autocorrelation function becomes equal to the signal energy: B u (0)=E u .
The autocorrelation function is even: B u (τ)=B u (-τ).
For any value of the time shift τ, the ACF module does not exceed the signal energy |В u (τ)|≤B u (0)=E u .
The ACF is related to the signal spectrum by the following relation:
.
The reverse is also true:
.
For a discrete signal, the ACF is defined as follows:
and has the following properties.
Discrete ACF is even: B u (n)=B u (-n).
At zero shift, the ACF determines the energy of a discrete signal:
.
Sometimes a cross-correlation function (CCF) of signals is introduced, which describes not only the shift of the signals relative to each other in time, but also the difference in the shape of the signals.
VKF is defined as follows
for continuous signals and
for discrete signals.
Consider the ACF of some signals.
1. Sequence of rectangular pulses
Rice. 2.4. ACF of a sequence of rectangular pulses.
2. 7-position Barker signal
B u (0)=7, B u (1)= B u (-1)=0, B u (2)= B u (-2)=-1, B u (3)= B u (-3 )=0, B u (4)= B u (-4)=-1, B u (5)= B u (-5)=0, B u (6)= B u (-6)=-1 , B u (7)= B u (-7)=0.
Rice. 2.5. ACF of a 7-position Barker signal.
3. 8-Position Walsh Functions
2nd order Walsh function
B u (0)=8, B u (1)= B u (-1)=3, B u (2)= B u (-2)=-2, B u (3)= B u (-3 )=-3, B u (4)= B u (-4)=-4, B u (5)= B u (-5)=-1, B u (6)= B u (-6)= 2, B u (7)= B u (-7)=1, B u (8)= B u (-8)=0.
Rice. 2.6. ACF of the Walsh function of the 2nd order.
7th order Walsh function
B u (0)=8, B u (1)= B u (-1)=-7, B u (2)= B u (-2)=6, B u (3)= B u (-3 )=-5, B u (4)= B u (-4)=4, B u (5)= B u (-5)=-3, B u (6)= B u (-6)=2 , B u (7)= B u (-7)=-1, B u (8)= B u (-8)=0.
Rice. 2.7. ACF of the Walsh function of the 7th order.
2.3 Types of complex signals
A signal is a physical process that can carry useful information and propagate along a communication line. Under the signal s(t) we mean the function of time, which reflects the physical process, which has a finite duration T.
Signals whose base B, which is equal to the product of the signal duration T and the width of its spectrum, is close to unity, are called "simple" or "ordinary". These signals can be distinguished by frequency, time (delay) and phase.
Complex, multidimensional, noise-like signals are formed according to a complex law. During the duration of the signal T, it undergoes additional manipulation (or modulation) in frequency or phase. Additional amplitude modulation is rarely used. Due to additional modulation, the spectrum of the signal Δf (while maintaining its duration T) expands. Therefore, for such a signal B=T Δf>>1.
Under certain laws of formation of a complex signal, its spectrum turns out to be continuous and practically uniform, i.e. close to the bandwidth-limited noise spectrum. In this case, the signal autocorrelation function has one main spike, the width of which is determined not by the signal duration, but by the width of its spectrum, i.e. has a form similar to the bandlimited noise autocorrelation function. In this regard, such complex signals are called noise-like.
Noise-like signals have been used in broadband communication systems, as: they provide high noise immunity of communication systems; allow organizing the simultaneous operation of many subscribers in a common frequency band; allow you to successfully deal with multipath propagation of radio waves by separating the beams; provide better use of the frequency spectrum in a limited area compared to narrowband communication systems.
A large number of different noise-like signals (NLS) are known. However, the following main NPSs are distinguished: frequency-modulated signals; multi-frequency signals; phase-shift keyed signals; discrete frequency signals; discrete composite frequency signals.
Frequency modulated signals (FM) are continuous signals, the frequency of which changes according to a given law (Fig. 2.8.).
Rice. 2.8. FM signal.
In communication systems, it is necessary to have many signals. In this case, the need for a quick change of signals and switching of the formation and processing equipment leads to the fact that the law of frequency change becomes discrete. In this case, FM signals are transferred to HF signals.
Multi-frequency (MF) signals are the sum of N harmonics u 1 (t) ... u N (t), the amplitudes and phases of which are determined in accordance with the laws of signal formation (Fig. 2.9.).
Rice. 2.9. MF signal.
MF signals are continuous and it is difficult to adapt digital technology methods for their formation and processing.
Phase-shift keyed (PM) signals are a sequence of radio pulses, the phases of which change according to a given law (Fig. 2.10., a). Usually the phase takes two values (0 or π). In this case, the RF FM signal corresponds to the video-FM signal (Fig. 2.10., b).
Rice. 2.10. FM signal.
FM signals are very common, because. they allow extensive use of digital methods in shaping and processing, and it is possible to implement such signals with relatively large bases.
Discrete frequency (DF) signals represent a sequence of radio pulses (Fig. 2.11.), The carrier frequencies of which change according to a given law.
Rice. 2.11. DC signal.
Discrete composite frequency (DSF) signals are DC signals in which each pulse is replaced by a noise-like signal.
On fig. 2.12. a video-frequency FM signal is shown, the individual parts of which are transmitted at different carrier frequencies.
Rice. 2.12. DCH signal.
2.4 Derived signal systems
A derivative signal is a signal that results from the multiplication of two signals. In the case of PM signals, the multiplication must be carried out element by element or, as it is more commonly called, symbol by symbol. A system composed of derivative signals is called a derivative. Among derivative systems, systems constructed as follows are of particular importance. As a basis, some system of signals is used, the correlation properties of which do not fully meet the requirements for CF, but which has certain advantages in terms of ease of formation and processing. Such a system is called the original. Then a signal is selected that has certain properties. Such a signal is called generating. Multiplying the generating signal by each signal of the original system, we obtain a derivative system. The generating signal should be chosen so that the derived system is really better than the original one, i.e. to have good correlation properties. The complex envelope of the derivative signal S μ m (t) is equal to the product of the complex envelopes of the original signals U m (t) and the generating signal V μ (t), i.e. S μ m (t)= U m (t) V μ (t). If the indices change within m=1..M, μ=1..H, then the volume of the derivative signal system is L=MH.
The choice of generating signals is determined by a number of factors, including the original system. If the original system signals are broadband, then the producing signal may be broadband and have small levels of side peaks of the uncertainty function close to the rms value. If the signals of the original system are narrow-band, then it suffices to fulfill the inequality F V >>F U (F V is the width of the spectrum of generating signals, F U is the width of the spectrum of the original signals) and the requirement that the side peaks of the ACF be small.
Let us take the Walsh system as the initial one. In this case, the generating signals should be broadband and have good ACFs. In addition, the generating signal must have the same number of elements as the original signals, i.e. N=2 k elements, where k is an integer. These conditions are generally satisfied by non-linear sequences. Since the main requirement is the smallness of the side peaks of the ACF, the best signals with the number of elements N = 16, 32, 64 were selected in the class of nonlinear sequences. These signals are shown in fig. 2.13. On fig. 2.13. the values of the number of blocks μ for each generating signal are also indicated. They are close to the optimal value μ 0 =(N+1)/2. This is a necessary condition for obtaining a good ACF with small side peaks.
Rice. 2.13. Producing FM signals.
The volume of the derivative system is equal to the volume of the Walsh system N. Derivative systems have better correlation properties than Walsh systems.
3. Modulation of complex signals
3.1 Geometric representation of signals
Consider a geometric or vector representation of signals. We define an N-dimensional orthogonal space as a space defined by a set of N linearly independent functions (ψ j (t)), called basis functions. Any function of this space can be expressed in terms of a linear combination of basis functions that must satisfy the condition
,
where the operator is called the Kronecker symbol. For non-zero constants K j the space is called orthogonal. If the basis functions are normalized so that all K j =1, the space is called orthonormal. The main orthogonality condition can be formulated as follows: each function ψ j (t) of the set of basis functions must be independent of the other functions of the set. Each function ψ j (t) must not interfere with other functions during the detection process. From a geometric point of view, all functions ψ j (t) are mutually perpendicular.
In orthogonal signal space, the Euclidean distance measure used in the detection process is most easily defined. If the waves carrying the signals do not form such a space, they can be converted into a linear combination of orthogonal signals. It can be shown that an arbitrary finite set of signals (s i (t)) (i=1…M), where each element of the set is physically realizable and has duration T, can be expressed as a linear combination of N orthogonal signals ψ 1 (t), ψ 2 ( t), …, ψ N (t), where NM, so that
where
The form of the basis (ψ j (t)) is not specified; these signals are chosen for convenience and depend on the waveform of the signal transmission. The set of such waves (s i (t)) can be considered as a set of vectors (s i )=(a i 1 , a i 2 , …,a iN ). The mutual orientation of the signal vectors describes the relationship between the signals (with respect to their phases or frequencies), and the amplitude of each set vector (s i ) is a measure of the signal energy transferred during symbol transmission time. In general, after choosing a set of N orthogonal functions, each of the transmitted signals s i (t) is completely determined by the vector of its coefficients s i =(a i 1 , a i 2 , …,a iN) i=1…M.
3.2 Phase-shift keying methods (PM2, PM4, RPM)
Phase Shift Keying (PSK) was developed early in the development of the deep space exploration program; now the PSK scheme is widely used in commercial and military communications systems. The signal in PSK modulation has the following form:
Here, the phase φ i (t) can take M discrete values, usually defined as follows:
The simplest example of phase shift keying is binary phase shift keying (PSK2). Parameter E is symbol energy, T is symbol transmission time. The operation of the modulation scheme is to shift the phase of the modulated signal s i (t) by one of two values, zero or π (180 0). A typical view of the PM2 signal is shown in fig. 3.1.a), where characteristic sharp phase changes are clearly visible during the transition between symbols; if the modulated data stream consists of alternating zeros and ones, such abrupt changes will occur at each transition. The modulated signal can be represented as a vector on a graph in polar coordinates; the length of the vector corresponds to the amplitude of the signal, and its orientation in the general M-ary case corresponds to the phase of the signal relative to other M - 1 signals of the set. When modulating FM2 (Fig. 3.1.b)) the vector representation gives two antiphase (180 0) vectors. Sets of signals that can be represented by such out-of-phase vectors are called antipodes.
Rice. 3.1. Binary phase shift keying.
Another example of phase shift keying is PM4 modulation (M=4). With PM4 modulation, the parameter E is the energy of two symbols, the time T is the transmission time of two symbols. The phase of the modulated signal takes one of four possible values: 0, π/2, π, 3π/2. In vector representation, the FM4 signal has the form shown in Fig. 3.2.
Rice. 3.2. FM4 signal in vector representation.
Let's consider another type of phase shift keying - relative phase shift keying (RPK) or differential phase shift keying (DPSK). The name differential phase shift keying requires some explanation, since two different aspects of the modulation/demodulation process are associated with the word "differential": the encoding procedure and the detection procedure. The term "differential encoding" is used when the encoding of binary characters is determined not by their value (ie, zero or one), but by whether the character is the same as or different from the previous one. The term "differential coherent detection" of signals in differential PSK modulation (it is in this meaning that the name DPSK is usually used) is associated with a detection scheme, which is often referred to as non-coherent schemes, since it does not require phase matching with the received carrier.
In non-coherent systems, no attempt is made to determine the actual value of the phase of the incoming signal. Therefore, if the transmitted signal is of the form
the received signal can be described as follows.
Here α is an arbitrary constant, usually assumed to be a random variable uniformly distributed between zero and 2π, and n(t) is noise.
For coherent detection, matched filters are used; for incoherent detection, this is impossible, since in this case the output of the matched filter will depend on the unknown angle α. But if we assume that α changes slowly with respect to an interval of two periods (2T), then the phase difference between two successive signals will not depend on α.
The basis of differential coherent signal detection in DPSK modulation is as follows. The demodulation process may use the carrier phase of the previous symbol interval as a phase reference. Its use requires differential coding of the message sequence in the transmitter, since the information is encoded by the phase difference between two successive pulses. To transmit the i-th message (i=1,2,…,M), the phase of the current signal must be shifted by φ i =2πi/M radians relative to the phase of the previous signal. In general, the detector calculates the coordinates of an incoming signal by determining its correlation with locally generated signals cosω 0 t and sinω 0 t. Then, as shown in Fig. 3.3., the detector measures the angle between the vector of the current received signal and the vector of the previous signal.
Rice. 3.3. Signal space for the DPSK scheme.
The DPSK scheme is less efficient than PSK, because in the first case, due to the correlation between signals, errors tend to propagate (to adjacent symbol times). It is worth remembering that the PSK and DPSK schemes differ in that in the first case the received signal is compared with an ideal reference signal, and in the second case two noisy signals are compared. Note that DPSK modulation produces twice as much noise as PSK modulation. Therefore, when using DPSK, you should expect twice the probability of error than in the case of PSK. The advantage of the DPSK scheme is the lower complexity of the system.
3.3 Modulation with minimal frequency shift.
One non-discontinuous modulation scheme is minimum frequency shift keying (MSK). MSK can be thought of as a special case of frequency shift keying without phase discontinuity. The MSK signal can be represented as follows.
Here, f 0 is the carrier frequency, d k =±1 represents bipolar data that is transmitted at a rate of R=1/T, and x k is the phase constant for the kth bit interval. Note that when d k =1 the transmitted frequency is f 0 +1/4T, and when d k =-1 is f 0 -1/4T. During each T-second data transmission interval, the value of x k is constant, i.e. x k =0 or π, which is dictated by the requirement of signal phase continuity at times t=kT. This requirement imposes a restriction on the phase, which can be represented by the following recursive relation for x k .
The equation for s(t) can be rewritten in the quadrature representation.
The in-phase component is denoted as ak cos(πt/2T)cos2πf 0 t, where cos2πf 0 t is the carrier, cos(πt/2T) is the sinusoidal symbol weighting, and k is the data-dependent term. Similarly, the quadrature component is b k sin(πt/2T)sin2πf 0 t, where sin2πf 0 t is the carrier quadrature term, sin(πt/2T) is the same sinusoidal symbol weighting, and b k is the information dependent term. It may seem that the values a k and b k can change their value every T seconds. However, due to the requirement of phase continuity, the value of a k can change only when the function cos(πt/2T) passes through zero, and b k only when the sin(πt/2T) passes through zero. Therefore, symbol weighting in an in-phase or quadrature channel is a sinusoidal pulse with a period of 2T and a variable sign. The in-phase and quadrature components are shifted relative to each other by T seconds.
The expression for s(t) can be rewritten in a different form.
Here d I (t) and d Q (t) have the same meaning of in-phase and quadrature data streams. An MSK scheme written in this form is sometimes referred to as a precoded MSK. The graphical representation of s(t) is given in fig. 3.4. On fig. 3.4. a) and c) shows the sinusoidal weighting of the in-phase and quadrature channel pulses, here multiplication by the sinusoid gives smoother phase transitions than in the original data representation. On fig. 3.4. b) and d) shows the modulation of the orthogonal components cos2πf 0 t and sin2πf 0 t by sinusoidal data streams. On fig. 3.4. e) the summation of the orthogonal components shown in fig. 3.4. b) and d). From the expression for s(t) and Fig.3.4. we can conclude the following: 1) the signal s(t) has a constant envelope; 2) RF carrier phase is continuous at bit transitions; 3) the signal s(t) can be considered as an FSK modulated signal with transmission frequencies f 0 +1/4T and f 0 -1/4T. Thus, the minimum tone spacing required for MSK modulation can be written as:
which is equal to half the bit rate. Note that the tone spacing required for MSK is half (1/T) the spacing required for incoherent detection of FSK modulated signals. This is because the carrier phase is known and continuous, which allows coherent demodulation of the signal.
Rice. 3.4. Minimal shift keying: a) modified in-phase bit stream; b) the product of the in-phase bit stream and the carrier; c) modified quadrature bitstream; d) the product of the quadrature bitstream and the carrier; e) MSK signal.
3.4 Quadrature modulation and its characteristics ( Q PSK , QAM )
Consider quadrature phase shift keying (QPSK). The initial data stream d k (t)=d 0 , d 1 , d 2 ,… consists of bipolar pulses, i.e. d k take the values +1 or -1 (Fig. 3.5.a)), representing a binary one and a binary zero. This pulse stream is divided into in-phase stream d I (t) and quadrature - d Q (t), as shown in fig. 3.5.b).
d I (t)=d 0 , d 2 , d 4 ,… (even bits)
d Q (t)=d 1 , d 3 , d 5 ,… (odd bits)
A convenient orthogonal implementation of the QPSK signal can be obtained using amplitude modulation of the in-phase and quadrature flows on the sine and cosine functions of the carrier.
Using trigonometric identities, s(t) can be represented as follows: s(t)=cos(2πf 0 t+θ(t)). The QPSK modulator shown in Fig. 3.5.c), uses the sum of the sinusoidal and cosine terms. The stream of pulses d I (t) is used for amplitude modulation (with an amplitude of +1 or -1) cosine. This is equivalent to shifting the phase of the cosine wave by 0 or π; therefore, the result is a BPSK signal. Similarly, the pulse stream d Q (t) modulates a sinusoid, which gives a BPSK signal orthogonal to the previous one. When these two orthogonal carrier components are summed, a QPSK signal is obtained. The value of θ(t) will correspond to one of four possible combinations of d I (t) and d Q (t) in the expression for s(t): θ(t)=0 0 , ±90 0 or 180 0 ; the resulting signal vectors are shown in the signal space in fig. 3.6. Since cos(2πf 0 t) and sin(2πf 0 t) are orthogonal, the two BPSK signals can be detected separately. QPSK has a number of advantages over BPSK: with QPSK modulation, one pulse transmits two bits, then the data rate is doubled, or at the same data rate as in the BPSK scheme, half the bandwidth is used; as well as increased noise immunity, tk. pulses are twice as long and therefore more powerful than BPSK pulses.
Rice. 3.5. QPSK modulation.
Rice. 3.6. Signal space for the QPSK scheme.
Quadrature amplitude modulation (KAM, QAM) can be considered a logical extension of QPSK, since the QAM signal also consists of two independent amplitude modulated carriers.
With quadrature amplitude modulation, both the phase and amplitude of the signal change, which makes it possible to increase the number of encoded bits and at the same time significantly increase noise immunity. The quadrature representation of signals is a convenient and fairly universal means of describing them. The quadrature representation consists in expressing the oscillation as a linear combination of two orthogonal components - sinusoidal and cosine (in-phase and quadrature):
s(t)=A(t)cos(ωt + φ(t))=x(t)sinωt + y(t)cosωt, where
x(t)=A(t)(-sinφ(t)),y(t)=A(t)cosφ(t)
Such discrete modulation (keying) is carried out over two channels, on carriers shifted by 90 0 relative to each other, i.e. in quadrature (hence the name).
Let us explain the operation of the quadrature circuit using the example of the formation of four-phase FM signals (FM-4) (Fig. 3.7).
Rice. 3.7. Scheme of quadrature modulator.
Rice. 3.8. Hexadecimal signal space (QAM-16).
The initial sequence of binary symbols of duration T is divided by the shift register into odd pulses y, which are fed into the quadrature channel (cosωt), and even pulses x, which enter the in-phase channel (sinωt). Both sequences of pulses are fed to the inputs of the corresponding shapers of manipulated pulses, at the outputs of which sequences of bipolar pulses x(t) and y(t) with an amplitude of ±U m and a duration of 2T are formed. Pulses x(t) and y(t) arrive at the inputs of channel multipliers, at the outputs of which two-phase (0, π) FM oscillations are formed. After summing, they form the FM-4 signal.
On fig. 3.8. 2D signal space and a set of hexadecimal QAM-modulated signal vectors depicted by dots arranged in a rectangular array are shown.
From fig. 3.8. it can be seen that the distance between the signal vectors in the signal space with QAM is greater than with QPSK, therefore, QAM is more noise-immune compared to QPSK,
3.5 Implementation of quadrature modems
The modem is designed to transmit / receive information via conventional telephone wires. In this sense, the modem acts as an interface between the computer and the telephone network. Its main task is to convert the transmitted information to a form acceptable for transmission over telephone channels, and to convert the received information to a form acceptable to a computer. As you know, a computer is able to process and transmit information in binary code, that is, in the form of a sequence of logical zeros and ones, called bits. A logic one can be assigned a high voltage level, and a logic zero can be assigned a low voltage level. When transmitting information over telephone wires, it is necessary that the characteristics of the transmitted electrical signals (power, spectral composition, etc.) meet the requirements of the receiving equipment of the exchange. One of the main requirements is that the signal spectrum should be in the range from 300 to 3400 Hz, that is, it should have a width of no more than 3100 Hz. In order to satisfy this and many other requirements, the data is subjected to appropriate encoding, which, in fact, is handled by the modem. There are several ways of possible encoding, in which data can be transmitted over subscriber switched channels. These methods differ from each other, both in terms of transmission speed and noise immunity. At the same time, regardless of the encoding method, data is transmitted via subscriber channels only in analog form. This means that a sinusoidal carrier signal is used to transmit information, which is subjected to analog modulation. The use of analog modulation results in a much smaller spectrum at a constant bit rate. Analog modulation is a physical coding method in which information is encoded by changing the amplitude, frequency, and phase of a sinusoidal carrier signal. There are several basic methods of analog modulation: amplitude, frequency and relative phase. Modems use the listed modulation methods, but not individually, but all together. For example, amplitude modulation can be used in conjunction with phase modulation (amplitude-phase modulation). The main problem that arises when transmitting information over subscriber channels is an increase in speed. The speed is limited by the spectral bandwidth of the communication channel. However, there is a way to significantly increase the rate of information transmission without increasing the width of the signal spectrum. The main idea of this method is to use multi-position coding. The sequence of data bits is divided into groups (symbols), each of which is associated with some discrete state of the signal. For example, using 16 different signal states (they can differ from each other, both in amplitude and in phase), it is possible to encode all possible combinations for sequences of 4 bits. Accordingly, 32 discrete states will encode a group of five bits in one state. In practice, to increase the information transfer rate, mainly multi-position amplitude-phase modulation is used with several possible values of the amplitude levels and phase shift of the signal. This type of modulation is called quadrature amplitude modulation (QAM). In the case of QAM, the signal states are conveniently depicted on the signal plane. Each point of the signal plane has two coordinates: the amplitude and phase of the signal and is an encoded combination of a bit sequence. To improve the noise immunity of quadrature amplitude modulation, the so-called Trellis modulation (Trellis Code Modulation, TCM) or, in other words, trellis coding, can be used. With trellis modulation, one extra trellis bit is added to each group of bits transmitted in one discrete signal state. If, for example, the information bits are divided into groups of 4 bits (a total of 16 different combinations are possible), then 16 signal points are placed in the signal plane. Adding a fifth trellis bit will result in 32 possible combinations, i.e. the number of signal points will double. However, not all combinations of bits are allowed, that is, they make sense. This is the idea of trellis coding. The value of the added trellis bit is determined by a special algorithm. The added trellis bit is calculated by a special encoder. On the receiving modem, a special decoder, the so-called Viterbi decoder, is designed to analyze incoming bit sequences. If the received sequences are legal, then the transmission is considered to be error-free and the trellis bit is simply removed. If among the received sequences there are forbidden sequences, then using a special algorithm, the Viterbi decoder finds the most suitable allowed sequence, thus correcting transmission errors. So, the meaning of trellised coding is to increase the noise immunity of the transmission at the cost of a relatively small redundancy. The use of trellis coding makes it possible mainly to protect from entanglement precisely the points adjacent in the signal space, which are just the most susceptible to the possibility of "entangling" under the influence of interference.
4. Characteristics of signal reception in telecommunication systems
4.1 Discrimination error probabilities M known signals
Signal detection in radio electronics is understood as an analysis of the received oscillation y(t), culminating in a decision on the presence or absence of some useful component in it, which is called a signal. Distinguishing M signals is defined as an analysis of the received fluctuation y(t), ending with a decision about which of the M signals belonging to the pre-specified set S(s 0 (t), s 1 (t), ..., s M -1 ( t)) is present in y(t). Signal detection is a special case of distinguishing two signals, one of which is equal to zero over the entire observation interval.
Let the observed fluctuation y(t) be the realization of a random process that has a distribution W y , i.e. n-dimensional probability density (PW) W(y) [or PW functional W(y(t))] belonging to one of M non-overlapping classes W i (W i ∩ W k =Ø, i≠k, i, k= 0, 1, …, M-1). It is necessary, after observing the implementation of y(t), to decide which of the classes belongs to W y . The assumption that W y W i is called the hypothesis H i: W y W i . Decisions that are the result of testing hypotheses will be denoted , where i(0, 1, …, M-1) is the number of the hypothesis, the truth of which is declared by the decision. The analyzed oscillation y(t) is the result of the interaction of the signal s i (t) present in it with an interfering random process (interference, noise) x(t): y(t)=F. Which of the M possible signals is present in y(t) determines the PV of the ensemble to which y(t) belongs, so that each s i (t) corresponds to some class W i of distributions of the ensemble represented by y(t). Thus, the hypotheses H i are treated as assumptions about the presence of the i-th (and only the i-th) signal in y(t). In this case, the solutions , one of which is the result of the discrimination procedure, are statements that the received oscillation contains exactly the i-th signal. Hypotheses H i correspond to classes W i . A hypothesis H i is called simple if the class W i contains one and only one distribution. Any other hypothesis is called complex. M complex hypotheses are called parametric if the classes corresponding to them differ from each other only in the values of a finite number of parameters of the same distribution described by a known law. Otherwise, the hypotheses are called parametric.
Consider distinguishing M deterministic non-zero signals of the same energy. In this case, the rule of maximum likelihood (ML) will be taken as the basis
optimal in the case when the quality criterion is the sum of the conditional error probabilities, or the total error probability with equal a posteriori probabilities of all signals p i =1/M.
For an arbitrary M, a discriminator adhering to the MP rule considers the signal present in y(t) to be the least distant from y(t) in the sense of the Euclidean distance 
or, which is equivalent for the same signal energies, having the maximum correlation with y(t) 
. If we consider signals s 0 (t), s 1 (t), ..., s M -1 (t) as a bundle of vectors located in the M-dimensional space, then in order to reduce the probability of confusing the i-th signal with k -th, it is necessary to “spread” the i-th and k-th vectors as much as possible. Thus, the optimal choice of M deterministic signals is reduced to finding such a configuration of the bundle M of vectors, in which the minimum Euclidean distance between a pair of vectors would be maximum: mind ik =max (i≠k). Since when the energies are equal, i.e. vector lengths
where ρ ik is the correlation coefficient of the i-th and k-th signals, E is the signal energy, then the requirement for the maximum minimum distance is identical to the condition for the minimum of the maximum correlation coefficient in the set of signals S(s 0 (t), s 1 (t), ..., s M -1 (t)). The maximum achievable minimum of the maximum correlation coefficient is established quite easily. Summing up ρ ik over all i and k, we obtain
where the inequality follows from the non-negativity of the square under the integral. In addition, in the sum on the left, M terms for i=k are equal to one, and the remaining M(M-1) is not more than ρ max =max ρ ik (i≠k). Therefore M+M(M-1)ρ max ≥0 and ρ max ≥-1/(M-1).
A configuration of M vectors in which the cosine of the angle between any pair of vectors is -1/(M-1) is called a regular simplex. If these vectors are taken as M signals, then the resulting deterministic ensemble, with the equiprobability of all s i (t), will provide a minimum of the total error probability P osh, which solves the problem of the optimal choice of M signals. When M>>1, the relation -1/(M-1)≈0 is fulfilled, and therefore, with a large number of distinguishable signals, the orthogonal ensemble practically does not lose to the simplex one in the value of P osh.
The sequence of derivation of the exact expression for the error probability of distinguishing M signals with arbitrary ρ ik is as follows. The probability density (PD) of a system of random variables z 0 , z 1 , …, z M -1 is an M-dimensional normal law, for setting which it is enough to know the averages of all z i and their correlation matrix. For the averages, if the hypothesis H l is true, we have . The correlation moment of the i-th and k-th correlations is equal to N 0 Eρ ik /2. After the M-dimensional PV is found, its M-fold integral over the area z l ≥z i , i=0, 1, ..., M-1, allows you to get the probability of a correct solution under the condition that H l is true. The sum of such probabilities, divided by M (taking into account the equiprobability of the signals), will be the total probability of the correct decision P pr, associated with P osh by the obvious equality P osh \u003d 1-P o. single. So, for any equally correlated (equidistant) signals (ρ ik =ρ, i≠k)
In practical calculations, this expression is rarely used because of the need for numerical integration. Its upper estimate is useful, for the derivation of which we assume that the hypothesis H l is true. In this case, an error always occurs when at least one of the events z i >z l , i≠l is true. Its probability P osh l , equal to the probability of combining events z i >z l , i≠l, according to the probability addition theorem,
and, by Boole's inequality, is at most the first sum on the right. Since each term of this sum is the probability of confusing two signals, then for equidistant signals
Here is the signal-to-noise ratio at the output of the filter, consistent with s i (t) under the hypothesis H i , 
- the probability of confusing two signals. With equiprobable signals (p i =1/M) we come to the so-called additive boundary of the total error probability
The use of this expression is justified, on the one hand, by the asymptotic convergence of its right side and P ref as the requirements for the quality of discrimination increase (P ref → 0), and on the other hand, by choosing the required signal energy (the minimum value of q) based on on the right side of the expression, the developer always acts with a certain safety margin, ensuring that the actual probability of error is kept below the figure accepted by him in the calculation.
4.2 Discrimination error probabilities M fluctuating signals
Far from always the observer is a priori aware of the distinguishable signals in detail. More often, he does not know in advance not only the number of the signal present in the analyzed implementation, but also the values of any parameters (amplitude, frequency, phase, etc.) of each of the M possible signals. In this case, the signals themselves are no longer deterministic, since their parameters are not set; the corresponding discrimination problem is called the discrimination of signals with unknown parameters.
Let us consider the solution of this problem using the example of distinguishing signals with random initial phases. Such signals are described by the model
s i (t; φ)=Re( i (t)exp),
where f 0 is the known center frequency; φ is a random initial phase with a priori PV W 0 (φ); (t) =S(t)e jγ (t) is the complex envelope of the signal s(t), which is the realization of s(t; φ) at φ=0: s(t)=s(t; 0); S(t) and γ(t) are known laws of amplitude and angle modulation. The application of the ML rule must be preceded by the calculation of the likelihood function (functional) W(y(t)|H i), i.e. averaging the FP W(y(t)|H i , φ) constructed for deterministic signals with a fixed phase φ over all its possible values, taking into account the a priori SW W 0 (φ). With a uniform phase PV W 0 (φ)=1/(2π), |φ|≤π, taking into account the equality of the energies of all distinguishable signals, W(y(t)|H i) is a modified zero-order Bessel function:
where c is a coefficient containing factors that do not depend on i, and 
is the correlation modulus of the complex envelopes of the received oscillation y(t) and the i-th signal. The monotonicity of the function I 0 ( ) on the positive semiaxis allows us to pass to the sufficient statistics Z i and write the MT rule in the form
Thus, the optimal discriminator of M signals of equal energy with random initial phases must calculate all M values of Z i and, if the maximum of them is Z k , make a decision about the presence of the k-th signal in y(t). This means that the signal contained in the observed oscillation y(t) is the one whose complex envelope has the highest absolute correlation with the complex envelope y(t).
Exact formulas for the error probabilities of distinguishing M arbitrary signals are quite cumbersome even for M=2, however, in applications, ensembles of signals that are orthogonal in the enhanced sense are more common in applications. The latter means that any two noncoinciding signals s i (t; φ i), s k (t; φ k) are orthogonal for any values of the initial phases:
∫s i (t; φ i)s k (t; φ k)dt=0 for any φ i , φ k and i≠k,
or, equivalently, the deterministic complex envelopes of these signals are orthogonal:
.
The orthogonality condition in a strong sense is more stringent than the usual orthogonality requirement that appeared earlier in application to deterministic signals. So, two segments of the cosine curve shifted by an angle of ±π/2, being orthogonal in the usual sense, are not orthogonal when the phase shift changes, i.e. in an enhanced sense. At the same time, signals that do not overlap in time or spectrum are also orthogonal in an enhanced sense.
If we first turn to the distinction between two signals, it is easy to understand that the opposite pair, minimizing P osh in the class of deterministic signals, is unacceptable in problems where the initial phases of the signals are random. Indeed, the only feature by which opposite signals are distinguished is the sign, i.e. the presence or absence of the term π in the initial phase. However, when before entering the discriminator each of the signals acquires a random phase shift, attempts to use the initial phase as a characteristic feature of the signal are meaningless, and the discriminator has to get rid of the non-informative value φ. Thus, we can conclude that in the class M≥2 of signals with random phases, simplex ensembles do not have optimal properties. It is the ensembles of signals that are orthogonal in the enhanced sense that turn out to be optimal: each of these signals causes a response at the output of only one of the filters of the receiving circuit, and therefore the mixing of the i-th signal with the k-th one will occur only if the noise envelope at the output k -th matched filter (SF) will have a value exceeding the value of the envelope of the sum of the signal with noise at the output of the i-th SF. Violation of the orthogonality condition in the enhanced sense will lead to a reaction to the i-th signal at the output not only of the i-th, but also of other SFs, for example, the k-th, resulting in an overshoot of the envelope at the output of the k-th SF, greater than the value of Z i , becomes more likely.
To find the probability of confusing p 01 s 0 (t; φ) with s 1 (t; φ) when distinguishing two signals, it is necessary to integrate the joint PV Z 0 , Z 1 under the hypothesis H 0 W(Z 0 , Z 1 |H 0) over the area Z 1 >Z 0 . For signals that are orthogonal in the amplified sense, the quantities Z 0 and Z 1 are independent, therefore W(Z 0 , Z 1 |H 0)=W(Z 0 |H 0)W(Z 1 |H 0). One-dimensional PVs Z 0 and Z 1 are known: if H 0 Z 0 is true, as the envelope of the sum of the signal with noise, it has a generalized Rayleigh PV; Z 1 as the noise only envelope is a Rayleigh random variable. Multiplying these PVs, after integrating the resulting PV W(Z 0 , Z 1 |H 0) and taking into account the obvious equality p 01 =p 10 for the total error probability of distinguishing two equally probable orthogonal in the enhanced sense signals with random phases, we obtain
Repetition of the reasoning of paragraph 4.2. (for deterministic signals) leads to an additive boundary
which, as a rule, is used to estimate the error probability if the number of equally probable signals, orthogonal in the enhanced sense, is M≥2.
4.3 Calculation of discrimination errors M signals with unknown non-energy parameters
Consider the problem of distinguishing "M" orthogonal signals with an unknown time position in asynchronous communication systems with code division of channels. The decision about the presence of a signal in the channel is made using the maximum likelihood method. Let us find the probability of discrimination error, taking into account noise emissions on the interval of possible time delays of the signals.
Let's assume that there are "M" subscribers of the communication system, each of which uses a different signal. The greatest noise immunity in the transmission of information under such conditions is provided by simplex signals. When M>>1, the noise immunity of such a system of signals practically coincides with the noise immunity of a system of orthogonal signals, for which
Here E kf is the energy of the signal f k . The condition of orthogonality, which can be called "orthogonality at a point", in practice requires a system of common time to organize synchronous communication. In asynchronous systems, signals orthogonal in the amplified sense are used, for which, for all values of τ k and τ m
If R km (τ k , τ m)<0.25 – 0.3, то можно считать ансамбль сигналов практически удовлетворяющим условию ортогональности.
We will consider a system of complex signals (f k (t)), k=1…M, orthogonal with an arbitrary shift. Among complex signals, phase-shift keyed (PM) signals with a complex envelope of the form
where a i is the sequence code, u 0 (t) is the shape of the elementary parcel envelope, Δ is its duration. In the case of a rectangular shape of the elementary parcel envelope, the autocorrelation function (ACF) has the form:
Here R 0 (τ)=(1-|τ|/Δ). In the vicinity of the ACF maximum R(τ)= R 0 (τ)=(1-|τ|/Δ). At the receiver input, after passing through the multipath channel, the useful signal can be written as
δ n is the relative delay of the signal along the beam with number n, τ is the unknown time of arrival, which is within the interval . ε n =A n /A 0 is the relative amplitude of the "n"-th beam, the parameter ν has the meaning of the number of additional propagation beams. Relative delays δ n >Δ, i.e. the beams are separated when processing a complex signal. When ν=0, the signal has the form s(t)=A 0 f(t-τ 0).
Consider the processing algorithm. The receiver receives a mixture
x(t)=s k (t-τ 0k)+η(t), (t),
where s k (t) is one of the possible signals, k=1…M, τ 0 k is the time delay of the signal, η(t) is white Gaussian noise with zero mean and power spectral density N 0 /2. It is necessary to make a decision which of the M possible signals is present at the input of the receiver. Consider a receiver without multipath compensation. The linear part of such a receiver contains M channels in which statistics of the form
The expression for L k (τ k) can be rewritten in a more convenient form for analysis
Here and in the following formulas, the index k is omitted for brevity if the characteristics of one channel are being studied, z 0 2 =2A 0 2 E f /N 0 is the signal-to-noise power ratio, S(τ-τ 0)=∫f(t-τ ) f(t-τ 0)dt/E f – normalized signal function, N(τ)=∫n(t)f(t-τ)dt – normalized noise function with zero mean, unit variance and correlation function
According to the maximum likelihood algorithm, the decision in favor of the signal with number m is made if supL m (τ m)≥supL k (τ k). To find the probabilities of correct and incorrect decisions according to this rule, it is necessary to calculate the distribution of the absolute maxima of the processes L(τ) on the interval [Т 1 ,Т 2 ].
Let us consider a method for calculating the error probability of distinguishing M signals with unknown parameters in the case of single-beam propagation of signals (or in the scheme of optimal signal summation). Let us denote by H k =supL k (τ k) the value of the absolute maximum statistics at the output of the k-th channel of the receiver. We write the joint distribution of random variables (H 1 ,H 2 ,..H M ) as w(u 1 ,u 2 ,..u M). The condition of orthogonality for signals f k (t) in the statistical sense means the independence of random variables H k , k=1..M. Then the probability of a correct solution using the maximum likelihood algorithm can be written as
If we take into account the condition of orthogonality of the system of signals (s k (t)), then
Let us assume that the system of signals (s k (t)) has the same energy, that is, z 0 m =z 0 k =z 0 . Then the formulas for H m and H k can be rewritten as
The distribution function of the absolute maximum h k of the implementation of the Gaussian process with the correlation function R(τ) can be approximated by the formula
ξ=(T 2 -T 1)/Δ is the reduced length of the a priori interval [T 1 ,T 2 ], which has the meaning of the resolution number of PM signals in this interval. The approximation is asymptotically exact as ξ→∞, u→∞. For finite values of ξ and u, one can use a more accurate approximation
Probability integral. For ξ>>1 and z 0 >>1, the absolute maximum distribution function h m can be written as F m (u)=F s (u)F N (u)≈Φ(u-z 0)F N (u). Substituting the expressions F N (u) and F m (u) into the relation for P rights, we obtain after appropriate transformations
The first term corresponds to the a priori probability of a correct solution for M equally probable events. The second term determines the change in probability due to the decision. As z 0 →∞, the integral in the expression for P is right tends to 1 and, accordingly, P is right →1.
The total error probability of distinguishing M signals with unknown parameters is equal to
It can be seen from the formulas that with an increase in the number of distinguishable signals, the probability of a decision error P e (z 0) increases. With an increase in the a priori interval of time delays of signals ξ, the probability of discrimination error P e (z 0) increases significantly.
4.4 Comparison of synchronous and asynchronous communication systems
Typically, when considering the performance of a receiver or demodulator, some level of signal synchronization is assumed. For example, in coherent phase demodulation (PSK scheme), it is assumed that the receiver can generate reference signals whose phase is identical (perhaps up to a constant offset) to the phase of the transmitter signal alphabet elements. Then, in the process of deciding on the value of the received symbol (based on the maximum likelihood principle), the reference signals are compared with the incoming ones.
When generating such reference signals, the receiver must be synchronized with the received carrier. This means that the phase of the incoming carrier and its copy at the receiver must match. In other words, if no information is encoded on the incoming carrier, the incoming carrier and its copy at the receiver will pass through zero at the same time. This process is called phase locked loop (this is a condition that must be met as closely as possible if we want to accurately demodulate coherently modulated signals in the receiver). As a result of phase locked loop, the local oscillator of the receiver is synchronized in frequency and phase with the received signal. If the carrier signal modulates directly not the carrier, but the subcarrier, both the carrier phase and the subcarrier phase need to be determined. If the transmitter does not perform carrier-subcarrier phase synchronization (which it usually does), the receiver will be required to generate a copy of the subcarrier, with the phase control of the subcarrier copy being separate from the phase control of the copy carrier. This allows the receiver to obtain phase lock on both the carrier and the subcarrier.
In addition, it is assumed that the receiver knows exactly where an incoming character starts and where it ends. This information is needed to know the appropriate symbol integration interval - the energy integration interval before deciding on the symbol value. Obviously, if the receiver integrates over an interval of inappropriate length, or over an interval spanning two symbols, the ability to make an accurate decision will be reduced.
It can be seen that symbol and phase synchronization have in common that both involve making a copy of part of the faithful signal in the receiver. For phase lock, this will be an exact copy of the carrier. For a symbolic one, this is a meander with a zero crossing simultaneously with the transition of the incoming signal between symbols. A receiver capable of doing this is said to have symbol timing. Since there are typically a very large number of carrier periods per symbol period, this second level of synchronization is much coarser than phase synchronization and is usually performed using a different scheme than that used in phase synchronization.
Many communication systems require an even higher level of synchronization, commonly referred to as frame synchronization. Frame synchronization is required when information is delivered in blocks, or messages containing a fixed number of characters. This occurs, for example, when a block code is used to implement a forward error protection scheme, or if the communication channel is time-divisional and is used by multiple users (TDMA technology). With block coding, the decoder must know the location of the boundaries between the code words, which is necessary for the correct decoding of the message. When using a time division channel, one needs to know the location of the boundaries between the users of the channel, which is necessary for the correct direction of information. Like symbol sync, framing is equivalent to being able to generate a square wave at a frame rate with zero transitions coinciding with transitions from one frame to another.
Most digital communications systems using coherent modulation require all three levels of synchronization: phase, symbol, and frame. Non-coherent modulation systems typically require only symbol and frame synchronization; since the modulation is non-coherent, precise phase synchronization is not required. In addition, incoherent systems require frequency synchronization. Frequency synchronization differs from phase synchronization in that the copy of the carrier generated by the receiver may have arbitrary phase shifts from the received carrier. The structure of the receiver can be simplified if there is no requirement to determine the exact value of the phase of the incoming carrier. Unfortunately, this simplification entails a deterioration in the dependence of the reliability of the transmission on the signal-to-noise ratio.
Until now, the focus of discussion has been the receiving end of the communication channel. However, sometimes the transmitter takes a more active role in synchronization - it changes the timing and frequency of its transmissions to match the receiver's expectations. An example of this is a satellite communications network where multiple ground terminals send signals to a single satellite receiver. In most of these cases, the transmitter uses the return link from the receiver to determine timing accuracy. Therefore, two-way communication or a network is often required for transmitter synchronization to succeed. For this reason, transmitter timing is often referred to as network timing.
The need to synchronize the receiver is associated with certain costs. Each additional level of synchronization implies a greater cost to the system. The most obvious investment is the need for additional software or hardware for the receiver to receive and maintain synchronization. Also, and less obviously, we sometimes pay with the time it takes to synchronize before communication begins, or the energy required to transmit signals that will be used in the receiver to obtain and maintain synchronization. At this point, the question may arise why a communications system designer should consider a system design requiring a high degree of synchronization at all. Answer: improved performance and versatility.
Consider a typical commercial analog AM radio, which can be an important part of a broadcast communication system including a central transmitter and multiple receivers. This communication system is not synchronized. At the same time, the receiver bandwidth must be wide enough to include not only the information signal, but also any carrier fluctuations due to the Doppler effect or drift of the transmitter reference frequency. This transmitter bandwidth requirement means that additional noise energy is delivered to the detector in excess of the energy theoretically required to transmit information. Slightly more sophisticated carrier-tracking receivers can include a narrow band-pass filter centered on the carrier, which will significantly reduce noise energy and increase the received signal-to-noise ratio. Therefore, although conventional radio receivers are quite suitable for receiving signals from large transmitters at a distance of several tens of kilometers, they may be incapacitated under lesser quality conditions.
For digital communications, trade-offs between performance and receiver complexity are often considered when choosing a modulation. The simplest digital receivers include those designed for use with binary FSK with non-coherent detection. The only requirement is bit synchronization and frequency tracking. However, if you choose coherent BPSK as the modulation, you can get the same bit error probability, but with a lower signal-to-noise ratio (approximately 4 dB). The disadvantage of BPSK modulation is that the receiver requires accurate phase tracking, which can be a difficult design problem if the signals have high Doppler rates or are fading.
Another trade-off between cost and performance involves error correction coding. Significant performance improvements can be achieved with the right error protection techniques. At the same time, the cost in terms of receiver complexity can be high. Proper operation of a block decoder requires that the receiver achieve block sync, frame sync, or message sync. This procedure is in addition to the normal decoding procedure, although there are certain error correction codes that have built-in block synchronization. Convolutional codes also require some additional synchronization to get optimal performance. Although performance analysis of convolutional codes often assumes an infinite length of the input sequence, in practice this is not the case. Therefore, to ensure a minimum error probability, the decoder must know the initial state (usually all zeros) from which the information sequence begins, the final state, and the time to reach the final state. Knowing the end of the initial state and reaching the final state is equivalent to the presence of frame synchronization. In addition, the decoder must know how to group the channel symbols to make a split decision. This requirement also applies to synchronization.
The above discussion of trade-offs has been in terms of the relationship between performance and complexity of individual channels and receivers. It is worth noting that the ability to synchronize also has significant potential implications for system efficiency and versatility. Frame synchronization allows the use of advanced, general-purpose multiple access techniques such as demand-based multiple access (DAMA) schemes. In addition, the use of spread spectrum techniques, both multiple access schemes and interference suppression schemes, require a high level of system synchronization. These technologies offer the possibility of creating very versatile systems, which is a very important feature when changing the system or when exposed to intentional or unintentional interference from various external sources.
Conclusion
The first section of my work describes the principles of building wireless telecommunication communication systems: a scheme for building a cellular communication system is given, methods for separating subscribers in cellular communication are indicated and the advantages (confidentiality and noise immunity) of code separation compared to time and frequency are noted, and common wireless standards are also considered. DECT, Bluetooth and Wi-Fi (802.11, 802.16) communications.
Further, the correlation and spectral properties of signals are considered and, for example, calculations of the spectra of some signals (rectangular pulse, Gaussian bell, smoothed pulse) and autocorrelation functions of Barker signals common in digital communications and Walsh functions are given, and types of complex signals for telecommunication systems are indicated.
The third chapter presents modulation methods for complex signals: phase shift keying methods, modulation with a minimum frequency shift (one of the continuous phase modulation methods), quadrature amplitude modulation; and their advantages and disadvantages are indicated.
The last part of the work contains a consideration of the error probabilities of distinguishing M known and M fluctuating signals against the background of noise, as well as an algorithm for calculating errors in distinguishing M orthogonal signals with an unknown time position in asynchronous code division communication systems.
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2. Sklyar B. Digital communication. Theoretical foundations and practical application, 2nd edition.: Per. from English. – M.: Williams Publishing House, 2003. – 1104 p.
3. Shakhnovich I. Modern technologies of wireless communication. Moscow: Technosfera, 2004. - 168 p.
4. Baskakov S.I. Radio circuits and signals: Proc. for universities on special "Radio engineering". - 3rd ed., revised. and additional - M .: Higher. school, 2000. - 462 p.
5. Noise-like signals in information transmission systems. Ed. prof. V.B. Pestryakova. M., "Owls. radio”, 1973. – 424 p.
6. Varakin L.E. Communication systems with noise-like signals. - M.: Radio and communication, 1985. - 384 p.
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8. Radchenko Yu.S., Radchenko T.A. Efficiency of code separation of signals with unknown time of arrival. Proceedings of the 5th int. conf. "Radar, navigation, communications" - RLNC-99, Voronezh, 1999, v.1, p. 507-514.
9. Radio engineering systems: Proc. for universities on special "Radio Engineering" / Yu.P. Grishin, V.P. Ipatov, Yu.M. Kazarinov and others; Ed. Yu.M. Kazarinov. - M .: Higher. school, 1990. - 469 p.
At present, the management process cannot be imagined without the rapid exchange of various information. The modern level of development of communication facilities provides ample opportunities for organizing such information interaction.
By a telecommunications system we mean a set of means and communication channels operating according to certain principles inherent in them (physical, organizational, technological, etc.) and designed to transmit information over long distances.
There are the following types of telecommunication systems:
Telegraph communication;
Telephone communications;
Radio communication;
Satellite connection;
Computer networks.
Telegraph communication can rightly be considered one of the oldest ways of transmitting information by technical means over long distances. Introduced at the beginning of the 19th century The system of electric telegraph communication is still used to this day for data transmission. However, at present, telegraph communications are being replaced by others - more modern, convenient and high-speed information exchange systems.
The invention of the telephone in 1876 marked the beginning of the development of telephone networks, which have not ceased to be improved to this day.
Now, not only voice information (during a conversation between two subscribers), but also facsimile messages and digital data are transmitted through the channels of the public telephone network.
Telephone networks are designed to transmit analog signals over them. The analog signal is continuous and can take values from a certain range. For example, the analog signal is human speech; in the telephone, TV, radio, information also exists in analog form. The disadvantage of this form of information presentation is its susceptibility to interference.
The digital form of information representation is characterized by the presence of only two specific values. In a computer, information is encoded with two values: "1" - the presence of an electrical signal, "0" - its absence.
In order to transmit digital information using telephone communication channels, which is necessary, for example, for organizing computer networks, special devices should be used to convert signals from one type to another. Such devices are modems (modulators / demodulators), which allow converting a digital signal coming from a computer into an analog one - for transmitting it over telephone lines. On the receiving side, inverse transformations are performed.
Thus, telephone networks are the basis for building another type of telecommunication systems - computer networks.
Another direction in the development of telephone communications also arose at the junction of two different methods of data transmission: telephone communications proper and radio communications. This is how mobile telephone networks, also called "cellular" communications, appeared. A similar name arose in connection with some features of the organization of such communication networks. A cellular network is a system consisting of a large number of transmitters, each of which covers a certain limited space of the entire communication zone - a "cell". Moving within the network, the subscriber enters the zone of operation of one transmitter, then another, while the connection is not interrupted and the subscriber himself does not have to make any switching. It should also be noted that cellular communication systems can use sections of the public telephone network, satellite communications, etc. as data transmission channels. These channels are used for communication between different Transmitting nodes of the network, while for communication of the end user with the the radio channel is used by the transmitter.
Modern mobile phones are convenient multifunctional devices. They allow not only to communicate with another subscriber from almost anywhere in the world, but also have a lot of other useful features. So, using a mobile phone, you can access the Internet, send text messages (SMS).
Radio communication in the activities of most organizations is rarely used directly to transfer information between two end users. However, radio communication channels are an important component of computer networks - primarily the Internet and long-distance corporate networks.
Satellite communication systems have now received great development. New satellite networks are constantly emerging. They are used as a data transmission channel in other communication systems (for example, when building global computer networks). Also, satellite systems are widely used in the organization of television broadcasting.
Satellite communication networks are built on the basis of three types of artificial satellites. These types differ in the type of orbit and the altitude at which the given satellite is located. So, satellites are distinguished in low circular orbits (low-flying satellites); in elliptical orbits and geostationary satellites.
Low-flying satellites have an orbit height of no more than 2000 km. Since one such satellite is located above a certain point on the Earth for a very short time, several dozens of such satellites are needed to ensure constant communication. When one of them leaves the reception area, then communication is carried out through the next satellite located in this area. At any given time, there are two or three satellites in the "line of sight" zone.
Satellite communication systems in elliptical orbits allow radio and television broadcasting throughout Russia. The typical orbit of such satellites is an ellipse with the smallest distance to the Earth's surface of the order of 400-600 km and the largest distance - up to 60,000 km. These satellites allow communication over large areas. However, due to the elliptical orbit, they leave the hanging zone at a certain time, and communication with satellites at this point in time is not carried out. When a satellite appears in the reception area, communication is restored.
Satellites in geostationary orbits allow for stable communication with almost any point on the globe (except for areas close to the poles). To build such a system, three satellites are sufficient, which are located above the equator at an altitude of about 36,000 km and at each moment of time “hang” over a certain point on the Earth. However, the high altitude of the orbit allows such a system of satellites to view almost the entire surface of the Earth. They do not cover only areas close to the poles (due to the curvature of the Earth).
Satellite communication systems are rarely used for direct communication between two network subscribers. Usually they are an intermediate link for the transmission of information that comes to the end user through other telecommunication networks (telephone, television, computer, etc.).
The main means of telecommunications, i.e., the organization of information exchange, for modern enterprises are computer networks.
This type of telecommunications is currently undergoing a period of rapid development and growth. Now every reputable organization has its own local area network, usually with Internet access.
In this regard, it is necessary to pay special attention to the organization, construction and use of various computer networks.
Through the interaction of computers on a network, a number of new possibilities become available.
The first is the sharing of hardware and software resources. So, with shared access to an expensive peripheral device (printer, plotter, scanner, fax, etc.), the costs for each individual user are reduced. Similarly, network versions of application software are used.
The second is the sharing of data resources. With centralized storage of information, the processes of ensuring its integrity, as well as backup, are greatly simplified, which ensures high reliability. The presence of alternative copies on two machines at the same time allows you to continue working when one of them is unavailable.
The third is to speed up data transfer and provide new forms of user interaction in the same team when working on a common project.
Fourth, the use of common means of communication between different application systems (communication services, data, video, voice, etc.).
Most often, networks are classified in terms of the territory they cover. It is on this basis that networks are divided into local and global.
Local networks(Local Area Network - LAN) consist of computers concentrated in a small area and, as a rule, belonging to one organization. Due to the fact that the distances between individual computers are small, there are ample opportunities for the use of expensive telecommunications equipment, which ensures high speed and quality of data transfer. Thanks to this, local network users can enjoy a wide range of services. In addition, in local networks, as a rule, simple methods of interaction between individual computers on the network are used.
According to the method of management, networks are divided into peer-to-peer and with a dedicated server (centralized management). In peer-to-peer networks, all nodes are equal - each node can act as both a client and a server. A client is a software and hardware object that requests some services. And under the server is a combination of hardware and software that provides these services. A computer connected to a local network, depending on the tasks performed on it, is called a workstation (workstation) or a server (server).
Peer-to-peer local area networks (LANs) are quite easy to maintain, but they cannot provide adequate information protection with a large network size. The costs of organizing peer-to-peer computing networks are relatively small. However, with an increase in the number of workstations, the efficiency of using the network decreases sharply. Therefore, peer-to-peer LANs are used only for small workgroups - no more than 20 computers.
A dedicated server implements network management (administration) functions in accordance with specified policies - sets of rules for separating and restricting the rights of network participants. Dedicated server LANs have good data security, can support thousands of users, but require constant skilled maintenance by the system administrator.
Depending on the data transmission technology used, broadcast networks and networks with transmission from node to node are distinguished. Broadcast transmission is used mainly in small networks, and in large networks, transmission from node to node is used.
In broadcast networks, all network nodes share a single communication channel. Messages sent by one computer, called packets, are received by all other machines. Each packet contains the address of the recipient of the message. If the packet is addressed to another computer, then it is ignored. Thus, after checking the address, the recipient processes only those packets that are intended for it.
Node-to-node networks consist of machines connected in pairs. In such a network, a packet passes through a series of intermediate machines to reach its destination. In this case, there are often alternative paths from the source to the recipient.
global networks(Wide Area Network - WAN) consist of a large number of host computers located in different cities, regions, countries. To create global networks, existing communication lines are usually used. This allows you to significantly reduce the cost, since it is not required to lay special communication lines over long distances. In addition, this approach makes it possible to make global networks accessible to a huge number of users.
However, the principle of using public communication systems has significant drawbacks. The low speeds of the channels used significantly narrow the range of services offered. For stable data transmission over low-quality communication lines, special methods and means are used (in particular, complex procedures for integrity control and data recovery). Such methods are the hallmarks of global networks.
The basis of the global network is made up of high-power computing systems designed for the simultaneous operation of many users - the so-called host-nodes. Special computers - communication nodes - are also a necessary component of global networks.
Urban (regional) networks (Metropolitan Area Network - MAN) are designed to connect local networks within a single city, as well as connect local networks to global ones. Urban networks represent a kind of intermediate link between high-speed, but territorially limited local networks and operating over long distances, but low-speed global networks. The use of urban networks will allow organizations to get high-quality and high-speed communications for much less money than when creating their own local area network. In Russia, computer networks of this type have not yet become widespread.
Separately, the so-called corporate networks. They are organized by enterprises that have a large number of branches located far from each other, between which it is necessary to organize an operational exchange of data. Such networks are created for the own needs of a particular organization and the fulfillment of tasks within the framework of its activities. At the same time, the network itself is virtual, and direct data transmission is carried out through other networks: the public telephone network, local networks of the organization and its branches, the Internet, etc.
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Telecommunication - communication at a distance (lat.)
Communication( the process of information exchange) is a necessary condition for the existence of living organisms, ecological communities and human society. Social development is accompanied by the development of telecommunication technologies. Telecommunication technologies have been developing especially intensively over the past few decades.
Telecommunications can be defined as technologies dealing with communication at a distance and this can be explained in various ways. Figure 8.2 shows one of the possible views of the various sections of telecommunications.
Figure 8.2. Telecommunications: forms and types
Telecommunications are divided into two types: unidirectional and bidirectional. Unidirectional, such as mass broadcasting and television broadcasting, involve the transmission of information in one direction - from the center to the subscribers. Bidirectional support a dialogue between two subscribers.
Telecommunications use mechanical and electrical means because, historically, telecommunications has evolved from a mechanical to an electrical form, using more and more complex electrical systems. This is the reason why many traditional carriers in telecommunications such as the national postal, telegraph and telephone companies use both forms. The share of mechanical telecommunications such as regular mail and press (newspaper distribution) is expected to decrease, while the share of electrical, especially bidirectional, will increase and become the main one in the future. Already in our time, corporations and the press are primarily interested in electrical telecommunications (telecommunications) as a profitable business opportunity.
Along the edges of Figure 8.2. telecommunication services are shown, initially mechanical: the press (forwarding newspapers), mail; then electrical: telegraph, telex (subscriber telegraph), telephone, radio, television, computer networks, leased networks, cable television and mobile phones.
Approximately in this order, telecommunications developed historically.
Telecommunication system- a set of technical objects, organizational measures and subjects that implement processes consisting of: connection processes, transfer processes and access processes.
Telecommunication systems use the natural or artificial environment to exchange information. Telecommunication systems, together with the medium that is used for transmission, form telecommunication networks. The most important telecommunication networks are (Fig. 8.2.): Postal service; public telephone network (PSTN); mobile telephone networks; telegraph network; Internet - a global network of interaction of computer networks; wire broadcasting network; cable television networks; television and radio broadcasting networks; departmental communication networks that provide communication services to government agencies, air and maritime traffic control systems, large industrial complexes; global networks of rescue and safety.
The telecommunication systems listed above, as a rule, closely interact with each other and use common resources for communication. To organize such interaction in each state and on a global scale, there are special bodies that regulate the use of common resources; determine the general rules of interaction (protocols) of telecommunication systems; develop advanced telecommunication technologies.
To implement communication at a distance, telecommunication systems use: switching systems; transmission systems; systems of access and control of transmission channels.
2 Two roots of computer networks Computing and telecommunication technologies Evolution of telecommunications Evolution of computer technology Evolution of computer networks Evolution of computer networks at the intersection of computer technology and telecommunication technologies
3 Telecommunication systems 1. Basic information about telecommunication systems The main function of telecommunication systems (TCS), or territorial communication networks (TCN), is to organize an efficient and reliable exchange of information between subscribers, as well as to reduce data transmission costs. The term "territorial" means that the communication network is distributed over a large area. It is created in the interests of the entire state, institution, enterprise or firm with branches in the district, region or throughout the country. The main indicator of the effectiveness of the functioning of telecommunication systems is the time of information delivery. It depends on a number of factors: the structure of the communication network, the throughput of communication lines, methods of connecting communication channels between interacting subscribers, information exchange protocols, subscriber access methods to the transmission medium, packet routing methods, etc.
4 Telecommunication systems 1. Basic information about telecommunication systems Characteristic features of territorial communication networks: diversity of communication channels from wired tone frequency channels (telephone) to fiber optic and satellite; limited number of communication channels between remote subscribers, through which it is necessary to provide data exchange, telephone, video, fax messages; the availability of such a critically important resource as the bandwidth of communication channels. Therefore, a territorial communication network (TCN) is a geographically distributed network that combines the functions of traditional data transmission networks (DTN), telephone networks and is designed to transmit traffic of various nature, with different probabilistic and temporal characteristics.
5 Telecommunication systems 1. Basic information about telecommunication systems Types of networks, lines and communication channels. TVS uses telephone, telegraph, television, and satellite communication networks. The following communication lines are used: cable (telephone lines, twisted pair, coaxial cable, fiber-optic lines), radio relay and radio lines. Among cable communication lines, light guides (i.e. fiber-optic lines) have the best performance. Their main advantages are: high bandwidth (hundreds of megabits per second); insensitivity to external fields and absence of own radiations; low labor intensity of laying an optical cable; spark, explosion and fire safety; increased resistance to aggressive environments; small specific gravity; various fields of application. Disadvantages: signaling is carried out only in one direction; connecting additional computers significantly weakens the signal; high-speed modems necessary for light guides are expensive; the light guides connecting the computers must be supplied with converters of electrical signals into light and vice versa.
6 Telecommunication systems 1. Basic information about telecommunication systems In telecommunication systems, the following types of communication channels have been used: simplex, when the transmitter and receiver are connected by one communication channel, through which information is transmitted only in one direction (this is typical for TV communication networks); half-duplex, when two communication nodes are also connected by one channel, through which information is transmitted alternately in one direction, then in the opposite direction (this is typical for information-reference, request-response systems); duplex, when two communication nodes are connected by two channels (forward and reverse), through which information is simultaneously transmitted in opposite directions. Duplex channels are used in systems with decision and information feedback.
7 Telecommunication systems 1. Basic information about telecommunication systems Switched and dedicated communication channels. In networks (TCS, TSS) there are dedicated (non-switched) communication channels and switched channels for the duration of information transmission over them. When using dedicated communication channels, the transceiver equipment of communication nodes is constantly connected to each other. This ensures a high degree of readiness of the system for information transfer, higher quality of communication, and support for a large amount of traffic. Due to the relatively high operating costs of networks with dedicated communication channels, their profitability is achieved only if the channels are fully loaded. Switched communication channels created only for the period of transmission of a fixed amount of information are characterized by high flexibility and relatively low cost. The disadvantages of such channels are: loss of time for switching (establishing communication between subscribers), the possibility of blocking due to the busyness of individual sections of the communication line, lower communication quality, high cost with a significant amount of traffic.
8 Telecommunication systems 1. Basic information about telecommunication systems Analogue and digital coding of digital data. The transfer of data from one network node to another is carried out by serial transmission of all bits of the message from the source to the destination. Physically, information bits are transmitted in the form of analog or digital electrical signals. Analog signals are signals that can represent an infinite number of values of some quantity within a limited range. Digital (discrete) signals can have a single value or a finite set of values. When working with analog signals, an analog carrier signal of a sinusoidal form is used to transmit encoded data, and when working with digital signals, two and a multi-level discrete signal are used. Analog signals are less sensitive to distortion due to attenuation in the transmission medium, but data encoding and decoding is easier for digital signals.
10 Telecommunication systems 1. Basic information about telecommunication systems Synchronization of network elements is part of the communication protocol. The synchronization process ensures synchronous operation of the receiver and transmitter equipment, in which the receiver samples the incoming information bits strictly at the moments of their arrival. A distinction is made between synchronous transmission, asynchronous transmission, and self-tuning transmission. Synchronous transmission is characterized by the presence of an additional communication line (in addition to the main one) for the transmission of synchronization pulses (SI) of a stable frequency. The issuance of data bits by the transmitter and the sampling of signals by the receiver are performed at the moments of the appearance of SI. This is reliable, but an additional line is needed. Asynchronous transmission does not require an additional line. The transmission is carried out in small fixed blocks, and the start bit is used for synchronization. In lockshift transmission, synchronization is achieved through the use of self-synchronizing codes (SCs). The coding of transmitted data using SC is to ensure regular and frequent changes in signal levels in the channel. Each transition is used to tune the receiver.
11 Satellite communication networks (SCN). Communication spacecraft (SC) are launched to a height of km and are in geostationary orbit, the plane of which is parallel to the plane of the equator. Three such spacecraft provide coverage of almost the entire surface of the Earth. Interaction between CCC subscribers is carried out along the chain: AS-sender of information > transmitting ground station >> satellite > receiving ground station > AS-receiver. One ground station serves a group of nearby speakers. The following methods are used to manage data transmission between the satellite and ground stations. 1. Conventional multiplexing with frequency and time division. 2. Regular primary/secondary discipline, with or without the use of survey/selection methods and tools. 3. Equal rank control disciplines with equal access to the channel in the conditions of competition for the channel. Telecommunication systems 1. Basic information about telecommunication systems transmitting ground station >> satellite > receiving ground station > receiving AC. One ground station serves a group of nearby speakers. The following methods are used to manage data transmission between the satellite and ground stations. 1. Conventional multiplexing with frequency and time division. 2. Regular primary/secondary discipline, with or without the use of survey/selection methods and tools. 3. Equal rank control disciplines with equal access to the channel in the conditions of competition for the channel. Telecommunication systems 1. Basic information about telecommunication systems "> 
12 Telecommunication systems 1. Basic information about telecommunication systems The main advantages of satellite communication networks: high bandwidth due to the operation of satellites in a wide range of gigahertz new frequencies. A satellite can support several thousand voice communication channels; providing communication between stations located at very large distances, and the possibility of servicing subscribers in the most inaccessible points; independence of the cost of information transfer from the distance between subscribers; the possibility of building a network without physically implemented switching devices. Disadvantages of satellite communication networks: the need to spend money and time to ensure the confidentiality of data transmission; the presence of a delay in the reception of a radio signal by a ground station due to large distances between the satellite and the communication station; the possibility of mutual distortion of radio signals from ground stations operating at adjacent frequencies; the susceptibility of signals to the influence of various atmospheric phenomena.
13 Telecommunication systems 2. Switching in networks Switching is a vital element of communication between subscriber systems (SS) and with control centers, processing and storage of information in networks. Network nodes are connected to some switching equipment, thus avoiding the need to create special communication lines. A switched transport network is a network in which communication is established between two (or more) end points on demand. An example of such a network is the switched telephone network. There are the following switching methods: switching circuits (channels); store-and-forward switching, divided into message switching and packet switching.
15 Telecommunication systems 2. Communication in networks Switching channels (circuits). When switching channels (circuits) between connected end points throughout the entire time interval of the connection, real-time exchange is provided, and bits are transmitted at a constant rate over a channel with a constant bandwidth. Advantages of the circuit switching method: the development of circuit switching technology; work in interactive mode and in real time; ensuring transparency regardless of the number of connections between AS; wide scope. Disadvantages of the circuit switching method: a long time to establish an end-to-end communication channel due to the possible waiting for the release of its individual sections; the need to retransmit the call signal due to the employment of the switching device in the signal chain; lack of choice of information transfer rates; the possibility of channel monopolization by one source of information; increasing the functions and capabilities of the network is limited; uniform loading of communication channels is not ensured.
17 Telecommunication systems 2. Communication in networks Message switching is an early method of data transmission (used in e-mail, news). Technology - "remember and send." The entire message retains its integrity as it passes from one node to another up to the destination, and the transit node cannot start further transmission of part of the message if it is still being received. Advantages of the method: no need to establish a channel in advance; formation of a route from sections with different throughput; implementation of systems for servicing requests, taking into account their priorities; the ability to smooth peak loads by storing streams; no loss of service requests. Disadvantages: the need to implement serious requirements for memory capacity in communication nodes to receive large messages; insufficient opportunities to implement an interactive mode and work in real time when transmitting data; channels are used less efficiently than other methods.
18 Telecommunication systems 2. Communication in networks Packet switching combines the advantages of circuit switching and message switching. Its main goals are: ensuring full network availability and acceptable response time to a request for all users, smoothing asymmetric flows between users, providing multiplexing capabilities of communication channels and network computer ports, dispersing critical network components. The data is broken into short packets of a fixed length. Each packet is supplied with protocol information: packet start and end codes, sender and recipient addresses, packet number in the message, information to control the reliability of transmitted data. Independent packets of the same message can be transmitted simultaneously along different routes as part of datagrams. The packets are delivered to their destination, where they form the initial message. Unlike message switching, packet switching allows you to: increase the number of connected stations; it is easier to overcome difficulties with connecting additional communication lines; implement alternative routing, which creates increased convenience for users; significantly reduce the time for data transfer, increase the bandwidth and efficiency of the use of network resources. Now packet switching is the main one for data transmission.
20 Telecommunication systems 2. Communication in networks Conclusion of the section The analysis of the considered switching technologies allows us to conclude that it is possible to develop a combined switching method based on the use of message and packet switching principles in a certain combination and providing more efficient management of heterogeneous traffic.
21 Telecommunication systems 3. Packet routing in networks Essence, goals and methods of routing. The task of routing is to choose a route for transmission from the sender to the recipient. First of all, we are talking about networks with an arbitrary (mesh) topology in which packet switching is implemented. However, in modern networks with a mixed topology (star-ring, star-bus, multi-segment), the problem of choosing a route for transmitting frames is really worth and solved, for which appropriate tools are used, for example, routers. In virtual networks, the task of routing when transmitting a message that is divided into packets is solved only once, when a virtual connection is established between the sender and the recipient. In datagram networks, where data is transmitted in the form of datagrams, routing is performed on a per-packet basis. The choice of routes in communication nodes of telecommunication networks is made in accordance with the implemented routing algorithm (method).
24 Telecommunication systems 3. Packet routing in networks The routing algorithm is a rule for assigning an output communication line for packet transmission, based on the information contained in the packet header (sender and recipient addresses), information about the load of this node (packet queue length) and the network as a whole . The main goals of routing are to ensure: the minimum delay of the packet during its transmission from the sender to the recipient; maximum network bandwidth; maximum protection of the package against threats for the information contained in it; reliability of package delivery to the addressee; the minimum cost of sending a packet to the destination. There are the following routing methods: - centralized routing; - distributed (decentralized) routing; - mixed routing
25 Telecommunication systems 3. Packet routing in networks 1. Centralized routing is implemented in networks with centralized control. The choice of the route for each packet is carried out in the network control center, and the nodes of the communication network only perceive and implement the results of solving the routing problem. This routing control is vulnerable to central node failures and is not very flexible. 2. Distributed (decentralized) routing is performed in networks with decentralized control. Routing control functions are distributed among network nodes that have the appropriate means for this. Distributed routing is more complex than centralized routing, but is more flexible. 3. Mixed routing is characterized by the fact that it implements the principles of centralized and distributed routing in a certain ratio. The problem of routing in networks is solved under the condition that the shortest route that ensures the transmission of a packet in the minimum time depends on the network topology, bandwidth and load on the communication line.
26 Telecommunication systems 3. Packet routing in networks Routing methods - simple, fixed and adaptive. The difference between them is in the degree to which topology changes and network load are taken into account when choosing a route. 1. Simple routing differs in that when choosing a route, neither a change in the network topology nor a change in its load is taken into account. It does not provide directional packet transmission and has low efficiency. Its advantages are ease of implementation and ensuring the stable operation of the network in the event of failure of its individual elements. Practical application received: random routing - one random free direction is selected for packet transmission. The packet "wanders" through the network and reaches the destination with a finite probability. flooding involves the transmission of a packet from a node over all free output lines. There is a phenomenon of "propagation" of the package. The main advantage of this method is the guaranteed optimal delivery time of the packet to the addressee. The method can be used in unloaded networks, when the requirements for minimizing the time and reliability of packet delivery are quite high.
27 Telecommunication systems 3. Packet routing in networks 2. Fixed routing - when choosing a route, changes in the network topology are taken into account and changes in its load are not taken into account. For each destination node, the direction of transmission is selected from a table of shortest routes. Lack of adaptation to load changes leads to network packet delays. A distinction is made between single-path and multi-path fixed routing. The first is built on the basis of a single packet transmission path between two subscribers, which is associated with instability to failures and overloads, and the second is based on several possible paths between two subscribers, from which the most preferred path is selected. Fixed routing is used in networks with little changing topology and steady packet flows. 3. Adaptive routing is different in that the decision on the direction of packet transmission is carried out taking into account changes in both the topology and the network load. There are several modifications of adaptive routing, which differ in what information is used when choosing a route. Local, distributed, centralized, and hybrid adaptive routing (the meaning is clear from the name) has become widespread.
28 Telecommunication systems 4. Protection against errors in networks When transmitting data, one error per thousand transmitted signals can seriously affect the quality of information. There are many methods for ensuring the reliability of information transmission (error protection), which differ: in the means used, in the time spent on their application, in the degree of ensuring the reliability of information transmission. The practical implementation of the methods consists of two parts: software and hardware. The ratio between them can be very different, up to the almost complete absence of one of the parts. The main causes of transmission errors in networks are failures in some part of the network equipment or the occurrence of adverse events in the network. The data transmission system is ready for this and eliminates them with the help of the means provided for in the plan; interference caused by external sources and atmospheric phenomena.
29 Telecommunication systems 4. Protection against errors in networks Among the numerous methods of protection against errors, three groups of methods are distinguished: group methods, error-correcting coding and error protection methods in feedback transmission systems. Of the group methods, the majority method and the method of transmitting information blocks with a quantitative characteristic of the block have been widely used. The essence of the majority method is that each message is transmitted several times (usually three times). Messages are remembered and compared, the correct one is chosen by coincidence "2 out of 3". Another group method, which also does not require information recoding, involves data transmission in blocks with a quantitative characteristic of the block (number of ones or zeros, checksum of symbols, etc.). At the receiving point, this characteristic is again calculated and compared with that transmitted over the communication channel. If the characteristics match, it is considered that the block does not contain errors. Otherwise, the transmitting side receives a signal with the requirement to retransmit the block. In modern fuel assemblies, this method is the most widely used.
30 Telecommunication systems 4. Protection against errors in networks Noise-immune (redundant) coding involves the development and use of corrective (noise-immune) codes. Transmission systems with feedback are divided into systems with decision feedback and systems with information feedback. A feature of systems with decisive feedback is that the decision on the need to retransmit information is made by the receiver. Error-correcting coding is used, with the help of which the received information is checked at the receiving station. If an error is detected, a re-request signal is sent to the transmitting side via the feedback channel, through which the information is retransmitted. In systems with information feedback, information is transmitted without error-correcting coding. The receiver, having received the information over the direct channel and storing it, transmits it back, where it is compared. If there is a match, the transmitter sends an acknowledgment signal, otherwise, all information is retransmitted, i.e. the decision to transmit is made by the transmitter.
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