Temporary separation of communication channels. Frequency division

Multichannel telephony and channel separation methods

Multichannel telephone communication (MTS)

At normal telephone communication the number of simultaneously operating connections must be less than or equal to the number of communication channels provided, and this increases the cost of constructing cable lines with a large number of subscribers. The solution in this case is to organize multichannel communication in some parts of the telephone network.

SPI - information conversion system;

TLF - telephone;

GK - group channel;

D - divisor;

GS - group signal.

The TA voice frequency channels have a range of 0.4 - 3.1 kHz and are combined into a group signal, which occupy the N frequency band (3.1 kHz + guard interval). The guard interval is approximately 0.3 kHz.

If we draw a frequency grid f, we will see that the channels are located as follows

1, 2, …, N - numbers of telephone channels.

The advantage of multi-channel telephone communication is the reduction in the cost of laying communication lines, since several conversations can be transmitted simultaneously over one pair of wires. Bandwidth overhead line communication with steel conductors is 30 kHz, with copper - 150 kHz, for cable communication lines - 10 MHz, for coaxial cable approximately - 1000 MHz.

The following options for the number of channels are actually used:

1st level - 12 telephone channels.

2nd level - 60 channels.

3rd level - 300 channels.

Channel separation methods

1. Frequency division channels(CHK) - FDMA

This method is based on the use of multi-channel filters and frequency converters.

PF - bandpass filter;

IF - frequency converter;

TLF - telephone set;

C - adder.

Frequency converter number i produces amplitude modulation from i-th telephone set, a bandpass filter selects the upper or lower side bands of the amplitude-modulated signal. And in the adder the formation takes place group signal. After transmission over a common channel, the processing process occurs in the opposite direction.

2. Time division of channels(VRK) - TDMA

When channels are temporarily separated, the signal from each telephone set is converted into digital form. In this case, data packets are formed containing a certain number of bits ( bit- unit of information in digital form). The generated packets for each telephone channel are transmitted to specially designated time slots, which are divided into time channels. Individual slots are separated by guard time intervals.

The principle of time division of channels is widely used in modern systems ah information transfer, since it allows you to reduce information redundancy during data compression digital methods. Time division of channels is used not only in wired networks for general use, but also in cellular systems communications.

3. Code division of channels(KRK) - CDMA

The principle of code channel separation is to separate channels by code.

4. Spectral channel separation(IBS) - WDMA

The principle of spectral separation is to separate channels by wavelength.

Topic No. 7

Principles of constructing multi-channel transmission systems

Topic lesson No. 2

Time division of channels

First study question

Time division of channels

Multichannel transmission systems with time division of channels (TDK) are widely used for transmitting analog and discrete information.

Time division of channels is possible only in the case of pulse modulation.

With a large duty cycle between pulses of one channel, a large period of time remains in which pulses of other channels can be placed. All channels occupy the same frequency band, but the communication line is used alternately to periodically transmit channel signals. The repetition frequency of channel signals is selected according to Kotelnikov’s theorem. To synchronize the operation of the transmitter and receiver switches, auxiliary synchronizing pulses are transmitted, for which one or more channels are allocated. With VRC, various types of pulse modulation are used in channels: PIM, PWM, PCM, DM, etc. For radio links, double modulation is used: PCM-OPSK, PIM-FM, etc.

Figure 7.2.1 shows a block diagram of a multichannel system (MCS) with time division of channels (TDC), where it is indicated:

M - modulator, PB - intermediate block, GI generator pulses, ST - counter, DS - decoder, GN - carrier generator, PRD - transmitter, LS - communication line, IP - source of interference, PRM - receiver, D - detector, VSI - sync pulse extractor, I - coincidence circuit.

Fig.7.2.1. Block diagram multi-channel time division system

The TI, ST, DS blocks form the RL distribution line, which is outlined by a dashed dotted line.

The first GI pulse appears on the first DS tap, the second on the second, etc. Nth pulse- on the Nth (last). The next pulse N + 1 will appear again at the first input of the DS and then the process is repeated. At the DS taps, periodic sequences of pulses are formed, shifted in time relative to each other. The first sequence of pulses arrives at the control input of the FSI clock pulse shaper, the rest - at the inputs of channel modulators M (the first stage of modulation). Their second inputs receive transmitted information signals, which modulate high-frequency pulses from the DS according to one of their parameters (amplitude, duration, etc.).

The operating principle of the presented circuit is illustrated by timing diagrams (Fig. 7.2.2 a-d) for the case of AIM in Mi channel modulators.

Fig.7.2.2. Timing diagram of the operation of the ISS circuit with the VRK

The latter are samplers made on key circuits or multiplexers. Let us first consider AIM modulators on keys, the number of which is N = 4. Moreover, the first channel is allocated for the synchronization pulse, and the other three for information signals. The SS synchronization signal differs from information pulses in some parameter, for example, duration or amplitude. The first pulse from the GI (Fig. 7.2.2 d) opens the first key, forming a CC at its output, the second pulse - the second key and passes the corresponding part of the signal of the first channel to its output, the third pulse - part of the signal of the second channel, and so on until the fourth pulse . The fifth pulse again forms the CC, etc. Since the outputs of all keys are connected to each other in parallel, the total (group) signal consists of pulses that do not overlap in time. In this case, the channels are said to be compacted in time. Next, the group signal (Fig. 7.2.2 e), after amplification in the PB block, is supplied as a modulating signal to the second modulation stage M, after which it is amplified in the PRD block and is supplied to the receiving side via the communication line.

In practice, most often it is not the AIM that is used, but the PCM, which also includes the AIM. The remaining PCM operations (quantization by level, coding) must be carried out in the PB block.

On the receiving side, the signal from the line enters the RRM, where it is filtered, amplified, and then detected in block D (see Fig. 12.5) to obtain a group signal (see Fig. 7.2.2 e). If AIM is used in the channels, then the group signal, after amplification in the PB block, is immediately sent to some inputs of all coincidence circuits AND, to the other inputs of which pulses of the CC synchronizing signal are supplied (Fig. 7.2.2 g) from the output of the RL distributor. The operation of the latter is the same as on the transmitting side, with the exception that the GI is synchronized by SI pulses isolated from the group signal. Each AND matching circuit opens for a time determined by the distributor pulse duration and passes the signal of its channel to its output. In schemes I and VRK is carried out (Fig. 7.2.2 h-k). At the output of each such circuit there is a low-pass filter, which performs the functions of the second demodulation stage, converting the AIM signal into a transmitted analog signal. If the channel signals are digital (with PCM), then decoding must take place in the PB block of the receiver, converting PCM into AIM. Next, the group signal from the AIM is separated in the manner described above.

The receiver AND circuits act as temporary parametric filters or switches.

With VRC, mutual interference also occurs, which is caused by two reasons: linear distortions and imperfect synchronization. Indeed, when the pulse spectrum is limited ( linear distortion) their fronts “fall over”, and the pulses of one channel are superimposed on the pulses of the other, which is what produces transient interference. To reduce their level, guard intervals are introduced, which corresponds to some expansion of the signal spectrum.

The efficiency of using the frequency spectrum with VRK is practically (not theoretically) worse than with FDC: as the number of channels increases, the frequency band increases. But with VRK there is no interference of non-linear origin and the equipment is much simpler, and the signal crest factor is lower than with FRK. A significant advantage of the VRK is the high noise immunity of pulsed transmission methods (PCM, FIM, etc.).

With VRC, it is easy to select channels on the receiving side without any limitation on their quality. The equipment is small in size and weight, which is due to its widespread use integrated circuits, elements of digital computer technology, microprocessors.

The main disadvantage of the TRC is the need to ensure synchronization of the transmitting and receiving sides of the transmission system.

Note that with TRC, the channel signals are orthogonal to each other, since they do not overlap in time. This means that when transmitting them, phase division of channels (PDCD) can also be used. An example of this could be single-sideband transmission of digital signals, minimum frequency shift keying, etc.

Frequency Division Multiplexing, Frequency Division Multiplexing ( English Frequency-Division Multiplexing (FDM)

Channels are separated by frequency. Since a radio channel has a certain spectrum, the sum of all transmitting devices produces modern radio communications. For example: signal spectrum for mobile phone 8 MHz. If a mobile operator gives a subscriber a frequency of 880 MHz, then the next subscriber can occupy the frequency of 880+8=888 MHz. Thus, if the operator mobile communications has a licensed frequency of 800-900 MHz, it is capable of providing about 12 channels, with frequency division.

Frequency division of channels is used in X-DSL technology. Telephone wires transmit signals of different frequencies: a telephone conversation is 0.3-3.4 kHz, and a band from 28 to 1300 kHz is used for data transmission.

It is very important to filter signals. Otherwise, signal overlaps will occur, which can cause the connection to deteriorate significantly.

The practice of building modern information transmission systems shows that the most expensive links in communication channels are communication lines: cable, waveguide and light guide, radio relay and satellite, etc. Since it is not economically feasible to use an expensive communication line to transmit information between a single pair of subscribers, the problem arises of constructing multi-channel transmission systems in which one common communication line is compressed by a large number of individual channels. This ensures increased efficiency in the use of communication line capacity. Messages A 1 (t), ..., A N (t) from N sources IS 1, ..., IS N using individual modulators M 1, ..., M N are converted into channel signals U 1 (t), ..., U N (t ). The sum of these signals forms a group channel signal U L (t), which is transmitted over a communication line (LC). The group receiver P converts the received signal Z L (t) into the original group signal Z(t)=U(t). Individual receivers P 1, ..., P N select the corresponding channel signals Z 1 (t), ..., Z N (t) from the group signal Z(t) and convert them into messages. Blocks M 1, ..., M N and the adder form the compaction equipment, blocks M, LS and P form the group channel. Compaction equipment, a group channel and individual receivers form a multi-channel communication system.

In order for separating devices to distinguish between signals of individual channels, the corresponding characteristics unique to a given signal must be determined. In the case of continuous modulation, such signs can be frequency, amplitude, phase, and in the case of discrete modulation, also the shape of the signal. In accordance with the characteristics used for separation, separation methods also differ: frequency, time, phase, etc.

23.Frequency separation of signals. Time separation of signals. Separation of signals by form (code).

In telemechanics systems for transmitting many signals over one communication line, the use of conventional coding is shown to be insufficient. Either additional signal separation or special coding that includes signal separation elements is required. Signal separation is the provision of independent transmission and reception of many signals over one communication line or in one frequency band, in which the signals retain their properties and do not distort each other.

The following methods are currently used:

Time division, in which signals are transmitted sequentially in time, alternately using the same frequency band;

Code-address separation, carried out on the basis of time (less often frequency) separation of signals with sending an address code;

Frequency division, in which each signal is assigned its own frequency and the signals are transmitted sequentially or parallel in time;

Time-frequency division, which allows you to take advantage of both frequency and time division of signals;

Phase separation, in which signals differ from each other in phase.

Time division (TS). Each of the n - signals is provided with a line in turn: first, for a period of time t 1 signal 1 is transmitted, for t 2 - signal 2, etc. In this case, each signal occupies its own time interval. The time allotted for transmitting all signals is called a cycle. The frequency band for signal transmission is determined by the shortest pulse in the code combination. Between information time intervals, guard time intervals are necessary to avoid mutual influence of channel on channel, i.e. pass-through distortion.

To implement temporary separation, distributors are used, one of which is installed at the control point, and the other at the control point.

Code-address separation of signals (CAR). Time code-address separation of signals (TCAR) is used, in which a synchronizing pulse or code combination (sync combination) is first transmitted to ensure coordinated operation of the distributors at the control point and the controlled point. Next, a code combination called the address code is sent. The first characters of the address code are intended to select the controlled item and object, the latter form the function address, which indicates which TM - operation (function) should be performed (TU, TI, etc.). This is followed by the code combination of the operation itself, i.e. command information is transmitted or notification information is received.

Frequency separation of signals. For each of the n-signals, its own band is provided in frequency range. At the receiving point (RP), each of the sent signals is first isolated by a bandpass filter, then fed to the demodulator, then to the executive relays. Signals can be transmitted sequentially or simultaneously, i.e. parallel.

Phase separation of signals. Several signals are transmitted at one frequency in the form of radio pulses with different initial phases. For this purpose, relative or phase manipulation is used.

Time-frequency separation of signals. The shaded squares with numbers are signals transmitted in a certain frequency band and in a selected time interval. There are guard time intervals and frequency bands between signals. The number of generated signals increases significantly.

Let us consider the features of the structure of signal transmission and reception paths and the sequence of signal conversion in PDM-FM systems. For this purpose, let us turn to Fig. 2.1 and 2.3 and find out what the elements shown on them are in relation to systems with PDM-FM.

The multiplexing equipment (EA) is built on the principle of frequency division of channels (FDM) or, in other words, according to the principle of frequency division multiplexing (FC), which is widely used for compacting cable communication lines. The BC principle is (Fig. 3.2 and 3.3) that in the transmission treaty the PM spectra individual messages with the help of individual transmission converters (ITCs) and then group transfer converters (GTCs) are transported to the region of higher frequencies, and the group conversion can have several stages.

Spectrum transfer is carried out using the single-sideband modulation method, and therefore systems with PRK-FM are sometimes called OB-FM, OBP-FM (one sideband), and the group signal is called a group or linear single-sideband signal (in Fig. 3.2.):

Individual transmission converter IPP (as well as the group transmission converter GPP) is a ring modulator which, on the one hand, receives the frequency spectrum of the converted signal (PM signal), and on the other, a harmonic oscillation of the carrier frequency. After the ring converter, a bandpass filter (BPF) is included, which selects one of the sidebands, upper or lower, and suppresses the remainder of the carrier and the second sideband. By choosing the value and frequency band of the PF filter, the transposed position and frequency bandwidth of the far channel signal on the frequency axis of the group (linear) signal is determined. On the receiving side, spectrum conversion occurs in the reverse order in group reception converters (GRPr) and in individual reception converters (IRPC). With individual conversion of signal spectra of standard PM channels lying within subcarrier frequencies that are multiples of 4 kHz. In this case, guard bands = 0.9 kHz are provided between adjacent channels, which are necessary for reliable filtering of the spectra of adjacent channels. As a result of individual conversion, primary channel groups (PGs) are formed, usually including 3,6 or 12 channels. Thus, for field small-channel military systems, 3-channel primary groups are most often used, occupying the frequency spectrum of 12.3 - 23.4 kHz - the so-called 3-channel ShK, formed using subcarriers 12,16,20 kHz with the allocation of the upper side . To form a linear spectrum, three conversion stages are used. In individual equipment, conversion is applied low frequency signals With

using carrier frequencies of 12, 16 and 20 kHz. for the first second and third channels, respectively, using the upper sidebands from 12.3 to 15.4 kHz, from 16.3 to 19.4 kHz, from 20.3 to 23.4 kHz. The signals of the fourth, fifth and sixth channels are subject to similar formation.

On second stage of conversion the spectra of two three-channel groups of 12.3-12.4 kHz are transferred to the frequency range from 68 to 96 kHz using carrier frequencies of 92 and 108 kHz. The frequency bands used are from 68 to 80 kHz (first group) and from 84 to 96 kHz (second group) using a third conversion stage, group, at a carrier frequency of 64 kHz. are transferred to the linear frequency spectrum 4-32 kHz.

In addition to the received frequency spectrum, signals from the service communication channel and a control frequency of 18 kHz are transmitted to the line.

In the receiving path, the conversion of linear spectrum signals into tonal frequency spectra is carried out in the reverse order. In small-channel stations with PRK-FM operating mainly in the meter wavelength range, a frequency-modulated signal (FM) is formed directly at radio frequency (Fig. 3.6) in a frequency-modulated generator (FMG), not stabilized by quartz. FGM oscillations are further amplified in the amplifier high frequency(UHF) at the output of which a multi-channel frequency-modulated signal (MCFMS) is formed, or is previously multiplied in frequency (usually no more than 2-4 times, i.e. fper = fchmg or fper = nfchmg. Modulation of the FMG oscillations is carried out with using a varicap or other reactive element included in oscillatory circuit HMG. The modulating group signal (GS) comes from the output of the transmitting path of the AC (Fig. 3.6.) and is fed to the reactive element of the HMG, having previously passed through a group amplifier (GA) and a predictor circuit. The latter helps to equalize the quality of channels based on noise. In order to ensure high stability of the HMG frequency, its frequency is stabilized by the fluctuation of the corresponding reference frequency from the set of frequencies generated by the reference frequency synthesizer (RFS). Frequency adjustment is carried out by comparing the HMG frequency (fHMG) with the reference frequency (fOR) in the system (SM). When fine-tuning the HMG, the intermediate frequency (fIF), obtained as the difference fF=fHMG-fF, is equal to its nominal value and the AFC ring, which includes an intermediate frequency amplifier (IFA) and a frequency detector (FD),

do not affect the frequency of HMG (the system is in a state of equilibrium). When the HMG detuning occurs, the value differs from the nominal value and the AFC system adjusts the HMG frequency, bringing its residual detuning to a certain small permissible value. A low-pass filter (LPF) sharply limits the frequency band, practically highlighting only the DC component.

In radio relay stations with PRK-FM operating in the microwave range, the transmitting part of the group path and radio path is built, as a rule, in accordance with the principle shown in Fig. 3.6. Here fPER = f1 ± fIF, and f1 = fGET ± fSDV, where fSDV is the shift frequency between the frequencies of the transmitter fPER and the receiver fPR of this half-set of the station. The shift frequency is usually constant, and the local oscillator frequency fGET, generated in the frequency synthesizer (MF), when the station is rebuilt

changes its purpose, as a result of which f1 changes, and therefore fPER. The intermediate frequency in the absence of modulation is always constant. During modulation by a group signal, the value of fIF changes proportionally to the voltage and in accordance with the sign of the voltage of the group signal.

At an intermediate relay station, when relaying over HF (HF transit), the group path is turned off and an intermediate frequency signal is received at the mixer input from a receiver in a different direction of communication. The signal of the service communication channel (CAC) is introduced into the frequency or phase modulator contained in the shift generator (GSDV).

The structure of the reception path is explained in principle using Fig. 3.7. A superheterodyne type receiver is built as an FM signal receiver. In small-channel RRS operating in the meter wave range, double frequency conversion is usually used. In midrange systems, single frequency conversion is used. In this case, when relaying over HF a multi-channel frequency-modulated intermediate frequency signal in transit mode (HFTr) without demodulation to a transmitter in a different direction of communication. Since the local oscillator in this mode is used simultaneously both for the operation of the transmitter and for the operation of the receiver (different directions of communication). The magnitude of the instability of the local oscillator frequency is excluded from the relayed signal, and where, respectively, is the transmission frequency and the reception frequency of opposite directions of communication on a given intermediate RRS.

When operating in the final mode (Ok), the intermediate frequency signal, after amplitude limitation in the limiter (Limit), is demodulated by a frequency detector. Next, the group signal is amplified by a group amplifier and, after an equalizing circuit (EC), enters the compaction equipment.

Advantages of the CHK-FM method:

– the ability to interface with wire lines of multi-channel telecommunications via a group path and along paths of standard broadband channels (BC), which makes it possible to easily obtain composite radio relay-cable communication lines and provide working together such means of communication with a minimum number of transits via PM;

– the possibility of using the external compaction method, which allows, if necessary, to place the RRS at a considerable distance from the communication center (up to 14-16 km);

– no need to use a synchronization system;

– the universality of broadband group and radio paths, in principle, suitable for transmitting not only multi-channel signals combining several signals of standard PM channels, but for transmitting high-speed streams of binary information, television signals etc.

Disadvantages of the CFM-FM method:

– the bulkiness of the compaction equipment with the number of channels equal to tens or more; in relation to military mobile RRLs, this leads to the need to allocate additional transport units to accommodate the AU;

– the impossibility of isolating any number of PM channels without demodulating all or part of the channels to PM, the need to allocate channels only in groups (threes, sixes, etc. Figure 3.8.d shows the principle of pulsed transmission of a continuous signal.);

– the need to maintain individual hardware seals with their own crews;

– the relative high cost of AC and RRS in general.

Channel separation methods: spatial, linear (frequency, temporal), by shape. Condition for linear channel separation.

In multi-channel systems, all signal paths must be separated in some way so that each source signal can reach its corresponding receiver. This procedure is called channel separation or channel signal separation.

Multiplexing(eng. MUX) – a procedure for combining (compressing) channel signals in an MSP.

The reverse procedure to multiplexing is associated with channel separation - demultiplexing(eng. DMX or DeMUX).

MUX + DMX = MULDEX - "muldex"

Classification of channel separation methods

All used channel separation methods can be classified into linear And nonlinear(see picture).

Figure - Classification of channel separation methods

In SMEs, the following methods of channel separation are distinguished:

- spatial (schematic);

- linear: frequency – PRK, time – VRK, channel separation according to shape – RKF;

- nonlinear: reducible to linear and majority.

Spatial separation.

This simplest form separation, in which each channel is assigned individual line connections:

Figure - SME with spatial division of channels

AI is a source of information

PI – information receiver

LAN - communication line

Other forms of channel sharing involve transmitting messages over a single communication line. In this regard, multi-channel transmission is also called sealing the channels.

Generalized block diagram of MSP with linear separation of channel signals

M i – modulator of the i-th channel

П i – multiplier of the i-th channel

And i is the integrator of the i-th channel

D i – modulator of the i-th channel

СС – clock signal of the transmitting side

PS – clock signal receiver on the receiving side

LAN – communication line

On the transmitting side the primary signals C 1 (t), C 2 (t),...,C N (t) arrive at the entrance M 1, M 2,..., M N, the other input of which receives linearly independent or orthogonal carriers from carrier generators ψ 1 (t), ψ 2 (t),...,ψ N (t), transferring primary signals to channel signals S 1 (t), S 2 (t),.., S N (t). Then the channel signals are summed and a group multichannel signal is formed S gr (t).

On the receiving side, the group signal S" gr (t), changed under the influence various types interference and distortion n(t), goes to multipliers P 1, P 2,..., P N, above the entrance of which carriers arrive from carrier generators ψ 1 (t), ψ 2 (t),..., ψ N (t). The multiplication results are sent to the integrators And 1, And 2,..., And N, at the output of which channel signals are obtained, taking into account interference and distortion, S" 1 (t), S" 2 (t),..., S" N (t). Next, the channel signals are sent to D 1,D 2,...,D n, which convert channel signals into primary ones, taking into account interference and distortion C" 1 (t), C" 2 (t),..., C" N (t).

The operation of the transmission system is possible with synchronous (and sometimes in-phase) influence of carriers on the devices for converting M at transmission and multiplying P at reception. To do this, on the transmitting side a clock signal (SS) is introduced into the group signal, and on the receiving side it is separated from the group signal by a clock signal receiver (RS).

Multichannel telecommunication systems with frequency division of channels. Methods for generating channel signals.

Telecommunications system frequency division called a system in the linear path of which for the transmission of channel signals non-overlapping frequency bands are allocated.

Let's consider the principle of frequency division of channels, using the diagram of an N-channel system and frequency plans at its characteristic points.

Figure - Block diagram of an N-channel SME with FDC

Harmonic oscillations with different frequencies are used as carriers in SMEs with FDC f 1, f 2, …f n(carrier oscillations):

ψ i(t) = S i

Channel signals are formed as a result of modulation of one of the carrier parameters by primary signals C i (t). Apply amplitude, frequency And phase modulation. The carrier oscillation frequencies are selected so that the spectra of the channel signals S1(t) And S2(t) did not overlap . Group signal S gr (t), received into the communication line, is the sum of channel signals

S gr(t) = S 1 (t) + S 2 (t) + ...+ S n(t)

When transmitted along a linear path, the signal S gr(t) undergoes linear and nonlinear distortion and interference n(t) is superimposed on it, i.e., a distorted signal arrives at the receiving part .

In the receiving part, channel signals are separated using channel bandpass filters KPF-1, KPF-2, KPF-n, i.e. from group signal allocate channel signals .

Primary signals are restored by demodulators D 1, D 2, ... D n using frequencies equal to the frequencies of the carriers in the transmission.

Frequency plans at its characteristic points (see diagram)

In FRC, the dominant position is occupied by the AM-OBP modulation type, since it is the most compromise.

Figure - Bandpass filtering options for AM-OBP

The formation of an AM-OBP signal in communications technology is carried out in two ways:

1) Filter method

2) Phase difference method

The filter method is more often used in SME technology, while the phase difference method is usually used in small-channel transmission systems.

Filter method

On the transmitting side

Example:

Signal spectrum 0.3 – 3.4 kHz. Determine the result of AM-OBP if a harmonic oscillation with a frequency of 100 kHz is used as a carrier.

On the receiving side

Note: Frequency instability (mismatch) between the generating equipment of the transmitting and receiving sides for the primary signal group (12x CFC) should be no more than 1.5 Hz.

Phase difference method

Working principle: the circuit consists of two arms connected at the input and output using decoupling devices (ID). To the modulator (M 2) of one arm, the original signal and carrier frequency are supplied phase-shifted by π/2 relative to the signal and carrier frequency supplied to the modulator (M 1) of the other arm. As a result, the output of the circuit will only oscillate one sideband. Phase contours (FC 1, FC FC 2) provide a phase shift of π/2.

The condition for separability of channel signals in SMEs with CBR is their orthogonality, i.e.

Where energy spectrum of the i-th channel signal;

the boundaries of the frequency band allocated in the linear path for the i-th channel signal.

Frequency spectrum width of the group signal D f S is determined by the number of channels in the transmission system (N); spectrum width of channel signals D f i and also frequency characteristics attenuation of channel bandpass filters KPF-1, KPF-2, KPF-n.

Crossover filters provide low attenuation in the passband ( Apr) and the required amount of attenuation in the range of effective delay ( apod). Between these bands are the defiltering bands of the separation filters. Therefore, channel signals must be separated by guard gaps (D fз), the values ​​of which must be no less than the filtering bands of the filters.

Hence, baseband width can be determined by the formula

D f gr= N×(D fi+D f z)

since the attenuation of crossover filters in the stopband is finite ( apod), then complete separation of channel signals is impossible. As a result, there appear interchannel crosstalk.

In modern telephony SMEs, each CTC is allocated a 4 kHz frequency band, although frequency spectrum transmitted sound signals limited to the band from 300 to 3400 Hz, i.e. the spectrum width is 3.1 kHz. Intervals of 0.9 kHz width are provided between frequency bands of adjacent channels, designed to reduce the level of mutual interference when filtering signals. This means that in multi-channel frequency division communication systems, only about 80% of the communication link bandwidth is effectively used. In addition, it is necessary to ensure a high degree of linearity of the entire group signal path.

Figure – Block diagram of the formation equipment

Topic 5. Channel separation methods

5.1 Methods of channel separation: spatial, linear (frequency, time), by shape. Condition for linear channel separation. Signals carriers and modulation of their parameters.

5.2 Multi-channel telecommunication systems with frequency division of channels. Methods for generating channel signals.

5.3 Multichannel telecommunication systems with time division of channels. Comparative analysis analog-pulse modulation methods.