Principle time division of channels (VRK) is that a group path is provided in turn for transmitting signals of each channel of a multichannel system

The transmission uses time sampling (pulse modulation). First, the pulse of the 1st channel is transmitted, then the next channel, etc. until the last channel numbered N, after which the pulse of the 1st channel is again transmitted and the process is repeated periodically. At the reception, a similar switch is installed, which alternately connects the group path to the corresponding receivers. In a certain short period of time, only one receiver / transmitter pair is connected to the group communication line.

This means that for the normal operation of a multichannel system with a VRK, synchronous and in-phase operation of the switches on the receiving and transmitting sides is necessary. To do this, one of the channels is occupied for the transmission of special synchronization pulses.

In fig. the timing diagrams are given, explaining the principle of VRK. In fig. a-c shows graphs of three continuous analog signals u 1 (t), u 2 (t) and u 3 (t) and the corresponding PIM signals. The pulses of different PAM signals are time shifted relative to each other. When individual channels are combined in a communication channel (line), a group signal is formed with a pulse repetition rate N times higher than the repetition rate of individual pulses.

The time interval between the nearest pulses of the group signal T K is called timeslot... The time interval between adjacent pulses of one individual signal is called transmission cycle T Ts. The number of pulses that can be placed in the cycle depends on the ratio of T Ts and T K, i.e. number of time channels.

There is mutual interference in time division, mainly due to two reasons.

The first is that linear distortions arising from the limited frequency band and imperfection of the amplitude-frequency and phase-frequency characteristics of any physically feasible communication system violate the impulse nature of the signals. With time division of signals, this will result in the pulses of one channel being superimposed on the pulses of other channels. Mutual crosstalk or intersymbol interference.

In the general case, to reduce the level of mutual interference, it is necessary to introduce "guard" time intervals, which corresponds to a certain spread of the signal spectrum. Time division systems have an undeniable advantage due to the fact that due to the difference in timing of transmission of signals from different channels, there is no non-linear crosstalk.

Principles of multichannel transmission The channel separation (RC) methods used can be classified into linear and non-linear (combinational). In most cases of channel splitting, each message source is assigned a special signal called channel. The message-modulated channel signals are combined to form a baseband signal (GC). If the combining operation is linear, then the resulting signal is called a linear baseband signal. For a standard channel, a voice frequency channel (PM channel) is taken, which ensures the transmission of messages with an efficiently transmitted frequency band of 300 ... 3400 Hz, corresponding to the main spectrum of the telephone signal.

Multichannel systems are formed by combining PM channels into groups, usually multiples of 12 channels. In turn, often use "secondary multiplexing" of PM channels by telegraph data transmission channels. Generalized block diagram of a multichannel communication system

The channel transmitters together with the adder form the combining equipment. The group transmitter M, the communication line of the LAN and the group receiver P constitute the group communication channel (transmission path), which together with the combination equipment and individual receivers constitutes a multi-channel communication system. In other words, separation equipment must be provided on the receiving side.

In order for the separating devices to be able to distinguish between the signals of individual channels, there must be certain features inherent only to this signal. In the general case, such features can be the parameters of the carrier, for example, amplitude, frequency or phase in the case of continuous modulation of a harmonic carrier. With discrete types of modulation, the waveform can also serve as a distinguishing feature. Accordingly, the methods of signal separation are also different: frequency, time, phase and others.

Thus, at the output of the four-port network, along with the frequencies of the input signals (ω, Ω), there appeared: the constant component; the second harmonics of the input signals; the components of the total (ω + Ω) and difference (ω - Ω) frequencies. (2ω, 2Ω); Information will also take place in signals with frequencies (ωn + Ω) and (ωn - Ω), which are mirror-like with respect to ω and are called upper (ω + Ω) and lower (ω - Ω) side frequencies. If a carrier frequency signal U 1 (t) \u003d Um ∙ Cosωнt and a tone frequency signal in the Ωn ... Ωw band (where Ωn \u003d 0.3 kHz, Ωw \u003d 3.4 kHz) are applied to the modulator, then the signal spectrum on the output of a four-port network will look like:

Signal spectrum at the output of a four-port network. Useful conversion (modulation) products are the upper and lower side bands. To restore the signal at reception, it is enough to supply the carrier frequency (ωн) and one of the side frequencies to the demodulator input.

In ISP-CHRK, only one sideband signal is transmitted over the channel, and the carrier frequency is taken from the local generator. At the output of each channel modulator, a bandpass filter with a passband ∆ω \u003d Ωw - Ωn \u003d 3.1 kHz is switched on. In order to reduce the influence of adjacent channels (crosstalk) caused by the imperfect frequency response of the filters, guard intervals are introduced between the signal message spectra. For PM channels, they are equal to 0. 9 kHz. Group signal spectrum with guard intervals

Principles of constructing FDD equipment In FDD systems with a number of channels of 12 or more, the principle of multiple frequency conversion is implemented. First, each of the PM channels is “tied” to one or another 12-channel group, called the primary group (PG). Terminal equipment (including AOK and ARC) is built in such a way that at each stage of frequency conversion, more and more enlarged groups of PM channels are formed. Moreover, in any group, the number of channels is a multiple of 12.

Each channel contains the following individual devices: on the transmission amplitude limiter OA, modulator M and bandpass filter PF; at the reception of the bandpass filter PF, demodulator DM, low-pass filter LPF and low-frequency amplifier ULF. To convert the original signal, carrier frequencies that are multiples of 4 kHz are supplied to the modulators and demodulators of each channel. When organizing telephone communication, you can use either a two-lane two-wire transmission system or a single-lane four-wire transmission system. The diagram shown in the figure refers to the second option.

If the channel is used for telephone communication, then the two-wire section of the circuit from the subscriber is connected to the four-wire channel through a differential system (DS). In case of transmission of other signals (telegraph, data, sound broadcasting, etc.), which require one or more one-way channels, the DS is disabled. Amplitude limiters prevent the group amplifiers from overloading (and, therefore, reducing the likelihood of nonlinear interference) at the moments when the voltage peaks of several speech signals appear.

Identical frequency bands of five PGs are spread in frequency in the 312 ... 552 kHz band and form a 60-channel (secondary) group (SH). With the help of bandpass filters PF 1 - PF 5, connected to the outputs of the group converters, signals of the SSB type with a frequency band of 48 kHz each are formed. As a result of the addition of these five signals that do not overlap in the spectrum, the SH spectrum with a frequency band of 240 kHz is formed.

To reduce the transient effects between SH signals transmitted through adjacent paths, both direct and inverse spectra of PG 2 - PG 5 can be used in the SH spectrum. In the first case, carrier frequencies 468, 516, 564, 612 are applied to the GP 2 - GP 5 kHz, and the corresponding bandpass filters emphasize the lower sidebands (as shown in the figure above). In the second case, carrier frequencies of 300, 348, 396, 444 kHz are fed to GP 2 - GP 5, and the upper side bands are highlighted by bandpass filters PF 2 - PF 5. The carrier frequency for PG 1 is the same in both cases (420 kHz), and the spectrum of PG 1 is not inverted.

Basic characteristics of group messages These parameters are determined by the corresponding frequency, information and energy characteristics. On the recommendation of the CCITT, the average message power in the active channel at the point with the zero relative level is set equal to 88 microns. W0 (- 10.6 in. Bm 0). However, when calculating Pav, the CCITT recommends taking the value P 1 \u003d 31.6 microns. W0 (- 15 in. Bm 0) If N ≥ 240, then the average power of the group message at the point of zero relative level is Pav \u003d 31.6 N, μ. W, and the corresponding average power level pav \u003d - 15 + 10 lg N, d. Bm 0.

If N

Time division multiplexing (TDM), analog transmission methods With TDM on the transmitting side, continuous signals from subscribers are transmitted alternately. Time division principle

To do this, these signals are converted into a series of discrete values \u200b\u200bthat are periodically repeated at certain time intervals Td, which are called the sampling period. According to the theorem of V.A.Kotelnikov, the sampling period of a continuous, spectrum-limited signal with an upper frequency Fw \u003e\u003e Fn should be equal to Td \u003d 1 / Fd, Fd ≥ 2 Fw The time interval between the nearest pulses of the group signal Tk is called a channel interval or a time slot (Time Slot).

From the principle of temporal combining of signals, it follows that transmission in such systems is carried out in cycles, that is, periodically in the form of groups of Ngr \u003d N + n pulses, where N is the number of information signals, n is the number of service signals (synchronization pulses - IC, service communication, control and calls). Then the value of the channel interval ∆tk \u003d Td / Ngr Thus, with the TDM, messages from N subscribers and additional devices are transmitted through a common communication channel in the form of a sequence of pulses, the duration of each of which is τi

Group signal at CPM with PPM With time division of channels, the following types of pulse modulation are possible: AIM - pulse-amplitude modulation; PWM - pulse width modulation; FIM - pulse phase modulation.

Each of the listed methods of pulse modulation has its own advantages and disadvantages. AIM - easy to implement, but poor noise immunity. It is used as an intermediate form of modulation for converting an analog signal to digital. With PWM, the signal spectrum changes depending on the pulse duration. The minimum signal level corresponds to the minimum pulse duration and, accordingly, the maximum signal spectrum. With a limited channel bandwidth, such pulses are highly distorted.

In equipment with VRM and analog modulation methods, PPM has received the greatest application, since when using it, it is possible to reduce the interfering effect of additive noise and interference by two-way limiting of pulses in amplitude, and also to optimally match the constant pulse duration with the channel bandwidth. Therefore, in transmission systems with VRK, PPM is mainly used. A characteristic feature of the signal spectra with pulse modulation is the presence of components with frequencies Ωn ... Ωw of the transmitted message uк (t) This spectrum feature indicates the possibility of demodulating the AMM and PWM low-pass filter (LPF) with a cutoff frequency equal to Ωv.

Demodulation will not be accompanied by distortions if the low sideband components (ωd - Ωw) ... (ωd - Ωn) do not fall into the low-pass filter passband, and this condition will be met if Fd\u003e 2 Fw is selected. Usually take ωd \u003d (2.3 ... 2.4) Ωv and when sampling a telephone message with a frequency band of 0.3 ... 3.4 kHz, the sampling frequency Fd \u003d ωd / 2π is chosen equal to 8 kHz, kHz a sampling period Td \u003d 1 / Fd \u003d 125 µs With PPM, the components of the spectrum of the modulating message (Ωn… Ωw) depend on its frequency and have a small amplitude, therefore PPM demodulation is performed only by converting into AMM or PWM with subsequent filtering in a low-pass filter.

To ensure the operation of channel modulators and additional devices, the sequences of pulses with a sampling frequency Fd are shifted relative to the first channel by i · ∆tk, where i is the channel number. Thus, the moments of the start of the CM operation are determined by the triggering pulses from the RC, which determines the moments of connection to the common broadband channel of the corresponding subscriber or additional device. The received group signal ugr (t) is fed to the input of the regenerator (P), which gives the discrete signals of different channels the same characteristics, for example, the same pulse shape.

All devices intended for generating a signal ugr (t): KM 1 ... KMN, RK, GIS, DUV, DSS, R - are included in the signal combining equipment (AO). To ensure correct channel separation, the RK ′ AR must operate synchronously and in phase with the RK AO, which is carried out using synchronization pulses (IS), allocated by the appropriate selectors (SIS) and the synchronization unit (BS). Messages from the CD outputs go to the corresponding subscribers through differential systems.

The noise immunity of transmission systems with a VDK is largely determined by the accuracy and reliability of the synchronization system and channel distributors installed in the equipment for combining and separating channels. group signal u * gr (t). The most expedient in FIM turned out to be the use of dual ICs, for the transmission of which one of the time slots ∆tk is allocated in each sampling period Td.

Let us determine the number of channels that can be obtained in a system with FIM. Td \u003d (2∆tmax + tg) Ngr, where tg is the guard interval; ∆tmax - maximum displacement (deviation) of impulses. In this case, we assume that the duration of the pulses is short compared to tg and tmax. , Maximum pulse deviation for a given number of channels. Accept, therefore

Taking into account that for telephone transmission Td \u003d 125 μs, we will obtain: at Ngr \u003d 6 ∆tmax \u003d 8 μs, at Ngr \u003d 12 ∆tmax \u003d 3 μs, at Ngr \u003d 24 ∆tmax \u003d 1.5 μs. The higher the ∆tmax, the higher the noise immunity of the system with PPM. When transmitting signals from PPM over radio channels at the second stage (in a radio transmitter), amplitude (AM) or frequency (FM) modulation can be used. In systems with PPM - AM are usually limited to 24 channels, and in a more noise-immune system PPM - FM - 48 channels.

Time division multiplexing (time division multiplexing)

The time division multiplexing method is used in time division multiplex communication lines. These links carry pulsed signals, while continuous signals are typical for frequency division links. With slowly changing telemetry data, the signal will be narrowband (for example, temperature data can be transmitted at a low speed; say, once every 10 s), and it is extremely uneconomical to occupy the entire radio link with such a signal. To increase transmission efficiency, the same communication line can be used to transmit other measurements in the pauses between temperature transmissions. It is clear that efficient use of the communication line can be achieved by temporarily dividing the communication channel between several measured parameters, each of which is transmitted with a frequency corresponding to its rate of change. With this time division, each measured value is assigned its own repeating time interval. In our example, a number of different data groups must be transmitted within 10 seconds. Values \u200b\u200bof various measured quantities. are transmitted one after another through the same communication line, each value at its own intervals. The receiving device must be able to divide the stream of values \u200b\u200binto channels so that in each of the channels there are sequences of values \u200b\u200bcorresponding to the primary measured value. For this, it is necessary to provide time synchronization or tag each time interval so that each data source can be recognized at the receiving end. In fig. 16 shows the time division multiplexing and functional diagram of a typical time division telemetry system.

A common method for identifying each time interval is to count its position in relation to the clock pulses that are present at the beginning of the cycle of transmitted data values \u200b\u200b- "clock pulses". In fig. 17, a shows more detailed functional diagrams of the switch and de-switch.

Figure: sixteen.

a-distribution of time intervals (10 channels); b-simplified functional diagram of the system.

The switch gathers multiple input channels from signal sources into a single transmission line. The counter specifies each time frame and therefore the location in the loop for each data source. For example, the fifth data channel in the above diagram is connected to the radio communication line when the counter is in position 5, or when counting 5. In fig. 17, b shows a simplified switching and de-switching diagram. When the switch of the switch is in position 1, the switch of the de-switch is in the same position, which is played by the switch working in the opposite direction. Therefore, the data of the first channel is transmitted and received. Both switches work synchronously.

Figure: 17.

a - functional diagram; b - interaction scheme. The sync signal in the receiver can be extracted from the sync pulses transmitted over the communication line or generated by a local generator.

The sync clock provides precise timing at the start of the cycle, ensuring consistent switch and de-switch switching. Note that the switch and de-switch use the same hardware; the difference is only in the direction of movement of the data.

Since commutation and de-commutation are controlled by a fixed frequency synchronization, the switching frequency is also stable and the duration of each time interval is the same. However, this can be disadvantageous in cases where significantly different frequency bands are required for different data sources. In order to understand the relationship between bandwidth and switching frequency, it is necessary to consider the process of data sampling.

As noted earlier, a sinusoid can be reconstructed from a sequence of samples of its instantaneous values. To reproduce a 1 kHz sine wave with high fidelity (less than 1% distortion), at least 5 samples from each waveform period are required. Consequently, a 1 kHz signal must be sampled at 5000 values \u200b\u200bper second, i.e. 5 samples per measured value period. If we intend to switch signals from 10 data sources (having 1 kHz bandwidth), each of which requires a sampling rate of 5000 samples per second, then a switching speed of 10 × 5000 samples / s is required. = 50,000 samples / s. The switch must switch from source to source at 50 kHz (every 20 ms), so each signal source will be polled once every 10 switches, that is, once every 20 ms, but at 5 kHz. The clock rate, that is, the number of clock cycles per second, will be 5000 clock / s. The switching frequency is equal to the clock frequency multiplied by the number of data sources in the system, or the clock frequency multiplied by the number of pulses per clock cycle (5000 × 10 \u003d 50,000 pulses / s). The communication line must be able to transmit pulsed data at this high rate (50,000 pulses / s) without perceivable distortion. This means that a communication system is needed. with a bandwidth well over 50,000 Hz.

Samples of data from various sources in the system shown in Fig. 16, b, directly modulate the carrier. In addition to this direct modulation, it is often the case that data samples are used to modulate a subcarrier, which in turn modulates the carrier, as shown by the dashed lines in Fig. 16, b. Data samples from a group of sources are thus transmitted on one of the subcarriers in a frequency division multiplexed system. This allows you to use both methods of multiplexing channels in the same communication line. The data samples themselves are nothing more than the pulse values \u200b\u200bof the signal with pulse amplitude modulation (PAM), i.e. the information is amplitude-and-pulse-modulated. Since such PAM signals modulate a subcarrier (eg, FM), which then modulates the carrier (eg, also FM), the result is a PAM / FM / FM system.

Now consider an example that demonstrates the effect of signal sampling on the bandwidth of a communication system.

Consider a 100 MHz carrier that is modulated (FM) by a subcarrier with a center frequency of 70 kHz. Information is carried using frequency modulation of a 70 kHz subcarrier. Thus, we have an FM / FM communication channel. To comply with standards, the subcarrier frequency deviation must be limited to ± 15%. This means that with a modulation index of 5, the information bandwidth is limited to 2100 Hz, i.e., much narrower than the 50,000 Hz bandwidth required for the proposed system with channel division is obtained. If the number of samples per clock were reduced to one, which means leaving one of the data sources, then a switching frequency of 5 kHz would be required, that is, still wider than the 2100 Hz bandwidth that the 70 kHz subcarrier has. Note that in the case of a single data source, no multiplexing is required and hence direct continuous transmission (no sampling) is possible. In this case, the 2,100 Hz bandwidth is twice the bandwidth required for a single source signal (1 kHz in the previous example). This degradation in bandwidth efficiency (sampling requires 5 kHz bandwidth, non-sampling only 1 kHz) is due to the properties of the signal sampling itself. When generating five samples of instantaneous signal values \u200b\u200bfor each period of a continuous signal, we expand the signal bandwidth by more than five times, and, consequently, the required channel bandwidth. Although using a single subcarrier to transmit signals from a large number of sources, bandwidth is not used efficiently, it also has its advantages, which are manifested in narrowband signals from sources. Therefore, time division, requiring signal sampling, is mainly used in applications with low bandwidth requirements. However, wideband signals can also be transmitted using long samples. The duration of each sample in this method is much longer than the information period, and is 5 or more of its periods. This simply means that the sample contains not one instantaneous value, but a finite segment of signal values \u200b\u200btransmitted in a given clock interval. With this method, it is necessary to be sure that there is no data loss during the interruption of the transmission of niformacin from a specific source.

Above, it was assumed that the transmission method is FM / FM. Therefore, at each discrete time slot, the varying subcarrier frequency represents the measured value sampled at that time. During this time interval, the frequency offset from the center of the subcarrier corresponds to the sample voltage that modulates the subcarrier frequency. The width of these time intervals is fixed, and the clock of their sequence is set by the sync pulse. The sync pulse causes the maximum frequency deviation and has a duration equal to twice the normal time interval. The broadening is necessary to separate the synchronization pulse from the signal sample pulses.

The setting of standards and control of transmission line characteristics is carried out by various state or international bodies (depending on the nature of the lines: satellite telemetry - by international agreements, industrial telemetry - by state control bodies, etc.). For example, the clock frequency must be kept constant with an accuracy of ± 5% (long term stability); the beat length is limited to no more than 128 time intervals, etc. (IRIG, "Telemetry Standards"). Note also that the bandwidth is often wider at high subcarrier frequencies; hence the switching frequency may be higher.

It is sometimes useful to have different sampling rates for different sources to improve efficiency.

The broadband source should be polled more often than the narrowband source. This is easily achieved by simple changes to the internal connections of the switch and de-switch. For example, if we connect positions 1 and 5 in a ten-point commutator (channel splitter), then the data source connected to positions 1 and 5 will be polled twice in one clock cycle, that is, with twice the frequency. It is also possible to make sub-commutation, i.e. allocate one or more time slots, the duration of which is divided into parts for data transmission from an additional number of sources. In this case, the duration of the main clock interval becomes a sub-clock for the sub-switch.

These techniques make it easy to adapt the system to a wide range of bandwidth requirements.

The communication line is the most expensive element of the communication system. Therefore, it is advisable to carry out multichannel information transmission via it, since with an increase in the number of channels N, its throughput increases S. Poich. the condition must be met:

Н К - productivity of the k-th channel

The main problem of multichannel transmission is the separation of channel signals on the receiving side. Let us formulate the conditions for this separation.

Let it be necessary to organize the simultaneous transmission of several messages over a common (group) channel, each of which is described by the expression

(7.1.1)

Taking into account formula (7.1.1.), We obtain:

In other words, the receiver has selective properties with respect to the Sk (t) signal.

Considering the issue of signal separation, a distinction is made between frequency, phase, time division of channels, as well as division of signals by shape and other characteristics.

Second tutorial question

Frequency division multiplexing

The block diagram of a multichannel communication system (ISS) with frequency division multiplexing (FDM) is shown in Figure 7.1.1, where it is indicated: IS - signal source, Мi - modulator, Фi - filter of the ith channel, Σ - signal adder, GN - carrier generator, PRD - transmitter, LAN - communication line, IP - source of interference, PRM - receiver, D - detector, PS - message receiver.

Figure 7.1.1. Block diagram of a multichannel communication system

With FDM, the carrier signals have different frequencies fi (subcarriers) and are spaced apart by an interval greater than or equal to the bandwidth of the modulated channel signal. Therefore, the modulated channel signals occupy non-overlapping frequency bands and are orthogonal to each other. The latter are summed up (multiplexed in frequency) in the Σ block, forming a group signal, which modulates the oscillation of the main carrier frequency fн in block M.

All known techniques can be used to modulate the channel bearers. But more economically the bandwidth of the communication line is used in single-sideband modulation (SSB AM), since in this case the spectrum width of the modulated signal is minimal and equal to the spectrum width of the transmitted message. In the second stage of modulation (group signal), AM SSB is also used more often in wired communication channels.

Such a signal with double modulation, after amplification in the transmission unit, is transmitted through the communication line to the receiver of the PRM, where it undergoes the inverse transformation process, i.e. demodulation of the signal along the carrier in unit D to obtain a group signal, separating channel signals from it with bandpass filters Фi and demodulation of the latter in blocks Di. The central frequencies of the bandpass filters Фi are equal to the frequencies of the channel carriers, and their transparency bands are equal to the spectrum width of the modulated signals. The deviation of the real characteristics of the bandpass filters from the ideal should not affect the quality of signal separation, therefore, guard frequency intervals between channels are used. Each of the receiving filters F should pass without attenuation only those frequencies that belong to the signal of this channel. The filter should suppress the frequencies of signals of all other channels.

where g k is the impulse response of an ideal band-pass filter that passes the frequency band of the k-th channel without distortion.

The main advantages of the CHRK: simplicity of technical implementation, high noise immunity, the ability to organize any number of channels. Disadvantages: the inevitable expansion of the used frequency band with an increase in the number of channels, relatively low efficiency of the communication line bandwidth due to filtering losses; bulkiness and high cost of the equipment, mainly due to the large number of filters (the cost of filters reaches 40% of the cost of a system with a PFC). In railway transport, an ISS with a K-24T type ChRK has been developed, in which small-sized electromechanical filters are used.

Third study question