Time division of channels

The principle of time division multiplexing (TDM) is that a group path is provided in turn for transmitting signals from each channel of a multichannel system (Figure 6.5). In foreign sources, the term is used to denote the principle of time division of channels Time Division Multiply Access (TDMA).

Figure 6.5 - Principle of time division of channels

The transmission uses time sampling (pulse modulation). First, the pulse of the 1st channel is transmitted, then the next channel, etc. to the last channel numbered N, after which the pulse of the first 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.

Figure 6.6, a, b, c shows graphs of three continuous analog signals S 1 (t), S 2 (t) and S 3 (t) and the corresponding PAM signals. The pulses of different PAM signals are time shifted relative to each other. When individual channels are combined, a group signal is formed S G ( t) (Figure 6.6, d) with a pulse repetition rate in N times the repetition rate of individual impulses. The time interval between the nearest pulses of the TK group signal is called timeslot or time slot (Time slot). The time interval between adjacent pulses of one individual signal is called transfer cycle of the shopping center ... The ratio of TC and TK determines the number of pulses that can be placed in the cycle, i.e. number of time channels.

Figure 6.6 - Timing diagrams of signal conversion at VRK

With time division, as well as with FDC, there is mutual interference, 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 pulses from one channel being superimposed on pulses from other channels. In other words, mutual crosstalk or intersymbol interference ... In addition, mutual interference can occur due to imperfect timing of the clock pulses on the transmitting and receiving sides.

For these reasons, time division of channels based on AIM has not received practical application. Time division is widely used in digital transmission systems for plesiochronous and synchronous hierarchies.

In the general case, to reduce the level of mutual interference, it is necessary to introduce "guard" time intervals, which corresponds to a certain spreading of the signal spectrum. So, in transmission systems, the bandwidth of the effectively transmitted frequencies F= 3100 Hz; in accordance with the Kotelnikov theorem, the minimum value of the sampling rate f 0 =1/T D = 2 F= 6200 Hz. However, in real systems, the sampling rate is chosen with some margin: f 0 = 8 kHz. With time division of channels, the signal of each channel occupies the same frequency band, which is determined under ideal conditions according to the Kotelnikov theorem from the relation (excluding the synchronization channel) Dt K = T 0 / N = 1/( 2NF) = 1/( 2F TOTAL), where F COMM = FN, which is the same as the total frequency division bandwidth of the system.

Although, in theory, time and frequency division can achieve the same efficiency in the use of the frequency spectrum, nevertheless, time division systems are inferior to frequency division systems in this indicator. At the same time, 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 are no crosstalk of nonlinear origin. In addition, time division equipment is much simpler than frequency division, where appropriate bandpass filters are required for each individual channel.

For signal separation, not only such obvious features as frequency, time and phase can be used. A common feature of signals is shape. Signals differing in shape can be transmitted simultaneously and have overlapping frequency spectra, and yet such signals can be separated if the orthogonality condition is satisfied. In foreign sources, to denote this principle, the concept is used code division Code Division Multiply Access(CDMA). In recent years, digital methods for separating signals according to their shape have been successfully developed, in particular, discrete orthogonal sequences in the form of Walsh, Rademacher functions and others are used as carriers of various channels. The wide development of methods of separation according to the shape of signals has led to the creation of communication systems with separation of "almost orthogonal" signals, which are pseudo-random sequences, the correlation functions and energy spectra of which are close to the analogous characteristics of "limited" white noise. Such signals are called noise-like (SHPS).

4. Principles of multichannel transmission. Basics of building telecommunication systems and networks

4. Principles of multichannel transmission

4.1. Fundamentals of multichannel messaging theory

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 a channel signal. The message-modulated channel signals are combined to form group signal(HS). If the combining operation is linear, then the resulting signal is called linear group signal.

To unify multichannel communication systems, the main or standard channel is taken tone channel(PM channel), providing 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 channels and data transmission channels.

Figure 4.1 shows a generalized block diagram of a multichannel communication system.

Figure 4.1. Generalized block diagram of a multichannel communication system

Implementation of messages of each source a 1 (t), a 2 (t), ..., and N (t) with the help of individual transmitters (modulators) M 1, M 2, ..., M N are converted into the corresponding channel signals s 1 (t) , s 2 (t),…, s N (t). The set of channel signals at the output of the channel combination equipment (CCA) forms a group signal s (t). Finally, in the group transmitter M, the signal s (t) is converted into a linear signal s L (t), which is fed into the LAN communication line. Let us assume that the line passes the signal practically without distortion and does not introduce noise. Then, at the receiving end of the communication line, the linear signal s L (t) can be converted again into a group signal s (t) using the channel separation equipment (DAC). Channel or individual receivers P 1, P 2, ..., P N from the group signal s (t) select the corresponding channel signals s 1 (t), s 2 (t), ..., s N (t) and then converted into intended recipients messages a 1 (t), a 2 (t),…, a N (t).

Channel transmitters together with a summing device form unification equipment... The group transmitter M, the LAN communication line and the group receiver P make up group communication channel(transmission path), which, together with the combination equipment and individual receivers, constitutes multichannel communication system.

Individual receivers of the multichannel communication system P K, along with performing the usual operation of converting signals s K (t) into the corresponding messages a K (t), must ensure the separation of signals s K (t) from the group signal s (t). In other words, the composition of technical devices on the transmitting side of a multichannel system should include unification equipment, and on the receiving side - separation apparatus.

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.

4.2. Frequency division multiplexing

The functional diagram of the simplest multichannel communication system with frequency separation of channels is shown in Figure 4.2.

Figure 4.2. Functional diagram of a multichannel system with frequency division multiplexing

In foreign sources, the term Frequency Division Multiply Access (FDMA) is used to denote the principle of frequency division Multiply Access (FDMA).

First, in accordance with the transmitted messages, the primary (individual) signals having the energy spectra G 1 (ω), G 2 (ω), ..., G N (ω) μ modulate the sub-carriers of the frequencies ω K of each channel, respectively. This operation is performed by modulators M 1, M 2, ..., M N channel transmitters ..

Modulators- these are four-port networks with a nonlinear amplitude characteristic, which in the general case is approximated by a polynomial of the n-th degree.

where а 1, ... а n - coefficients of approximation

For simplicity, let's take a polynomial of the 2nd degree, that is:

, (4.2)

Let's send signals of two frequencies to such a four-port network, that is

where ω> Ω. Then

, (4.4)

After the appropriate transformations, we get:

, (4.5)

The spectrum of the signal at the output of the four-port network will look like (Figure 4.3):

Figure 4.3. Signal spectrum at the output of a four-port network

Thus, at the output of the four-port network, along with the frequencies of the input signals (ω, Ω), there appeared: the constant component ; second harmonics of input signals (2ω, 2Ω); ρ components of the total (ω + Ω) θ difference (ω - Ω) frequencies.

If we assume that the signal with a frequency Ω contains information, then it will also take place in signals with frequencies (ω n + Ω) θ (ω n - Ω), which are mirror-like with respect to ω and are called upper (ω + Ω ) θ with the lower (ω - Ω) current frequencies.

If a carrier frequency signal U 1 (t) = U m ∙ Cosω n t and a tone frequency signal in the band Ω n ... Ω in (where Ω n = 0.3 kHz, Ω in = 3.4 kHz) are applied to the four-terminal, then the signal spectrum at the output a four-port network will look like (Figure 4.4)

Figure 4.4. Signal spectrum at the output of a four-port network

Useful conversion (modulation) products are the upper and lower sidebands. To restore the signal at reception, it is enough to apply the carrier frequency (ω n) and one of the side bands to the demodulator input.

In multichannel transmission systems with frequency division multiplexing (MSP-CHRK), only one sideband signal is transmitted over the channel, and the carrier frequency is taken from the local generator. Thus, at the output of each channel modulator, a band-pass filter with a passband ∆ω = Ω in - Ω n = 3.1 kHz is turned on. Spectra G 1 (ω), G 2 (ω) ... GN (ω) ο after transposition (transfer) to different frequency intervals and inversion (this operation is optional in principle, but usually performed to simplify equipment) are added and form the group spectrum G gr ( ω).

In order to reduce the influence of adjacent channels (reduce crosstalk) caused by the imperfection of the frequency response of the filters, between the signal message spectra, guard intervals... For PM channels, they are 0.9 kHz. Thus, the bandwidth of the PM channel, taking into account the guard interval, is 4 kHz (Figure 4.5)

4.3. Principles of constructing the equipment of the CHRK

In FDC systems with 12 or more channels, the principle of multiple frequency conversion is implemented. The construction of a multichannel system is based on a standard tone channel (PM). In accordance with the recommendations of the CCITT, the terminal equipment (including the AOK and the ARC) is built in such a way that at each stage of frequency conversion with the help of unified blocks, more and more enlarged groups of PM channels are formed. Moreover, in any group, the number of channels is a multiple of 12.

Initially, each of the PM channels is "tied" to a 12-channel group called the primary group (PG). Diversity of signals of 12 different telephone messages across the spectrum (formation of PG) is carried out using individual frequency conversion in a standard 12-channel unit. These blocks provide both direct and feedback in each of the 12 duplex channels (Figure 4.6, a).

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 fed to the modulators and demodulators of each channel.

Figure 4.6. Structural diagram of an individual transformation unit (a) and a diagram of the formation of a primary group (b)

The spectrum of the PG group signal is shown in Figure 4.6, b.

In the given version of the formation of the GHG, the principle of a single transformation of the spectrum of the PM channel is used (Figure 4.7, a)

Since the individual equipment in all 12 channels is of the same type, this figure shows only devices related to one channel (the first). As noted earlier, when organizing telephone communication, you can use either a two-lane two-wire transmission system or a single-lane four-wire transmission system. The circuit shown in Figure 4.6 refers to the second option. Here, each channel has a separate transmission path and a receive path (operating in the same frequency band), that is, each channel is four-wire. 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 the case of transmission of other signals (telegraph, data, sound broadcasting, etc.), which require one or more one-way channels, the DS is disabled.

In the transmission mode, the message from the subscriber (Ab) through the DS and the amplitude limiter (OA) is fed to one of the inputs of the individual frequency converter (modulator M 11). The other input M 11 is supplied with a subcarrier signal with a frequency of F 12. As a result of multiplying these signals, a signal is formed, the spectrum of which consists of two side (relative to F 12) bands. The signal of the lower of these bands is selected by the filter PF 12 and fed to one of the adder inputs. The other inputs of the adder receive signals from the output of similar transmission paths of 11 other channels.

Amplitude limiters prevent the group amplifiers from overloading (and, therefore, reducing the likelihood of nonlinear interference) when the voltage peaks of several speech signals appear.

In the receive mode, the channel signal is extracted using the bandpass filter PF 12 from the spectrum of the primary group (with a band of 60 ... 108 kHz) and fed to an individual converter DM 12. The other input of the DM 12 receives the same signal of the subcarrier frequency F 12, which also feeds M 11. The spectrum of the output signal DM 12 consists of two side (relative to F 12) bands. The signal from the lower of these bands is highlighted by a low-pass filter, amplified and fed through the DS to the subscriber. The receiving paths of 11 other channels are constructed in the same way. In equipment with 60 or more channels, individual equipment is placed in special racks of individual converters SIP-60 or SIP-300.

In practice, another option is also used: the formation of a primary group of four preliminary groups (Figure 4.8), each of which combines three PM channels. The two-fold transformation principle is implemented here (Figure 4.7, b)

Figure 4.7. Structural diagrams and diagrams of single (a) and double (b) transformation of the spectrum of the PM channel

Figure 4.8. Structural diagram of the formation of steam generator using a double transformation

The further process of enlarging the groups of channels takes place in the group equipment and is explained in Figure 4.3.4. Identical frequency bands of five PGs, using the primary group conversion, are spread in frequency in the 312 ... 552 kHz band and form a 60-channel (secondary) group (SH). Figure 4.9 shows a simplified block diagram of VG group equipment. Messages from five primary groups PG 1 - PG 5 are fed to five group converters ГП 1 - ГП 5, to the second inputs of which signals of subcarriers are received from the generator equipment.

Figure 4.9. Block diagram of VG group equipment

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

To reduce the transient influences 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, the carrier frequencies of 468, 516, 564, 612 kHz are supplied to the GP 2 - GP 5, and the corresponding bandpass filters select the lower side bands (as shown in Figure 4.9). In the second case, carrier frequencies of 300, 348, 396, 444 kHz are fed to the GP 2 - GP 5, and the upper side bands are highlighted by the 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. The equipment of the primary group conversion is located in special racks of the primary converters USPP or SPP. The next steps of the group transformation are performed in the same way.

The equipment for the formation of group paths can consist of various combinations of standard blocks in which one or another stage of frequency conversion is carried out. For example, in the currently widely used equipment of the K-1920 system, PM channels are combined into two 60-channel groups (VG) and six 300-channel groups (TG). In this case, the total number of channels N = 60 ∙ 2 + 300 ∙ 6 = 1920.

After the nominal number of channels is reached by serial combining, another frequency conversion is usually carried out: the total (group) spectrum is converted into a linear spectrum, that is, into the frequency band in which the multichannel signal of this system is transmitted over the line. This takes into account the features of each line.

If individual and group conversion is usually carried out in standard blocks and racks, then the interface of this equipment (in particular, the formation of a linear spectrum) with a linear path is performed in equipment specific to each given wired or radio system.

Consider basic characteristics of group messages.

When designing and developing multichannel transmission systems, it becomes necessary to quantify the parameters of group messages at various stages of conversion, in particular, signals at the input of the linear path. These parameters, as well as for any communication signals, are determined by the corresponding frequency, information and energy characteristics.

According to the CCITT recommendation, the average message power in the active channel at the point with the zero relative level is set equal to 88 μW0 (- 10.6 dBm0). However, when calculating P av, the CCITT recommends taking the value P 1 = 31.6 μW0 (- 15 dBm0) (in addition to the activity of the channels, other factors are also taken into account, for example, the organization of TT channels in some PM channels, the imperfection of individual equipment, and the like). If N ≥ 240, then the average power of the group message at the point of the zero relative level is P avg = 31.6N, μW, and the corresponding average power level is p avg = - 15 + 10 lg N, dBm0.

According to the standards adopted in the Russian Federation with N ≥ 240

P 1 = 50 μW0 (- 13 dBm0); p av = - 13 + 10 lg N, dBm0. (4.6)

If N< 240, то приходится учитывать существенную зависимость коэффициента активности от N. В этом случае Р 1 представляют как функцию N, и уровень средней мощности группового сообщения определяют иначе:

Рср = - 1 + 4 log N, dBm0. (4.7)

Some parameters and area of ​​application of typical equipment for cable transmission systems with FDCs are shown in Table 4.1.

Table 4.1. Parameters of a typical equipment for cable transmission systems with a frequency converter

4.4. Time division multiplexing (TDM), analog transmission methods

Formation of the signal of the linear path of the transmission systems with the VRM and analog transmission methods. With a VRM on the transmitting side, continuous signals from subscribers are transmitted alternately (Figure 4.9)

To do this, these signals are converted into a series of discrete values ​​that are periodically repeated at certain time intervals T d, which are called the sampling period (see Figure 4.10). According to the theorem of V.A. Kotelnikov, the sampling period of a continuous, spectrum-limited signal with an upper frequency F in >> F n should be equal to

T d = 1 / F d, F d ≥ 2F in, (4.8)

The time interval between the nearest pulses of the baseband signal T to is called a timeslot or 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 N gr = 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 ∆t to = T d / N gr.

Thus, in the case of TDM, messages from N subscribers and additional devices are transmitted over a common communication channel in the form of a sequence of pulses, the duration of each of which is τ and< ∆τ к (смотри рисунок 4.10 и 4.11) .

Figure 4.11. Group signal at VRK with PPM

With time division of channels, the following types of pulse modulation are possible (Figure 4.12): AIM - amplitude-pulse modulation; PWM - pulse width modulation; FIM - Pulse Phase Modulation.

Figure 4.12. Modulation of channel impulses at the VRK: a) continuous message; b) AIM; c) PWM; d) FIM

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 type of modulation when converting an analog signal to digital,.

With PWM, the signal spectrum changes depending on the pulse width. 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 a 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 VDK, PPM is mainly used.

A characteristic feature of the signal spectra with pulse modulation is the presence of components with frequencies Ω n… Ω in the transmitted message u to (t) (Figure 4.3). This spectrum feature indicates the possibility of PWM and PWM demodulation with a low-pass filter (LPF) with a cutoff frequency equal to Ω in. Demodulation will not be accompanied by distortions if the components of the lower sideband (ω d - Ω in) ... (ω d - Ω n) do not fall into the low-pass filter passband, and this condition will be fulfilled if you choose

F d> 2F in,

which corresponds to condition (4.11). Usually take ω d = (2.3 ... 2.4) Ω in and when sampling a telephone message with a frequency band of 0.3 ... 3.4 kHz, the sampling frequency F d = ω d / 2π β is chosen equal to 8 kHz, and the sampling period T d = 1 / F d = 125 μs.

With PPM, the components of the spectrum of the modulating message (Ω n ... Ω in) 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.

4.5. Principles of constructing equipment with a VRK

Figure 4.13 shows a simplified block diagram of a terminal station of a multichannel system with a VDC. A continuous message from each of the subscribers u 1 (t) ... u N (t) through the corresponding differential systems DS 1 ... DS N are fed to the inputs of the channel modulators KM 1 ... KM N. In channel modulators, in accordance with the transmitted message, the pulses are modulated, following through the sampling period T d, according to one of the parameters, for example, PPM. In accordance with the value of the transmitted continuous message (Figure 4.12, a), at the moment of counting with PPM, the position of the pulse of constant amplitude and duration changes relative to the middle of the channel interval from + ∆t m to - ∆t m (Figure 4.12, d). Modulated pulses from the CM output, synchronization pulses from the synchronization generator (GIS), as well as the impulses of the service communication sensor (DSS), the control and call signal sensor (OUV) are combined. The result is a group signal u gr (t). To ensure the operation of channel modulators and additional devices, sequences of pulses with a sampling frequency of F d, shifted relative to the first channel by i∆t to, where i is the channel number. Thus, the moments of the start of the CM work 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 u gr (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 designed to generate a signal u gr (t): KM 1 ... KM N, RK, GIS, DUV, DSS, R - are included in the signal combining equipment (AO), which combines all signals in time and forms a group signal. Further, the signal can be transmitted to the next station via wired connecting lines or using radio communication.

Figure 4.13. Simplified block diagram of the terminal station of the communication system with the VRK

At reception, the selected signal u * gr (t) is fed to the inputs of all channel demodulators KD 1 ... KD N and receivers of intercom (MSS), control and call (PUV).

Channel demodulators divide u * gr (t) into separate channel signals, which are discrete samples, and restore from these samples continuous messages u * 1 (t) ... u * N (t), corresponding to those fed to the inputs of the CM in the AO. To ensure the time separation of channel signals, it is necessary that each of the CDs should be opened one by one only (!) At the time intervals ∆t k corresponding to the given channel. the transmitting end of the communication line. To ensure correct channel separation, the RK ′, which is in the AR, must work 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 appropriate subscribers through differential systems.

The noise immunity of transmission systems with a VRK is largely determined by the accuracy and reliability of the synchronization system and channel distributors installed in the equipment for combining and separating channels. To ensure the accuracy of the synchronization system, synchronization pulses (IS) must have parameters that allow the most simple and reliable separation of them from the sequence of pulses of the group signal u * gr (t). The most expedient for PPM turned out to be the use of dual ICs, for the transmission of which one of the time slots ∆t k is allocated in each sampling period T d (see Figure 4.11).

Let us determine the number of channels that can be obtained in a system with FIM. Figure 4.11 shows the pulse sequence for multichannel PPM transmission. It follows from the figure that

T d = (2∆τ max + τ s) N gr, (4.9)

where τ s - guard interval; ∆τ max - maximum displacement (deviation) of impulses. In this case, we assume that the duration of the pulses is small in comparison with τ s and ∆τ max.

From formula (4.9) we obtain

; (4.10)

Maximum pulse deviation for a given number of channels

, (4.11)

We accept, therefore

... (4.11, a)

Considering that for telephone transmission T d = 125 μs, we obtain at N gr = 6 ∆τ max = 8 μs, with N gr = 12 ∆τ max = 3 μs and at N gr = 24 ∆τ max = 1.5 μs. The higher the ∆τ max, the higher the noise immunity of the PPM system.

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.

Control questions:

1. What does a multichannel communication system include? Explain how it works.
2. What is the principle of frequency division of channels?
3. Define a modulator. What are useful modulation products?
4. How long is the cycle time when transmitting telephone messages from the VRK, why?
5. What are clippers for FDC transmission systems for?
6. What are frequency filters used for in transmission systems with VRM?
7. What is the principle of time division of channels?
8. Explain the purpose of the diffsystem (simplified block diagram of the terminal station of the communication system with the VRK), what requirements should such devices satisfy?
9. What types of pulse modulation are possible with time division multiplexing?
10. What signal parameter is the carrier of information in signals with AMM, PPM, PWM?
11. Why are synchronization pulses transmitted?
12. List the types of synchronization by purpose.
13. What causes the mutual interference arising from channel separation? What is done to reduce the level of mutual interference?

So let's take a look at how a mobile phone call is made. As soon as the user dials the number, the handset (HS - Hand Set) begins to search for the nearest base station (BS - Base Station) - the transceiver, control and communication equipment that makes up the network. It includes a base station controller (BSC-Base Station Controller) and several repeaters (BTS - Base Transceiver Station). The base stations are controlled by the Mobile Switching Center (MSC - Mobile Service Center). Thanks to the cellular structure, the repeaters cover the area with a zone of reliable reception in one or several radio channels with an additional service channel through which synchronization takes place. More precisely, the communication protocol of the device and the base station is negotiated by analogy with the procedure of modem synchronization (handshacking), during which the devices agree on the transmission rate, channel, etc. When the mobile device finds a base station and synchronization occurs, the base station controller generates a full duplex channel to the mobile switching center via the fixed network. The Center transmits information about the mobile terminal to four registers: Visitor Register of Mobile Subscribers or "Guests" (VLR - Visitor Layer Register), "Home" Register of Local Mobile Subscribers (HRL - Home Register Layer), Subscriber or Authentication Register (AUC - AUthentiCator) and an Equipment Identification Register (EIR). This information is unique and is located in a plastic subscriber microelectronic telecard or module (SIM - Subscriber Identity Module), which is used to verify the subscriber's eligibility and billing. Unlike landline telephones, the use of which is charged depending on the load (the number of busy channels) received over a fixed subscriber line, the mobile communication fee is charged not from the telephone set in use, but from the SIM card, which can be inserted into any apparatus.

The card is nothing more than an ordinary flash chip made using smart technology (SmartVoltage) and having the necessary external interface. It can be used in any device, and the main thing is to match the operating voltage: earlier versions used a 5.5V interface, while modern cards usually have 3.3V. The information is stored in the standard of a unique international subscriber identifier (IMSI - International Mobile Subscriber Identification), which eliminates the possibility of the appearance of "doubles" - even if the card code is randomly selected, the system will automatically exclude a fake SIM, and you will not have to pay for other people's calls later. When developing the standard for the cellular communication protocol, this moment was initially taken into account, and now each subscriber has his own unique and unique identification number in the world, which is encoded with a 64-bit key during transmission. In addition, by analogy with scramblers designed to encrypt / decrypt a conversation in analog telephony, 56-bit coding is used in cellular communications.

Based on this data, the system forms an idea of ​​the mobile user (his location, status in the network, etc.) and a connection is made. If a mobile user during a conversation moves from the coverage area of ​​one repeater to the coverage area of ​​another, or even between the coverage areas of different controllers, the connection is not interrupted or deteriorated, since the system automatically selects the base station with which communication is best. Depending on the channel load, the phone chooses between the 900 and 1800 MHz networks, and switching is possible even during a conversation, absolutely imperceptible for the speaker.

A call from a regular telephone network to a mobile user is carried out in the reverse sequence: first, the location and status of the subscriber is determined based on constantly updated data in the registers, and then the connection and communication are maintained. The maximum radiation power of a mobile device, depending on its purpose (permanent or portable automobile, wearable or pocket), can vary within 0.8-20 W (respectively 29-43 dBm). As an example, in table 4.9. the classes of stations and subscriber devices according to the applied power, adopted in the GSM-900 system, are given.

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

The division of channels is carried out by frequency. Since the radio channel has a certain spectrum, in the sum of all transmitting devices, modern radio communication is obtained. For example: the signal spectrum for a mobile phone is 8 MHz. If a mobile operator gives a subscriber a frequency of 880 MHz, then the next subscriber can occupy a frequency of 880 + 8 = 888 MHz. Thus, if a mobile operator has a licensed frequency of 800-900 MHz, then it is able to provide about 12 channels, with frequency division.

Frequency division multiplexing is used in X-DSL technology. Signals of various frequencies are transmitted over telephone wires: telephone conversation - 0.3-3.4 kHz and a band from 28 to 1300 kHz is used for data transmission.

It is very important to filter the signals. Otherwise, signal overlaps will occur, which can severely degrade communication.

The practice of building modern information transmission systems shows that the most expensive links in communication channels are communication lines: cable, waveguide and fiber optic, radio relay and satellite, etc. Since it is economically inexpedient to use an expensive communication line to transfer information between a single pair of subscribers, the problem arises of constructing multichannel transmission systems in which one common communication line is compressed by a large number of individual channels. This ensures an increase in the efficiency of using the bandwidth of the communication line. Messages А 1 (t), ..., А N (t) from N sources IC 1, ..., IC N with the help of individual modulators М 1, ..., М N are converted into channel signals U 1 (t), ..., UN (t ). The sum of these signals forms a group channel signal U L (t), which is transmitted over a communication line (LAN). 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 from the group signal Z (t) the corresponding channel signals Z 1 (t), ..., Z N (t) and convert them into messages. Blocks M 1, ..., M N and the adder form the compaction equipment, blocks M, LS and P - the group channel. Compression equipment, group channel and individual receivers form a multichannel communication system.

In order for the separating devices to be able to distinguish between the signals of individual channels, the corresponding features inherent only to this signal must be determined. In the case of continuous modulation, such signs can be frequency, amplitude, phase, in the case of discrete modulation, also the waveform. In accordance with the signs used for separation, the separation methods are also different: frequency, time, phase, etc.

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

In telemechanics systems for the transmission of many signals over one communication line, the use of conventional coding is shown to be insufficient. Either additional signal separation is required, or special coding that includes signal separation elements. Separation of signals - ensuring the independent transmission and reception of many signals on the same communication line or in the same 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 division, carried out on the basis of time (less often frequency) division of signals with the sending of an address code;

Frequency division, in which each of the signals is assigned a different frequency and the signals are transmitted sequentially or in parallel in time;

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

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

Time division (VR). A line is provided to each of the n - signals in turn: first, for a period of time t 1, signal 1 is transmitted, for t 2 - signal 2, etc. Moreover, each signal occupies its own time interval. The time allotted for the transmission of all signals is called a cycle. The signal bandwidth is determined by the shortest pulse in the pattern. Guard slots are needed between the information slots to avoid channel interference on the channel, i.e. pass-through distortion.

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

Code - address division of signals (KAR). Time code-address division of signals (VKAR) is used, in this case, a synchronizing pulse or code combination (synchrocombination) is first transmitted to ensure the coordinated operation of the valves at the control point and the controlled point. Next, a code combination is sent, called the address code. 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 (TC, 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 division of signals. For each of the n - signals, its own band is output in the frequency range. At the receiving point (CP), each of the sent signals is selected first 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, relative or phase-difference keying is used.

Time-frequency division of signals. The shaded squares with numbers are signals transmitted in a specific frequency band and in a designated time interval. There are guard time intervals and frequency bands between the signals. In this case, the number of generated signals increases significantly.

Channel separation methods: spatial, linear (frequency, time), shape. Linear channel separation condition.

In multichannel systems, all signal paths must be separated in some way so that the signal of each source can reach the corresponding receiver. This procedure is called channel separation or separating channel signals.

Multiplexing(eng. MUX) - a procedure for combining (multiplexing) channel signals in the MRP.

The inverse procedure of multiplexing is related to channel splitting - demultiplexing(English 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 figure).

Figure - Classification of channel separation methods

In SMEs, the following channel separation methods are distinguished:

- spatial (schematic);

- linear: frequency - FDK, time - FDK, channel separation by form - RKF;

- nonlinear: reducible to linear and majority.

Spatial separation.

This is the simplest type of separation, in which an individual communication line is assigned to each channel:

Figure - ISP with spatial separation of channels

AI is a source of information

PI - information receiver

LAN - communication line

Other forms of channel separation involve the transmission of messages over a single communication line. In this regard, multichannel transmission is also called channel sealing.

Generalized block diagram of an MSP with linear separation of channel signals

M i - modulator of the i-th channel

P 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

SS - transmitting side sync signal

PS - the receiver of the sync signal 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, at the other input of which linearly independent or orthogonal carriers arrive from the 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), which has changed under the influence of various types of interference and distortions n (t), is fed to the multipliers P 1, P 2, ..., P N, above the entrance of which the carriers come from the generators of the vectors 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 noise and distortion, S "1 (t), S" 2 (t), ..., S "N (t). Then the channel signals are fed to D 1, D 2, ..., D n that convert channel signals into primary ones, taking into account interference and distortion С "1 (t), С" 2 (t), ..., С "N (t).

The operation of the transmission system is possible with a synchronous (and sometimes in-phase) effect of carriers on the conversion devices M in transmission and multiplication P in reception. To do this, on the transmitting side, a sync signal (SS) is introduced into the baseband signal, and on the receiving side, it is separated from the baseband signal by the sync signal receiver (SS).

Multichannel telecommunication systems with frequency division multiplexing. Methods for the formation of channel signals.

Telecommunication system frequency division multiplexing is 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 scheme of the N-channel system and frequency plans at its characteristic points.

Figure - Structural diagram of an N-channel MRP with a frequency converter

Harmonic vibrations with different frequencies are used as carriers in MSF with PRC. f 1, f 2, ... f n(carrier fluctuations):

ψ i(t) = S i

Channel signals are formed as a result of modulation of one of the parameters of the carriers with primary signals C i (t)... Are applied amplitude, frequency and phase modulation. The frequencies of the carrier oscillations are chosen so that the spectra of the channel signals S 1 (t) and S 2 (t) did not overlap ... Group signal S gr (t) received in the communication line is the sum of the channel signals

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

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

In the receiving part, the channel signals are separated using channel band-pass crossover filters KPF-1, KPF-2, KPF-n, i.e. from group signal extract channel signals .

The primary signals are restored by the 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 the FDC, the dominant position is occupied by the AM-SSB modulation, since it is the most compromise.

Figure - Variants of filtering band for AM-SSB

Formation of the AM-SSB signal in communication technology is carried out in two ways:

1) Filter method

2) Phase difference method

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

Filter method

On the transmitting side

Example:

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

On the receiving side

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

Phase difference method

Principle of operation: the circuit consists of two arms, connected at the input and output by means of decoupling devices (RU). To the modulator (M 2) of one arm, the original signal and the carrier frequency are supplied phase-shifted by π / 2 with respect to the signal and carrier frequency supplied to the modulator (M 1) of the other arm. As a result, only one sideband will oscillate at the output of the circuit. Phase contours (ФК 1, ФК ФК 2) provide a phase shift of π / 2.

Separability condition for channel signals in SMEs with CHRK 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); bandwidth of channel signals D f i, as well as the frequency characteristics of the attenuation of the channel band-pass crossover filters KPF-1, KPF-2, KPF-n.

Crossover filters provide low passband attenuation ( apr) and the required amount of attenuation in the range of effective retention ( apod). Between these bands are the filtering bands of the crossover filters. Therefore, the channel signals must be separated by guard gaps (D fz), the values ​​of which must not be less than the filtering bands.

Hence, group signal width can be determined by the formula

D f gr= N× (D fi+ D f s)

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

In modern SMEs of telephone communication, each KTCH is allocated a frequency band of 4 kHz, although the frequency spectrum of transmitted audio signals is limited to a band from 300 to 3400 Hz, i.e. the spectrum width is 3.1 kHz. Between the frequency bands of adjacent channels, intervals of 0.9 kHz are provided, designed to reduce the level of mutual interference when filtering signals. This means that in multichannel communication systems with frequency division signals, only about 80% of the bandwidth of the communication line 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. Methods for splitting channels

5.1 Methods of channel separation: spatial, linear (frequency, temporal), in shape. Linear channel separation condition. Carrier signals and modulation of their parameters.

5.2 Multichannel telecommunication systems with frequency division of channels. Methods for the formation of channel signals.

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