- - [A.S. Goldberg. English Russian Energy Dictionary. 2006] Topics energy in general EN time sharing …
waveform separation- - [L.G. Sumenko. English Russian Dictionary of Information Technologies. M.: GP TsNIIS, 2003.] Topics Information Technology in general EN waveform separation … Technical Translator's Handbook
separation of signals by frequency- - [A.S. Goldberg. English Russian Energy Dictionary. 2006] Energy topics in general EN frequency sharing … Technical Translator's Handbook
separation of GNSS navigation signals coded- Source: GOST R 52928 2010: Global satellite navigation system. Terms and definitions original ...
frequency separation of GNSS navigation signals- Source: GOST R 52928 2010: Global satellite navigation system. Terms and definitions origin ... Dictionary-reference book of terms of normative and technical documentation
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time separation of digital data signals temporal division The reverse process of temporal combining of digital data signals. Note In accordance with the method of temporal combining of digital signals, the data acquires its name and the method of temporal division ... ... Technical Translator's Handbook
time separation of digital telecommunication signals- temporal division The reverse process of the temporal division of digital telecommunication signals. Note In accordance with the method used for the temporary combination of digital signals, telecommunications acquires its name and method of temporary ... ... Technical Translator's Handbook
Time separation of digital data signals- 40. Time division of digital data signals Time division E. Time demultiplexing The reverse process of time division of digital data signals. Note. In accordance with the method used to temporarily combine digital ... ... Dictionary-reference book of terms of normative and technical documentation
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If we consider the simplest network consisting of two points A and B, between which N digital channels are organized (it is not specified here how), then independent signal transmission over these channels is possible if these channels divided between themselves. The following ways of dividing channels between two points are possible:
Spatial division (space division), using different transmission media to organize channels;
Time division, which transmits digital signals in different time intervals in different channels;
Code division, in which division occurs by applying specific code values for each signal;
Wavelength separation at which digital signals transmitted over digital channels organized at different wavelengths in an optical cable;
Separation by fashion when organizing a channel on various types electromagnetic wave (modes) of hollow waveguides and optical cable;
Separation by polarization of an electromagnetic wave of hollow waveguides and an optical cable.
In all cases, the division of channels between two nodes does not imply the presence of a single medium for the propagation of an electromagnetic signal. To transmit signals in one propagation medium, channels separated by one or another criterion (except for spatial) are grouped by means of a combination (multiplexing) operation, forming a digital transmission system (DTS).
AT digital systems switching (CSC) such combination and separation of signals most often occurs using time division multiplexing. Time multiplexing is currently an important component of not only DSPs, but also CSCs, and plays a decisive role especially at the interface of these systems. In telephony, time multiplexing is defined as a tool for distributing (separating and combining) telephone channels in time when transmitted over a single physical communication line. In this case, one of the types of pulse modulation is used. Each pulse corresponds to the signal of one of the channels, signals from different channels are transmitted sequentially.
The principle of temporal combining of signals is shown in fig. 1.8, which shows a rotating commutator To(center) connected alternately to the outputs of the channel sequence. The switch is connected to the output of channel 1 at the time t, to channel 2 output at time t2, to the output of channel N at time t N , after which the process is repeated. The resulting output signal will consist of a sequence of signals different channels, shifted relative to each other for a while At.
Separation of signals on the receiving side will occur in a similar way: the rotating commutator is connected to the channels in turn, transmitting the first signal to channel number 1, the second signal to channel number 2, and so on. Obviously, the operation of the switches on the receiving and transmitting sides must be synchronized in a certain way so that the signals that come along the line are directed to the necessary channels. On fig. 1.9 shows the timing diagrams for the case of combining three channels through which amplitude-pulse modulated signals are transmitted.
As mentioned above, the DSP uses PCM signals, which are digital code sequences consisting of several bits.
temporary association several PCM signals is a combination of code sequences coming from different sources for joint transmission over a common line, in which the line at any time is provided for the transmission of only one of the received code sequences.
The temporal combining of PCM signals is characterized by a number of parameters. Cycle temporal combining is a set of successive time intervals allocated for the transmission of PCM signals from different sources. In the time combining cycle, each PCM signal is allocated a specific time interval, the position of which can be uniquely determined. Since usually each signal corresponds to its own transmission channel, such a time interval allotted for the transmission of one channel is called timeslot(KI). There are two types of cycle - basic, whose duration is equal to the sampling period of the signal, and supercycle - a repeating sequence of successive main cycles, in which the position of each of them is uniquely determined.
Rice. 1.8. Circular Interpretation of Time Multiplexing
Rice. 1.9. temporary association
When building PCM equipment, they use homogeneous temporary association PCM signals, in which the code word rates of the combined PCM signals are the same. This makes it possible to produce group association PCM signals and build on this basis hierarchical systems for transmitting PCM signals.
At frequency division channels(FDM) each of the messages to be transmitted occupies the frequency band of a standard PM channel. In the process of forming a group signal, each channel signal is assigned a frequency band that does not overlap with the spectra of other signals. Then the total bandwidth N-channel group will be equal to . Assuming that single-sideband modulation is used and each channel signal occupies the frequency band , for the spectrum of the group signal we obtain
Group signal is converted to line signal s l (t) and is transmitted over the communication line (transmission path). On the receiving side, after converting a linear signal into a group signal, the latter with the help of band-pass channel filters Ф To(see Fig. 11.1) with bandwidth and demodulators D To is converted into a channel message that is sent to the recipients of the messages.
To the input of the receiving device i-th channel signals of all N channels. In order to separate the signals without mutual interference, each of the filters Ф i must pass without attenuation only those frequencies that belong to a given i-th channel; frequencies of signals of all other channels filter Ф i must suppress. Due to the non-ideal characteristics of the bandpass channel filters, mutual crosstalk occurs between the channels. To reduce these interferences to acceptable level it is necessary to introduce guard frequency intervals between channels. AT modern systems multichannel telephone connection each channel is allocated a 4 kHz bandwidth, although frequency spectrum transmitted speech signals is limited to a band of 300 ... 3400 Hz, i.e. the width of the signal spectrum is 3.1 kHz. Thus, in this case = 0.9 kHz. This means that in multi-channel FDM systems, approximately 80% of the bandwidth of the transmission path is effectively used. In addition, it is necessary to ensure a high degree linearity of the entire group path.
At time division channels(TSC) a group path with the help of synchronous switches of the transmitter and receiver is alternately provided for signal transmission of each channel of the multichannel system. Structural scheme multi-channel transmission system with TRC is shown in Fig. 11.2.
Sequences of modulated pulses that do not overlap in time (for example, in amplitude) are used as channel signals in systems with TDM. The set of channel signals forms a group signal.
With time division, crosstalk between channels is also possible, which is mainly due to two reasons. The first reason is the imperfection of the frequency response and phase response of the transmission path, and the second is the imperfection of the synchronization of the switches on the transmitting and receiving sides. In order to reduce the level of mutual interference during TRC, it is also necessary to introduce protective time intervals. This requires a reduction in the pulse duration of each channel and, as a consequence, an expansion of the signal spectrum. So, in multichannel telephone communication systems, the band of effectively used frequencies F B=3100 Hz. In accordance with the Kotelnikov sampling theorem minimum value sample rate f D = 2f V= 6200 Hz. However, in real systems one chooses f D\u003d 8 kHz (with a margin).
Theoretically, TDM and FDM are equivalent in terms of the efficiency of using the frequency spectrum, however, in real conditions, systems with TDM are somewhat inferior to systems with FDM in this indicator due to the difficulties in reducing the level of mutual interference when separating signals. However, systems with VRC have undeniable advantage, due to the fact that due to the difference in the timing of the transmission of signals from different channels, they do not have crosstalk of non-linear origin. In RTO systems, the crest factor is lower. In addition, the RMC equipment is much simpler than the PMC equipment. TDM finds the widest application in digital transmission systems with PCM.
A special case of time separation is phase separation of signals, at which only two-channel transmission can be provided.
In the general case, signals occupying a common frequency band and transmitted simultaneously can be separated if the condition of their linear independence or the orthogonality condition.
These requirements are met signals that differ in shape. Wave-separated digital multi-channel systems use orthogonal sequences in the form of Walsh functions. A generalization of division by form, are asynchronous-address communication systems(AACC). In such systems, bandwidth reserves arising from "little active" subscribers are easily realized. So, for example, it is possible to organize a 1000-channel communication system in which any 50-100 subscribers out of a thousand are simultaneously transmitting.
At combined separation method the group signal is a display of certain combinations of discrete channel messages by means of numbers corresponding to the combination number. These numbers can be transmitted using discrete modulation signals of any kind. For example, for binary codes (m=2) and number of channels N=2 group message can receive possible values, corresponding to various combinations of zeros and ones: 00, 01, 10, 11. For N-channel systems will require different values of the modulated parameter (frequency, phase). In the general case, several parameters of the carrier can be modulated simultaneously, for example, amplitude and phase, frequency and phase, etc. The block diagram of a multichannel system with combinational (code) separation (compression) is shown in Fig. 11.3.
Fig.11.3. Structural diagram of a multichannel system with a combination seal
Recently, there has been a lot of interest in systems amplitude-phase modulation(APM), which can be implemented by a quadrature modulation scheme. In AFM systems, during the transmission interval of one elementary signal, its phase and amplitude take values selected from a number of possible discrete values of amplitudes and phases. Each combination of amplitude and phase values represents one of the multi-position group signal signals with code base . APM signals can also be generated by multilevel amplitude and phase modulation of two quadrature (phase-shifted by) oscillations of the carrier frequency.
AT last years theory is also successfully developed signal-code structures(SKK), aimed at increasing the transmission speed and noise immunity with significant restrictions on energy and the occupied frequency band. Questions of the theory of QCM are discussed in Chapter 11.
L E C T I O N 16
Topic:
The text of the lecture on the discipline:"Theory of electrical communication"
Kaliningrad 2013
Text of Lecture No. 27
by discipline:"Theory of electrical communication"
"Frequency, time and phase separation of signals"
Introduction
The most expensive element of a communication system is the communication line. In transmission systems, the common medium may be coaxial, balanced or optical cables, overhead communication cables or radio links. There is a need to condense physical circuits, simultaneously transmitting information from several terminal means of communication along them. The sealing of the communication line is carried out by means of sealing equipment, which, together with the transmission medium, forms multichannel transmission system.
Multi-channel transmission system(SME) is called the totality technical means, providing simultaneous and independent transmission of two or more signals over one physical circuit or communication line.
In multi-channel telecommunications, FDM and TDM are used. Code division of channels finds application in mobile radio communication systems.
With FDM, a certain spectrum (band) of frequencies is assigned to each communication channel. During TRC, pulse sequences of very short pulses containing information about the primary signals and shifted relative to each other in time are transmitted to the communication line.
FDM MSPs are analog systems, while VDM MSPs are digital systems.
For these purposes, systems with multiple access and compaction are being created. It is these systems that underlie modern communications.
Frequency division of signals
Functional diagram the simplest system multi-channel communication with channel division by frequency is shown in Fig. one
In foreign sources, the term Frequency Division Multiply Access (FDMA) is used to denote the principle of frequency division of channels (FCD).
First, in accordance with the transmitted messages, the primary (individual) signals having energy spectra , ,..., modulate the subcarrier frequencies of each channel. This operation is performed by modulators , ,..., of channel transmitters. The spectra of channel signals obtained at the output of the frequency filters , ,..., respectively occupy the frequency bands , ,..., , which in the general case can differ in width from the message spectra , ,..., . With wideband modulations, such as FM, the spectrum width 
, i.e. in general . For simplicity, we will assume that AM-OBP is used (as is customary in analog SPs with FDM), i.e. and .
Let's trace the main stages of signal formation, as well as the change of these signals in the process of transmission (Fig. 2).
We will assume that the spectra of individual signals are finite. Then it is possible to choose subcarriers w K so that the bands ,..., do not overlap in pairs. Under this condition, the signals ; 
mutually orthogonal.
Then the spectra , ,..., are summed up and their totality goes to the group modulator (). Here, the spectrum is transferred to the frequency range allocated for the transmission of a given group of channels by means of carrier frequency oscillations, i.e. the group signal is converted to a line signal. In this case, any kind of modulation can be used.
At the receiving end, the line signal is fed to the group demodulator (receiver P), which converts the spectrum of the line signal into the spectrum of the group signal. The spectrum of the group signal is then divided again into separate bands corresponding to the individual channels using frequency filters , ,.... Finally, channel demodulators D convert signal spectra into message spectra intended for recipients.
From the above explanations, it is easy to understand the meaning of the frequency method of channel separation. Since any real communication line has a limited bandwidth, then with multi-channel transmission, each individual channel is assigned certain part total bandwidth.
On the receiving side, the signals of all channels operate simultaneously, differing in the position of their frequency spectra on the frequency scale. In order to separate such signals without mutual interference, receivers must contain frequency filters. Each of the filters must pass without attenuation only those frequencies that belong to the signal of this channel; the frequencies of the signals of all other channels, the filter must suppress.
In practice, this is not feasible. The result is mutual interference between channels. They arise both due to the incomplete concentration of the signal energy of the k-th channel within a given frequency band , and due to the imperfection of real band-pass filters. In real conditions, it is also necessary to take into account mutual interference of non-linear origin, for example, due to the non-linearity of the characteristics of the group channel.
To reduce crosstalk to an acceptable level, it is necessary to introduce guard frequency intervals (Fig. 3).
So, for example, in modern systems of multichannel telephone communication, each telephone channel is allocated a band frequencies kHz, although the frequency spectrum of the transmitted sound signals limited to the band
The communication line is the most expensive element of the communication system. Therefore, it is advisable to conduct multi-channel information transmission over it, since with an increase in the number of channels N, its throughput S. Poich. condition must be met:
H K - performance 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 division.
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 the formula (7.1.1.) we obtain:
In other words, the receiver has selective properties with respect to the signal Sk(t).
Considering the issue of separation of signals, frequency, phase, time separation of channels, as well as separation of signals by shape and other features are distinguished.
Second study question
Frequency division channels
A block diagram of a multichannel communication system (MCS) with frequency division channels (FDM) is shown in Fig. 7.1.1, where it is indicated: IS - signal source, Mi - modulator, Фi - filter of the i-th channel, Σ - signal adder, GN - carrier generator, TX - transmitter, LS - communication line, IP - source of interference, PFP - receiver, D - detector, PS - message receiver.
Fig.7.1.1. Structural diagram of a multichannel communication system
With FDM, the carrier signals have different frequencies fi (subcarriers) and are separated by an interval greater than or equal to the width of the spectrum 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 (compacted in frequency) in block Σ forming a group signal, which modulates the oscillation of the main carrier frequency fn in block M.
All known techniques can be used to modulate the channel carriers. But more economically, the bandwidth of the communication line is used in single-sideband modulation (SSB AM), since in this case the width of the spectrum of the modulated signal is minimal and equal to the width of the spectrum of the transmitted message. In the second stage of modulation (group signal), OBP AM is also more often used in wired communication channels.
Such a signal with double modulation, after amplification in the TX block, is transmitted over the communication line to the RX receiver, where it undergoes an inverse conversion process, i.e. demodulation of the signal along the carrier in block D to obtain a group signal, extracting channel signals from it by bandpass filters Fi and demodulation of the latter in blocks Di. The central frequencies of the bandpass filters Fi are equal to the frequencies of the channel carriers, and their transparency bands are equal to the width of the spectrum of the modulated signals. The deviation of the real characteristics of the bandpass filters from the ideal ones should not affect the quality of signal separation, therefore, guard frequency intervals between channels are used. Each of the filters Ф reception must pass without attenuation only those frequencies that belong to the signal of a given channel. The signal frequencies of all other channels must be suppressed by the filter.
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The frequency separation of signals by ideal bandpass filters can be mathematically represented as follows:
where g k is the impulse response of an ideal bandpass filter that passes without distortion the frequency band of the k-th channel.
The main advantages of the CHRK: simplicity of technical implementation, high noise immunity, the possibility of organizing any number of channels. Flaws: the inevitable expansion of the used frequency band with an increase in the number of channels, the relatively low efficiency of using the bandwidth of the communication line due to filtering losses; cumbersome and high cost of equipment, mainly due to a large number of filters (the cost of filters reaches 40% of the cost of a system with FDM). In railway transport, an MCS with a K-24T type PMC has been developed, in which small-sized electromechanical filters are used.
Third study question