RU2541199C1 - Device for decoding discrete signals propagating in multibeam channel - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: device includes three correlators, a unit for determining a cyclic time shift value corresponding to the maximum of a cyclic correlation function and a decision device.

EFFECT: reduced computational costs.

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Description

The invention relates to the field of transmission of discrete (digital) information and is intended for use in decoders of communication systems operating in the conditions of multipath channels.

One of the main characteristics of a digital communication system is the transmission speed, and one of the ways to ensure a high transmission speed is the use of multi-position (N-position) coding. Moreover, each transmitted information pulse contains 2 ^N bits of information. The disadvantage of this way in the general case of a code variant is the need to implement when receiving (decoding) a N-channel correlator message (see, for example, [1]) in each spatial and Doppler reception channel (if any), which entails technical complexity and the high cost of the decoder.

This drawback is overcome by using N-position coding based on signals phase-manipulated (FM) by m-sequences, as follows: for a selected working m-sequence, coding is performed by its cyclic time shift [2]. Using this encoding option, when decoding a message, it is limited to the low-channel (in each spatial and Doppler reception channel, if any) equivalent of the correlator, which computes the cyclic convolution of the received signal with the transmitted FM signal read in reverse time. (Reference: if the initial signal of duration τ = M · Δ (where Δ is the sampling period of both the input signal and the reference functions of all correlators) has the form S (n) = S (t _n = n · Δ) (in, so to speak , natural time, that is, without time inversion), then the signal read in reverse time (or with time inversion) has the form S (Mn · Δ); Synonymic variants of the term “cyclic convolution” are the terms “circular ...” or “periodic convolution”; for an explanation of this term, see [3, Section 2.23]; it should also be noted that, to a significant degree, they are synonyms mines are “correlation” and “convolution”; the only difference between them is that if, in calculating the correlation, both signals are read in natural time, then in the calculation of convolution, one of the signals is read in inverse time; therefore, both of these terms are used below. The decoding in this coding is based on measuring the value of the temporary cyclic shift of the maximum of the result of the calculation of the cyclic convolution. nical convolution.

However, when transmitting information through the multipath channel, the indicated equivalent of the correlator that calculates the specified cyclic convolution becomes insufficient to solve the decoding problem, since when the signal waveforms propagate in the multipath channel, the received information pulses with the original transmitted FM signal are substantially decorrelated. It should be noted that this effect takes place regardless of the magnitude of the cyclic time shift introduced into each of these pulses. Under such conditions, decoding the message becomes impossible.

The closest in technical essence to the claimed object is a device for decoding described in [4, Fig. 3] (prototype). The block diagram of the prototype is presented in figure 2 (the features appearing in [4, Fig. 3], which are not, from the point of view of the authors, essential, are omitted in the present description), where indicated (the adopted numbering in the designation of the features of the prototype corresponds to the through numbering of similar signs in the following block diagram of the claimed device):

- 1, 2 - the first and second correlators, respectively;

- 4 - unit for determining the magnitude of the cyclic time shift corresponding to the maximum of the correlation function (in the terminology of the prototype description in [4] - “the identifier of number i in the l-th ray”);

- 5 - a decisive device (in the description of the prototype in [4, Fig. 3] it is absent, but implied);

- 6 - buffer memory device;

- 7 - block determining the delay of rays (in the terminology of the description of the prototype in [4] - "address determinant l (KFM)");

- 8 - drive number i on (all) l beams.

The principle of operation of the prototype is as follows. The received signal mixture with noise is recorded in the buffer memory 6 and from it goes to the first correlator 1. The correlator 1 calculates the correlation function between the implementation of the input signal and its own reference function, which coincides with the clock signal S _c (n). Hereinafter, in cases where the correlator reference function is stable, i.e. not updated over time, this function is stored in the memory included in the correlator; the reference input of each such correlator in figure 2 (as well as in the future and figure 1) is not shown. In the prototype, this applies to both correlators 1 and 2.

. (Справка: под циклической корреляционной функцией понимаем циклическую свертку, вычисляемую при чтении опорной функции в естественном времени). В связи с тем, что опорной функцией коррелятора 1 является синхросигнал S_c(n), временная реализация, формируемая на выходе этого коррелятора, есть полученная по синхросигналу оценка импульсной реакции канала (ИРК) распространения. Этот результат поступает на вход блока определения задержек лучей 7; функция данного блока реализуется путем сравнения с порогом всех временных отсчетов сигналов, формируемых на выходе коррелятора 1 при приеме каждого синхросигнала в отдельности, и определения тех моментов времени, в которые этот порог отсчетом сигнала превышен, т.е. моментов прихода сигнала в отдельных лучах. В связи с тем, что нумерация моментов времени прихода лучей однозначно соответствует адресам выборок сигнала, хранящихся в буферной памяти 6, в описании прототипа в [4] этот блок 7 назван как «определитель адресов l (ИРК)» (здесь l - количество лучей, по которым в точку приема пришел сигнал). Эти номера по мере приема синхроимпульсов (которые многократно передаются наряду с информационными импульсами) могут накапливаться. В связи с этим между выходом блока определения задержек лучей 7 и управляющим входом буферной памяти 6 в описании прототипа в [4] включен накопитель ИРК. В связи с тем, что этот признак прототипа обязательным не является, в настоящем описании прототипа он опущен.The correlator 2 calculates a cyclic (or, what is the same thing, periodic or circular) correlation function between a fragment of a received signal coming from a buffer memory device 6 and a reference function equal to the original (transmitted) information signal at its zero time cyclic shift $$S_{\mathrm{and}}{}^{(0)}\left(n\right)$$

. (Reference: by a cyclic correlation function we mean a cyclic convolution calculated when reading the support function in natural time). Due to the fact that the reference function of the correlator 1 is the clock signal S _c (n), the temporal realization formed at the output of this correlator is the estimate of the pulse response of the propagation channel (IRF) obtained from the clock signal. This result is fed to the input of the ray delay determination unit 7; the function of this unit is implemented by comparing with the threshold all time samples of signals generated at the output of correlator 1 when each clock signal is received separately, and determining those times at which this threshold is exceeded by the signal sample, i.e. moments of arrival of the signal in individual beams. Due to the fact that the numbering of the times of arrival of the rays unambiguously corresponds to the addresses of the samples of the signal stored in the buffer memory 6, in the description of the prototype in [4] this block 7 is called “address determinant l (KFM)” (here l is the number of rays, by which a signal arrived at the receiving point). These numbers as they receive clock pulses (which are repeatedly transmitted along with information pulses) can accumulate. In this regard, between the output of the unit for determining the delays of the rays 7 and the control input of the buffer memory 6 in the description of the prototype in [4] the IRF drive is included. Due to the fact that this feature of the prototype is not required, in the present description of the prototype it is omitted.

, сформированный в отличие от всех передаваемых информационных сигналов так, что порождающая его m-последовательность характеризуется фиксированным (и нулевым) циклическим временным сдвигом. Приходящий в каждом отдельном луче информационный сигнал, соответствующий передаваемому символу, например «i» $$S_{и}{}^{(i)}\left(n\right)$$

, коррелирован с опорной функцией $$S_{и}{}^{(0)}\left(n\right)$$

коррелятора 2 при циклическом сдвиге времени, равном ni=i·Δ. При передаче символа «i» каждый из l результатов вычисления циклической свертки (корреляции) в корреляторе 2 будет иметь максимум при временном аргументе этой свертки (корреляционной функции), равном ni. При наличии шумов возможны и такие ситуации, когда указанное условие выполнено не будет; оно выполняется лишь в смысле статистической тенденции, причем тем более часто, чем выше отношение сигнал/шум.Information about the ray delays from block 7 is fed to the control input of the buffer memory device 6 as a set of commands for sequentially reading l fragments of temporary realizations, each of which contains an information impulse that came one (from the 1st to the lth) from the rays. Each of these l fragments sequentially receives a signal at the input of the second correlator 2, to the reference input of which an information signal is supplied $$S_{\mathrm{and}}{}^{(0)}\left(n\right)$$

, formed in contrast to all transmitted information signals so that the m-sequence generating it is characterized by a fixed (and zero) cyclic time shift. The information signal arriving in each individual ray corresponding to the transmitted symbol, for example, “i” $$S_{\mathrm{and}}{}^{(i)}\left(n\right)$$

correlated with reference function $$S_{\mathrm{and}}{}^{(0)}\left(n\right)$$

correlator 2 with a cyclic time shift equal to ni = i · Δ. When transmitting the symbol “i”, each of l results of the calculation of cyclic convolution (correlation) in correlator 2 will have a maximum with the time argument of this convolution (correlation function) equal to ni. In the presence of noise, such situations are possible when the specified condition is not fulfilled; it is fulfilled only in the sense of a statistical trend, and the more often, the higher the signal-to-noise ratio.

The function of determining the temporal convolution argument that corresponds to the aforementioned maximum is performed by the unit 4 for determining the magnitude of the cyclic time shift corresponding to the maximum of the correlation function. Due to the fact that the maxima of the results of calculating the correlation by the correlator 2 when processing signals in all beams, as a statistical trend, coincide, their accumulation over all beams in block 8 increases the reliability of estimating the desired time argument of the cyclic convolution. The indicated estimate is actually the result of decoding the current message element. The output to the decoder shown in the description of the prototype in [4, Fig. 3] is formal, because for the final decoding of the message element after forming the estimate of the temporary convolution argument, it remains only to read the information symbol corresponding to this argument from the table of the same name (i.e., from the table correspondence of time shifts to transmitted characters). The specified function is performed by the solving device 5 omitted in the description of the prototype [4, Fig. 3].

In the prototype, the problem of having the number of correlators equal to the positioning of code N (not counting the correlator, whose support function coincides with the clock), is largely circumvented. However, in it, the second correlator 2, upon receiving the next information signal, fulfills its function l times, i.e. it is practically equivalent to l correlators in terms of required computing resources. Thus, in the prototype, the need for large computational resources remains, although (under the usually occurring condition l <N or even l << N) and to a lesser extent than in other analogues.

The aim of the proposed technical solution is to reduce the necessary computing resources for decoding.

The goal is achieved in that in a device for decoding discrete signals propagating in a multipath channel, containing the first and second correlators, a unit for determining the magnitude of the cyclic time shift corresponding to the maximum of the cyclic correlation function, and a solving device, wherein the output of the device for decoding is the output of the decoding device, a third correlator is introduced, connected between the output of the second correlator and the input of the cyclic time shift value determining unit, corresponding to maximizing the cyclic correlation function, the output of which is connected to the input of the resolver, the combined signal inputs of the first and second correlators are the input of the decoding device, and the output of the first correlator is connected to the reference input of the second correlator.

A block diagram of the inventive device is shown in figure 1, where indicated:

- 1, 2 and 3 - the first, and second, and third correlators, respectively;

- 4 - unit for determining the magnitude of the cyclic time shift corresponding to the maximum of the correlation function;

- 5 - decisive device.

Each of the correlators 1 ... 3 is implemented, for example, in accordance with [5, the block diagram in Fig.5.14, p.295]. In this case, the signal input of the correlator is the lower input in the indicated Fig. 5.14, to which the received signal x (n) is supplied. The reference function of the correlator of the stage (in the indicated fig. 5.14 it is designated as h (n) is stored in its memory, not shown in fig. 5.14 for simplicity. In the claimed device, the supporting functions of the correlators have the form:

- correlator 1 - h1 (n) = S _c (n);

- оценка ИРК;- correlator 2 - $$h2(n)=\stackrel{\wedge }{H}\left(n\right)$$

- assessment of the KFM;

.- correlator 3 - $$h3(n)=S_{\mathrm{and}}{}^{(0)}\left(n\right)$$

.

Each of the correlators 1 and 2 calculates a linear (or aperiodic) correlation function between the input signal and its own reference function. When each of these correlators is implemented in the spectral region (ie, based on the fast aperiodic convolution procedure; see [3, Section 2.23]), the support function of each of them, supplemented by the “right” M time axis with zero samples, performs the operation discrete Fourier transform (DFT), and the array of the DFT result (the result of its complex conjugation) is stored in the memory of the corresponding correlator. Moreover, the reference function of the correlator 1 is not updated in time, and therefore, the DFT operation on it is performed in advance, and the memory in which the result of this DFT is recorded is long-term. The reference function of correlator 2 is updatable, and therefore, the DFT operation on it is performed as this update is made, and the memory in which the result of this DFT is entered is operational.

(где $$S_{и}{}^{(0)}\left(n\right)$$

- информационный сигнал при нулевом сдвиге времени; длительности указанных сигналов совпадают). При этом длина окна всех реализуемых в корреляторах 1 и 2 операциях ДПФ (2М отсчетов) составляет двойную длительность каждого из этих сигналов. Два независимо работающих коррелятора 1 и 2 показаны на фиг.1 условно. При их реализации в спектральной области входящая в состав этих корреляторов процедура ДПФ от входного сигнала может быть для этих двух корреляторов общей.Above the arrays of samples of the input signal x (n) of size 2M, DFT is also performed when updating in the adjacent time cycles of computing this DFT by M samples, then elementwise multiplication (i.e., multiplication of the same samples) of the arrays of DFT results over the reference function and input signal is performed and the inverse of the DFT (DFT) from the array of results of the specified multiplication. The period of updating the array of samples of the input signal at adjacent time cycles of calculating the correlation in each of the correlators of the first stage is usually chosen equal to the duration of each of the signals (pulses) S _c (n) and $$S_{\mathrm{and}}{}^{(0)}\left(n\right)$$

(Where $$S_{\mathrm{and}}{}^{(0)}\left(n\right)$$

- information signal at zero time shift; the durations of the indicated signals coincide). In this case, the window length of all DFT operations implemented in correlators 1 and 2 (2M samples) is the double duration of each of these signals. Two independently working correlators 1 and 2 are shown in FIG. 1 conditionally. When implemented in the spectral region, the DFT procedure of the input signal, which is part of these correlators, can be common for these two correlators.

Correlators 1 and 2 can also be implemented in the time domain. A variant of the correlator block diagram implemented in the time domain is given in [5], Fig. 6.18b, p. 418, where (in accordance with the current state of the art) instead of a recirculating delay line that stores an array of time samples of the reference signal when it is strictly limited , a multi-bit shift register is implemented storing the same samples represented by multi-bit code words.

The correlator 3 calculates a cyclic correlation function between the input signal and its own reference function. The difference between this correlator and correlators 1 and 2 is as follows. The reference function of the correlator 3 is not supplemented with zero samples. This reference function is not updated over time, and therefore, as in the case of correlator 1, the DFT operation on it is performed in advance, and the memory in which the result of this DFT is stored is long-term. All DFT procedures in correlator 3 are performed on M samples of the input and reference signals.

Block 4 (determining the magnitude of the cyclic time shift corresponding to the maximum of the correlation function) performs a function that fully corresponds to its name. It contains a memory in which the temporary implementation is written, which is the result of calculating the cyclic correlation function by the correlator 3, as well as a block for determining the time argument (time index n), which corresponds to the maximum of the specified function. The last block is a programmable device.

The solving device 5 contains a table of correspondence of temporary cyclic shifts of the information signal {i} to the alphabet of symbols of the discrete communication system {A _i } for i = 1 ... N. When applying to its input the result of evaluating the cyclic time shift of the next received information signal in the decider 5, the symbol corresponding to this shift is read from the indicated table and its output to the consumer.

The inventive device is designed for use in a synchronous communication system. In such a system, at the receiving end, the arrival times of each information signal and the clock signal are known. There are basically three possible synchronization options.

a) A variant of the operation of the transmitter and receiver in a single time system; while the propagation time of the signal from the transmitter to the receiver is known. In this case, the decoder includes a timer that issues a synchronization signal to blocks 3 ... 5 at the time of the arrival of the next clock signal; the moments of arrival of information signals are tightly connected with this moment; in this case, the clock signals can be transmitted either simultaneously with each k-th information signal (for k, an integer, in particular, equal to 1), or alternately (i.e., with time diversity) with a block of k information signals (see [6 ], Fig. 3.1 a on p.108). At the moment of supplying the synchronization signal, the execution of its function by block 3 begins and then with a small delay (necessary for a single calculation of the cyclic correlation function by block 3) perform its functions block 4 and then (also with a small delay) block 5. Correlators 1 and 2 are invariant in time of the function (actually this linear filtering function), and therefore the “linking” of their work to the moments of arrival of the signals is not necessary.

b) The option of working with synchronization at the time of arrival of the clock signal. In this case, a comparison is made with the threshold of the signal samples at the output of the correlator 2 (the reference function of which coincides in shape with the clock signal), which ensures the detection of the clock signal that came in the first beam. The moment of the first (in the time interval of receiving the clock signal) detection of the clock signal (i.e., the clock signal that came in the first beam) is completely analogous to the synchronization option a) a clock pulse is issued in blocks 3 ... 5.

c) A variant of operation with synchronization at the time of arrival of the previous information signal [4]. Moreover, the difference between this synchronization option and variant b) consists only in the fact that the previous information signal plays the role of the clock signal.

These means of synchronization are not included in the composition of the claimed object, since the vast majority of digital (discrete) communication systems are synchronous, and therefore, a specialist does not need to specify synchronization means to reproduce the claimed object.

It should be noted that according to the functions performed, the correlators 1 and 2 of the prototype in the inventive device correspond to the correlators 1 and 3.

The process of the claimed device in dynamics during the transmission of clock signals in turn with a block k of information signals (see [6], Fig. 3.1 on p.108) is illustrated in Fig.3. On figa) conventionally shows a set of temporary implementations of the envelope of the clock signal that arrived at the receiving point for 3 beams with different amplitudes and propagation times, while the signal in the first beam arrives at time t ₁ ; Fig.3b) shows the KFM estimate generated by the indicated clock signal at the output of the correlator 1 (more precisely, the module of this assessment; each KFM sample is also characterized by a phase), this estimate is formed on the time interval t ₂ ... t ₂ + T _irk , where T _irk - the duration of the KFM; on figv) conventionally shows a set of temporary implementations of the envelope of the information signal that arrived at the receiving point on the same 3 beams as the clock signal, and this information signal came to the receiving point at time t ₃ ; on fig.3d) shows the result of working out its function correlator 2, which led to the compensation of the temporal delay of the signal in the multipath channel, i.e. to the formation of a single-beam information signal, the intensity of which is equal to the sum of the intensities of the information signal that came along each of the three rays, this single-beam signal is formed on the time interval t ₄ ... t ₄ + τ (τ is the duration of each information and clock pulse); on fig.3d shows the result of calculating the cyclic correlation by block 3, which is a peak at a time offset determined by the transmitted symbol i. Further, the magnitude of this shift is determined by block 4, after which it is converted by block 5 into a symbol corresponding to the received information signal.

It should be noted that the result of working out its function with the correlator 2, along with the (main) pulse shown in Fig. 3d, side pulses not shown in Fig. 3g are formed, however, the level of each of them is much lower than the level of the main pulse, and the development of its function by the correlator 3 is carried out it is over this pulse (shown in FIG. 3d) that is provided by the synchronization of the operation of the decoding device. Thus, in terms of synchronization, it is fundamentally important that block 3 performs its function on a temporary implementation formed at the output of block 2 precisely in the time interval t ₁ ... t ₄ + τ.

When the communication system operates in the single time mode, all time instants t ₁ ... t ₄ (note that t ₂ = t ₁ + τ), as well as the time parameters τ and T _irc (its predicted value) at the receiving point are known in advance. When synchronizing at the time of arrival of the clock signal, such time parameters as the difference t ₃ -t ₁ and also τ and T _{irk are known in advance} ; the instant of time t ₂ is determined by detecting the clock signal that came in the first ray and measuring the time of its arrival (as noted above, the technical means that implement this function are standard and go beyond the scope of the claimed device). When synchronizing by the moment of arrival of the previous information signal, the role of the clock signal in determining the moment t _{2 is} played by this information signal (the operation of the communication system in this synchronization option begins with a single transmission of the clock signal and initial synchronization by this clock signal, after which synchronism is maintained by estimating the moments of arrival at the first ray of information signals).

In the prototype, an estimate of the time shift of the code for each of the l rays separately was developed, and then these estimates were averaged (accumulated). In this regard, in it, the correlator 2, upon receipt of each information pulse, performed its function l times. In the claimed device, the function of quasi-coherent signal accumulation is realized (this accumulation is not fully coherent, since the correlator 2 forms an estimate of the KFM with finite accuracy) for all the rays. In this case, the multipath signal is transformed into a single-beam signal, characterized by the total (i.e., summed over all the rays) intensity, which makes it possible to calculate the cyclic correlation function on the time interval of receiving one information signal once. Thus, the technical effect of saving the necessary computing resources of the decoding device in the inventive device is achieved.

Literature.

1. A device for transmitting and receiving multi-position signals. Auth. testimonial. USSR No. 649164.

2. Kwon H.M., Birdsal T.G. Digital Waveform Codings For Ocean Acoustic Telemetry. IEEE Journal of Oceanic Engineering, vol. 16, No. 1, January 1991. P.56-65.

3. Rabiner L., Gould B. Theory and application of digital signal processing. M .: World. 1978. 848 s, ill.

4. V.Z. Krantz, V.V. Sechin. Using information symbols to synchronize a communication system with complex signals // Hydroacoustics. Vol. No. 15, 2012. P.36-41.

5. The use of digital signal processing. Ed. E. Oppenheim. M .: Mir, 1980.552 s., Ill.

6. Klovsky D.D. Transmission of discrete messages over the air. M .: Communication. 1969.

Claims (1)

A device for decoding discrete signals propagating in a multipath channel, containing the first and second correlators, a unit for determining the magnitude of the cyclic time shift corresponding to the maximum of the cyclic correlation function, and a resolver, the output of the device for decoding is the output of the resolver, characterized in that the third a correlator connected between the output of the second correlator and the input of the cyclic time shift value determining unit corresponding to about the maximum of the cyclic correlation function, the output of which is connected to the input of the resolver, the combined signal inputs of the first and second correlators are the input of the decoding device, and the output of the first correlator is connected to the reference input of the second correlator.

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