RU2541908C1 - Device for decoding signals passing through multibeam communication channel - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: device includes two first step correlators, two delay units, two second step correlators, two additional second step correlators and a decision device.

EFFECT: providing high quality of decoding regardless of the number of symbols in the transmitted message.

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Description

The invention relates to the field of transmission of discrete information and is intended for use in decoders of communication signals transmitted in multipath channels.

When transmitting communication signals through a multipath channel, distortions of their shape occur, and for the coherent processing and decoding of received signals, control (estimation) of the channel impulse response (ICC) is necessary. In this regard, most of the known solutions to these problems are based on radiation along with information (i.e., unknown at the receiving end of the communication system) pulses and test pulses, which are used to evaluate the ICC. Such a transmission principle is referred to as a “system with a test pulse and prediction” (SIIP) (see, for example, [1], section 3.1). This principle underlies, in particular, objects [2] - [4].

The disadvantage of the principle of constructing a communication system, implemented in well-known analogues, is the relatively low quality of reception (decoding) of messages, caused either by a loss of time when the transmission of test and information pulses is separated in time (as is the case in [1]), or by the need to expend part ( as a rule, half) of the energy of the transmitting equipment per test pulse while transmitting test and information pulses. In the end, both of these situations lead to the loss of part of the radiation energy due to the need to control the current form of the ICC.

The closest in technical essence to the claimed object is the device described in [5]. It is considered as a prototype. The prototype solves the following problem. One of two possible signals (or symbols) is transmitted - S0 (t) or S1 (t) (as applied to the digital embodiment of the decoding device, we further consider all signals as functions of discrete time, i.e., consider signals S0 (n) and S1 ( n), where n is the index of the discrete time argument). As noted above, the shape of the transmitted symbol during propagation in the multipath channel was distorted, described as a convolution of this symbol with the KFM, the shape of which is a priori unknown. At the receiving point, a decision must be made about which of the two characters was transmitted (i.e., decode the transmitted message).

The block diagram of the prototype is shown in figure 1; explanations on it are given below when describing the principle of operation of the prototype.

The principle of operation of the prototype is as follows. For each possible moment of arrival of the connected signal, a cross-correlation between the received signal and each of the two possible symbols S0 (n) or S1 (n) is calculated. This action is performed by the correlators of the first stage (respectively, positions 1-1 and 1-2 in figure 1). In this case, the correlators of the first stage are essentially matched filters, i.e. each correlator of the first stage plays the role of a filter consistent with the corresponding symbol. As a result of performing this function, temporary implementations are formed at the outputs of the correlators of the first stage 1-1 and 1-2, and at the output of that correlator whose reference oscillation coincides with the actually transmitted symbol, this temporary implementation is an estimate of the ICR in the working frequency band (with noise) , and at the output of another correlator - only noise. Due to the fact that the prototype information about which of the two possible symbols was transmitted is unknown, it sums up (the same time samples) time realizations generated at the outputs of both correlators of the first stage (adder 2 in figure 1). This result of summing up contains an estimate of the CPC. The adder 2 is also cumulative over a moving time interval, i.e. it accumulates arrays of estimates of the ICC (mixed with noise), which are formed sequentially in time as a series of connected symbols arrives. Such accumulation is necessary, in particular, in ensuring a decrease in the correlation of noise at the inputs of each of the second-stage correlators. The number of indicated accumulated implementations is limited both by the number of characters contained in the transmitted message and (this is a more stringent condition) by the interval of stability of the ICC.

Next, the calculation of the correlation between the estimate of the KFM (it is formed at the output of the adder 2 and fed to the reference inputs of the correlators of the second stage) and each of the temporary realizations formed at the outputs of the correlators of the first stage 1-1 and 1-2 is implemented. This function is performed in the correlators of the second stage (positions 3-1 and 3-2 in figure 1).

Further, for concreteness, we assume that the symbol S0 (n) is transmitted. Moreover, in the correlator of the second stage 3-1, a response is actually formed proportional to the energy of the transmitted multipath signal (i.e., the symbol S0 (n) distorted in the shape of the symbol), which corresponds to the effect of coherent addition of rays. This response is characterized by a high level. At the output of the correlator of the second stage 3-2 in this situation, only the realization of noise is formed. It has a low level, mainly because the noise coming to the inputs of the correlator of the second stage with 3-2 noises is practically not correlated with a large number of implementations accumulated in the adder 2. Further, when comparing (in the solving device 4) with each other and / or with a threshold of response levels formed at the outputs of the correlators of the second stage 3-1 and 3-2, a decision is made on the actually transmitted symbol. So, in the case under consideration, as a rule, the level at the output of the correlator of the second stage 2-1 will be large and / or higher than the threshold, which will lead (as a rule, i.e., as a trend) to the decision that the symbol S0 (n), which in the situation under consideration is the correct solution.

Thus, in the prototype, the problem of temporal dispersion of the energy of the communication signal is solved, because, as noted above, an effect is achieved that is equivalent to the coherent addition of all the rays (more precisely, the signals that came along all the rays).

The disadvantage of the prototype is the low quality of decoding that occurs when receiving a message consisting of a small number of characters, or with a relatively small interval of stability of the ICC. So, when receiving a message consisting of a single character, in the adder 2 of the prototype, the result of summing the responses of the correlators of the first stage 1-1 and 1-2 to the received additive mixture of the information pulse and noise is formed. In this case, at both inputs of each of the correlators of the second stage, the noise is largely correlated. This correlation of noise significantly eliminates the differences in the responses of the correlators of the second stage 2-1 and 2-2 to the received signal and thereby reduces the tendency to make the right decision. As the number of characters in the message increases, the effect of the above factor, which reduces the quality of decoding, although decreases, but very smoothly. A small interval of stability of the ICC in the light of the above explanation of the reasons for the low quality of decoding in the prototype leads to the same consequences as a small number of characters in the message.

The purpose of the claimed device is to eliminate the specified disadvantage of the prototype, i.e. providing the ability to receive (decode) the message regardless of these factors. The goal is achieved in that in a device for decoding signals containing two correlators of the first stage and two correlators of the second stage, as well as a solver, the common input of the correlators of the first stage being the input of the decoding device, the outputs of the first and second correlators of the first stage are connected to the first inputs, respectively the first and second correlators of the second stage, the outputs of the correlators of the second stage are connected to the inputs of the solving device, and the output of the solving device is the output of the device For coding, two additional correlators of the second stage are introduced, as well as two delay units, the output of the first correlator of the first stage being additionally connected to the first input of the first additional correlator of the second stage and to the input of the first delay unit, the output of which is connected to the second inputs of the first and second correlators of the second stage , the output of the second correlator of the first stage is additionally connected to the first input of the second additional correlator of the second stage and to the input of the second delay unit, the output of which connected to the second inputs of the first and second additional correlators of the second stage, and the outputs of the additional correlators of the second stage are connected to the inputs of the solver.

The listed set of features of the inventive decoding device (which is the minimum necessary) corresponds to its version, designed to receive a two-position code (ie, a code that takes one of two values, for example, "0" or "1"). In the general case, when receiving an N-position code, the decoder contains N correlators of the first stage and delay blocks, as well as N ² correlators of the second stage.

The block diagram of the claimed object is shown in figure 2, where indicated:

- 5-1 and 5-2 - correlators of the first stage;

- 6-1 and 6-2 - delay blocks;

- 7-1 and 7-3 - correlators of the second stage;

- 7-2 and 7-4 - additional correlators of the second stage;

- 8 - decisive device.

Each correlator of the first stage (5-1, 5-2) is implemented, for example, in accordance with the block diagram in Fig.5.14, p.295 of the book [6]. 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 first stage (in the indicated Fig. 5.14 it is designated as h (n)) is stored in its long-term memory, which is not shown in Fig. 5.14 for simplicity. In the claimed device, the support functions of the correlators of the first stage h (n) are:

- correlator 5-1 - h1 (n) = S0 (n);

- correlator 5-2 - h2 (n) = S1 (n).

When the correlator of the first stage is implemented in the spectral region (i.e., based on the fast convolution procedure), the discrete Fourier transform (DFT) operation is performed in advance on the support function of each of these correlators, and the array of the DFT result (the result of its complex conjugation) is stored in long-term memory corresponding correlator of the first stage. DFT is also performed on the arrays of samples of the input signal x (n), then elementwise multiplication (i.e., multiplication of the same samples) of the arrays of DFT results over the reference function and input signal and the inverse DFT (DFT) from the array of the results of the specified multiplication are performed. The period of updating the array of samples of the input signal for 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 pulses (signals) S0 (n), S1 (n) (the durations of all these pulses coincide in the simplest case), when the length of the DFT window is equal to the double duration of each of these signals. Two independently working correlators of the first stage are shown in FIG. 2 conditionally. When implemented in the spectral region, the DFT procedure leading to the composition of these correlators from the input signal may be common to all of them.

A variant of the block diagram of the correlator of the first stage in the time domain equivalent to the one considered is also possible; A description of this correlator option is given in [6], Fig. 6.186, p.418, where (in accordance with the current capabilities of the technique) 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 stored that stores the samples represented by multi-bit code words.

The dynamics of updating the input and output data of the correlator under consideration is illustrated, for example, in [7, pp. 76-78].

Each of blocks 6-1 and 6-2 delays the signal arriving at its input for a time equal to the period of the signals in the transmitted sequence T (this period, as a rule, is equal to the duration of each of these signals), and is implemented, for example, as a multi-bit shift register.

Each (including additional) correlator of the second stage 7-1 ... 7-4 is implemented similarly to the correlator of the first stage (preferably in the time-domain variant), with the only difference being that it does not have a long-term memory that stores the reference oscillation. The duration of the signal update cycle at the output of each correlator of the second stage can be, for example, one sampling period of the input signals.

The solving device 8 is a comparison chart of the current signal levels at its inputs with each other and / or with a predetermined threshold stored in its long-term memory: In case of exceeding the maximum of the four input signals (for device 8) of the threshold, the code is generated at the output of the solving device 8 combinations of their two received and decoded signals. So, for example, when transmitting code positions “0” and “1”, respectively, with signals S0 (n) and S1 (n), if the signal at the output of the correlator of the second stage 7-1, 7-2 is maximum and exceeds the threshold, 7-3 or 7-4 (in the inventive device, all these correlators for the function performed and their implementation coincide), the solving device 8 as a solution generates a code sequence, respectively, “0-0” (ie, the first of the received signals is decoded as “ 0 "and the second - also as" 0 ")," 0-1 "," 1-0 "or" 1-1 ".

The principle of operation of the claimed device is as follows. When transmitting a sequence of pulses, for example, S1 (n) -S0 (n) (ie, the code combination “1-0”) at the moment of the arrival of the first signal of this sequence (more precisely, at the moment of arrival of its trailing edge) at the output of the correlator of the first steps 5-2 (the reference oscillation of which coincides with the information pulse S1 (n)), a response is formed equal to the convolution of the ICC with the autocorrelation function (ACF) of the pulse S1 (n) in a mixture with noise. We further consider this moment to be conditionally zero. At the same moment, at the output of the correlator of the first stage 5-1 (whose reference oscillation coincides with the information pulse S0 (n)), a response is formed containing only noise. Then, at the moment T of the arrival of the second signal of this sequence, the output of the correlator of the first stage 5-1 (the reference oscillation of which coincides with the information pulse S0 (n)) generates a response equal to the convolution of the ICR with the ACF of the pulse S0 (n) mixed with noise. (We believe that the ACF of both signals coincide.) At the same moment, at the output of the correlator of the first stage 5-2 (whose reference oscillation coincides with the information pulse S1 (n)), a response is formed containing only the implementation of noise. Then, at the moment T, at the inputs of the (additional) correlator of the second stage 7-3, two convolute ICH convolutions with ACF of transmitted pulses (signals) S0 (n) and S1 (n) are formed, each mixed with noise; as a result, the signal at the output of this second stage correlator 7-3 has a relatively high level. At the same time, at one of the inputs of the correlator of the second stage 7-4, a convolution of the ICC with the ACF of the pulse S0 (n) in a mixture with noise is formed, and at its second input, only the implementation of noise; at the same time, only noise implementations are formed at all inputs of the second-stage correlators 7-1 and 7-2. Noise implementations at all inputs of all second-stage correlators are uncorrelated. Thus, there is a statistical tendency for the reception of the transmitted combination of signals S1 (n) -S0 (n) (ie, the code sequence “1-0”) the signal level at the output of the correlator of the second stage 7-3 will significantly exceed such at the output of the correlator of the second stage 7-4 and to an even greater extent - at the outputs of the correlators of the second stage 7-1 and 7-2. In this case, the decision device 8 generates a decision corresponding to the transmitted code combination.

When transmitting a code combination, for example, “0-0”, the convolution of the ICC with the ACF of the transmitted pulses (signals) S0 (n) and again S0 (n) will be simultaneously (taking into account the delay introduced in this situation by the delay unit 6- 1) to be formed only at the inputs of the correlator of the second stage 7-1, and at the same time, the decisive device 8 will develop a solution in favor of the code combination “0-0”.

The inventive device provides high quality decoding even with a small (but not less than two) number of characters transmitted in the message and a small interval of stability of the ICC (an interval of stability of the ICC is sufficient, not less than double the duration of the character, i.e. 2T).

Thus, the object of the invention is achieved.

Literature

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

2. A device for receiving discrete signals in a multipath communication channel. RF patent No. 2048701.

3. Digital device for demodulating discrete signals in a multipath communication channel. RF patent No. 2267230.

4. Device for transmitting discrete signals in a multipath communication channel. RF patent No. 959291.

5. Sussman S.M. A matched filter communication system for multipath channels // IEEE Trans. IT - 6. N 3. June 1960.

6. “The use of digital signal processing”, ed. E. Oppenheim. M .: World. 1980.

7. L. Rabiner, B. Gould. Theory and application of digital signal processing. M .: World. 1978.

Claims (1)

A device for decoding signals transmitted through a multipath communication channel containing two correlators of the first stage and two correlators of the second stage, as well as a solver, the common input of the correlators of the first stage being the input of the decoding device, the outputs of the first and second correlators of the first stage are connected to the first inputs of the first and the second correlators of the second stage, the outputs of the correlators of the second stage are connected to the inputs of the solver, and the output of the solver is the output of the decoding triples, characterized in that two additional correlators of the second stage are introduced into it, as well as two delay units, the output of the first correlator of the first stage being additionally connected to the first input of the first additional correlator of the second stage and to the input of the first delay unit, the output of which is connected to the second the inputs of the first and second correlators of the second stage, the output of the second correlator of the first stage is additionally connected to the first input of the second additional correlator of the second stage and to ode second delay unit, whose output is connected to the second inputs of the first and second additional correlators of the second stage and the outputs of additional correlators of the second stage are connected to the inputs of the decision unit.

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