Spread spectrum underwater acoustic communication coding and decoding method

Abstract

The invention relates to a spread spectrum underwater acoustic communication coding and decoding method, which comprises the following steps of: performing dual-channel coding modulation, sequentially performing CRC (cyclic redundancy check) coding, serial-parallel conversion, multi-system error correction coding and interleaving operation on an information bit stream at a transmitting end, then performing spread spectrum modulation on a code element by using a pseudorandom sequence, and finally mapping modulation output of an I channel and a Q channel into a baseband signal, and a spread spectrum underwater acoustic communication signal is formed through pulse shaping and up-conversion processing. According to the method, the communication accuracy is improved through a dual-channel incoherent and coherent redundancy decoding mode and an opportunity cross validation decoding mode.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of underwater acoustic communication, and particularly relates to a spread spectrum underwater acoustic communication encoding and decoding method. BACKGROUND

[0002] With the acceleration of ocean development, underwater acoustic communication, as a main cable-free transmission means underwater, is widely used in the process of ocean development. However, the complexity of the underwater acoustic channel has the characteristics of time-varying, space-varying, strong multipath and Doppler effect, which seriously limits the performance of underwater acoustic communication. Spread spectrum underwater acoustic communication uses a pseudo-random sequence to expand the information bandwidth by several times, thereby obtaining spread spectrum gain, and thus has strong anti-multipath and noise capability, becoming an underwater acoustic communication system capable of working in low signal-to-noise ratio, even negative signal-to-noise ratio conditions. Due to the expansion of the information bandwidth, the frequency utilization rate of the spread spectrum communication is low. In order to improve the frequency utilization rate, the CSK (Cyclic Shift Keying) and M-ary orthogonal sequence spread spectrum methods are usually used to increase the carrier signal rate of the code element, and the frequency utilization rate is doubled compared with the direct sequence spread spectrum method. Whether it is CSK or M-ary orthogonal sequence spread spectrum, the information is recovered by correlating the received sequence with the pseudo-random sequence matrix during the despreading, which essentially belongs to a multi-hypothesis testing process. When the frequency utilization rate is higher, the number of hypotheses is more, and under certain signal-to-noise ratio conditions, the detection performance is poor, and the bit error rate will rise. Therefore, how to further combine the encoding and decoding technology to improve the bit error rate performance of the spread spectrum underwater acoustic communication is a technical problem to be solved. SUMMARY

[0003] The technical problem to be solved by the application is to provide a spread spectrum underwater acoustic communication encoding and decoding method. The method forms a code block for concurrent transmission of two channels through independent error correction encoding, modulation and interleaving operations of the in-phase I channel and the quadrature Q channel, and the receiving end improves the communication accuracy through the non-coherent and coherent redundant decoding mode and the opportunity cross-validation decoding mode.

[0004] The technical solution of the application is to provide a spread spectrum underwater acoustic communication encoding and decoding method, comprising the following steps:

[0005] Double-channel encoding modulation, at the sending end, the information bit stream is sequentially subjected to CRC encoding, serial-parallel conversion, multi-ary error correction encoding, interleaving operation, and then the code element is modulated by a pseudo-random sequence, and finally the modulation outputs of the I and Q channels are mapped into baseband signals, which are subjected to pulse shaping and up-conversion processing to form a spread spectrum underwater acoustic communication signal;

[0006] The dual-channel non-coherent and coherent redundant decoding, at the receiving end, firstly, the I and Q channels are sequentially subjected to coherent demodulation, de-interleaving, multi-ary error correction decoding and CRC check, if the CRC checks of both channels are correct, the decoding ends; otherwise, the non-coherent demodulation, de-interleaving, error correction decoding and CRC check are sequentially performed, if the CRC checks of both channels are correct, the decoding ends, otherwise, the opportunity cross verification decoding is performed;

[0007] The opportunity cross verification decoding, the four CRC check results of the I and Q channels of the coherent and non-coherent decoding are comprehensively utilized, firstly, it is determined whether the I and Q channels are both correct, if yes, the decoding ends; otherwise, it is determined whether only the I channel or only the Q channel is correct, if yes, the bit stream decoded and output by the correct channel is subjected to the same encoding and modulation processing as that of the sending end, the baseband signal of the channel is reconstructed, the symbol phase estimation and correction are performed on the received baseband signal by using the reconstructed signal, then the coherent demodulation is performed again, the de-interleaving, multi-ary error correction decoding and CRC check are performed on the other error channel, and the decoding ends.

[0008] As preferred, in the dual-channel encoding and modulation process, the pseudo-random sequence is utilized to perform the CSK element or M element spread spectrum modulation on the symbol.

[0009] As preferred, in the dual-channel encoding and modulation process, the pseudo-random sequence is utilized to perform the CSK element and M element combined spread spectrum modulation on the symbol.

[0010] As preferred, in the dual-channel encoding and modulation process, the information bit stream is divided into two paths, and firstly subjected to the CRC encoding, serial-parallel conversion, multi-ary error correction encoding and interleaving operation.

[0011] As preferred, in the dual-channel non-coherent and coherent redundant decoding process, the likelihood probability calculation of the coherent demodulation I and Q channels is as follows:

[0012]

[0013] Wherein are the real part and imaginary part operations respectively, and y l (k) is the lth chip signal of the kth received symbol, is the lth chip of the nth pseudo-random sequence used for I channel mapping, is the lth chip of the nth pseudo-random sequence used for Q channel mapping, and c is a constant obtained through calculation, and the likelihood probability calculation of the coherent demodulation I and Q channels is as follows:

[0014]

[0015] Wherein |·| is the modulo operation.

[0016] Further, in the opportunity cross-validation decoding process, the reconstructed signal is used to calculate the symbol phase estimation of the received baseband signal as follows:

[0017]

[0018] wherein is the angle of the phase deviation of the kth symbol caused by channel phase fluctuation or inaccurate time-frequency synchronization, is the baseband signal of the kth symbol reconstructed when the I channel CRC check is correct, is the baseband signal of the kth symbol reconstructed when the Q channel CRC check is correct, and ∠(·) is the angle taking operation.

[0019]

[0020] Compared with the prior art, the present application has the following advantages:

[0021] ①Through the independent error detection and correction coding, modulation and interleaving operation of the I and Q channels, the receiving end can use the CRC check result to improve the decoding accuracy through the opportunity cross-validation decoding method. In addition, the present application proposes to use a multi-ary error correction code for spread spectrum underwater acoustic communication in the CSK or M-ary or combined manner. Since the correlation despreading result can be easily obtained, the likelihood probability of the symbol is obtained, the likelihood probability calculation is simplified, and the error correction performance of the former is better than that of the latter, and the decoding accuracy is higher than that of the latter; ②The coherent and non-coherent decoding methods are adopted, and when one of them fails, the other decoding method can be selected. This redundant decoding method improves the decoding accuracy to some extent. When the phase fluctuation of the underwater acoustic channel is large, the non-coherent decoding effect is better than the coherent decoding, and when the phase fluctuation is small and the time-frequency synchronization is accurate, the coherent decoding effect is better than the non-coherent decoding. Therefore, the coherent and non-coherent decoding methods of the present application also improve the robustness to the phase fluctuation changes of the underwater acoustic channel; ③When the coherent and non-coherent decoding has errors, the present application also uses the decoding result of the correct channel to realize phase estimation and correction through signal reconstruction, and re-uses the coherent decoding method for the error channel. After phase correction, the probability likelihood ratio of the demodulation output is greatly improved, which greatly improves the decoding accuracy. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 It is a double-channel encoding and modulation block diagram for spread spectrum underwater acoustic communication.

[0023] Figure 2 It is a double-channel non-coherent and coherent redundant decoding process.

[0024] Figure 3 It is an opportunity cross-validation decoding process. Detailed implementation method:

[0025] The present invention will be further described below with reference to the accompanying drawings and specific embodiments:

[0026] Figure 1 The diagram shows the dual-channel coding and modulation block diagram for spread spectrum underwater acoustic communication. At the transmitting end of the spread spectrum underwater acoustic communication, the information bit stream is divided into two paths. First, CRC encoding, serial-to-parallel conversion, multi-level error correction encoding, and interleaving are performed sequentially. Then, the symbols are spread spectrum modulated using a pseudo-random sequence in CSK, M-ary, or a combination of both. Finally, the I and Q channel modulation outputs are mapped to the baseband signal. After pulse shaping and up-conversion processing, the spread spectrum underwater acoustic communication signal is formed. Equation (1) is the baseband signal output after spread spectrum modulation of the k-th symbol.

[0027]

[0028] in Let i be the l-th chip of the pseudo-random sequence with symbol value i in channel I. Let be the l-th chip of a pseudo-random sequence with Q-channel symbol value q. N is the number of pseudo-random sequences, and L is the length of the pseudo-random sequence.

[0029] Figure 2 The dual-channel non-coherent and coherent redundant decoding process is as follows: At the receiving end, the I and Q channels first undergo coherent demodulation, deinterleaving multi-level error correction decoding, and CRC check. If both channels' CRC checks are correct, decoding ends; otherwise, non-coherent demodulation, error correction decoding, and CRC check are performed sequentially. If both channels' CRC checks are correct, decoding ends; otherwise, opportunistic cross-validation decoding is performed. Compared to a single decoding method, by employing both coherent and non-coherent decoding methods, if one decoding method fails, the other is selected. This redundant decoding method improves the decoding accuracy to some extent.

[0030] The likelihood probability of the k-th symbol I channel code value in coherent demodulation is given by equation (2).

[0031]

[0032] In equation (2), y l (k) represents the l-th chip signal of the received k-th symbol. For the l-th chip of the n-th pseudo-random sequence used for I-channel mapping, further simplification yields:

[0033]

[0034] In formula (3) For the real part operation, c for P I(n, k) are constants, which can be obtained through... We obtain the result. Similarly, the likelihood probability of the Q-channel symbol value of the k-th symbol in coherent demodulation being n is:

[0035]

[0036] In formula (4) To perform the imaginary part operation, Let be the l-th chip of the n-th pseudo-random sequence used for Q-channel mapping. The symbol likelihood probability of incoherent demodulation can be calculated as follows:

[0037]

[0038] In equation (5), |·| represents the modulus operation.

[0039] It can be seen from equations (3), (4), and (5) that The results of correlation processing on the received sequence and the pseudo-random sequence matrix of the demodulated output from the I and Q channels are shown. The likelihood probability of the symbol can be directly obtained from the correlation results, making the calculation relatively simple. Likelihood probability is often used as soft information input for multi-level error correction decoding, effectively improving error correction capability. Furthermore, compared to binary error correction decoding, the symbol likelihood probability needs to undergo a series of transformations to obtain the bit likelihood probability, and multi-level error correction capability is superior to binary. It can also be seen that the difference between coherent demodulation and incoherent demodulation lies in the fact that the former involves taking the real or imaginary part of the correlation result, while the latter involves a modulo operation. Due to the modulo operation, incoherent demodulation introduces inter-channel crosstalk, resulting in inferior decoding performance compared to coherent demodulation when channel phase fluctuations are small and time-frequency synchronization is accurate. When channel phase fluctuations are large or time-frequency synchronization is inaccurate, coherent demodulation introduces greater inter-channel crosstalk than incoherent demodulation due to the operation of taking the real or imaginary parts. In extreme cases, when phase fluctuations or inaccurate time-frequency synchronization cause a deviation from the true phase angle to approach 90 degrees, the likelihood probabilities of the I and Q channels become confused, severely degrading error correction decoding performance. Therefore, this invention improves robustness to phase fluctuations in underwater acoustic channels to a certain extent through redundant decoding methods combining coherent and uncorrelated demodulation.

[0040] Figure 3The opportunistic cross-validation decoding process is initiated when both coherent and incoherent decoding fail to decode correctly. This process utilizes all four CRC checksums from the I and Q channels of both coherent and incoherent decoding. If all four CRC checksums are incorrect, there is no further decoding opportunity, and decoding ends. Otherwise, it checks if both the I and Q channels have correct CRC checksums. If so, selecting the correct channel's decoding result as the final output will result in a correct overall decoding, and decoding ends. Otherwise, it checks if only the I channel or only the Q channel has a correct CRC checksum. If so, the bitstream output from the correct channel is processed using the same encoding and modulation as the sending end (e.g., ...). Figure 1 As shown, the process involves sequentially performing CRC encoding, serial-to-parallel conversion, multi-level error correction encoding, interleaving, and spread spectrum modulation to reconstruct the baseband signal of the correct channel. The reconstructed signal is then used to estimate and correct the symbol phase of the received baseband signal. After that, coherent demodulation is performed again, followed by deinterleaving, multi-level error correction decoding, and CRC verification of the erroneous channel. The decoding process is then complete.

[0041] Symbol phase estimation of the received baseband signal using the reconstructed signal can be expressed as:

[0042]

[0043] In formula (6) The angle represents the phase deviation of the k-th symbol caused by channel phase fluctuations or inaccurate time-frequency synchronization. This refers to the baseband signal of the k-th symbol that passes the reconstructed CRC check when the I-channel CRC check is correct. Let be the baseband signal of the k-th symbol reconstructed when the Q-channel CRC check is correct, and ∠(·) represent the angle operation. The likelihood probability calculated after phase correction using the estimated symbol phase deviation angle is as follows:

[0044]

[0045] Once the phase is corrected, the likelihood probability will be significantly improved compared to before the correction, and coherent decoding will greatly improve the decoding accuracy.

[0046] Therefore, as long as one of the four CRC check results is correct, there is a chance that the overall decoding result will be correct through cross-validation decoding. As long as the I channel and Q channel CRC checks are correct at the same time, the overall decoding result can be ensured to be correct. If only one check result is correct, the overall decoding result can also be ensured to be correct with a high probability through phase estimation and correction, recoherent decoding, etc.

[0047] The above description only illustrates preferred embodiments of the present invention and should not be construed as limiting the scope of the claims. Any equivalent procedural modifications made using this specification are included within the patent protection scope of this invention.

Claims

1. A spread spectrum underwater acoustic communication encoding and decoding method, characterized in that: Includes the following steps: Dual-channel coding modulation involves sequentially performing CRC encoding, serial-to-parallel conversion, multi-level error correction encoding, and interleaving on the information bit stream at the transmitting end. Then, pseudo-random sequences are used to spread spectrum modulation of the symbols. Finally, the modulation outputs of the I and Q channels are mapped to the baseband signal, which is then processed by pulse shaping and up-conversion to form a spread spectrum underwater acoustic communication signal. Dual-channel non-coherent and coherent redundant decoding: At the receiving end, the I and Q channels first undergo coherent demodulation, deinterleaving, multi-level error correction decoding, and CRC check in sequence. If both channels' CRC checks are correct, the decoding ends; otherwise, non-coherent demodulation, deinterleaving, error correction decoding, and CRC check are performed in sequence. If both channels' CRC checks are correct, the decoding ends; otherwise, opportunistic cross-validation decoding is performed. Opportunistic cross-validation decoding utilizes four CRC check results from the I and Q channels of coherent and non-coherent decoding. First, it determines whether both the I and Q channels are correct in the CRC check results. If so, decoding ends. Otherwise, it determines whether only the I channel or only the Q channel has a correct CRC check. If so, the bitstream output from the correct channel is modulated using the same encoding as the transmitter to reconstruct the baseband signal of that channel. The reconstructed signal is then used to estimate and correct the symbol phase of the received baseband signal. After that, coherent demodulation is performed again. Then, deinterleaving, multi-level error correction decoding, and CRC check are performed on the other incorrect channel, and decoding ends.

2. The spread spectrum underwater acoustic communication encoding and decoding method according to claim 1, characterized in that: During dual-channel coding and modulation, pseudo-random sequences are used to spread spectrum modulation of the symbols using CSK or M-ary elements.

3. The spread spectrum underwater acoustic communication encoding and decoding method according to claim 1, characterized in that: In the dual-channel coding and modulation process, pseudo-random sequences are used to spread spectrum modulation of the symbols using a combination of CSK and M elements.

4. The spread spectrum underwater acoustic communication encoding and decoding method according to claim 1, characterized in that: In the dual-channel coding and modulation process, the information bit stream is divided into two paths, and CRC encoding, serial-to-parallel conversion, multi-level error correction encoding, and interleaving are performed sequentially.

5. The spread spectrum underwater acoustic communication encoding and decoding method according to claim 1, characterized in that: In the dual-channel non-coherent and coherent redundant decoding process, the likelihood probabilities of the I and Q channels for coherent demodulation are calculated as follows: in These are operations that extract the real part and the imaginary part, respectively. l (k) represents the l-th chip signal of the received k-th symbol. For the l-th chip of the n-th pseudo-random sequence used for I-channel mapping, Let c be the l-th chip of the n-th pseudo-random sequence used for Q-channel mapping, and c be a constant. The likelihood probabilities of the coherent demodulation I and Q channels are calculated as follows: Where |·| represents the modulo operation.

6. The spread spectrum underwater acoustic communication encoding and decoding method according to claim 1, characterized in that: During the opportunistic cross-validation decoding process, the symbol phase estimation of the received baseband signal is calculated using the reconstructed signal as follows: in The angle represents the phase deviation of the k-th symbol caused by channel phase fluctuations or inaccurate time-frequency synchronization. This refers to the baseband signal of the k-th symbol that passes the reconstructed CRC check when the I-channel CRC check is correct. Let be the baseband signal of the k-th symbol reconstructed when the Q-channel CRC check is correct, and ∠(·) be the angle operation. The likelihood probability after phase correction is calculated as follows:

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