Underwater sound DPPM signal soft demodulation method, storage medium and device

Abstract

The invention discloses an underwater acoustic DPPM signal soft demodulation method, a storage medium and a device, and belongs to the field of underwater acoustic communication. The method comprises the following steps of: after data at a transmitting end are scrambled, coded by LDPC (Low Density Parity Check) and interleaved, carrying out Gray code mapping and DPPM modulation, after a receiving end is synchronized to a signal, carrying out cross-correlation on the acquired DPPM signal and a local signal, calculating the probability of judging each DPPM code element as each time slot according to a cross-correlation result, taking the probability as code element soft information, carrying out Gray mapping, and sending the code element soft information to the receiving end; and obtaining the probability that each information bit is judged to be 0 or 1 as bit soft information, performing error correction decoding by using an LDPC soft decoding algorithm after de-interleaving, and finally performing descrambling and restoring transmitted data. Compared with a traditional hard solution method, the method provided by the invention has the advantages of stronger anti-interference capability, better communication effect and lower bit error rate.

Description

Technical Field

[0001] The present invention belongs to the field of underwater acoustic communication, and relates to an underwater acoustic signal modulation and demodulation method, and in particular to an underwater acoustic DPPM signal soft demodulation method, a storage medium and a device. Background Art

[0002] Pulse position modulation (PPM) is to use the modulation signal to control the relative position of each pulse in the transmitted pulse sequence, which changes with the change of the modulation signal. It has the advantages of simple coding, high transmission efficiency, and strong anti-interference ability, which can greatly improve the information security of the communication system. However, PPM modulation and demodulation occupies a large amount of bandwidth when transmitting information, causing unnecessary waste. Therefore, a new modulation and demodulation method, differential pulse position modulation and demodulation technology (DPPM), is proposed on this basis. Under the same communication rate, DPPM modulation and demodulation occupies less channel bandwidth than PPM modulation, which greatly improves the efficiency of information transmission in the communication system. Therefore, underwater communication mostly adopts DPPM format.

[0003] Since the underwater acoustic channel is a very complex multipath transmission channel, with high environmental noise, narrow bandwidth, low applicable carrier frequency and large transmission delay, this further affects the performance of the underwater acoustic DPPM communication system. At present, there is no research on DPPM soft demodulation, and the method for PPM soft demodulation is only effective for low-order PPM modulation such as 2PPM and 4PPM. Summary of the invention

[0004] The purpose of the present invention is to address the problem that the DPPM underwater acoustic communication system is greatly affected by underwater background noise and multipath interference, to study the underwater acoustic communication soft demodulation technology based on DPPM, to provide an underwater acoustic DPPM signal soft demodulation method and storage medium, and to combine the obtained soft demodulation information with LDPC soft decoding, and apply it to the underwater acoustic DPPM communication system, so as to improve the DPPM underwater acoustic communication quality and reduce the bit error rate of underwater communication.

[0005] The technical solution adopted by the present invention is:

[0006] A method for soft demodulation of underwater acoustic DPPM signals, comprising:

[0007] The underwater acoustic DPPM communication signal frame structure of the present invention is as follows: Figure 1 As shown, it is known that n binary information can be mapped to M time slots. For a DPPM signal with N code elements:

[0008] (1) The received DPPM signal is cross-correlated with the local signal, and the maximum value of the correlation result Sc is taken, the maximum value position is recorded, and a certain range around the maximum value is set to 0. The maximum value of the result is taken again, the maximum value position is recorded, and a certain range around the maximum value is set to 0. The above process is repeated N times to obtain N maximum value positions. The communication signal position obtained by synchronization and the N maximum value positions are combined into an array A of length N+1, and the difference is calculated as follows to obtain the number of time slots occupied by the N code elements T:

[0009] T(i)=A(i+1)-A(i)i=1,2...N;

[0010] (2) The cross-correlation results are further processed as follows:

[0011] Nos=K*sum(Sc) / length(Sc)

[0012] S=Sc-Nos

[0013] Where Sc is the result after cross-correlation, K is a constant, and S is the result after processing;

[0014] (3) The probability that the i-th symbol width is judged to be j time slots is Ts(i,j), j = 1, 2, 3...M

[0015] (a) If the width of the i-th code element T(i) < 0.5, then Ts(i,j) = 1;

[0016] (b) If the width of the i-th symbol T(i) ≥ 0.5, then A(i)-ns / 2 is used as the starting point to divide the code into M subintervals, and the subintervals are S[A(i)-j*ns / 2,A(i)+j*ns / 2], where ns is the number of sampling points in a time slot;

[0017] Then the probability that each code element is judged as j time slots is

[0018] Ts(i,j)=max(S[A(i)-j*ns / 2,A(i)+j*ns / 2]);

[0019] (4) The probability of Ts of each code element is normalized. The normalization process can be expressed as:

[0020]

[0021] Define the set of time slots corresponding to the sequence where the i-th bit of the Gray code is 1 as Q i = {Q i1 ,Q i2 ...Q i16}, then the likelihood ratio of the kth bit of the ith codeword is:

[0022]

[0023] The beneficial effects of the present invention are:

[0024] In the underwater acoustic channel, the DPPM soft demodulation method of the present invention has stronger anti-interference ability, better communication effect and lower bit error rate compared with the traditional hard demodulation method. Figure 4 It can be seen that when the communication bit error rate is 10 -3 When the soft demodulation method proposed in the present invention is compared with the traditional hard demodulation method, the communication effect is improved by about 1.5dB. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a signal frame structure diagram of the method of the present invention.

[0026] Figure 2 It is an overall flow chart of the DPPM soft demodulation communication algorithm of the present invention.

[0027] Figure 3 It is the result of cross-correlation between the local signal and the received signal before DPPM demodulation of the present invention.

[0028] Figure 4 It is the relevant result after the processing of the present invention.

[0029] Figure 5 The simulation results of the DPPM soft solution method and the hard solution method of the present invention are compared. DETAILED DESCRIPTION

[0030] The present invention is mainly verified by simulation experiment, and all steps and conclusions are verified to be correct in Matlab2020. In order to facilitate those skilled in the art to understand the technical content of the present invention, the content of the present invention is further explained below in conjunction with the accompanying drawings. The specific parameters of the simulation implementation are shown in Table 1.

[0031] Table 1 Simulation communication system parameters

[0032] parameter Parameter Value parameter Parameter Value Sampling rate 96000Hz Maximum delay 31ms Signal bandwidth 8000Hz Temporal resolution 1ms Center frequency 12000Hz Signal time domain envelope Rayleigh Window Symbol time 20ms LDPC matrix row number 90 Number of code elements 36 LDPC matrix columns 180 Guard Interval 5ms LDPC matrix column weight 3

[0033] A method for soft demodulation of underwater acoustic DPPM signals comprises the following steps:

[0034] Step 1: See Figure 1As shown, 32 time slots can be mapped to 5 binary information. For a DPPM signal with 10 code elements, the received DPPM signal is cross-correlated with the local signal. The maximum value of the correlated result Sc is taken, the maximum value position is recorded, and a certain range centered on the maximum value is set to 0. The maximum value of the result is taken again, the maximum value position is recorded, and a certain range centered on the maximum value is set to 0. The above process is repeated N times to obtain N maximum value positions. The synchronized communication signal position and the N maximum value positions are combined into an array A of length 11, and the difference is made according to the following formula to obtain the number of time slots T occupied by N code elements:

[0035] T(i)=A(i+1)-A(i)i=1,2...11

[0036] Step 2: Continue to process the cross-correlation results as follows, with K set to 4:

[0037] Nos=4*sum(Sc) / length(Sc)

[0038] S=Sc-Nos

[0039] Where Sc is the result of cross-correlation, such as Figure 3 As shown, S is the result after processing, such as Figure 4 shown.

[0040] Step 3: The probability that the width of the i-th code element is j milliseconds is Ts(i,j), j=1,2,3...32, then:

[0041] (a) If the width of the i-th code element T(i) < 0.5, then Ts(i,j) = 1;

[0042] (b) If the width T(i) of the i-th code element is ≥ 0.5, then 32 sub-intervals are divided with A(i)-24 as the starting point, and the sub-intervals are S[A(i)-j*24,A(i)+j*24], where 24 is the number of sampling points in half a time slot.

[0043] Then the probability that each code element is judged as j time slots is

[0044] Ts(i,j)=max(S[A(i)-j*24,A(i)+j*24])

[0045] Step 4: Normalize the probability of Ts of each code element. The normalization process can be expressed as:

[0046]

[0047] Define the set of time slots corresponding to the sequence where the i-th bit of the Gray code is 1 as Q i = {Q i1 ,Q i2...Q i16}, then

[0048] Q1={17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32}

[0049] Q2={9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24}

[0050] Q3={5,6,7,8,9,10,11,12,21,22,23,24,25,26,27,28}

[0051] Q4={3,4,5,6,11,12,13,14,15,16,17,18,19,20,21,22,27,28,29,30}

[0052] Q5={2,3,6,7,10,11,14,15,18,19,22,23,26,27,30,31}

[0053] Table 2 Comparison table of natural code and Gray code

[0054] Binary Gray code Binary Gray code 00000 00000 10000 11000 00001 00001 10001 11001 00010 00011 10010 11011 00011 00010 10011 11010 00100 00110 10100 11110 00101 00111 10101 11111 00110 00101 10110 11101 00111 00100 10111 11100 01000 01100 11000 10100 01001 01101 11001 10101 01010 01111 11010 10111 01011 01110 11011 10110 01100 01010 11100 10010 01101 01011 11101 10011 01110 01001 11110 10001 01111 01000 11l11 10000

[0055] Then the likelihood ratio of the kth bit of the ith symbol is

[0056]

[0057] The simulation effect comparison between the DPPM soft solution method and the hard solution method is shown in Figure 5 shown.

[0058] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific variations and combinations that do not deviate from the essence of the present invention based on the technical revelations disclosed by the present invention, and these variations and combinations are still within the protection scope of the present invention.

Claims

1. A method for soft demodulation of an underwater acoustic DPPM signal, wherein the frame structure of the underwater acoustic DPPM signal includes n known binary information that can be mapped to M time slots, and the number of code elements of the DPPM signal is N; characterized in that: The following steps are involved: (1) The received DPPM signal is cross-correlated with the local signal, and the maximum value of the correlation result Sc is taken, the maximum value position is recorded, and a certain range around the maximum value is set to 0. The maximum value of the result is taken again, the maximum value position is recorded, and a certain range around the maximum value is set to 0. The above process is repeated N times to obtain N maximum value positions. The communication signal position obtained by synchronization and the N maximum value positions are combined into an array A of length N+1, and the difference is calculated as follows to obtain the number of time slots occupied by the N code elements T: T(i)=A(i+1)-A(i)i=1,2...N (2) Continue processing the cross-correlation results: Nos=K*sum(Sc) / length(Sc) S=Sc-Nos Among them: Sc is the result after cross-correlation, K is a constant, and S is the result after processing; (3) The probability that the i-th symbol width is judged to be j time slots is Ts(i,j), j = 1, 2, 3...M (a) If the width of the i-th code element T(i) < 0.5, then Ts(i,j) = 1; (b) If the width of the i-th symbol T(i) ≥ 0.5, then A(i)-ns / 2 is used as the starting point to divide the code into M subintervals, and the subintervals are S[A(i)-j*ns / 2,A(i)+j*ns / 2], where ns is the number of sampling points in a time slot; Then the probability Ts of each code element being judged as j time slots is: Ts(i,j)=max(S[A(i)-j*ns / 2,A(i)+j*ns / 2]); (4) The probability of Ts of each code element is normalized. The normalization process is expressed as: Define the set of time slots corresponding to the sequence where the i-th bit of the Gray code is 1 as Q i = {Q i1 ,Q i2 ...Q i16 }, then the likelihood ratio LP of the kth bit of the i-th codeword is:

2. The underwater acoustic DPPM signal soft demodulation method according to claim 1, characterized in that: Said K=4.

3. The underwater acoustic DPPM signal soft demodulation method according to claim 1, characterized in that: The number of sampling points in one time slot is ns=48.

4. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the underwater acoustic DPPM signal soft demodulation method as described in any one of claims 1 to 3 are implemented.

5. A soft demodulation device for underwater acoustic DPPM signals, comprising a processor, characterized in that: Also included is a computer-readable storage medium as claimed in claim 4.

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