Gold sequence spread spectrum underwater acoustic modulation and demodulation method against strong multipath interference

Abstract

The application relates to a Gold sequence spread spectrum underwater acoustic modulation and demodulation method against strong multipath interference, relating to underwater acoustic communication. The method comprises the design of a transmitting end modulation and a receiving end demodulation; on the basis of a traditional Gold sequence spread spectrum modulation and demodulation technology, a molecule set method is adopted to divide the traditional GSSS code set into multiple subsets, and different subsets are adopted in the modulation process of each symbol, so that weak correlation is ensured between each symbol. The demodulation process is based on the molecule set modulation method of the modulation process, energy detection is carried out according to the corresponding local GSSS code subset symbol by symbol, the number of bits carried by each symbol is calculated, each bit soft information is calculated, the soft information is input into an interleaver and a channel decoder, and the original information is correctly demodulated. The method overcomes the problem that the traditional GSSS modulation and demodulation technology cannot be correctly demodulated in a strong multipath environment, improves the robustness of the GSSS modulation and demodulation technology to the strong multipath underwater acoustic channel, and realizes stable and robust communication in the strong multipath environment.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of underwater acoustic communication, and in particular to an underwater acoustic modulation and demodulation method based on a Gold sequence spread spectrum (ESMR-GSSS) against extremely strong multipath interference. BACKGROUND

[0002] Underwater acoustic communication modulation and demodulation schemes can be divided into non-coherent and coherent underwater acoustic modulation and demodulation schemes based on whether phase information is used for information transmission. Coherent underwater acoustic modulation and demodulation schemes use phase information for information transmission, which can achieve a high communication rate. However, the underwater environment is much more complex than the terrestrial environment, and the signal will inevitably be severely distorted after passing through the underwater channel, including multipath effect, Doppler effect, etc., which requires a high complexity Doppler compensation and channel equalization technology to design the receiver. In contrast, the non-coherent underwater acoustic communication modulation and demodulation scheme generally uses an energy detection method, which has high robustness to many challenges in the underwater acoustic channel and avoids high complexity receiver design. Therefore, non-coherent underwater acoustic communication is widely used in the engineering field of underwater acoustic communication technology.

[0003] Direct sequence spread spectrum underwater acoustic modulation and demodulation technology spreads the information to be sent with a pseudo-random sequence to a wider frequency band, and uses the same pseudo-random sequence at the transmitting end to correlate the received spread spectrum signal at the receiving end, and then restores the transmitted information. At present, the commonly used pseudo-random sequences in direct sequence spread spectrum underwater acoustic modulation and demodulation technology include m sequences, Gold sequences, Welsh sequences, etc. Gold sequences are widely used in application scenarios that require a large number of spread spectrum codes due to their excellent cross-correlation characteristics and rich code types. The interference signal in the transmission of Gold sequence spread spectrum (GSSS) technology is not correlated with the Gold sequence, and is expanded in the receiving end, so that the power of the interference signal falling into the signal frequency band is greatly reduced, thereby improving the received signal-to-noise ratio of the system, which is beneficial to data transmission in poor channels, and has a natural low probability of interception. The excellent strong autocorrelation and weak cross-correlation of Gold sequences are used to achieve reliable and stable communication performance under low signal-to-noise ratio conditions, so that the quasi-orthogonal cross-correlation property of GSSS codes can be used to distinguish different GSSS codes, and different GSSS codes represent different information. Due to its simple principle and easy implementation, the correlation energy detection method has low computational complexity, and has extremely high robustness in poor underwater acoustic environments, so it is the most commonly used robust communication technology in non-coherent underwater acoustic communication modulation and demodulation schemes.

[0004] Due to the complex and dynamic underwater environment, extremely strong multipath interference may be encountered during signal transmission, that is, multiple secondary paths have energy basically the same as or stronger than the main path energy, and the multipath time delay exceeds one symbol length, which occurs especially in deep sea. In the traditional direct sequence spread spectrum modulation and demodulation scheme, due to the inter-symbol interference caused by extremely strong multipath, that is, the multipath components of the previous several symbols are superimposed with the following symbols and have basically the same energy or are stronger, and this strong aliasing state will cause that after the received symbol is correlated with the local symbol set, correct demodulation cannot be performed according to the energy detection method, resulting in serious bit error. In the current public technology, there is no solution to the problem of invalidation of direct spread spectrum sequence modulation and demodulation technology under such strong multipath. Literature (Direct sequence spread spectrum point-to-point communication scheme in underwater acoustic sparse channels, A. Jamshidi, IET Comunications, vol. 5 no. 4, pp. 456-466, Nov. 2011, doi: 10.1049 / iet-com.2010.0031.) and literature (Hybrid passive time reversal and RAKE receiver for spread spectrum underwater acoustic communication, Zhou Yuehai, Zeng Kun, Tong Feng, Journal of Xiamen University: Natural Science Edition, 2015, 54(2): 270-275.) respectively propose to use noise sequences and passive time reversal sequences to combine with RAKE receivers to process multipath interference and overcome inter-symbol interference. Chinese patent CN115720117A discloses a packet direct sequence spread spectrum underwater modulation and demodulation method resistant to long multipath time delay, which can overcome the inter-symbol interference caused by long time delay multipath, but does not consider the case that the multipath amplitude is equal to or higher than the first-arriving path in extremely strong multipath interference. Chinese patent CN104753561B discloses a direct sequence spread spectrum modulation method for suppressing multipath interference in underwater acoustic communication, which specifically constructs two sequences satisfying the complementary sequence pair relationship and performs orthogonal combination, so as to generate a pseudo-random sequence as a spread spectrum code, thereby enhancing the robustness of the direct sequence spread spectrum system to multipath. Under the condition of extremely strong multipath in underwater acoustic channel, that is, the multipath time delay is longer than one symbol length, and the multipath energy is similar to or higher than the first-arriving path, the current direct sequence spread spectrum underwater modulation and demodulation technology cannot correctly determine the correlation peak of the received signal and the local spread spectrum sequence by the maximum energy criterion, resulting in demodulation failure. SUMMARY

[0005] The present application aims at extremely strong multipath robust Gold sequence spread spectrum (ESMR-GSSS) underwater modulation and demodulation method to solve the problem that traditional GSSS technology cannot correctly demodulate due to extremely strong multipath interference in underwater acoustic channel.

[0006] To achieve the above technical purposes, the present application provides the following technical solutions.

[0007] An extremely strong multipath robust Gold sequence spread spectrum underwater modulation and demodulation method comprises the following steps:

[0008] 1) Channel coding and interleaving: the original binary sequence is first encoded by a Polar channel encoder to enhance the anti-noise performance and error correction capability of the system data, and then converted in an interleaver to effectively disperse continuous errors caused by the channel; the Polar code is a modern channel coding technology with excellent performance, which can provide coding gain close to the Shannon limit, thereby effectively enhancing the anti-noise performance of the data, and the interleaver disperses continuous errors to make it easier to detect and correct these errors in subsequent processing;

[0009] 2) Base conversion: the binary sequence processed in step 1) is converted to a decimal sequence through base conversion for subsequent GSSS code selection;

[0010] 3) Spread spectrum code subset division: based on the traditional Gold sequence spread spectrum modulation and demodulation technology, the traditional GSSS code set is divided into multiple subsets by using the subset method, and each symbol corresponds to a different GSSS code subset; which helps to achieve more accurate symbol recognition and error correction at the receiving end;

[0011] 4) Signal modulation and transmission: in the signal modulation stage, a specific GSSS code is selected in the corresponding GSSS code subset based on the decimal number to be modulated for each symbol, and the modulated GSSS signal is transmitted through the underwater acoustic channel; by using GSSS code for spread spectrum modulation, the anti-interference ability and transmission reliability of the signal can be improved;

[0012] 5) Receiver processing: The received signal is first symbol divided for subsequent symbol-by-symbol processing; the correlation despreading technique based on energy detection is used, and each symbol uses the corresponding different GSSS code subset for the correlation operation of the despreading process to extract the original symbol information from the received signal, and the maximum absolute value of the correlation result is saved for subsequent soft demodulation;

[0013] 6) Soft information calculation and decoding restoration: based on the number of bits carried by each symbol and the maximum value saved by each symbol despreading, soft information calculation is performed to improve the accuracy of decoding; then input into the interleaver and Polar channel decoder to restore the original transmitted binary information sequence; the role of the interleaver is to recombine the previously scattered errors so that the Polar decoder can more effectively correct errors. Finally, the Polar decoder restores the original transmitted binary information sequence.

[0014] In step 1), the specific steps of channel coding and interleaving are:

[0015] Assume that the binary sequence has a length of L b , and after passing through the Polar channel encoder with a code rate of R = L b / L a and the interleaver, it is converted into a binary sequence

[0016] In step 2), the specific steps of the binary-to-decimal conversion are: convert each L m bits of the binary sequence a into a decimal sequence , i.e. L d = L a / L m , and the decimal value after binary conversion satisfies

[0017] In step 3), the specific steps of the subset division of the spreading code are: assuming that there are L G GSSS codes with a length of L g in the GSSS code set, i.e. Each GSSS code subset contains L c GSSS codes, where The GSSS code set is divided into N c = L G / L c GSSS subsets, i.e. where N c represents the total number of symbols in a frame of signal; each subset is represented as to ensure that the GSSS codes in each subset have orthogonality or weak correlation, i.e.

[0018]

[0019] In step 4), the specific steps of the signal modulation are:

[0020] For each decimal number to be modulated for each symbol, based on the specific GSSS code subset of each symbol, select the corresponding GSSS code from the subset for modulation; the modulation of each symbol uses a different subset to ensure weak correlation between symbols; the decimal number corresponding to the ith symbol is d i The corresponding sequential subset is C i Then the ith ESMR-GSSS symbol is represented as:

[0021] x i = C i (:, d i );

[0022] Arrange all the modulated symbols in order to form a final frame of ESMR-GSSS transmission signal, represented as And then perform digital-to-analog conversion, power amplification, and transmit to the underwater acoustic channel through the underwater acoustic transducer.

[0023] In step 4), the GSSS signal modulated symbol by symbol is transmitted through the underwater acoustic channel, specifically:

[0024] The transmitted signal is converted to digital-to-analog, amplified by a power amplifier, and transmitted to the underwater acoustic channel through the underwater acoustic transducer. Considering that the signal passes through a double selective fading underwater acoustic channel, and assuming that all paths have the same Doppler spread factor, the baseband received signal is represented as:

[0025]

[0026] Where h(t; τ) represents the time-varying double spread channel, p represents the multi-path index, N p represents the total number of multi-paths, A p represents the amplitude of the pth path, τ represents the multi-path delay, τ p represents the pth multi-path delay, r(t) represents the time-domain received signal, δ(t) represents the impulse response, N h represents the channel length, w(t) represents the Gaussian white noise in the underwater acoustic channel.

[0027] Considering the extremely strong multi-path interference in the underwater acoustic channel, i.e. some multi-paths have the same or higher amplitude than the first arriving path, and the GSSS system has excellent robustness to Doppler effect, therefore only considering the multi-path effect and noise, the above double spread channel is simplified as:

[0028]

[0029] where h(τ) represents the multipath propagation channel.

[0030] In step 5), the receiving end processes, specifically including:

[0031] The received signal represents that a frame of ESMR-GSSS received signal after the underwater acoustic channel is represented by a vector as:

[0032]

[0033] where h = [h(0), h(1), …, h(N-1)], h represents convolution calculation, r represents a frame of received signal; the received signal of each symbol contains the multipath signal of the current symbol and the very strong multipath interference of the previous K symbols;

[0034] The received i-th ESMR-GSSS symbol received signal is represented as:

[0035] y i = r(1+(i-1)L g :iL g );

[0036] where y i represents the i-th received symbol;

[0037] Demodulation process: for each received symbol, select the corresponding local GSSS code subset for energy detection, calculate the maximum absolute value of the correlation result of the received signal and each GSSS code in the subset, and further calculate the soft information demodulation result of each bit based on this;

[0038] The demodulation of the i-th ESMR-GSSS symbol uses the i-th local GSSS code subset C i , that is, the maximum absolute value of the correlation result of the i-th

[0039] ESMR-GSSS symbol and the k-th spread spectrum code in the i-th local GSSS code subset C i is calculated, represented as:

[0040]

[0041] where |·| represents absolute value calculation, represents cross-correlation calculation, max(·) represents maximum value operation, v i,k represents the maximum absolute value of the correlation result of the i-th ESMR-GSSS symbol and the k-th spread spectrum code in the i-th local GSSS code subset C i ;

[0042] Consider the i-th received symbol y i ​The previous K symbols are severely interfered by multipath signals, all GSSS codes are orthogonal to each other and different GSSS code subsets are used in different symbols, that is, the GSSS code x i i i is orthogonal to the GSSS codes modulated by the previous K interfering symbols, therefore, the multipath interference of the previous K interfering symbols on the current i-th symbol is greatly reduced compared with the conventional GSSS technology, and the strong spreading gain is maintained due to the orthogonality between different GSSS code subsets, so that robust underwater acoustic communication under severe multipath interference is realized.

[0043] In step 6), the soft information calculation, the demodulation of the i-th symbol only needs to be based on the i-th ESMR-GSSS symbol and the local GSSS code subset C i The maximum absolute value of the correlation result is calculated to obtain the soft information, which is represented as:

[0044]

[0045] Wherein, represents the soft information of the β-th bit of the i-th ESMR-GSSS symbol, α represents the index of the combination matrix, B β1 and B β0 respectively represent the combination matrix corresponding to the β-th bit value of 1 and the bit value of 0, and are defined as:

[0046]

[0047] Wherein, bitshift(γ,β) and bitand(γ,1) respectively represent right shifting the binary number γ by β bits and performing AND operation on the binary number γ and 1.

[0048] In step 6), the decoding restores: after the bit soft information corresponding to all symbols is demodulated, it is input into the Polar channel decoder through deinterleaving, and the restored binary information

[0049] Compared with the prior art, the present application has the following outstanding technical effects and advantages:

[0050] ​​1. The application divides the traditional GSSS set into multiple subsets based on the traditional GSSS modulation and demodulation technology, and uses different GSSS code subsets for each symbol in the modulation process to ensure weak correlation between each symbol. The demodulation process is based on the sub-set modulation method of the modulation process, and the energy detection is performed according to the corresponding local GSSS code subset symbol by symbol, and the weak cross-correlation between the spread spectrum code subsets is used to make the correlation peak of the extremely strong multipath interference symbol of the non-current symbol remain a very small value, only the current GSSS symbol has a very large value, and the soft information of each bit is calculated based on the maximum absolute value of the correlation result, and finally the original binary information is correctly demodulated through the interleaver and the decoder. Soft information calculation can provide additional information about the reliability of each symbol, thereby improving the accuracy of decoding. The application solves the problem that the traditional GSSS technology cannot be correctly demodulated in an extremely strong multipath environment, greatly improves the robustness of the GSSS technology to the multipath environment, and realizes robust communication in an extremely strong multipath environment.

[0051] 2. In order to overcome the problem that the extremely strong multipath environment leads to signal aliasing and incorrect correlation peak, the application does not design a complex receiver to complete the processing of the extremely strong multipath, but divides the spread spectrum set into multiple subsets based on the traditional GSSS technology, uses the weak cross-correlation, strong autocorrelation and rich sequence set of Gold sequence, calculates the maximum absolute value of the correlation result based on energy detection, and obtains the soft information of each bit, and finally combines the interleaver and the channel decoder to further improve the system performance. Without increasing the complexity of the receiver processing, the problem of extremely strong multipath interference is ingeniously solved. BRIEF DESCRIPTION OF DRAWINGS

[0052] Figure 1 The ESMR-GSSS technology schematic diagram.

[0053] Figure 2 The extremely strong multipath interference underwater acoustic channel schematic diagram.

[0054] Figure 3 The ESMR-GSSS and the traditional GSSS underwater acoustic modulation and demodulation technology simulation experiment result diagram. DETAILED DESCRIPTION

[0055] In order to make the purpose, technical scheme and advantages of the application more clear and understandable, the following embodiments will be further described in combination with the drawings. It should be understood that the specific embodiments described herein are only used to explain the application and do not limit the application.

[0056] The ESMR-GSSS underwater acoustic modulation and demodulation technology schematic diagram of the application is shown in Figure 1 The process of transmitting end modulation and receiving end demodulation is shown.

[0057] First, the original binary sequence is input into the Polar channel encoder and interleaver to improve the anti-noise performance and error correction capability of the system. The encoded binary sequence is converted into a decimal sequence for subsequent GSSS code selection. In the signal modulation stage, each symbol selects a corresponding spreading code subset based on the current decimal number, which is generated from the total GSSS code set and the subset parameter. The modulated GSSS signal is transmitted through the underwater acoustic channel. At the receiving end, the received signal is first divided into symbols for subsequent symbol-by-symbol processing. Using energy detection-based correlation despreading technology, each symbol is despread using the corresponding different GSSS code subset, and the maximum absolute value of the correlation result is saved for subsequent soft demodulation. After despreading, soft information is calculated based on the number of bits carried by each symbol and the maximum value saved by each symbol, and finally input into the interleaver and Polar channel decoder to restore the original transmitted binary information sequence. Specifically:

[0058] I. Transmitter:

[0059] Assume a binary sequence with length L b , after Polar channel encoding and interleaving with code rate R = L b / L a , it is converted into a binary sequence and then converted into a decimal sequence by converting the binary sequence a every L m , i.e. L d = L a / L m . It can be seen that the decimal number after binary conversion satisfies

[0060] In traditional GSSS modulation and demodulation technology, GSSS code set is generated for information modulation, where the spreading code sequence can be defined as Each spreading code satisfies orthogonality, which can be expressed as:

[0061]

[0062] In the ESMR-GSSS technology proposed in the present application, it is assumed that each GSSS code subset contains L c spreading codes, where Therefore, the GSSS spreading code set can be divided into N c = L G / L c spreading code subsets, i.e. where N cdenotes the total number of symbols in a frame. Therefore, each GSSS code subset may be denoted as To ensure the orthogonality or weak correlation of GSSS codes within each subset, since the modulation process of each ESMR-GSSS symbol is the same, the modulation process of the ith symbol is given as follows.

[0063] The decimal number corresponding to the ith symbol is d i , and the corresponding spreading code subset is C i . Therefore, the ith ESMR-GSSS symbol is denoted as:

[0064] x i = C i (:, d i ) (2)

[0065] Arranging all the modulated symbols in order, the final transmitted signal in a frame can be denoted as:

[0066]

[0067] And then, digital-to-analog conversion, power amplification, and transmission to the underwater acoustic channel through the underwater acoustic transducer are performed.

[0068] II. Underwater acoustic channel:

[0069] The transmitted signal is converted to digital-to-analog, amplified by a power amplifier, and transmitted to the underwater acoustic channel through the underwater acoustic transducer. Considering that the signal passes through a double-selective fading underwater acoustic channel, and assuming that all paths have the same Doppler spread factor, the baseband received signal can be denoted as:

[0070]

[0071] where h(t; τ) denotes the time-varying double spread channel, p denotes the multi-path index, N p denotes the total number of multi-paths, A p denotes the amplitude of the pth path, τ denotes the multi-path delay, τ p denotes the pth multi-path delay, r(t) denotes the time-domain received signal, δ(t) denotes the impulse response, N h denotes the channel length, and w(t) denotes the Gaussian white noise in the underwater acoustic channel.

[0072] Considering the underwater acoustic channel with extremely strong multi-path interference, i.e., some multi-paths have the same or higher amplitude than the first-arrival path, as shown in Figure 2 Figure 2 ​The amplitude relationship between the multipath signal and the first-arrival signal in the underwater acoustic channel is shown. It can be seen that the amplitude of the first-arrival signal is not always stronger than the multipath signal, and the amplitude of some multipath signals is even higher than that of the first-arrival signal, which will lead to the fact that the traditional GSSS technology cannot correctly demodulate the current symbol modulation information.

[0073] Since the common Doppler factor can be easily removed by resampling, and the GSSS system has excellent robustness to Doppler effect, only considering the multipath effect and noise, the above formula can be simplified as:

[0074]

[0075] where h(τ) represents the multipath spread channel.

[0076] III. Receiver:

[0077] A frame of ESMR-GSSS received signal after the underwater acoustic channel is represented by a vector as follows:

[0078]

[0079] where h = [h(0), h(1), …, h(N-1)], h represents convolution calculation, and r represents a frame of received signal. The received signal of each symbol contains the multipath signal of the current symbol and the extremely strong multipath interference of the previous symbol.

[0080] Since there is no mutual dependence between symbols in the demodulation process of the received signal at the receiver, the processing is performed symbol by symbol, therefore, the received i ESMR-GSSS symbol signals can be represented as:

[0081] y i = r(1+(i-1)L g :iL g ) (8)

[0082] where y i represents the i-th received symbol.

[0083] For each ESMR-GSSS symbol, the corresponding local GSSS code subset is selected for energy detection, the maximum absolute value of the correlation result of the received signal with each GSSS code in the subset is calculated, and based on this, the soft information of each bit is further calculated;

[0084] Since different GSSS code subsets are used for each symbol in the ESMR-GSSS modulation technology, the demodulation of the i-th ESMR-GSSS symbol uses the i-th local GSSS code subset C i , that is, the i-th ESMR-GSSS symbol and the i-th local GSSS code subset C i ​The maximum absolute value of the correlation result of the k-th spreading code can be expressed as:

[0085]

[0086] Where |·| represents absolute value calculation, ⊙ represents cross-correlation calculation, max(·) represents the maximum value operation, and v i,k This represents the i-th ESMR-GSSS symbol and the i-th local GSSS code subset C. i The maximum absolute value of the correlation result of the kth spreading code in the matrix;

[0087] Consider the received i-th symbol y i Due to the extremely strong multipath interference from the preceding K symbols, and because all GSSS codes are mutually orthogonal and each symbol uses a different subset of GSSS codes, i.e., the GSSS code x modulated by the current i-th symbol... i =C i (:,d i The GSSS code modulated by the first K interference symbols is orthogonal to the first K interference symbols. Therefore, the multipath interference of the first K interference symbols in the current i-th symbol is greatly reduced compared to the traditional GSSS technology. Even under extremely strong multipath interference, due to the orthogonality between the subsets of each GSSS code, it can maintain extremely strong spreading gain and realize robust underwater acoustic communication under extremely strong multipath interference.

[0088] Therefore, the demodulation of the i-th ESMR-GSSS symbol only needs to be based on the i-th ESMR-GSSS symbol and the local GSSS code subset C. i The soft information can be calculated from the maximum absolute value of the relevant results, and is expressed as:

[0089]

[0090] in, The β-th bit represents the soft information of the i-th ESMR-GSSS symbol, α represents the index of the combination matrix, and B... β1 and B β0 Let the combination matrix representing the values ​​of the β-th bit as 1 and the β-th bit as 0 be defined as follows:

[0091]

[0092] in, This represents the integer field. bitshift(γ,β) and bitand(γ,1) represent shifting the binary number γ right by β bits and performing an AND operation between the binary number γ and 1, respectively.

[0093] After demodulating the bit soft information corresponding to all symbols, the data is de-interleaved and input into the Polar channel decoder, which outputs the restored binary information.

[0094] The application divides the traditional spread spectrum set into multiple subsets based on the traditional GSSS modulation and demodulation technology, and uses different spread spectrum code subsets for each symbol in the modulation process to ensure weak correlation between each symbol.

[0095] The demodulation process of the application is based on the sub-set modulation method of the modulation process, and the energy detection is performed according to the corresponding local spread spectrum code subset for each symbol, the soft information of each bit is calculated based on the maximum absolute value of the correlation result, and the interleaver and channel decoder are inputted, so that the original information is correctly demodulated.

[0096] The improved modulation and demodulation technology can overcome the problem that the traditional GSSS technology cannot correctly demodulate in the extremely strong multipath environment, and realizes robust communication in the extremely strong multipath environment.

[0097] The GSSS modulation and demodulation technology utilizes the spread spectrum gain, strong autocorrelation of the spread spectrum code and weak cross-correlation between the spread spectrum codes, and has high robustness to multipath effect and low signal-to-noise ratio in the underwater acoustic environment, but due to the complexity of the underwater acoustic channel, there is an extremely strong multipath, that is, the time delay exceeds the symbol length, and the multipath amplitude is the same as or higher than the first arriving path. Under the influence of the extremely strong multipath interference of the underwater acoustic channel, the traditional GSSS modulation and demodulation technology cannot distinguish the correct spread spectrum code in the current symbol due to the superposition of multiple strong amplitude spread spectrum codes, and therefore cannot normally perform underwater acoustic communication.

[0098] Compared with the traditional GSSS technology, the application adopts a sub-set method, that is, different spread spectrum code subsets are used for information modulation for each symbol, so that even if the extremely strong multipath interference causes serious symbol aliasing problem, the energy detection is performed by using the spread spectrum code subset corresponding to the current symbol at the demodulation end, and the weak cross-correlation between the spread spectrum code subsets is utilized to keep the correlation peak of the extremely strong multipath symbol of the non-current symbol small, so that the spread spectrum code in the spread spectrum code subset corresponding to the current symbol is correctly demodulated, the problem that the traditional GSSS technology cannot correctly demodulate is solved, the robustness of the GSSS technology to the multipath environment is greatly improved, and the scheme does not increase the complexity of signal processing.

[0099] In order to verify the effectiveness of the proposed system, simulation experiments are carried out, and the bit error rate is used as the performance evaluation index to evaluate the communication performance of the traditional GSSS underwater acoustic modulation and demodulation technology under different signal-to-noise ratios in the extremely strong underwater acoustic multipath interference.

[0100] The average value is taken by 1000 times of Monte Carlo simulation experiment, and the simulation result is as follows Figure 3It can be seen that in the case of extremely strong multipath interference of underwater sound, the traditional GSSS underwater sound modulation and demodulation technology cannot be correctly demodulated at any signal-to-noise ratio, and has a very high bit error rate, because the root cause of the error is that the current symbol is severely distorted by the superposition of the extremely strong multipath spread signal of the previous symbol, and the superposition of the extremely strong multipath signal leads to the error of the correlation peak, which is irrelevant to the level of signal-to-noise ratio. The ESMR-GSSS technology proposed in the application avoids the problem of error of the correlation peak caused by the extremely strong multipath spread signal of the previous symbol by using the diversity idea, and effectively solves the problem that the traditional GSSS underwater sound modulation and demodulation technology cannot be correctly used in the underwater sound channel with extremely strong multipath interference.

[0101] The traditional GSSS underwater sound modulation and demodulation technology adopts an energy detection method, that is, the received signal is correlated with the local spread spectrum code to calculate the correlation peak, the soft information of each bit is calculated based on the correlation peak, and the binary information transmitted is restored by combining the channel decoder. Due to the multipath effect of the underwater sound channel, the received symbol signal is superimposed by the current symbol signal and the previous symbol signal, and the demodulation process is disturbed by the previous symbol, but since the spread spectrum code has strong autocorrelation and weak cross-correlation between spread spectrum codes, it has certain robustness to multipath effect. However, in the marine environment, there will be an extremely strong multipath underwater sound channel, that is, the multipath time delay is longer than one symbol length, and the multipath energy is similar to or higher than the first arrival path, which will cause the maximum peak value to appear in the energy detection method in the demodulation process of the above-mentioned traditional spread spectrum technology, and the modulation information cannot be correctly demodulated.

[0102] Although the traditional GSSS underwater sound modulation and demodulation technology has good robustness to multipath effect, in the marine environment, there will be an extremely strong multipath underwater sound channel, that is, the multipath time delay is longer than one symbol length, and the multipath energy is similar to or higher than the first arrival path, which will cause the maximum peak value to appear in the energy detection method in the demodulation process of the above-mentioned traditional spread spectrum technology, and the modulation information cannot be correctly demodulated.

[0103] Based on the traditional GSSS modulation and demodulation technology, the spread spectrum set is divided into subsets according to the set diversity parameters by using the diversity idea, and the information to be transmitted is modulated on the spread spectrum signal based on different subsets in turn, and the weak correlation between different GSSS code subsets is used to overcome the extremely strong multipath interference, and the robustness of the traditional GSSS modulation and demodulation technology to extremely strong multipath interference is improved.

[0104] The above embodiments are only preferred embodiments of the present application, and cannot be considered as limiting the scope of the present application. Any equivalent changes and improvements made within the scope of the present application should still belong to the patent coverage of the present application.

Claims

1. A Gold sequence spread spectrum underwater acoustic modulation and demodulation method against extremely strong multipath interference, Gold sequence spread spectrum against extremely strong multipath interference is ESMR-GSSS, characterized in that The method comprises the following steps: 1) Channel coding and interleaving: the original binary sequence is first encoded by a Polar channel encoder to enhance the anti-noise performance and error correction capability of system data, and then converted in an interleaver to effectively disperse continuous errors caused by the channel; 2) Conversion: the binary sequence processed in step 1) is converted into a decimal sequence through conversion, which is used for subsequent GSSS code selection; 3) Sub-set division of spread spectrum code: on the basis of the traditional Gold sequence spread spectrum modulation and demodulation technology, a sub-set method is adopted to divide the traditional GSSS code set into multiple subsets, and each symbol corresponds to a different GSSS code subset; 4) Signal modulation and transmission: in the signal modulation stage, a specific GSSS code in the corresponding GSSS code subset is selected for each symbol based on the decimal number to be modulated, and the modulated GSSS signal of each symbol is transmitted through the underwater acoustic channel; 5) Receiver processing: the received signal is first divided into symbols for subsequent symbol-by-symbol processing; Using the correlation despreading technology based on energy detection, each symbol uses the corresponding different GSSS code subset for the cross-correlation operation of the despreading process, and the maximum absolute value of the correlation result is saved for subsequent soft demodulation; 6) Soft information calculation and decoding restoration: based on the number of bits carried by each symbol and the maximum value saved by each symbol despreading, soft information calculation is performed to improve the accuracy of decoding; then, the original transmitted binary information sequence is restored by inputting into the interleaver and the Polar channel decoder.

2. The Gold sequence spread spectrum underwater acoustic modem method against very strong multipath interference according to claim 1, characterized in that In step 1), the specific steps of the channel coding and interleaving are: Assuming a binary sequence Length L b After the code rate is R=L b / L a After being converted into a binary sequence by a Polar channel encoder and interleaver, the sequence is...

3. The Gold sequence spread spectrum underwater acoustic modem method against very strong multipath interference according to claim 1, characterized in that In step 2), the specific step of the base conversion is: converting the binary sequence a every L m bits into a decimal sequence , i.e. L d = L a / L m , and the decimal number after the binary conversion has a value range satisfying 4. The Gold sequence spread spectrum underwater acoustic modem method against very strong multipath interference according to claim 3, characterized in that In step 3), the specific steps of the subset division of the spread spectrum code are as follows: assuming that the GSSS code set has L G GSSS codes with length L g , i.e. each GSSS code subset contains L c GSSS codes, wherein the GSSS code set is divided into N c =L G / L c GSSS subsets, i.e. wherein N c represents the total number of symbols of a frame of signals; each subset is represented as to ensure that the GSSS codes in each subset have orthogonality or weak correlation, i.e.

5. The Gold sequence spread spectrum underwater acoustic modem method against very strong multipath interference according to claim 4, characterized in that In step 4), the specific steps of the signal modulation are: For the decimal number to be modulated for each symbol, a corresponding GSSS code is selected from the specific GSSS code subset for each symbol for modulation; different subsets are used for the modulation of each symbol to ensure the weak correlation between symbols; The decimal number corresponding to the ith symbol is d i The subset of the corresponding order is C i The ith ESMR-GSSS symbol is represented as x i = C i (d i ) All modulated symbols are arranged in order into a final frame of ESMR-GSSS transmission signal, denoted as And then, digital-to-analog conversion, power amplification, and through the underwater acoustic transducer to the underwater acoustic channel.

6. The Gold sequence spread spectrum underwater acoustic modem method that resists very strong multipath interference according to claim 1, characterized in that In step 4), the GSSS signal modulated symbol by symbol is transmitted through the underwater acoustic channel, specifically: The transmitted signal is converted from digital to analog, amplified by a power amplifier, and transmitted to the underwater acoustic channel through an underwater transducer. Considering that the signal passes through a double-selective fading underwater acoustic channel, and assuming that all paths have the same Doppler spread factor, the baseband received signal is represented as: where h(t; τ) represents a time-varying double spread channel, p represents a multipath index, N p represents a total number of multipaths, A p represents a pth path amplitude, τ represents a multipath delay, τ p represents a pth multipath delay, r(t) represents a time-domain received signal, δ(t) represents an impulse response, N h represents a channel length, w(t) represents a Gaussian white noise in the underwater acoustic channel; Considering the underwater acoustic channel under extremely strong multipath interference, that is, part of the multipath has the same amplitude as the first-arriving path or higher amplitude than the first-arriving path, and the GSSS system has excellent robustness to the Doppler effect, therefore, only the multipath effect and noise are considered, and the double-expansion channel is simplified as: Where h(τ) represents the multipath expansion channel.

7. The Gold sequence spread spectrum underwater acoustic modem method against very strong multipath interference according to claim 5, characterized in that In step 5), the receiver processing specifically includes: The received signal is represented as: a frame of ESMR-GSSS received signal after passing through the underwater acoustic channel is represented as a vector: where h = [h(0), h(l),..., h(N h -1)]. denotes a convolution operation, r denotes a received signal of a frame; the received signal of each symbol contains the multipath signal of the current symbol and the very strong multipath interference of the previous K symbols. The i-th ESMR-GSSS symbol received signal is represented as: where y i represents the ith received symbol; Demodulation process: for each received symbol, a corresponding local GSSS code subset is selected for energy detection, the maximum absolute value of the correlation result of the received signal and each GSSS code in the subset is calculated, and the soft information demodulation result of each bit is further calculated based on this; The demodulation of the ith ESMR-GSSS symbol employs the ith local GSSS code subset C i , i.e. calculates the maximum absolute value of the correlation result between the ith ESMR-GSSS symbol and the kth spreading code in the ith local GSSS code subset C i , and is expressed as: where |·| denotes an absolute value calculation, denotes a cross-correlation calculation, max(·) denotes a maximum value operation, v i,k denotes the maximum absolute value of the correlation result of the ith ESMR-GSSS symbol and the kth spread spectrum code in the ith local GSSS code subset C i ​ Consider the received i-th symbol y i Affected by the strong multipath signal interference of the previous K symbols, since all GSSS codes are orthogonal to each other and different GSSS code subsets are used for each symbol, i.e. the GSSS code x i = C i (:,d i ) is orthogonal to the GSSS code modulated by the previous K interfering symbols, therefore, the multipath interference of the previous K interfering symbols on the current i-th symbol is greatly reduced compared to the traditional GSSS technology, and even under strong multipath interference, due to the orthogonality between each GSSS code subset, strong spreading gain is still maintained, realizing robust underwater acoustic communication under strong multipath interference.

8. The Gold sequence spread spectrum underwater acoustic modem method against very strong multipath interference according to claim 1, characterized in that In step 6), the soft information calculation, the demodulation of the ith symbol only needs to be based on the ith ESMR-GSSS symbol and the local GSSS code subset C i The relevant results maximum absolute value calculation of soft information can be expressed as: wherein, represents the soft information of the βth bit of the ith ESMR-GSSS symbol, χ represents the index of the combining matrix, B β1 and B β0 represent the combining matrix corresponding to the βth bit value of 1 and the bit value of 0, respectively, and are defined as: wherein denotes the field of integers; bitshift(γ, β) denotes a right shift of the binary number γ by β bits, and bitand(bitshift(γ, β), 1) denotes a right shift of the binary number γ by β bits followed by an AND operation with 1.

9. The Gold sequence spread spectrum underwater acoustic modem method that counteracts very strong multipath interference according to claim 1, characterized in that In step 6), the decoding restores: after demodulating the bit soft information corresponding to all symbols, input into the Polar channel decoder through deinterleaving, output the restored binary information

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