Gold sequence spread spectrum underwater acoustic modulation and demodulation technology for resisting extremely strong multipath interference

Abstract

The invention discloses a Gold sequence spread spectrum underwater acoustic modulation and demodulation technology for resisting extremely strong multipath interference, and relates to underwater acoustic communication. Comprising the design of transmitting end modulation and receiving end demodulation; on the basis of a traditional Gold sequence spread spectrum modulation and demodulation technology, a traditional GSSS code set is divided into a plurality of subsets by adopting a molecular set method, and different subsets are adopted for each symbol in the modulation process, so that weak correlation between the symbols is ensured. In the demodulation process, demodulation is carried out based on a molecular set modulation method of the modulation process, energy detection is carried out symbol by symbol according to a corresponding local GSSS code subset, soft information of each bit is calculated based on the number of bits carried by each symbol, the soft information is input into an interleaver and a channel decoder, and original information is correctly demodulated. The problem that the traditional GSSS modulation and demodulation technology cannot correctly demodulate in an extremely strong multipath environment is solved, the robustness of the GSSS modulation and demodulation technology to an extremely strong multipath underwater acoustic channel is improved, and robust communication in the extremely strong multipath environment is realized.

Description

Technical Field

[0001] The present invention relates to the field of underwater acoustic communication technology, and in particular to an extremely strong multipath robust Gold sequence spread spectrum (ESMR-GSSS) underwater acoustic modulation and demodulation technology for counteracting extremely strong multipath interference. Background Art

[0002] Based on whether phase information is used for information transmission, underwater acoustic communication modulation and demodulation schemes can be divided into incoherent and coherent underwater acoustic modulation and demodulation schemes. Coherent underwater acoustic modulation and demodulation schemes use phase information for information transmission and can achieve a higher communication rate. However, the underwater acoustic environment is much more complex than the land environment. The signal will inevitably be severely distorted when passing through the underwater acoustic channel, including multipath effect, Doppler effect, etc., requiring highly complex Doppler compensation and channel equalization technologies to design the receiver. On the contrary, incoherent underwater acoustic modulation and demodulation schemes generally use energy detection methods, which are highly robust to many challenges in the underwater acoustic channel and avoid highly complex receiver design. Therefore, incoherent 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 uses a pseudo-random sequence to expand the information to be sent to a wider frequency band, and uses the same pseudo-random sequence as the transmitting end to correlate the received spread spectrum signal at the receiving end, thereby recovering the sent information. At present, the commonly used pseudo-random sequences in direct sequence spread spectrum underwater acoustic modulation and demodulation technology include m sequence, Gold sequence, Welsh sequence, etc. Because the Gold sequence has excellent cross-correlation characteristics and rich code types, it is widely used in application scenarios that require a large number of spread spectrum codes. The interference signal in the transmission of the Gold sequence spread spectrum (GSSS) technology is unrelated to the Gold sequence and is expanded at the receiving end, so that the power of the interference signal falling into the signal frequency band is greatly reduced, thereby improving the system's receiving signal-to-noise ratio, which is conducive to data transmission under harsh channels and has a natural low interception probability. The excellent strong autocorrelation and weak cross-correlation of the Gold sequence are used to achieve reliable and stable communication performance under low signal-to-noise ratio conditions. Therefore, the quasi-orthogonal cross-correlation properties of the GSSS code can be used to distinguish different GSSS codes, and different GSSS codes represent different information. Because of its simple principle and easy implementation, low computational complexity using the correlation energy detection method, and extremely high robustness in harsh underwater acoustic environments, 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 ever-changing dynamic underwater environment, extremely strong multipath interference may be encountered during signal transmission, that is, the energy of multiple secondary paths is basically the same as or stronger than the main path, and the multipath delay exceeds the length of a symbol. This situation occurs especially in the 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 first few symbols are superimposed with the following symbols and the energy is basically the same or stronger, this strong aliasing state will cause the received symbols to be correlated with the local symbol set. After the operation, it is impossible to correctly demodulate according to the energy detection method, resulting in serious bit errors. In the current public technology, there is no solution to the failure of direct spread spectrum sequence modulation and demodulation technology under such strong multipath.

[0005] The literature (Direct sequence spread spectrum point-to-point communications cheme 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 the literature (Spread spectrum underwater acoustic communication scheme using a combination of passive time reversal and RAKE reception, Zhou Yuehai, Zeng Kun, Tong Feng, Journal of Xiamen University: Natural Science Edition, 2015, 54 (2): 270-275.) respectively proposed using noise sequence and passive time reversal sequence to combine with RAKE receiver to process multipath interference and overcome inter-symbol interference. Chinese patent CN115720117A discloses a packet direct sequence spread spectrum underwater acoustic modulation and demodulation method that is resistant to long multipath delays. It can overcome the inter-code interference caused by long delay multipath, but does not consider the situation where the multipath amplitude is equal to or higher than the first arrival path in extremely strong multipath interference. Chinese patent CN104753561B discloses a direct sequence spread spectrum modulation method that suppresses multipath interference in underwater acoustic communications. The specific method is to construct two sequences that satisfy the complementary sequence pair relationship and perform orthogonal combination, and use the pseudo-random sequence generated in this way as the spread spectrum code to enhance the robustness of the direct sequence spread spectrum system to multipath. In an underwater acoustic channel with extremely strong multipath, that is, the multipath delay is longer than one symbol length, and the multipath energy is similar to or higher than the first arrival path. The current direct sequence spread spectrum underwater acoustic modulation and demodulation technology cannot correctly judge the correlation peak between the received signal and the local spread spectrum sequence through the maximum energy criterion due to the extremely strong multipath, resulting in demodulation failure. Summary of the invention

[0006] The purpose of the present invention is to provide an Extremely strong multipathrobust Gold sequence spread spectrum (ESMR-GSSS) underwater acoustic modulation and demodulation technology that can combat extremely strong multipath interference, in order to address the problem that traditional GSSS technology cannot be correctly demodulated due to extremely strong multipath interference in underwater acoustic channels. A traditional GSSS code set is divided into multiple subsets based on subset parameters, and each symbol uses a different subset, which effectively solves the problem that traditional GSSS underwater acoustic modulation and demodulation technology cannot be correctly demodulated in underwater acoustic channels with extremely strong multipath interference, and has high robustness to extremely strong multipath effects.

[0007] In order to achieve the above technical objectives, the present invention provides the following technical solutions.

[0008] A Gold sequence spread spectrum underwater acoustic modulation and demodulation technology for resisting extremely strong multipath interference includes the following steps:

[0009] 1) Channel coding and interleaving: The original binary sequence is first encoded by the Polar channel encoder to enhance the noise resistance and error correction capabilities of the system data; then it is input into the interleaver for conversion to effectively disperse the continuous errors caused by the channel; 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 noise resistance of the data. The role of the interleaver is to disperse the continuous errors, making it easier to detect and correct these errors in subsequent processing;

[0010] 2) Base conversion: The binary sequence processed in step 1) is converted into a decimal sequence for subsequent GSSS code selection;

[0011] 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 using the molecular set method. Each symbol corresponds to a different GSSS code subset; this helps to achieve more accurate symbol recognition and error correction at the receiving end;

[0012] 4) Signal modulation and transmission: In the signal modulation stage, each symbol selects a specific GSSS code from the corresponding GSSS code subset based on the decimal number to be modulated, and the GSSS signal modulated symbol by symbol is transmitted through the underwater acoustic channel; by using the GSSS code for spread spectrum modulation, the signal's anti-interference ability and transmission reliability can be improved;

[0013] 5) Receiver processing: The received signal is first symbolized for subsequent symbol-by-symbol processing. The energy detection-based correlation despreading technique is used. Each symbol uses a local corresponding different GSSS code subset to perform cross-correlation operations in the despreading process. The information of the original symbol is extracted from the received signal, and the maximum absolute value of the correlation result is saved for subsequent soft demodulation.

[0014] 6) Soft information calculation and decoding restoration: Based on the number of bits carried by each symbol and the maximum value of each symbol despread, soft information calculation is performed to improve the accuracy of decoding; then it is 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 correct errors more effectively. Finally, the Polar decoder restores the original transmitted binary information sequence.

[0015] In step 1), the specific steps of channel coding and interleaving are:

[0016] Assuming the binary sequence , the length is , after the code rate is The Polar channel encoder and interleaver convert it into a binary sequence .

[0017] In step 2), the specific steps of the base conversion are: convert the binary sequence Every Convert bits to decimal sequence ,Right now , the decimal value range after binary conversion satisfies .

[0018] In step 3), the specific steps of dividing the spread spectrum code subset are: assuming that the GSSS code set has The length is The GSSS code is , ; Each GSSS code subset contains GSSS codes, of which , the GSSS code set is divided into GSSS subsets, namely ,in Represents the total number of symbols in a frame signal; each subset Expressed as ; Ensure that the GSSS codes in each subset have orthogonality or weak cross-correlation, that is

[0019] .

[0020] In step 4), the specific steps of signal modulation are:

[0021] For the decimal number to be modulated for each symbol, based on the GSSS code subset specific to each symbol, a corresponding GSSS code is selected from the subset for modulation; a different subset is used for modulation of each symbol to ensure weak correlation between symbols; The decimal number corresponding to the symbol is , the corresponding sequence subset is , then The ESMR-GSSS symbol is represented by:

[0022] ;

[0023] All modulated symbols are arranged in order to form a final frame of ESMR-GSSS transmission signal, expressed as , and then undergo digital-to-analog conversion, power amplification, and transmission into the underwater acoustic channel through an underwater acoustic transducer.

[0024] In step 4), the GSSS signal after symbol-by-symbol modulation is transmitted through an underwater acoustic channel, specifically:

[0025] The transmitted signal is converted by digital to analog and transmitted to the underwater acoustic channel through the power amplifier and the underwater acoustic transducer. Considering that the signal passes through the underwater acoustic channel with double selective fading and assuming that all paths have the same Doppler expansion factor, the baseband received signal is expressed as:

[0026] ;

[0027] ;

[0028] in, represents a time-varying doubly spread channel, represents the multipath index, represents the total number of multipaths, Indicates The path amplitude, represents the multipath delay, Indicates Multipath delay, represents the received signal in the time domain, represents the pulse impulse response, Indicates the channel length, represents Gaussian white noise in the underwater acoustic channel;

[0029] Considering an underwater acoustic channel with extremely strong multipath interference, that is, the amplitude of some multipaths is the same as or higher than that of the first arrival path, and the GSSS system has excellent robustness to the Doppler effect, so only considering the multipath effect and noise, the double extension channel above is simplified to:

[0030] ;

[0031] in, Indicates a multipath extension channel.

[0032] In step 5), the receiving end processes, specifically including:

[0033] Representation of received signal: A frame of ESMR-GSSS received signal after passing through the underwater acoustic channel is represented by a vector:

[0034] ;

[0035] in, , represents the convolution calculation, Represents a frame of received signal; the received signal of each symbol contains the multipath signal of the current symbol and the previous symbol. Very strong multipath interference of symbols;

[0036] The received The received signal of an ESMR-GSSS symbol is expressed as:

[0037] ;

[0038] in, Indicates received symbols;

[0039] Demodulation process: For each received symbol, the corresponding local GSSS code subset is selected for energy detection, and the maximum absolute value of the correlation result between the received signal and each GSSS code in the subset is calculated. Based on this, the soft information demodulation result of each bit is further calculated;

[0040] No. The demodulation of ESMR-GSSS symbols is carried out using the Local GSSS code subset , that is, calculate the The ESMR-GSSS symbol Local GSSS code subset The The maximum absolute value of the correlation result of the spreading code is expressed as:

[0041] ;

[0042] in, represents absolute value calculation, represents the cross-correlation calculation, Indicates the maximum value operation. Indicates The ESMR-GSSS symbol Local GSSS code subset The The maximum absolute value of the correlation results of the spreading codes;

[0043] Consider receiving Symbols In front The extremely strong multipath signal interference of the symbol, since all GSSS codes are orthogonal to each other and each symbol uses a different GSSS code subset, that is, the current GSSS code modulated by With the previous The GSSS codes modulated by the interference symbols are orthogonal, so the previous The interference symbol is in the current Compared with traditional GSSS technology, the multipath interference of each symbol is greatly reduced. Even under extremely strong multipath interference, due to the orthogonality between each GSSS code subset, a strong spread spectrum gain is still maintained, thus achieving robust underwater acoustic communication under extremely strong multipath interference.

[0044] In step 6), the soft information is calculated. The demodulation of the symbol is based on ESMR-GSSS symbols and local GSSS code subset The maximum absolute value of the correlation result can be used to calculate the soft information, which can be expressed as:

[0045] ;

[0046] in, Representative ESMR-GSSS symbol bits of soft information, represents the index of the combination matrix, and Respectively represent The combination matrix corresponding to the bit value of 1 and the bit value of 0 is defined as:

[0047] ;

[0048] ;

[0049] in, , , represents the domain of integers; and Represents binary numbers Shift Right Bits and Binary Numbers AND operation is performed with 1.

[0050] In step 6), the decoding and restoration: after demodulating the bit soft information corresponding to all symbols, the information is input into the Polar channel decoder through deinterleaving, and the restored binary information is output. .

[0051] Compared with the prior art, the present invention has the following outstanding technical effects and advantages:

[0052] 1. Based on the traditional GSSS modulation and demodulation technology, the present invention adopts the molecular set method to divide the traditional GSSS set into multiple subsets, and each symbol uses a different GSSS code subset in the modulation process to ensure that each symbol has a weak correlation. The demodulation process is based on the molecular set modulation method of the modulation process. Energy detection is performed symbol by symbol according to the corresponding local GSSS code subset. The weak mutual correlation between the spread spectrum code subsets is used to keep the correlation peak of the extremely strong multipath interference symbol of the non-current symbol to a very small value. Only the current GSSS symbol has a very large value. 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 decoder. Soft information calculation can provide additional information about the reliability of each symbol, thereby improving the accuracy of decoding. The present invention 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.

[0053] 2. In order to overcome the problem that the extremely strong multipath environment causes signal aliasing and obtains the wrong correlation peak, which leads to the inability to demodulate correctly, the present invention does not design a complex receiver to complete the processing of extremely strong multipath. Instead, it divides the spread spectrum set into multiple subsets based on the traditional GSSS technology, and uses the weak mutual correlation, strong autocorrelation and rich sequence set characteristics of the Gold sequence to calculate the maximum absolute value of the correlation result based on energy detection and obtain the soft information of each bit, and finally combines the interleaver and channel decoder to further improve the system performance. Without increasing the complexity of the receiver processing, the problem of extremely strong multipath interference is cleverly solved. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 Schematic diagram of ESMR-GSSS technology.

[0055] Figure 2 Schematic diagram of an underwater acoustic channel with extremely strong multipath interference.

[0056] Figure 3 This is a simulation experiment result diagram of the ESMR-GSSS of the present invention and the traditional GSSS underwater acoustic modulation and demodulation technology. DETAILED DESCRIPTION

[0057] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the following embodiments will further illustrate the present invention in conjunction with the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0058] The schematic diagram of the ESMR-GSSS underwater acoustic modulation and demodulation technology of the present invention is as follows Figure 1 As shown, the process of modulation at the transmitting end and demodulation at the receiving end is demonstrated.

[0059] First, the original binary sequence is input into the Polar channel encoder and interleaver to improve the system's anti-noise performance and error correction capability. The encoded binary sequence is converted into a decimal sequence for subsequent GSSS code selection. In the signal modulation stage, each symbol selects the corresponding spread spectrum code subset based on the current decimal number. The spread spectrum code subset is generated by the total GSSS code set and the subset parameters. The GSSS signal modulated symbol by symbol is transmitted through the underwater acoustic channel. At the receiving end, the received signal is first symbol-divided for subsequent symbol-by-symbol processing. Using the energy detection-based correlation despreading technology, each symbol is despread using the corresponding different GSSS code subsets, and the maximum absolute value of the correlation result is saved for subsequent soft demodulation. After despreading, soft information calculation is performed based on the number of bits carried by each symbol and the maximum value saved by the despreading of each symbol, and finally input into the interleaver and Polar channel decoder to restore the original transmitted binary information sequence. Specifically:

[0060] 1. Transmitter:

[0061] Assuming the binary sequence , the length is , after the code rate is The Polar channel encoder and interleaver convert it into a binary sequence , and then perform base conversion to convert the binary sequence Every Convert this to a decimal sequence ,Right now , it can be seen that the range of decimal numbers after binary conversion satisfies .

[0062] Considering the traditional GSSS modulation and demodulation technology, the GSSS code set is generated For information modulation, the spreading code sequence can be defined as , each spreading code satisfies the orthogonality and can be expressed as:

[0063] (1)

[0064] In the ESMR-GSSS technology proposed in the present invention, it is assumed that each GSSS code subset contains spreading codes, where , so the GSSS spreading code set can be divided into A subset of spreading codes, namely ,in Represents the total number of symbols in a frame signal. Therefore, each GSSS code subset It can be expressed as To ensure that the GSSS codes in each subset have orthogonality or weak cross-correlation, the modulation process of each ESMR-GSSS symbol is the same, so the following gives The modulation process of a symbol.

[0065] From the above parameter settings, we can know that The decimal number corresponding to the symbol is , the corresponding spreading code subset is , so we get The ESMR-GSSS symbol is represented by:

[0066] (2)

[0067] Arrange all modulated symbols in order, and the final frame of transmitted signal can be expressed as:

[0068] (3)

[0069] Then it undergoes digital-to-analog conversion, power amplification, and is transmitted into the underwater acoustic channel through an underwater acoustic transducer.

[0070] 2. Hydroacoustic Channel:

[0071] The transmitted signal is converted by digital to analog and transmitted to the underwater acoustic channel through the power amplifier and the underwater acoustic transducer. Considering that the signal passes through the underwater acoustic channel with double selective fading and assuming that all paths have the same Doppler expansion factor, the baseband received signal can be expressed as:

[0072] (4)

[0073] (5)

[0074] in, represents a time-varying doubly spread channel, represents the multipath index, represents the total number of multipaths, Indicates The path amplitude, represents the multipath delay, Indicates Multipath delay, represents the received signal in the time domain, represents the pulse impulse response, Indicates the channel length, Represents Gaussian white noise in the underwater acoustic channel.

[0075] Consider an underwater acoustic channel with extremely strong multipath interference, that is, the amplitude of some multipaths is the same as or higher than that of the first arrival path, such as Figure 2 shown. Figure 2 The amplitude relationship between the multipath signal and the first-arrival path signal in the underwater acoustic channel is shown. It can be seen that the amplitude of the first-arrival path is not always stronger than that of the multipath signal. Some multipath amplitudes are even higher than the first-arrival path, which will cause the traditional GSSS technology to be unable to correctly demodulate the current symbol modulation information.

[0076] Since the common Doppler factor can be easily removed by resampling and the GSSS system has excellent robustness to the Doppler effect, only the multipath effect and noise are considered, and the above formula can be simplified to:

[0077] (6)

[0078] in, Indicates a multipath extension channel.

[0079] 3. Receiving end:

[0080] A frame of ESMR-GSSS received signal after passing through the underwater acoustic channel is represented by a vector:

[0081] (7)

[0082] in, , represents the convolution calculation, 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.

[0083] Since there is no mutual dependence between the symbols in the demodulation process of the received signal at the receiving end, the processing process is performed symbol by symbol. The received signal of an ESMR-GSSS symbol can be expressed as:

[0084] (8)

[0085] in, Indicates Receive symbols.

[0086] For each ESMR-GSSS symbol, the corresponding local GSSS code subset is selected for energy detection, and the maximum absolute value of the correlation result between the received signal and each GSSS code in the subset is calculated, based on which the soft information of each bit is further calculated;

[0087] Since each symbol of ESMR-GSSS modulation technology uses different GSSS code subsets, The demodulation of ESMR-GSSS symbols is carried out using the Local GSSS code subset , that is, calculate the The ESMR-GSSS symbol Local GSSS code subset Middle The maximum absolute value of the correlation result of the spreading code can be expressed as:

[0088] (9)

[0089] in, represents absolute value calculation, represents the cross-correlation calculation, Indicates the maximum value operation. Indicates The ESMR-GSSS symbol Local GSSS code subset The The maximum absolute value of the correlation results of the spreading codes;

[0090] Consider receiving Symbols In front The extremely strong multipath signal interference of the symbol, since all GSSS codes are orthogonal to each other and each symbol uses a different GSSS code subset, that is, the current GSSS code modulated by With the previous The GSSS codes modulated by the interference symbols are orthogonal, so the previous The interference symbol is in the current Compared with the traditional GSSS technology, the multipath interference of each symbol is greatly reduced. Even under extremely strong multipath interference, due to the orthogonality between the GSSS code subsets, a strong spread spectrum gain can be maintained, thus achieving robust underwater acoustic communication under extremely strong multipath interference.

[0091] Therefore, the The demodulation of the ESMR-GSSS symbol is based on ESMR-GSSS symbols and local GSSS code subset The maximum absolute value of the correlation result can be used to calculate the soft information, which can be expressed as:

[0092] (10)

[0093] in, Representative ESMR-GSSS symbol bits of soft information, represents the index of the combination matrix, and Respectively represent The combination matrix corresponding to the bit value of 1 and the bit value of 0 can be defined as:

[0094] (11)

[0095] (12)

[0096] in, , , Represents a domain of integers. and Represents binary numbers Shift Right Bits and Binary Numbers AND operation is performed with 1.

[0097] After demodulating the bit soft information corresponding to all symbols, it is input into the Polar channel decoder through deinterleaving and outputs the restored binary information .

[0098] Based on the traditional GSSS modulation and demodulation technology, the present invention adopts a molecular set method to divide the traditional spread spectrum set into multiple subsets, and uses a different spread spectrum code subset for each symbol in the modulation process to ensure that each symbol has a weak correlation.

[0099] The demodulation process of the present invention is based on the molecular set modulation method of the modulation process for demodulation, performs energy detection symbol by symbol according to the corresponding local spread spectrum code subset, calculates each bit of soft information based on the maximum absolute value of the correlation result, inputs the interleaver and channel decoder, and thus correctly demodulates the original information.

[0100] The present invention can overcome the problem that the traditional GSSS technology cannot be correctly demodulated in an extremely strong multipath environment through the above-mentioned improved modulation and demodulation technology, and realize robust communication in an extremely strong multipath environment.

[0101] GSSS modulation and demodulation technology utilizes the spread spectrum gain, the strong autocorrelation of the spread spectrum code, and the weak cross-correlation between the spread spectrum codes. It has high robustness against multipath effects and low signal-to-noise ratio underwater acoustic environments. However, due to the complexity of the underwater acoustic channel, there are extremely strong multipath conditions, that is, the delay exceeds one symbol length and the multipath amplitude is the same as or higher than the first arrival path. Under the influence of the underwater acoustic channel with extremely strong multipath interference, 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, so it cannot perform underwater acoustic communication normally.

[0102] Compared with the traditional GSSS technology, the present invention adopts the molecular set method, that is, each symbol adopts a different spread spectrum code subset for information modulation. Therefore, even if encountering a serious symbol aliasing problem caused by extremely strong multipath interference, at the demodulation end, since the spread spectrum code subset corresponding to the current symbol is used for energy detection, the weak cross-correlation between the spread spectrum code subsets is used to keep the correlation peak of the extremely strong multipath symbol other than the current symbol at a small value, thereby correctly demodulating the spread spectrum code in the spread spectrum code subset corresponding to the current symbol, solving the problem that the traditional GSSS technology cannot demodulate correctly, greatly improving the robustness of the GSSS technology to the multipath environment, and the scheme does not increase the complexity of signal processing.

[0103] In order to verify the effectiveness of the proposed system, simulation experiments were carried out. The bit error rate was used as a performance evaluation indicator, and the communication performance of different signal-to-noise ratios under extremely strong underwater acoustic multipath interference was evaluated together with the traditional GSSS underwater acoustic modulation and demodulation technology.

[0104] The average value was obtained by 1000 Monte Carlo simulation experiments. The simulation results are as follows Figure 3 As shown. It can be seen that in the case of extremely strong underwater acoustic multipath interference, the traditional GSSS underwater acoustic modulation and demodulation technology cannot be correctly demodulated regardless of the signal-to-noise ratio, and has an extremely high bit error rate. This is because the fundamental reason for the bit error at this time is that the current symbol is severely distorted by the superposition of the extremely strong multipath extension signal of the previous symbol, and the superposition of the extremely strong multipath signal leads to the error in correlation peak judgment, which has nothing to do with the level of the signal-to-noise ratio. The ESMR-GSSS technology proposed in the present invention utilizes the idea of ​​diversity to avoid the problem of correlation peak errors caused by the extremely strong multipath extension signal of the previous symbol, and effectively solves the problem that the traditional GSSS underwater acoustic modulation and demodulation technology cannot be used correctly in an underwater acoustic channel with extremely strong multipath interference.

[0105] Traditional GSSS underwater acoustic modulation and demodulation technology uses the energy detection method, that is, the correlation peak between the received signal and the local spread spectrum code is calculated, the soft information of each bit is calculated based on the correlation peak, and the transmitted binary information is restored in combination with the channel decoder. Due to the multipath effect in the underwater acoustic channel, a received symbol signal is composed of the current symbol signal and the previous symbol signal superimposed, and the demodulation process is interfered by the previous symbol. However, since the spread spectrum code has strong autocorrelation and the spread spectrum codes have weak mutual correlation, it has a certain robustness to the multipath effect. However, in the marine environment, there will be extremely strong multipath underwater acoustic channels, that is, the multipath duration is longer than one symbol length, and the multipath energy is similar to or higher than the first path. This will cause the above-mentioned traditional spread spectrum technology to have an erroneous maximum peak in the energy detection method in the demodulation process, and the modulation information cannot be correctly demodulated.

[0106] Although the traditional GSSS underwater acoustic modulation and demodulation technology has good robustness against multipath effects, there will be extremely strong multipath underwater acoustic channels in the marine environment, that is, the multipath duration is longer than one symbol length, and the multipath energy is similar to or higher than the first-reaching path. This will cause the above-mentioned traditional spread spectrum technology to have an erroneous maximum peak in the energy detection method in the demodulation process, and the modulation information cannot be correctly demodulated.

[0107] The present invention is based on traditional GSSS modulation and demodulation technology and utilizes the diversity concept to divide the spread spectrum set into subsets according to set diversity parameters. The information to be transmitted is modulated on the spread spectrum signal in sequence based on different subsets, and the weak correlation between different GSSS code subsets is utilized to overcome extremely strong multipath interference, thereby improving the robustness of traditional GSSS modulation and demodulation technology against extremely strong multipath interference.

[0108] The above embodiments are only preferred embodiments of the present invention and cannot be considered to limit the scope of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of the present invention.

Claims

1. A Gold sequence spread spectrum underwater acoustic modulation and demodulation technology to combat extremely strong multipath interference, characterized in that The following steps are involved: 1) Channel coding and interleaving: The original binary sequence is first encoded by the Polar channel encoder to enhance the noise resistance and error correction capabilities of the system data; then it is input into the interleaver for conversion to effectively disperse the continuous errors caused by the channel; 2) Base conversion: The binary sequence processed in step 1) is converted into a decimal sequence for subsequent GSSS code selection; 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 using the molecular set method, and each symbol corresponds to a different GSSS code subset; 4) Signal modulation and transmission: In the signal modulation stage, each symbol selects a specific GSSS code from the corresponding GSSS code subset based on the decimal number to be modulated, and the GSSS signal modulated symbol by symbol is transmitted through the underwater acoustic channel; 5) Receiver processing: The received signal is first symbolized for subsequent symbol-by-symbol processing. The energy detection-based correlation despreading technique is used. Each symbol uses a local corresponding different GSSS code subset to perform cross-correlation operations in the despreading process. 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 after despreading each symbol, soft information calculation is performed to improve the accuracy of decoding; then it is input into the interleaver and Polar channel decoder to restore the original transmitted binary information sequence.

2. A Gold sequence spread spectrum underwater acoustic modulation and demodulation technology for resisting extremely strong multipath interference as claimed in claim 1, characterized in that In step 1), the specific steps of channel coding and interleaving are: Assuming the binary sequence , the length is , after the code rate is The Polar channel encoder and interleaver convert it into a binary sequence .

3. A Gold sequence spread spectrum underwater acoustic modulation and demodulation technology for resisting extremely strong multipath interference as claimed in claim 1, characterized in that In step 2), the specific steps of the base conversion are: convert the binary sequence Every Convert bits to decimal sequence ,Right now , the decimal value range after binary conversion satisfies .

4. A Gold sequence spread spectrum underwater acoustic modulation and demodulation technology for resisting extremely strong multipath interference as claimed in claim 1, characterized in that In step 3), the specific steps of dividing the spread spectrum code subset are: assuming that the GSSS code set has The length is The GSSS code is , ; Each GSSS code subset contains GSSS codes, of which , the GSSS code set is divided into GSSS subsets, namely ,in Represents the total number of symbols in a frame signal; each subset Expressed as ; Ensure that the GSSS codes in each subset have orthogonality or weak cross-correlation, that is 。 5. A Gold sequence spread spectrum underwater acoustic modulation and demodulation technology for resisting extremely strong multipath interference as claimed in claim 1, characterized in that In step 4), the specific steps of signal modulation are: For the decimal number to be modulated for each symbol, based on the GSSS code subset specific to each symbol, a corresponding GSSS code is selected from the subset for modulation; a different subset is used for modulation of each symbol to ensure weak correlation between symbols; The decimal number corresponding to the symbol is , the corresponding sequence subset is , then The ESMR-GSSS symbol is represented by: ； All modulated symbols are arranged in order to form a final frame of ESMR-GSSS transmission signal, expressed as , and then undergo digital-to-analog conversion, power amplification, and transmission into the underwater acoustic channel through an underwater acoustic transducer.

6. A Gold sequence spread spectrum underwater acoustic modulation and demodulation technology for resisting extremely strong multipath interference as claimed in claim 1, characterized in that In step 4), the GSSS signal after symbol-by-symbol modulation is transmitted through an underwater acoustic channel, specifically: The transmitted signal is converted by digital to analog and transmitted to the underwater acoustic channel through the power amplifier and the underwater acoustic transducer. Considering that the signal passes through the underwater acoustic channel with double selective fading and assuming that all paths have the same Doppler expansion factor, the baseband received signal is expressed as: ； ； in, represents a time-varying doubly spread channel, represents the multipath index, represents the total number of multipaths, Indicates The path amplitude, represents the multipath delay, Indicates Multipath delay, represents the received signal in the time domain, represents the pulse impulse response, Indicates the channel length, represents Gaussian white noise in the underwater acoustic channel; Considering an underwater acoustic channel with extremely strong multipath interference, that is, the amplitude of some multipaths is the same as or higher than that of the first arrival path, and the GSSS system has excellent robustness to the Doppler effect, so only considering the multipath effect and noise, the double extension channel above is simplified to: ； in, Indicates a multipath extension channel.

7. A Gold sequence spread spectrum underwater acoustic modulation and demodulation technology for resisting extremely strong multipath interference as claimed in claim 1, characterized in that In step 5), the receiving end processes, specifically including: Representation of received signal: A frame of ESMR-GSSS received signal after passing through the underwater acoustic channel is represented by a vector: ； in, , represents the convolution calculation, Represents a frame of received signal; the received signal of each symbol contains the multipath signal of the current symbol and the previous symbol. Very strong multipath interference of symbols; The received The received signal of an ESMR-GSSS symbol is expressed as: ； in, Indicates received symbols; Demodulation process: For each received symbol, the corresponding local GSSS code subset is selected for energy detection, and the maximum absolute value of the correlation result between the received signal and each GSSS code in the subset is calculated. Based on this, the soft information demodulation result of each bit is further calculated; No. The demodulation of the ESMR-GSSS symbol is carried out using the Local GSSS code subset , that is, calculate the The ESMR-GSSS symbol Local GSSS code subset The The maximum absolute value of the correlation result of the spreading code is expressed as: ； in, represents absolute value calculation, represents the cross-correlation calculation, Indicates the maximum value operation. Indicates The ESMR-GSSS symbol Local GSSS code subset The The maximum absolute value of the correlation results of the spreading codes; Consider receiving Symbols In front The extremely strong multipath signal interference of the symbol, since all GSSS codes are orthogonal to each other and each symbol uses a different GSSS code subset, that is, the current GSSS code modulated by With the previous The GSSS codes modulated by the interference symbols are orthogonal, so the previous The interference symbol is in the current Compared with traditional GSSS technology, the multipath interference of each symbol is greatly reduced. Even under extremely strong multipath interference, due to the orthogonality between each GSSS code subset, a strong spread spectrum gain is still maintained, thus achieving robust underwater acoustic communication under extremely strong multipath interference.

8. The Gold sequence spread spectrum underwater acoustic modulation and demodulation technology for resisting extremely strong multipath interference as claimed in claim 1, characterized in that In step 6), the soft information is calculated. The demodulation of the symbol is based on ESMR-GSSS symbols and local GSSS code subset The maximum absolute value of the correlation result can be used to calculate the soft information, which can be expressed as: ； in, Representative ESMR-GSSS symbol bits of soft information, represents the index of the combination matrix, and Respectively represent The combination matrix corresponding to the bit value of 1 and the bit value of 0 is defined as: ； ； in, , , represents the domain of integers; and Represents binary numbers Shift Right Bits and Binary Numbers AND operation is performed with 1.

9. The Gold sequence spread spectrum underwater acoustic modulation and demodulation technology for resisting extremely strong multipath interference as claimed in claim 1, characterized in that In step 6), the decoding and restoration: after demodulating the bit soft information corresponding to all symbols, the information is input into the Polar channel decoder through deinterleaving, and the restored binary information is output. .

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