A multi-user M-CSK-IDMA iterative receiving method for time-varying double-selection underwater acoustic channel

Abstract

This invention discloses a multi-user M-CSK-IDMA iterative reception method for time-varying dual-select underwater acoustic channels, belonging to the field of underwater acoustic communication and multi-user detection. Addressing the problems of frame-level Doppler compensation being prone to lock-out, strong multipath timing misjudgment, and poor adaptability to single hydrophones in traditional schemes, a four-stage cascaded processing method is adopted: first, macroscopic Doppler estimation and physical resampling are performed on the received signal; then, anchor points are recursively truncated based on the M-CSK symbol sequence, and synchronization parameters are jointly searched in a two-dimensional space of residual frequency offset and chip timing offset; after fixing the frequency offset, an effective delay set is selected, and the timing parameters are corrected using the earliest path minimum delay value; finally, the corrected parameters are despread, Turbo iterative detection and decoding are performed, and hard decision is output. This invention can achieve symbol-level time-frequency synchronization and anti-multipath locking with a single hydrophone, resist interference from time-varying dual-select channels, reliably separate multi-user signals, and is suitable for mobile underwater acoustic communication platforms with limited payloads, such as small AUVs.

Description

Technical Field

[0001] This invention belongs to the field of underwater acoustic communication and multi-user detection technology, and in particular relates to a multi-user M-CSK-IDMA iterative reception method for time-varying dual-select underwater acoustic channels. Background Technology

[0002] As underwater acoustic communication evolves towards multi-node collaboration, the M-CSK-IDMA system, which combines M-ary cyclic shift keying (CPSK) and interleaved multiple access (IPA), has attracted attention due to its ability to improve multi-user data rates while maintaining spread spectrum gain. However, in mobile platform applications, underwater acoustic channels exhibit dual selective fading caused by time-varying Doppler spread and frequency-selective multipath spread, and the received signal is simultaneously affected by the coupling effects of multiple access interference and channel time-varying distortion.

[0003] Traditional M-CSK-IDMA receiver schemes typically first use a frame preamble for one-time Doppler estimation and compensation, assume that the channel is quasi-static and unchanged, perform symbol-by-symbol despreading with fixed timing synchronization parameters, and then achieve multi-user separation through Turbo iteration between the basic signal estimator and the soft channel decoder. In terms of multipath resistance, the maximum correlation peak criterion is used for timing decision, and spatial diversity of multiple receiver array elements is relied upon to obtain gain.

[0004] However, existing solutions have the following prominent problems: one-time frame-level Doppler compensation cannot track symbol-by-symbol frequency offset changes, leading to timing drift accumulation and synchronization loss during long frame transmission; the maximum correlation peak criterion under strong multipath conditions is easily hijacked by false peaks of delayed reflection paths, causing M-CSK demodulation misjudgment; multi-element spatial diversity is difficult to apply on small AUV platforms equipped with only a single hydrophone, and there is a lack of effective means for time-frequency synchronization, anti-multipath locking, and iterative multi-user detection joint processing under single-element conditions. Summary of the Invention

[0005] Based on this, it is necessary to provide a multi-user M-CSK-IDMA iterative reception method for time-varying dual-select underwater acoustic channels that can achieve symbol-level time-frequency synchronization and anti-multipath locking in a single hydrophone configuration, and complete multi-user detection and decoding through Turbo iteration, in order to address the above-mentioned technical problems.

[0006] In a first aspect, this application provides a multi-user M-CSK-IDMA iterative reception method for time-varying dual-select underwater acoustic channels, including:

[0007] S1. Acquire the received passband signal, perform frame preamble coarse synchronization acquisition and pilot frequency estimation on the received passband signal to obtain the macroscopic Doppler frequency offset estimate; perform physical resampling on the received passband signal based on the macroscopic Doppler frequency offset estimate to obtain the resampled signal;

[0008] S2. Obtain the user's M-CSK symbol sequence, and recursively obtain the truncation anchor point based on the M-CSK symbol sequence. Extract the analysis window signal from the resampled signal with the truncation anchor point as the center. In the two-dimensional parameter space of residual frequency offset and chip timing offset, perform a joint search on the analysis window signal with the maximum value of the despreading correlation peak as the cost function to obtain the residual frequency offset estimate and the chip timing offset estimate.

[0009] S3. Fix the residual frequency offset estimate and calculate the cost function slice along the time delay dimension; based on the global maximum value of the slice, select the effective time delay candidate set through a preset threshold; select the minimum time delay value from the effective time delay candidate set to correct the chip timing offset estimate and obtain the corrected timing synchronization parameter;

[0010] S4. Based on the corrected timing synchronization parameters and the residual frequency offset estimate, despread the resampled signal to obtain the received chip vector of each user; perform Turbo iterative multi-user detection and decoding on the received chip vector to obtain the posterior log-likelihood ratio of the information bits of each user.

[0011] S5. Perform a hard decision on the posterior log-likelihood ratio of the information bits to obtain the decoded bit sequence for each user.

[0012] In one embodiment, Turbo iterative multi-user detection and decoding are performed on the received chip vector to obtain the posterior log-likelihood ratio of the information bits for each user, including:

[0013] S11. Set the prior expectation of each chip in the received chip vector for each user to zero and set the prior variance of the chip to a unit value to obtain the initial chip prior statistics; perform channel estimation based on the received chip vector and the initial chip prior statistics to obtain the initial instantaneous channel coefficients.

[0014] S12. Obtain the user's chip prior statistics and current instantaneous channel coefficients. Based on the chip prior statistics and corresponding instantaneous channel coefficients, calculate the multiple access interference statistics. Based on the multiple access interference statistics, perform soft interference cancellation on the current user's chip values ​​in the received chip vector to obtain the extrinsic information log-likelihood ratio of each chip for the current user. Among them, the corresponding instantaneous channel coefficients are the initial instantaneous channel coefficients in the first iteration and the current instantaneous channel coefficients in subsequent iterations.

[0015] S13. The external information log likelihood ratio is sequentially deinterleaved with chips and soft demodulated with M-CSK to obtain the coded bit soft information; the coded bit soft information is input into the BCJR decoder for decoding to obtain the posterior log likelihood ratio of the coded bit.

[0016] S14. Update the chip prior statistics of each user based on the posterior log-likelihood ratio of the encoded bits to obtain the updated chip prior statistics. Then, perform channel estimation symbol by symbol based on the updated chip prior statistics to obtain the updated instantaneous channel coefficients. Feed the updated chip prior statistics and the updated instantaneous channel coefficients back to S12 of the next Turbo iteration.

[0017] S15. Repeat S12 to S14 until the iteration termination condition is met, and obtain the posterior log-likelihood ratio of the information bits output by the BCJR decoder in the final round.

[0018] Based on the above embodiments, channel estimation is performed symbol-by-symbol based on the updated chip prior statistics to obtain the updated instantaneous channel coefficients, including:

[0019] S21. Based on the updated chip prior statistics, reconstruct the transmit chip soft expectation vector for each user's M-CSK symbol;

[0020] S22. Input the conjugate transpose of the transmitted chip soft expectation vector and the received chip vector corresponding to the same M-CSK symbol into the instantaneous channel coefficient calculation function to calculate the updated instantaneous channel coefficients; wherein, the calculation expression of the instantaneous channel coefficient calculation function is:

[0021] ;

[0022] in, For the first The user Updated instantaneous channel coefficients for each M-CSK symbol. Index for users, This is the index of the M-CSK symbol in the M-CSK symbol sequence. For the first The user The conjugate transpose of the soft expectation vector of the transmit chip of each M-CSK symbol For the first The user The received chip vector of each M-CSK symbol, This is the vector modulo square operation.

[0023] In one embodiment, a truncation anchor point is recursively obtained based on the M-CSK symbol sequence, and an analysis window signal is extracted from the resampled signal centered on the truncation anchor point. Within the two-dimensional parameter space of residual frequency offset and chip timing offset, a joint search is performed on the analysis window signal with the maximum value of the despreading correlation peak as the cost function to obtain estimates of residual frequency offset and chip timing offset, including:

[0024] S31. For the first M-CSK symbol in the M-CSK symbol sequence, the moment when the received passband signal is used for frame preamble coarse synchronization acquisition is taken as the interception anchor point.

[0025] S32. Using the interception anchor point as the center, extract a signal segment from the resampled signal whose length is the sum of the nominal duration of a single M-CSK symbol and the guard interval, to obtain the analysis window signal;

[0026] S33. Within the preset initial search range, the signal in the analysis window is sequentially subjected to frequency offset derotation, matched filtering, and chip extraction to obtain the chip vector; the optimal parameters are determined based on the maximum inner product modulus of the chip vector and the M-CSK spread spectrum waveform library to obtain the initial residual frequency offset estimate and the initial chip timing offset estimate of the M-CSK symbol.

[0027] S34. For other M-CSK symbols in the M-CSK symbol sequence, add the truncation anchor point of the previous M-CSK symbol, the nominal duration of the M-CSK symbol, and the chip timing offset estimate of the previous M-CSK symbol to obtain the current truncation anchor point.

[0028] S35. Using the current interception anchor point as the interception anchor point, repeat S32-S34 until all M-CSK symbols in the M-CSK symbol sequence have been processed, and integrate the initial residual frequency offset estimate and the initial chip timing offset estimate to obtain the residual frequency offset estimate and the chip timing offset estimate.

[0029] In one embodiment of the present invention, a fixed residual frequency offset estimate is used to calculate a slice of the cost function along the time delay dimension; based on the global maximum value of the slice, an effective time delay candidate set is selected through a preset threshold; the minimum time delay value is selected from the effective time delay candidate set to correct the chip timing offset estimate, thereby obtaining corrected timing synchronization parameters, including:

[0030] S41. Fix the residual frequency offset estimate, calculate the cost function value at each time delay point along the time delay dimension, with the residual frequency offset estimate as the condition, and obtain the cost function slice sequence corresponding to each time delay point.

[0031] S42. Perform value-by-value comparisons on the cost function slice sequence to obtain the global maximum value of the cost function slice sequence;

[0032] S43. Multiply the global maximum value and the relative threshold to obtain the screening threshold; integrate the candidate time delay values ​​in the cost function slice sequence that have a cost value greater than or equal to the screening threshold to obtain the effective time delay candidate set;

[0033] S44. Compare the values ​​of all candidate delay values ​​in the effective delay candidate set to obtain the minimum delay value, and use the minimum delay value as the correction timing synchronization parameter.

[0034] Secondly, this application also provides a multi-user M-CSK-IDMA iterative receiving apparatus for time-varying dual-select underwater acoustic channels, used to implement the method described in the first aspect, including:

[0035] The signal synchronization and resampling module is used to acquire the received passband signal, perform frame preamble coarse synchronization acquisition and pilot frequency estimation on the received passband signal to obtain the macroscopic Doppler frequency offset estimate; and perform physical resampling on the received passband signal based on the macroscopic Doppler frequency offset estimate to obtain the resampled signal.

[0036] The synchronization parameter joint search module is used to obtain the user's M-CSK symbol sequence, recursively obtain the truncation anchor point based on the M-CSK symbol sequence, and extract the analysis window signal from the resampled signal with the truncation anchor point as the center; in the two-dimensional parameter space of residual frequency offset and chip timing offset, a joint search is performed on the analysis window signal with the maximum value of the despreading correlation peak as the cost function to obtain the residual frequency offset estimate and the chip timing offset estimate.

[0037] The timing synchronization correction module is used to fix the residual frequency offset estimate and calculate the cost function slice along the time delay dimension; based on the global maximum value of the slice, it selects an effective time delay candidate set through a preset threshold; and selects the minimum time delay value from the effective time delay candidate set to correct the chip timing offset estimate, thereby obtaining the corrected timing synchronization parameters.

[0038] The iterative multi-user detection module is used to despread the resampled signal to obtain the received chip vector of each user based on the corrected timing synchronization parameters and residual frequency offset estimates; and to perform Turbo iterative multi-user detection and decoding on the received chip vector to obtain the posterior log-likelihood ratio of the information bits of each user.

[0039] The decoding output module is used to make a hard decision on the posterior log-likelihood ratio of the information bits to obtain the decoded bit sequence for each user.

[0040] Thirdly, this application also provides a computer device, including a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the steps of any of the methods in the first aspect of this application.

[0041] Fourthly, this application also provides a computer-readable storage medium having a computer program stored thereon, wherein the computer program, when executed by a processor, implements the steps of any of the methods in the first aspect of this application.

[0042] The aforementioned multi-user M-CSK-IDMA iterative reception method, apparatus, computer equipment, and storage medium for time-varying dual-select underwater acoustic channels construct a four-stage cascaded processing architecture of "macrophysical physical resampling—symbol-by-symbol two-dimensional blind search—earliest peak locking—Turbo iterative detection." This architecture first compresses the broadband Doppler effect into a narrowband residual frequency offset before performing symbol-by-symbol tracking, and then recursively updates the anchor points to generate a symbol-level phase-locked loop using the chip timing offset estimate. This fundamentally solves the problems of timing drift accumulation and synchronization loss in oscillating motion scenarios under traditional frame-level one-time compensation. Simultaneously, it uses the earliest peak criterion to select effective delay candidates from the cost function slice. By selecting the minimum delay value to correct the timing synchronization parameters, the receiver synchronization reference is always aligned with the first arriving valid signal component, avoiding the M-CSK demodulation misjudgment caused by the hijacking of false peaks in the reflection path under strong multipath conditions, which is a problem of the traditional maximum peak criterion. Based on this, the received chip vector is accurately despread using the corrected timing synchronization parameters and residual frequency offset estimates. Soft information is exchanged between the basic signal estimator and the BCJR decoder through Turbo iteration, and the instantaneous channel coefficients are estimated symbol by symbol. This achieves multi-user signal separation with near-single-user performance under the condition of only a single hydrophone, and is particularly suitable for mobile platforms with limited payload space, such as small AUVs. Attached Figure Description

[0043] To more clearly illustrate the technical solutions in the embodiments or related technologies of this application, the accompanying drawings used in the description of the embodiments or related technologies will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0044] Figure 1 A schematic diagram of an implementation environment provided for one embodiment of the present invention;

[0045] Figure 2 This is a flowchart of a multi-user M-CSK-IDMA iterative reception method for time-varying dual-select underwater acoustic channels according to one embodiment of the present invention.

[0046] Figure 3 This is a schematic diagram of a multi-user M-CSK-IDMA iterative receiving device for time-varying dual-select underwater acoustic channels according to one embodiment of the present invention.

[0047] Figure 4 This is a flowchart of the Turbo ESE / BCJR iterative detection process in one embodiment of the present invention. Detailed Implementation

[0048] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0049] The multi-user M-CSK-IDMA iterative reception method for time-varying dual-select underwater acoustic channels provided in this application can be applied to, for example... Figure 1 In the application environment shown, the iterative receiving terminal 100 communicates with the sensor 101 via a data transmission link. The sensor 101 is a single hydrophone deployed on an underwater receiving platform. Multiple user AUVs act as multi-user transmitting nodes, transmitting communication signals underwater using the M-CSK-IDMA multiple access method. These signals are transmitted through a time-varying dual-select underwater acoustic channel, received by the sensor 101, converted into electrical signals, and then transmitted to the iterative receiving terminal 100. The iterative receiving terminal 100 executes the iterative receiving method of this application to perform synchronous correction and multi-user detection decoding on the multi-user mixed signal transmitted through the time-varying dual-select underwater acoustic channel, thereby achieving reliable reception of multi-user data under the time-varying dual-select underwater acoustic channel. The sensor 101 can be a single hydrophone, and the iterative receiving terminal 100 can be, but is not limited to, various personal computers, laptops, smartphones, and tablets.

[0050] In one exemplary embodiment, such as Figure 2 As shown, a multi-user M-CSK-IDMA iterative reception method for time-varying dual-select underwater acoustic channels is provided, which is then applied to... Figure 1 Taking the iterative receiving terminal in the example, the method includes the following steps:

[0051] S1. Acquire the received passband signal, perform frame preamble coarse synchronization acquisition and pilot frequency estimation on the received passband signal to obtain the macroscopic Doppler frequency offset estimate; perform physical resampling on the received passband signal based on the macroscopic Doppler frequency offset estimate to obtain the resampled signal.

[0052] Specifically, the macroscopic Doppler frequency offset estimate can be a frequency offset estimate that characterizes the overall time-domain scaling ratio of the received signal, obtained by jointly solving the frame preamble LFM coarse synchronization and pilot single-tone DTFT maximization criteria for the broadband Doppler effect of the time-varying dual-select underwater acoustic channel. Physical resampling can be a physical layer sampling rate conversion operation that performs time-domain axis mapping on the received passband signal by calculating the resampling factor based on the macroscopic Doppler frequency offset and performing time-domain axis mapping on the received passband signal through spline interpolation. It is not a simple digital domain frequency offset derotation and can suppress the residual frequency offset to an extremely low level, which is suitable for the front-end preprocessing requirements of a single hydrophone platform.

[0053] For example, the iterative receiving terminal can acquire the received passband signal output by the underwater acoustic sensor through a sensor, extract the linear frequency modulated (LFM) signal segment in the frame preamble, and complete the sliding correlation coarse synchronization acquisition to locate the start time of each user data frame; the iterative receiving terminal can extract the pilot single-tone signal segment and use the discrete-time Fourier transform (DTFT) maximization criterion to complete the fine estimation of the pilot frequency, obtaining the macroscopic Doppler frequency offset estimate; the iterative receiving terminal can calculate the physical resampling factor based on the nominal carrier frequency and the macroscopic Doppler frequency offset estimate; wherein, the calculation expression of the physical resampling factor can be:

[0054]

[0055] in, This is the physical resampling factor. The nominal carrier frequency, This is the macroscopic Doppler frequency offset estimate; the iterative receiving terminal can perform cubic spline interpolation time-domain resampling of the received passband signal based on the physical resampling factor to eliminate the main Doppler scaling effect and obtain the resampled signal.

[0056] S2. Obtain the user's M-CSK symbol sequence, and recursively obtain the truncation anchor point based on the M-CSK symbol sequence. Extract the analysis window signal from the resampled signal with the truncation anchor point as the center. In the two-dimensional parameter space of residual frequency offset and chip timing offset, perform a joint search on the analysis window signal with the maximum value of the despreading correlation peak as the cost function to obtain the residual frequency offset estimate and the chip timing offset estimate.

[0057] Specifically, the truncation anchor point can be a time-domain reference point generated recursively based on the timing synchronization parameters of the previous symbol to adapt to the timing drift of time-varying channels. This truncation anchor point can be used to locate the center of the current M-CSK symbol analysis window, which is different from the fixed frame start anchor point and can dynamically track the instantaneous arrival time of the signal. The two-dimensional parameter space can be a joint search space composed of the residual frequency offset candidate dimension and the chip timing offset candidate dimension. The two-dimensional parameter space can simultaneously solve the symbol-level residual Doppler and timing jitter problems. The cost function of the maximum value of the despreading correlation peak can be a synchronous search criterion with the maximum value of the product modulus within the candidate spreading template and the window signal as the core, which is the core basis of the two-dimensional joint search.

[0058] For example, the iterative receiving terminal can acquire the target user's M-CSK symbol sequence and recursively calculate the truncation anchor point of the current symbol based on the corrected timing synchronization parameters of the previous symbol:

[0059]

[0060] in, For the first Anchor point for extracting a symbol, The anchor point for extracting the previous symbol. The nominal duration of the M-CSK symbol, This is the estimated timing offset of the previous symbol chip. For indexing purposes; the iterative receiving terminal can extract an analysis window signal containing a preset guard interval from the resampled signal, centered on the intercepted anchor point; wherein, the residual frequency offset candidate set includes all residual frequency offset candidate values ​​to be searched; the chip timing offset candidate set includes all chip timing offset candidate values ​​to be searched; the iterative receiving terminal can perform frequency offset derotation and M-CSK spread spectrum template matched filtering on the combined parameters of each set of residual frequency offset candidate values ​​and chip timing offset candidate values, and extract the despread chip vector and calculate the cost function; wherein the calculation expression of the cost function can be:

[0061]

[0062] in, The cost function for unspreading the correlation peak maximum value, For spread spectrum template index, For spread spectrum template index set, For window signals under candidate parameters, The first one in the M-CSK spread spectrum waveform library One candidate template, For residual frequency offset, The chip timing offset is used; the iterative receiving terminal obtains the residual frequency offset estimate and chip timing offset estimate of the current symbol by maximizing the cost function.

[0063] S3. Fix the residual frequency offset estimate and calculate the cost function slice along the delay dimension; based on the global maximum value of the slice, select the effective delay candidate set through a preset threshold; select the minimum delay value from the effective delay candidate set to correct the chip timing offset estimate and obtain the corrected timing synchronization parameter.

[0064] For example, the iterative receiving terminal can fix the residual frequency offset estimate, calculate a one-dimensional slice of the cost function along the time delay dimension, and extract the global maximum value; the iterative receiving terminal can set a preset relative threshold and filter to obtain a set of effective time delay candidates; wherein, the expression for filtering the set of effective time delay candidates can be:

[0065]

[0066] in, The effective delay candidate set can include all delay points that meet the energy requirements and are physically valid. As candidate values ​​for delay, The maximum value of the cost function in the frequency dimension under a fixed delay is given. The iterative receiving terminal can perform the earliest peak anti-multipath locking and select the minimum delay value from the effective delay candidate set. The iterative receiving terminal can correct the chip timing offset estimate based on the minimum delay value to obtain the corrected timing synchronization parameter adapted to the strong multipath environment.

[0067] Specifically, the earliest peak anti-multipath locking criterion is a timing correction criterion proposed for strong multipath underwater acoustic channels. It prioritizes the earliest arrival effective path delay to avoid the problem of false peak hijacking of reflection paths from a physical perspective. The effective delay candidate set is obtained by filtering based on the global maximum value of the cost function slice and a preset relative threshold. It includes the set of all delay points that meet the energy requirements and have physical validity. The one-dimensional cost function slice is a function slice that reflects only the relationship between delay and cost function after fixing the residual frequency offset. The global maximum value is the maximum value of the one-dimensional cost function slice, which serves as the benchmark for filtering effective delay. The preset relative threshold is the threshold used to filter effective delay candidate values ​​that meet the energy requirements. The corrected timing synchronization parameter is a precise timing parameter adapted to strong multipath environments and used for M-CSK despreading after being corrected by the earliest peak criterion.

[0068] S4. Based on the corrected timing synchronization parameters and the residual frequency offset estimate, despread the resampled signal to obtain the received chip vectors of each user; perform Turbo iterative multi-user detection and decoding on the received chip vectors to obtain the posterior log-likelihood ratio of the information bits of each user.

[0069] For example, the iterative receiving terminal can perform frequency offset de-rotation and M-CSK despreading from the resampled signal based on the corrected timing synchronization parameters and the residual frequency offset estimate, to obtain the received chip vectors of each user; the iterative receiving terminal can calculate the user instantaneous channel coefficients through decision-oriented instantaneous channel estimation; wherein, the expression for the user instantaneous channel coefficients can be:

[0070]

[0071] in, For the first The first user's The instantaneous channel coefficients of each M-CSK symbol, the user instantaneous channel coefficients can be used to characterize the instantaneous transmission characteristics of the time-varying dual-select channel; To reconstruct the soft symbol chip vector, To reconstruct the conjugate transpose of the soft symbol chip vector, The received signal vector for the current user and the current symbol. for The squared norm of the L2; the iterative receiving terminal can calculate the log-likelihood ratio of the chip-level external information using the basic signal estimator (ESE); where the formula for calculating the log-likelihood ratio of the chip-level external information is:

[0072]

[0073] in, The chip-level extrinsic information log-likelihood ratio of the basic signal estimator ESE output. For chip rate, For user number The conjugate of the instantaneous channel coefficients corresponding to each chip This is the average value of multiple access interference. This represents the variance of multi-access interference.

[0074] Specifically, decision-oriented instantaneous channel estimation can be a method for calculating the instantaneous coefficients of time-varying dual-selection channels in real time, adapting to the channel estimation requirements of single hydrophones without spatial diversity; Turbo iterative multi-user detection can be a closed-loop iterative detection mechanism that combines M-CSK soft demodulation, ESE multi-user interference cancellation, and BCJR decoding, achieving layer-by-layer suppression of multiple access interference under a single array element through external information interaction; the log-likelihood ratio can be a chip-level soft information generated based on the statistical characteristics of multi-user interference without prior information, and is the core interactive data of Turbo iteration; the received chip vector is the chip-level received signal obtained after despreading, and the posterior log-likelihood ratio is a soft information used to characterize the probability of the information bit being 1 or 0.

[0075] S5. Perform a hard decision on the posterior log-likelihood ratio of the information bits to obtain the decoded bit sequence for each user.

[0076] Specifically, hard decision can be an operation that uses the posterior soft information output by Turbo iteration to determine deterministic bits through threshold judgment, and it is a key step in obtaining the final transmissible decoding result; the decoded bit sequence is the user's original information bit sequence output after hard decision, free from multiple access interference and channel distortion, and is the final output result of the iterative reception method; the posterior log-likelihood ratio can be soft information used to characterize the probability of information bits being 1 or 0, and the decoded bits are the deterministic result of the posterior log-likelihood ratio after hard decision.

[0077] For example, the iterative receiving terminal can perform a hard decision on the posterior log-likelihood ratio based on the posterior log-likelihood ratio of each user information bit, with the decision rule being: At that time, the decoded bits ,when At that time, the decoded bits The iterative receiving terminal can concatenate all hard decision results according to bit timing to obtain the decoded bit sequence for each user.

[0078] The aforementioned multi-user M-CSK-IDMA iterative reception method for time-varying dual-select underwater acoustic channels constructs a four-stage cascaded processing architecture: "macrophysical physical resampling—symbol-by-symbol two-dimensional blind search—earliest peak locking—Turbo iterative detection." This architecture first compresses the broadband Doppler effect into a narrowband residual frequency offset before symbol-by-symbol tracking. Furthermore, it recursively updates the anchor point using the chip timing offset estimate to form a symbol-level phase-locked loop, fundamentally solving the problems of timing drift accumulation and synchronization loss in oscillating motion scenarios under traditional frame-level one-time compensation. Simultaneously, it selects the minimum delay value from the effective delay candidate set of the cost function slice using the earliest peak criterion. By modifying the timing synchronization parameters, the receiver synchronization reference is always aligned with the first arriving valid signal component, avoiding the M-CSK demodulation misjudgment caused by the hijacking of false peaks in the reflection path under strong multipath conditions, which is a problem of the traditional maximum peak criterion. Based on this, the received chip vector is accurately despread using the modified timing synchronization parameters and residual frequency offset estimates. Soft information is exchanged between the basic signal estimator and the BCJR decoder through Turbo iteration, and the instantaneous channel coefficients are estimated symbol by symbol. This achieves multi-user signal separation with near-single-user performance under the condition of only a single hydrophone, and is particularly suitable for mobile platforms with limited payload space, such as small AUVs.

[0079] In one embodiment of the present invention, Turbo iterative multi-user detection and decoding are performed on the received chip vector to obtain the posterior log-likelihood ratio of the information bits for each user, including:

[0080] S11. Set the prior expectation of each chip in the received chip vector for each user to zero and set the prior variance of the chip to a unit value to obtain the initial chip prior statistics; perform channel estimation based on the received chip vector and the initial chip prior statistics to obtain the initial instantaneous channel coefficients.

[0081] Specifically, the initial chip prior statistics can be the set of prior statistical parameters set for each chip in the received chip vector of each user during the first round of Turbo iteration initialization. The initial chip prior statistics can be used as the initial input for the first round of channel estimation. The chip prior expectation can be a parameter characterizing the average level of the chip prior information, and the chip prior variance can be a parameter characterizing the degree of dispersion of the chip prior information. The initial instantaneous channel coefficients can be the first-round channel coefficients obtained based on the initial chip prior statistics and the received chip vector, and used as the channel characteristic characterization parameters for the first round of Turbo iteration.

[0082] For example, the iterative receiving terminal can set the chip prior expectation and chip prior variance for each chip in the received chip vector for each user, set the prior expectation of each user for each chip in the received chip vector to zero, and set the prior variance of each chip to a unit value, thus obtaining the initialized chip prior statistics; the iterative receiving terminal can perform channel estimation based on the received chip vector and the initialized chip prior statistics to obtain the initialized instantaneous channel coefficients; wherein, the calculation expression of the initialized instantaneous channel coefficients can be:

[0083]

[0084] in, For the first Each user initializes instantaneous channel coefficients. For mathematical expectation, This is the reference chip vector reconstructed based on the prior statistics of the initialized chip. for The conjugate transpose of .

[0085] S12. Obtain the user's chip prior statistics and current instantaneous channel coefficients. Based on the chip prior statistics and corresponding instantaneous channel coefficients, calculate the multiple access interference statistics. Based on the multiple access interference statistics, perform soft interference cancellation on the current user's chip values ​​in the received chip vector to obtain the external information log-likelihood ratio of each chip for the current user. Among them, the corresponding instantaneous channel coefficients are the initial instantaneous channel coefficients in the first iteration and the current instantaneous channel coefficients in subsequent iterations.

[0086] Specifically, the chip prior statistics can be a set of statistical parameters characterizing chip prior information. These statistics may include chip prior expectation and chip prior variance, used for calculating multiple access interference statistics. The current instantaneous channel coefficient can be a parameter characterizing the instantaneous transmission characteristics of the channel in the current iteration round. In the first iteration, it is the initialized instantaneous channel coefficient; in subsequent iterations, it is the instantaneous channel coefficient obtained from the previous iteration. The multiple access interference statistics can be statistical parameters characterizing the level of multiple access interference, calculated based on the chip prior statistics and channel coefficients of other users. Soft interference cancellation can be a processing method that cancels the impact of multiple access interference from the received chip vector based on the multiple access interference statistics. The extrinsic information log-likelihood ratio can be soft information characterizing the reliability of chip information after multiple access interference cancellation, and is the core interactive data of the Turbo iteration.

[0087] For example, the iterative receiving terminal can calculate the mean and variance of multiple access interference based on the chip prior statistics and corresponding instantaneous channel coefficients of each user; wherein, the expression for calculating the mean can be:

[0088]

[0089] in, Indexing users other than the current user. For other users The current instantaneous channel coefficients of each chip, For the first The expression for calculating the mean and variance of multiple access interference on each chip can be:

[0090]

[0091] in, For the variance of multi-access interference, Index for the current user, The chip index is used; the iterative receiving terminal can calculate the multiple access interference statistics based on the mean and variance of the multiple access interference; based on the multiple access interference statistics, the iterative receiving terminal can perform soft interference cancellation on the chip values ​​of the current user in the received chip vector to offset the multiple access interference from other users, and obtain the external information log-likelihood ratio of each chip of the current user, where soft interference cancellation is as follows: Figure 4 The ESE soft information update step in the TurboESE / BCJR iterative detection flowchart shown.

[0092] S13. Perform chip deinterleaving and M-CSK soft demodulation on the external information log-likelihood ratio in sequence to obtain the coded bit soft information. Input the coded bit soft information into the BCJR decoder for decoding to obtain the posterior log-likelihood ratio of the coded bit.

[0093] Specifically, chip deinterleaving can be the operation of restoring the timing of the log-likelihood ratio of chip-level extrinsic information to the original timing of the coded bits. Chip deinterleaving can be used to match the input requirements of M-CSK soft demodulation. M-CSK soft demodulation can be a process of converting chip-level extrinsic information into coded bit soft information based on spread spectrum template matching. The coded bit soft information can be soft information characterizing the reliability of the coded bits and is used as input to the BCJR decoder. The BCJR decoder can be a module that decodes the coded bit soft information based on convolutional coding characteristics. The posterior log-likelihood ratio of the coded bits can be the posterior soft information output by the BCJR decoder, characterizing the reliability of the coded bits.

[0094] For example, the iterative receiving terminal can sequentially perform chip deinterleaving processing on the log-likelihood ratio of the extrinsic information of each chip of the current user to restore the original timing of the coded bits; the iterative receiving terminal can send the deinterleaved extrinsic information to the M-CSK soft demodulator for M-CSK soft demodulation, and calculate the soft information of the coded bits through spread spectrum template matching; the iterative receiving terminal can input the soft information of the coded bits into the BCJR decoder for decoding processing to obtain the posterior log-likelihood ratio of the coded bits; wherein, the deinterleaving, soft demodulation and decoding processes correspond to Figure 4The Turbo ESE / BCJR iterative detection flowchart includes the steps of ESE soft information update, M-CSK soft demodulation, and BCJR decoding.

[0095] S14. Update the chip prior statistics of each user based on the posterior log-likelihood ratio of the encoded bits to obtain the updated chip prior statistics. Then, perform channel estimation symbol by symbol based on the updated chip prior statistics to obtain the updated instantaneous channel coefficients. Feed the updated chip prior statistics and the updated instantaneous channel coefficients back to S12 of the next Turbo iteration.

[0096] Specifically, updating the chip prior statistics can be the set of chip prior parameters used for the next iteration, obtained based on the updated posterior log-likelihood ratio of the coded bits. The updated chip prior statistics can include the updated chip prior expectation and chip prior variance. The updated instantaneous channel coefficients can be the channel coefficients required for the next iteration, obtained by symbol-by-symbol channel estimation based on the updated chip prior statistics. The Turbo iterative feedback can be the process of feeding back the updated chip prior statistics and the updated instantaneous channel coefficients to the next iteration S12 to obtain a closed-loop iteration.

[0097] For example, the iterative receiving terminal can update the chip prior expectation and chip prior variance for each user based on the posterior log-likelihood ratio of the encoded bits through probabilistic statistical calculations, thereby obtaining updated chip prior statistics. Based on the updated chip prior statistics, the iterative receiving terminal can perform channel estimation symbol-by-symbol for each user, and calculate updated instantaneous channel coefficients by combining the received chip vectors. The iterative receiving terminal can then feed back the updated chip prior statistics and updated instantaneous channel coefficients to step S12 of the next Turbo iteration as input parameters for the next iteration. This feedback process corresponds to... Figure 4 The external feedback reconstruction step is shown in the Turbo ESE / BCJR iterative detection flowchart.

[0098] S15. Repeat S12 to S14 until the iteration termination condition is met, and obtain the posterior log-likelihood ratio of the information bits output by the BCJR decoder in the final round.

[0099] Specifically, the iteration termination condition can be a preset condition used to determine whether the Turbo iteration stops. The iteration termination condition can include the number of iterations reaching a preset maximum value, the decoding error rate reaching a preset threshold, or the external information converging. The posterior log-likelihood ratio of the final round information bits can be soft information output by the last round BCJR decoder after the Turbo iteration meets the termination condition, which characterizes the reliability of each user information bit and is used for subsequent hard decision to obtain the decoded bit sequence.

[0100] For example, the iterative receiving terminal can repeatedly execute operations S12 to S14. After each iteration, it determines whether the current iteration meets the preset iteration termination condition. The iterative determination process corresponds to... Figure 4 The iteration termination judgment step in the TurboESE / BCJR iterative detection flowchart; if the iteration termination condition is not met, the iterative receiving terminal can continue to execute the next round of iteration until the iteration meets the termination condition and then stop the iteration operation; the iterative receiving terminal can extract the posterior log-likelihood ratio of the information bits of each user output by the BCJR decoder in the final round as the final output of Turbo iterative multi-user detection and decoding.

[0101] This application provides a multi-user M-CSK-IDMA iterative reception method for time-varying dual-select underwater acoustic channels. Through a Turbo iterative multi-user detection and decoding process, it first completes iterative initialization and channel estimation. Then, it generates external information through multiple access interference statistics and soft interference elimination. After deinterleaving, soft demodulation, and decoding, it updates the iterative parameters and forms a closed loop until the termination condition is met, outputting the posterior log-likelihood ratio of the information bits. This effectively suppresses multiple access interference, adapts to the characteristics of time-varying dual-select underwater acoustic channels, improves the decoding reliability and synchronization stability of multi-user reception, and provides high-precision soft information support for subsequent hard decision-making.

[0102] Based on the above embodiments, channel estimation is performed symbol-by-symbol based on the updated chip prior statistics to obtain the updated instantaneous channel coefficients, including:

[0103] S21. Based on the updated chip prior statistics, reconstruct the transmit chip soft expectation vector for each user's M-CSK symbol.

[0104] Specifically, the transmit chip soft expectation vector can be a vector representing the transmit chip expectation level of each user for each M-CSK symbol, which is reconstructed based on the updated chip prior statistics and used for subsequent calculation of the updated instantaneous channel coefficients; the updated chip prior statistics can include the updated chip prior expectation and chip prior variance, which are the core basis for reconstructing the transmit chip soft expectation vector; the M-CSK symbol sequence can be the symbol transmission sequence of the target user, with each symbol corresponding to a set of transmit chips.

[0105] For example, the iterative receiving terminal can, based on the updated chip prior statistics, select each M-CSK symbol for each user, determine the range of all chips corresponding to the M-CSK symbol, extract the updated chip prior expectations of each chip, and arrange them into vectors according to the original transmission timing of each chip in the M-CSK symbol, thereby reconstructing the transmit chip soft expectation vector of the M-CSK symbol; finally, the transmit chip soft expectation vector of all users and all M-CSK symbols is reconstructed, providing input for the calculation of the updated instantaneous channel coefficients in the subsequent S22 step.

[0106] S22. Input the conjugate transpose of the transmitted chip soft expectation vector and the corresponding received chip vector into the instantaneous channel coefficient calculation function to calculate the updated instantaneous channel coefficient.

[0107] Specifically, the calculation expression for the instantaneous channel coefficient calculation function can be:

[0108]

[0109] in, For the first The user Updated instantaneous channel coefficients for each M-CSK symbol. Index for users, This is the index of the M-CSK symbol in the M-CSK symbol sequence. For the first The user The conjugate transpose of the soft expectation vector of the transmit chip of each M-CSK symbol For the first The user The received chip vector of each M-CSK symbol, This is the vector modulo square operation.

[0110] Specifically, the instantaneous channel coefficient calculation function can be a dedicated function for calculating and updating the instantaneous channel coefficients. This function can obtain coefficients characterizing the instantaneous transmission characteristics of the time-varying channel by performing correlation operations between the transmitted chip soft expectation vector and the received chip vector, combined with vector modulus square normalization. The updated instantaneous channel coefficients can be channel coefficients calculated based on the transmitted chip soft expectation vector and the received chip vector, used for the next Turbo iteration. The conjugate transpose of the transmitted chip soft expectation vector can be a vector obtained by performing a conjugate transpose operation on the reconstructed transmitted chip soft expectation vector, used for correlation calculation with the received chip vector. The vector modulus square operation can be an operation of summing the squares of the moduli of each element of the vector, used to normalize the channel coefficient calculation results.

[0111] For example, the iterative receiving terminal can perform a conjugate transpose operation on the soft expectation vector of the transmitted chip to obtain its conjugate transpose vector. The iterative receiving terminal can then input the conjugate transpose vector and the corresponding received chip vector into the instantaneous channel coefficient calculation function to complete the calculation of the updated instantaneous channel coefficients. During the calculation process, the iterative receiving terminal can achieve matching between the transmitted and received chips through vector correlation operations, and perform normalization processing by combining vector modulus square operations to obtain the updated instantaneous channel coefficients corresponding to each user and each M-CSK symbol.

[0112] This application provides a multi-user M-CSK-IDMA iterative reception method for time-varying dual-select underwater acoustic channels. By reconstructing the soft expectation vector of the transmitted chip based on the prior statistics of the updated chip, and combining it with the received chip vector, the method completes the accurate calculation of the instantaneous channel coefficients for updating. This provides reliable time-varying channel parameter support for Turbo iteration, effectively improves the channel estimation accuracy and iterative stability of multi-user detection under time-varying dual-select underwater acoustic channels, and ensures the efficient advancement of subsequent closed-loop iterations.

[0113] In one embodiment of the present invention, an anchor point is recursively obtained based on the M-CSK symbol sequence, and an analysis window signal is extracted from the resampled signal centered on the anchor point. Within the two-dimensional parameter space of residual frequency offset and chip timing offset, a joint search is performed on the analysis window signal with the maximum value of the despreading correlation peak as the cost function to obtain estimates of residual frequency offset and chip timing offset, including:

[0114] S31. For the first M-CSK symbol in the M-CSK symbol sequence, the moment when the received passband signal is used for frame preamble coarse synchronization acquisition is taken as the interception anchor point.

[0115] Specifically, anchor point extraction can be the reference time of the analysis window signal extracted from the resampled signal, used to accurately locate the signal segment corresponding to the M-CSK symbol to be processed; frame preamble coarse synchronization acquisition can be the operation of achieving preliminary signal synchronization and determining the approximate arrival time of the signal by receiving the frame preamble sequence in the passband signal.

[0116] For example, the iterative receiving terminal can select the first M-CSK symbol in the M-CSK symbol sequence as the initial processing object, perform a frame preamble coarse synchronization acquisition operation on the received passband signal, and determine the acquisition time of the frame preamble coarse synchronization acquisition by identifying the characteristics of the frame preamble sequence. The iterative receiving terminal can use the acquisition time as the truncation anchor point of the M-CSK symbol to provide an accurate reference time for the subsequent truncation of the analysis window signal.

[0117] S32. Using the intercepted anchor point as the center, extract a signal segment from the resampled signal whose length is the sum of the nominal duration of a single M-CSK symbol and the guard interval, to obtain the analysis window signal.

[0118] Specifically, the analysis window signal can be a signal segment extracted from the resampled signal for subsequent joint search of two-dimensional parameters. The analysis window signal can include the complete signal of a single M-CSK symbol and the guard interval; the nominal duration can be the preset standard transmission duration of a single M-CSK symbol, characterizing the basic time span of the symbol; the guard interval can be a redundant time set to avoid interference between adjacent symbols, used to cancel inter-symbol crosstalk caused by multipath propagation; the resampled signal can be the signal obtained after resampling the received passband signal, adapted to the rate requirements of subsequent signal processing.

[0119] For example, the iterative receiving terminal can determine the range of signal interception with the interception anchor point as the center: the interception length is set to the sum of the nominal duration of a single M-CSK symbol and the guard interval, ensuring that the intercepted signal segment contains both the complete and valid signal of the M-CSK symbol and avoids interference from adjacent symbols through the guard interval; the iterative receiving terminal can intercept the signal segment from the resampled signal according to this range to obtain the analysis window signal corresponding to the M-CSK symbol.

[0120] S33. Within the preset initial search range, the signal in the analysis window is sequentially subjected to frequency offset derotation, matched filtering, and chip extraction to obtain the chip vector. The optimal parameters are determined based on the maximum inner product modulus of the chip vector and the M-CSK spread spectrum waveform library to obtain the residual frequency offset estimate and chip timing offset estimate of the M-CSK symbol.

[0121] For example, the iterative receiving terminal can sequentially perform signal processing operations on the analysis window signal within a preset initial search range: first, frequency offset derotation is performed to eliminate the influence of frequency offset on the signal and keep the signal phase synchronized; then, matched filtering is performed to match the processed signal with the spread spectrum waveforms in the M-CSK spread spectrum waveform library to enhance the effective signal and suppress irrelevant interference; then, chip decimation is performed to extract the chip-level signal from the filtered signal to obtain the chip vector; the iterative receiving terminal can calculate the inner product magnitude of the chip vector and all spread spectrum waveforms in the M-CSK spread spectrum waveform library, and select the parameters corresponding to the spread spectrum waveform with the largest inner product magnitude as the residual frequency offset estimate and chip timing offset estimate of the M-CSK symbol according to the maximum inner product magnitude criterion.

[0122] S34. For other M-CSK symbols in the M-CSK symbol sequence, add the truncation anchor point of the previous M-CSK symbol, the nominal duration of the M-CSK symbol, and the chip timing offset estimate of the previous M-CSK symbol to obtain the current truncation anchor point.

[0123] Specifically, the truncation anchor point of the previous M-CSK symbol can be the reference time for signal truncation of the previous symbol, providing a basis for the recursion of the current symbol anchor point; the nominal duration of the M-CSK symbol can be a fixed symbol transmission duration, used to determine the time interval between adjacent symbols; the chip timing offset estimate of the previous M-CSK symbol can be the timing deviation parameter of the previous symbol, used to correct the anchor point position of the current symbol and improve the accuracy of the anchor point; the current truncation anchor point is the reference time determined for subsequent symbols in the M-CSK symbol sequence, used to truncate the signal in the analysis window.

[0124] For example, the iterative receiving terminal can use a recursive method to determine the current intercept anchor point: extract the intercept anchor point of the previous M-CSK symbol, the preset nominal duration of the M-CSK symbol, and the chip timing offset estimate obtained from the previous M-CSK symbol, and sum the intercept anchor point, the nominal duration of the M-CSK symbol, and the chip timing offset estimate. The result of the calculation is the intercept anchor point of the current M-CSK symbol.

[0125] S35. Using the current interception anchor point as the interception anchor point, repeat S32-S34 until all M-CSK symbols in the M-CSK symbol sequence have been processed, and integrate the initial residual frequency offset estimate and the initial chip timing offset estimate to obtain the residual frequency offset estimate and the chip timing offset estimate.

[0126] Specifically, iterative processing can be a process of processing all symbols in the M-CSK symbol sequence in sequence to ensure that each symbol can obtain the corresponding residual frequency offset estimate and chip timing offset estimate. The residual frequency offset estimate can be a precise parameter characterizing the signal frequency offset after correction, and the chip timing offset estimate is a precise parameter characterizing the chip transmission timing deviation.

[0127] For example, the iterative receiving terminal can use the current interception anchor point as the new interception anchor point and repeatedly execute operations S32 to S34: intercept the analysis window signal with the current anchor point as the center, perform a two-dimensional parameter joint search on the analysis window signal to obtain the residual frequency offset and chip timing offset estimates of the current symbol, and then recursively obtain the interception anchor point of the next symbol until all M-CSK symbols in the M-CSK symbol sequence have been processed, and obtain the residual frequency offset estimates and chip timing offset estimates of all symbols.

[0128] This application provides a multi-user M-CSK-IDMA iterative reception method for time-varying dual-select underwater acoustic channels. By recursively determining the anchor point and the analysis window signal including the guard interval, and jointly searching for the optimal parameters in the two-dimensional parameter space, the residual frequency offset and chip timing offset of all M-CSK symbols are accurately estimated. This effectively avoids multipath and adjacent symbol interference, avoids the accumulation of anchor point deviation, improves signal synchronization accuracy, and provides reliable synchronization parameter support for subsequent synchronization correction, multi-user detection, and demodulation decoding.

[0129] In one embodiment of the present invention, a fixed residual frequency offset estimate is used to calculate a slice of the cost function along the time delay dimension; based on the global maximum value of the slice, an effective time delay candidate set is selected through a preset threshold; the minimum time delay value is selected from the effective time delay candidate set to correct the chip timing offset estimate, thereby obtaining corrected timing synchronization parameters, including:

[0130] S41. Fix the residual frequency offset estimate, calculate the cost function value at each time delay point along the time delay dimension, with the residual frequency offset estimate as the condition, and obtain the cost function slice sequence corresponding to each time delay point.

[0131] For example, the iterative receiving terminal can fix the residual frequency offset estimate and calculate the cost function value point by point along the time delay dimension; wherein, the expression for calculating the cost function value can be:

[0132]

[0133] in, The cost function values ​​corresponding to different time delay points, This is the delay point of the current traversal. To receive the chip sequence, This is the sequence after the spreading code template has been shifted. For the length of the received chip sequence, The imaginary unit, This is the residual frequency offset estimate. Using time indexes, the iterative receiving terminal can sequentially traverse all delay points to obtain a cost function slice sequence composed of the cost function values ​​corresponding to each delay point.

[0134] S42. Perform value-by-value comparisons on the cost function slice sequence to obtain the global maximum value of the cost function slice sequence.

[0135] Specifically, the global maximum value can be the largest cost function value in the cost function slice sequence, which is used as the benchmark threshold for effective delay screening; value-by-value comparison can be the operation of comparing all values ​​in the cost function slice sequence one by one to determine the maximum value, which is the core method for screening the global maximum value.

[0136] For example, the iterative receiving terminal can perform a value-by-value comparison operation on the cost function slice sequence, and select the cost function value with the largest value from the cost function values ​​corresponding to all time delay points.

[0137] S43. Multiply the global maximum value and the relative threshold to obtain the screening threshold. Integrate the candidate delay values ​​in the cost function slice sequence that have a cost value greater than or equal to the screening threshold to obtain the effective delay candidate set.

[0138] Specifically, the filtering threshold can be a value obtained by multiplying the global maximum value by the preset relative threshold, used to determine whether the cost function value corresponding to the delay point meets the valid condition; the valid delay candidate set can be a set of delay points whose cost function value is greater than or equal to the filtering threshold, which is the core basis for subsequent correction of timing synchronization parameters.

[0139] For example, the iterative receiving terminal can calculate the filtering threshold by multiplying the global maximum value by a preset relative threshold; the iterative receiving terminal can traverse the cost function values ​​corresponding to all time delay points in the cost function slice sequence, filter out the time delay points whose cost value is greater than or equal to the filtering threshold, and integrate these time delay points to obtain an effective time delay candidate set.

[0140] S44. Compare the values ​​of all candidate delay values ​​in the effective delay candidate set to obtain the minimum delay value, and use the minimum delay value as the correction timing synchronization parameter.

[0141] Specifically, the minimum delay value can be the delay parameter with the smallest value in the effective delay candidate set, which can effectively avoid timing deviations caused by interference such as reflection paths; the corrected timing synchronization parameter can be the synchronization parameter obtained by correcting the original chip timing offset estimate by using the minimum delay value as a benchmark, which is used to improve the accuracy of subsequent signal processing.

[0142] For example, the iterative receiving terminal can compare all delay values ​​in the effective delay candidate set, select the delay value with the smallest value, use the smallest delay value as the correction benchmark, correct the chip timing offset estimate, and obtain the corrected timing synchronization parameters.

[0143] This application provides a multi-user M-CSK-IDMA iterative reception method for time-varying dual-select underwater acoustic channels. By calculating the cost function slice with a fixed residual frequency offset estimate, screening the effective delay candidate set, and selecting the minimum delay value to correct the timing synchronization parameters, this method effectively avoids timing deviations caused by multipath interference, improves chip timing synchronization accuracy, and provides accurate timing support for subsequent M-CSK soft demodulation, decoding, and multi-user detection, ensuring the stability and accuracy of reception processing.

[0144] It should be understood that although the steps in the flowcharts of the embodiments described above are shown sequentially according to the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless explicitly stated herein, there is no strict order restriction on the execution of these steps, and they can be executed in other orders. Moreover, at least some steps in the flowcharts of the embodiments described above may include multiple steps or multiple stages. These steps or stages are not necessarily completed at the same time, but can be executed at different times. The execution order of these steps or stages is not necessarily sequential, but can be performed alternately or in turn with other steps or at least some of the steps or stages of other steps.

[0145] Based on the same inventive concept, this application also provides a multi-user M-CSK-IDMA iterative receiving apparatus for implementing the multi-user M-CSK-IDMA iterative receiving method for time-varying dual-select underwater acoustic channels described above. The solution provided by this apparatus is similar to the implementation scheme described in the above method. Therefore, the specific limitations of one or more embodiments of the multi-user M-CSK-IDMA iterative receiving apparatus for time-varying dual-select underwater acoustic channels provided below can be found in the limitations of the multi-user M-CSK-IDMA iterative receiving method for time-varying dual-select underwater acoustic channels described above, and will not be repeated here.

[0146] In one exemplary embodiment, such as Figure 3 As shown, a multi-user M-CSK-IDMA iterative receiving device 500 for time-varying dual-select underwater acoustic channels is provided to implement the methods in the above-described method embodiments, including:

[0147] The signal synchronization and resampling module 501 is used to acquire the received passband signal, perform frame preamble coarse synchronization acquisition and pilot frequency estimation on the received passband signal to obtain the macroscopic Doppler frequency offset estimate; and perform physical resampling on the received passband signal based on the macroscopic Doppler frequency offset estimate to obtain the resampled signal.

[0148] The synchronization parameter joint search module 502 is used to obtain the user's M-CSK symbol sequence, recursively obtain the truncation anchor point based on the M-CSK symbol sequence, and extract the analysis window signal from the resampled signal with the truncation anchor point as the center; in the two-dimensional parameter space of residual frequency offset and chip timing offset, a joint search is performed on the analysis window signal with the maximum value of the despreading correlation peak as the cost function to obtain the residual frequency offset estimate and the chip timing offset estimate.

[0149] The timing synchronization correction module 503 is used to fix the residual frequency offset estimate and calculate the slice of the cost function along the time delay dimension; based on the global maximum value of the slice, it selects an effective time delay candidate set through a preset threshold; and selects the minimum time delay value from the effective time delay candidate set to correct the chip timing offset estimate, thereby obtaining the corrected timing synchronization parameters.

[0150] The iterative multi-user detection module 504 is used to despread the resampled signal to obtain the received chip vector of each user based on the corrected timing synchronization parameters and the residual frequency offset estimate; and to perform Turbo iterative multi-user detection and decoding on the received chip vector to obtain the posterior log-likelihood ratio of the information bits of each user.

[0151] The decoding output module 505 is used to make a hard decision on the posterior log-likelihood ratio of the information bits to obtain the decoded bit sequence for each user.

[0152] In one embodiment of the present invention, the iterative multi-user detection module 504 can also be used for:

[0153] S11. Set the prior expectation of each chip in the received chip vector for each user to zero and set the prior variance of the chip to a unit value to obtain the initial chip prior statistics; perform channel estimation based on the received chip vector and the initial chip prior statistics to obtain the initial instantaneous channel coefficients.

[0154] S12. Obtain the user's chip prior statistics and current instantaneous channel coefficients. Based on the chip prior statistics and corresponding instantaneous channel coefficients, calculate the multiple access interference statistics. Based on the multiple access interference statistics, perform soft interference cancellation on the current user's chip values ​​in the received chip vector to obtain the extrinsic information log-likelihood ratio of each chip for the current user. Among them, the corresponding instantaneous channel coefficients are the initial instantaneous channel coefficients in the first iteration and the current instantaneous channel coefficients in subsequent iterations.

[0155] S13. The external information log likelihood ratio is sequentially deinterleaved with chips and soft demodulated with M-CSK to obtain the coded bit soft information; the coded bit soft information is input into the BCJR decoder for decoding to obtain the posterior log likelihood ratio of the coded bit.

[0156] S14. Update the chip prior statistics of each user based on the posterior log-likelihood ratio of the encoded bits to obtain the updated chip prior statistics. Then, perform channel estimation symbol by symbol based on the updated chip prior statistics to obtain the updated instantaneous channel coefficients. Feed the updated chip prior statistics and the updated instantaneous channel coefficients back to S12 of the next Turbo iteration.

[0157] S15. Repeat S12 to S14 until the iteration termination condition is met, and obtain the posterior log-likelihood ratio of the information bits output by the BCJR decoder in the final round.

[0158] Based on the above embodiments, the iterative multi-user detection module 504 can also be used for:

[0159] S21. Based on the updated chip prior statistics, reconstruct the transmit chip soft expectation vector for each user's M-CSK symbol;

[0160] S22. Input the conjugate transpose of the transmitted chip soft expectation vector and the corresponding received chip vector into the instantaneous channel coefficient calculation function to calculate the updated instantaneous channel coefficients; wherein, the calculation expression of the instantaneous channel coefficient calculation function is:

[0161]

[0162] in, For the first The user Updated instantaneous channel coefficients for each M-CSK symbol. Index for users, This is the index of the M-CSK symbol in the M-CSK symbol sequence. For the first The user The conjugate transpose of the soft expectation vector of the transmit chip of each M-CSK symbol For the first The user The received chip vector of each M-CSK symbol, This is the vector modulo square operation.

[0163] In one embodiment of the present invention, the synchronization parameter joint search module 502 can also be used for:

[0164] S31. For the first M-CSK symbol in the M-CSK symbol sequence, the moment when the received passband signal is used for frame preamble coarse synchronization acquisition is taken as the interception anchor point.

[0165] S32. Using the interception anchor point as the center, extract a signal segment from the resampled signal whose length is the sum of the nominal duration of a single M-CSK symbol and the guard interval, to obtain the analysis window signal;

[0166] S33. Within the preset initial search range, the signal in the analysis window is sequentially subjected to frequency offset derotation, matched filtering, and chip extraction to obtain the chip vector; the optimal parameters are determined based on the maximum inner product modulus of the chip vector and the M-CSK spread spectrum waveform library to obtain the initial residual frequency offset estimate and the initial chip timing offset estimate of the M-CSK symbol.

[0167] S34. For other M-CSK symbols in the M-CSK symbol sequence, add the truncation anchor point of the previous M-CSK symbol, the nominal duration of the M-CSK symbol, and the chip timing offset estimate of the previous M-CSK symbol to obtain the current truncation anchor point.

[0168] S35. Using the current interception anchor point as the interception anchor point, repeat S32-S34 until all M-CSK symbols in the M-CSK symbol sequence have been processed, and integrate the initial residual frequency offset estimate and the initial chip timing offset estimate to obtain the residual frequency offset estimate and the chip timing offset estimate.

[0169] In one embodiment of the present invention, the timing synchronization correction module 503 can also be used for:

[0170] S41. Fix the residual frequency offset estimate, calculate the cost function value at each time delay point along the time delay dimension, with the residual frequency offset estimate as the condition, and obtain the cost function slice sequence corresponding to each time delay point.

[0171] S42. Perform value-by-value comparisons on the cost function slice sequence to obtain the global maximum value of the cost function slice sequence;

[0172] S43. Multiply the global maximum value and the relative threshold to obtain the screening threshold; integrate the candidate time delay values ​​in the cost function slice sequence that have a cost value greater than or equal to the screening threshold to obtain the effective time delay candidate set;

[0173] S44. Compare the values ​​of all candidate delay values ​​in the effective delay candidate set to obtain the minimum delay value, and use the minimum delay value as the correction timing synchronization parameter.

[0174] In one embodiment, this application provides a computer device including a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the steps in the above-described method embodiments.

[0175] In one embodiment, a computer-readable storage medium is provided having a computer program stored thereon, which, when executed by a processor, implements the steps in the above method embodiments.

[0176] For the device embodiments, since they basically correspond to the method embodiments, the relevant parts can be referred to in the description of the method embodiments. The device embodiments described above are merely illustrative. The components described as separate parts may or may not be physically separate, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed across multiple network units. Some or all of the modules can be selected to achieve the purpose of this disclosure according to actual needs. Those skilled in the art can understand and implement this without creative effort.

[0177] The above-described embodiments are merely illustrative of several implementation methods of the embodiments of this application, and their descriptions are relatively specific and detailed. However, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the embodiments of this application, and these modifications and improvements all fall within the protection scope of the embodiments of this application.

Claims

1. A multi-user M-CSK-IDMA iterative reception method for time-varying dual-select underwater acoustic channels, characterized in that, The method includes: S1. Acquire the received passband signal, perform frame preamble coarse synchronization acquisition and pilot frequency estimation on the received passband signal to obtain a macroscopic Doppler frequency offset estimate; perform physical resampling on the received passband signal based on the macroscopic Doppler frequency offset estimate to obtain a resampled signal; S2. Obtain the user's M-CSK symbol sequence, recursively obtain the truncation anchor point based on the M-CSK symbol sequence, and extract the analysis window signal from the resampled signal with the truncation anchor point as the center; in the two-dimensional parameter space of residual frequency offset and chip timing offset, perform a joint search on the analysis window signal with the maximum value of the despreading correlation peak as the cost function to obtain the residual frequency offset estimate and the chip timing offset estimate. S3. Fix the residual frequency offset estimate and calculate the slice of the cost function along the time delay dimension; based on the global maximum value of the slice, select an effective time delay candidate set through a preset threshold; select the minimum time delay value from the effective time delay candidate set to correct the chip timing offset estimate and obtain the corrected timing synchronization parameter; S4. Based on the corrected timing synchronization parameters and the residual frequency offset estimate, despread the resampled signal to obtain the received chip vector of each user; perform Turbo iterative multi-user detection and decoding on the received chip vector to obtain the posterior log-likelihood ratio of the information bits of each user. S5. Perform a hard decision on the posterior log-likelihood ratio of the information bits to obtain the decoded bit sequence for each user.

2. The method according to claim 1, characterized in that, The step of performing Turbo iterative multi-user detection and decoding on the received chip vector to obtain the posterior log-likelihood ratio of each user's information bits includes: S11. Set the prior expectation of each user corresponding to each chip in the received chip vector to zero and set the prior variance of the chip to a unit value to obtain the initial chip prior statistics; perform channel estimation based on the received chip vector and the initial chip prior statistics to obtain the initial instantaneous channel coefficients. S12. Obtain the user's chip prior statistics and current instantaneous channel coefficients. Based on the chip prior statistics and corresponding instantaneous channel coefficients, calculate the multiple access interference statistics. Based on the multiple access interference statistics, perform soft interference cancellation on the current user's chip values ​​in the received chip vector to obtain the extrinsic information log-likelihood ratio of each chip for the current user. Wherein, the corresponding instantaneous channel coefficient is the initial instantaneous channel coefficient in the first iteration and the current instantaneous channel coefficient in subsequent iterations. S13. The external information log-likelihood ratio is sequentially subjected to chip deinterleaving and M-CSK soft demodulation to obtain the coded bit soft information; the coded bit soft information is input into the BCJR decoder for decoding to obtain the coded bit posterior log-likelihood ratio. S14. Update the chip prior statistics of each user based on the posterior log-likelihood ratio of the encoded bits to obtain updated chip prior statistics, and perform channel estimation symbol by symbol based on the updated chip prior statistics to obtain updated instantaneous channel coefficients; feed the updated chip prior statistics and the updated instantaneous channel coefficients back to S12 of the next Turbo iteration. S15. Repeat S12 to S14 until the iteration termination condition is met, and obtain the posterior log-likelihood ratio of the information bits output by the BCJR decoder in the final round.

3. The method according to claim 2, characterized in that, The symbol-by-symbol channel estimation based on the updated chip prior statistics to obtain the updated instantaneous channel coefficients includes: S21. Based on the updated chip prior statistics, reconstruct the transmit chip soft expectation vector for each user's M-CSK symbol; S22. Input the conjugate transpose of the transmitted chip soft expectation vector and the received chip vector corresponding to the same M-CSK symbol into the instantaneous channel coefficient calculation function to calculate the updated instantaneous channel coefficients; wherein, the calculation expression of the instantaneous channel coefficient calculation function is: ； in, For the first The user The updated instantaneous channel coefficients of the M-CSK symbols, Index for users, The index of the M-CSK symbol in the M-CSK symbol sequence is given by the index of the M-CSK symbol. For the first The user The conjugate transpose of the soft expectation vector of the transmit chip of each M-CSK symbol For the first The user The received chip vector of each M-CSK symbol, This is the vector modulus square operation.

4. The method according to claim 1, characterized in that, The step involves recursively obtaining a truncation anchor point based on the M-CSK symbol sequence, and extracting an analysis window signal from the resampled signal centered on the truncation anchor point. Within the two-dimensional parameter space of residual frequency offset and chip timing offset, a joint search is performed on the analysis window signal with the maximum value of the despreading correlation peak as the cost function to obtain estimates of the residual frequency offset and chip timing offset, including: S31. For the first M-CSK symbol in the M-CSK symbol sequence, the moment when the received passband signal is captured by frame preamble coarse synchronization is taken as the interception anchor point. S32. Centered on the interception anchor point, extract a signal segment from the resampled signal with a length equal to the sum of the nominal duration and guard interval of a single M-CSK symbol to obtain the analysis window signal; S33. Within the preset initial search range, the analysis window signal is sequentially subjected to frequency offset derotation, matched filtering, and chip extraction to obtain a chip vector; the optimal parameters are determined based on the maximum inner product modulus of the chip vector and the M-CSK spread spectrum waveform library to obtain the initial residual frequency offset estimate and the initial chip timing offset estimate of the M-CSK symbol. S34. For other M-CSK symbols in the M-CSK symbol sequence, add the truncation anchor point of the previous M-CSK symbol, the nominal duration of the M-CSK symbol, and the chip timing offset estimate of the previous M-CSK symbol to obtain the current truncation anchor point. S35. Using the current interception anchor point as the interception anchor point, repeat S32-S34 until all M-CSK symbols in the M-CSK symbol sequence have been processed, and integrate the initial residual frequency offset estimate and the initial chip timing offset estimate to obtain the residual frequency offset estimate and the chip timing offset estimate.

5. The method according to claim 1, characterized in that, The residual frequency offset estimate is fixed, and the cost function is sliced ​​along the time delay dimension; based on the global maximum value of the slice, an effective time delay candidate set is selected through a preset threshold. The minimum delay value is selected from the candidate set of effective delays to correct the chip timing offset estimate, resulting in corrected timing synchronization parameters, including: S41. Fix the residual frequency offset estimate, calculate the cost function value at each time delay point along the time delay dimension, with the residual frequency offset estimate as a condition, and obtain the cost function slice sequence corresponding to each time delay point; S42. Perform value-by-value comparisons on the cost function slice sequence to obtain the global maximum value of the cost function slice sequence; S43. Multiply the global maximum value and the relative threshold to obtain the screening threshold; integrate the candidate time delay values ​​in the cost function slice sequence whose cost value is greater than or equal to the screening threshold to obtain an effective time delay candidate set; S44. Compare the values ​​of all candidate delay values ​​in the effective delay candidate set to obtain the minimum delay value, and use the minimum delay value as the corrected timing synchronization parameter.

6. A multi-user M-CSK-IDMA iterative receiving device for time-varying dual-select underwater acoustic channels, used to implement the method of any one of claims 1 to 5, characterized in that, The device includes: The signal synchronization and resampling module is used to acquire the received passband signal, perform frame preamble coarse synchronization acquisition and pilot frequency estimation on the received passband signal to obtain a macroscopic Doppler frequency offset estimate; and perform physical resampling on the received passband signal based on the macroscopic Doppler frequency offset estimate to obtain a resampled signal. The synchronization parameter joint search module is used to obtain the user's M-CSK symbol sequence, recursively obtain the truncation anchor point based on the M-CSK symbol sequence, and extract the analysis window signal from the resampled signal with the truncation anchor point as the center; in the two-dimensional parameter space of residual frequency offset and chip timing offset, the analysis window signal is jointly searched with the maximum value of the despreading correlation peak as the cost function to obtain the residual frequency offset estimate and the chip timing offset estimate. The timing synchronization correction module is used to fix the residual frequency offset estimate, calculate the slice of the cost function along the delay dimension, and based on the global maximum value of the slice, select an effective delay candidate set through a preset threshold; select the minimum delay value from the effective delay candidate set to correct the chip timing offset estimate, and obtain the corrected timing synchronization parameter. The iterative multi-user detection module is used to despread the resampled signal to obtain the received chip vector of each user based on the corrected timing synchronization parameters and the residual frequency offset estimate; and to perform Turbo iterative multi-user detection and decoding on the received chip vector to obtain the posterior log-likelihood ratio of the information bits of each user. The decoding output module is used to perform hard decision on the posterior log-likelihood ratio of the information bits to obtain the decoded bit sequence for each user.

7. A computer device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that, When the processor executes the computer program, it implements the steps of the method according to any one of claims 1 to 5.

8. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by a processor, it implements the steps of the method according to any one of claims 1 to 5.

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