Single-carrier underwater acoustic communication Doppler compensation method based on Turbo equalization

Abstract

The invention discloses a Turbo equalization-based single-carrier underwater acoustic communication Doppler compensation method. The Turbo equalization-based single-carrier underwater acoustic communication Doppler compensation method is combined with Turbo equalization to jointly design Doppler compensation, equalization, decoding and other processing of an underwater acoustic communication receiving end. Doppler invariance of front and back synchronous sequence signals of each frame of data segment is utilized to estimate a Doppler factor initial value, a Doppler factor candidate set is constructed by utilizing the Doppler factor initial value, the Doppler compensation effect is judged according to the size of an extrinsic information absolute value of a decoder, the optimal factor and soft symbol are selected to perform next iteration, and a Doppler compensation result is obtained. Therefore, a combined Turbo equilibrium iteration structure is formed. Compared with a traditional Doppler estimation compensation method, the method can cope with the influence of multipath Doppler in shallow sea mobile underwater acoustic communication, reduces Doppler factor estimation deviation caused by Doppler inconsistency of each path, improves the resolution of the estimated Doppler factor, and remarkably improves the decoding performance.

Description

Technical Field

[0001] The present invention relates to the field of underwater acoustic communication technology, especially a single - carrier signal transmission system in the case where both communication parties move relatively fast, and is also applicable to land fast - moving wireless communication. Specifically, it relates to a Doppler compensation method for single - carrier underwater acoustic communication based on Turbo equalization. Background Art

[0002] Underwater acoustic communication technology is the preferred technology for underwater wireless communication. However, the underwater acoustic environment is very complex, and the underwater acoustic channel exhibits characteristics such as fast time - variation, severe multipath, and Doppler effect. Since the transmission speed of underwater sound waves is about 1500 m / s, which is much lower than the propagation speed of electromagnetic waves, for fast - moving transceiver platforms, it will cause a severe Doppler effect, that is, the signal is extended or compressed in the time domain, seriously affecting the accurate decision of received symbols and further demodulation. Therefore, estimating and compensating the Doppler effect is crucial for the underwater acoustic communication system of fast - moving platforms.

[0003] In current communication systems, the commonly used Doppler estimation method is to utilize the Doppler - spread - insensitive characteristic of the hyper - frequency modulation (HFM) signal. Insert HFM signals as synchronization sequences before and after the data segment, estimate the Doppler factor through the time difference between the arrivals of the two HFM signals, and then perform Doppler factor compensation through methods such as linear interpolation.

[0004] Currently, the Doppler estimation and compensation algorithms based on HFM signals rarely consider the influence of multipath effects, and the resolution of the estimated Doppler factor is limited by the hardware sampling rate, and the compensation effect of the estimated Doppler factor is not good. Therefore, there is an urgent need to propose a Doppler compensation method for single - carrier underwater acoustic communication based on Turbo equalization for relatively fast mobile communication between the transceiver ends affected by multipath effects. Summary of the Invention

[0005] The main purpose of the present invention is to overcome the disadvantages and deficiencies of the prior art, and provide a Doppler compensation method for single - carrier underwater acoustic communication based on Turbo equalization.

[0006] To achieve the above - mentioned purpose, the present invention adopts the following technical solutions:

[0007] A Doppler compensation method for single - carrier underwater acoustic communication based on Turbo equalization, characterized in that the single - carrier underwater acoustic communication Doppler compensation method includes the following steps:

[0008] S1. Frame positioning. Locate the F - th frame from the band - pass sampled signal received from the underwater acoustic channel, where F = 1, 2, …, F max , F maxis the total number of data frames to be demodulated. The data is intercepted by a sliding window and cross-correlated with the local synchronization sequence to find the preamble sequence r1 and the postamble sequence r2 of the frame.

[0009] S2. Estimate the Doppler factor. Estimate the initial value α0 of the Doppler factor and the width w of the Doppler factor fluctuation range through the preamble sequence data r1 and the postamble sequence data r2 of the current frame, and initialize the number of Turbo equalization iteration rounds I = 0.

[0010] S3. Construct a candidate list of Doppler factors and perform Doppler compensation and downsampling. If F = 1 and I = 0, construct the current candidate list α of Doppler factors using the result α0 estimated in the previous step; if F > 1 and I = 0, construct the candidate list α using the optimal factor of the previous frame; if I > 0, construct the candidate list α using the optimal Doppler factor of the previous iteration; then use all the factors in the list to perform Doppler compensation, down-conversion, and downsampling on the data segment to obtain N groups of baseband signal data y F =[y F,1 ,y F,2 ,…,y F,k ,…,y F,N , where y F,k represents the k-th group of baseband signal data of the F-th frame, k = 1,…,N, and N is the number of elements in the factor list in this iteration;

[0011] S4. Perform Turbo equalization decoding. Input the baseband signal data y F =[y F,1 ,y F,2 ,…,y F,N into the Turbo equalizer for iterative equalization and decoding. If the number of iteration rounds I = 0, the soft symbol of each baseband signal data is set to for equalization. If I > 0, use the optimal soft symbol in the previous iteration for equalization; after each round of iterative decoding, calculate the extrinsic information matrix L F =[L e,1 ,L e,2 ,…,L e,k ,…,L e,N of each baseband signal data, and the 1-norm of each column vector. L e,k represents the extrinsic information column vector of each baseband signal data of the k-th group, k = 1,…,N;

[0012] S5. Select the optimal value. Select the corresponding factor in α as the optimal factor of this iteration according to the magnitude of the 1-norm of the extrinsic information and calculate the corresponding optimal soft symbol

[0013] S6. Determine whether the iteration end criterion is met. Decode the optimal result in this iteration and perform CRC check. If the check is successful, the end criterion is reached; otherwise, determine whether the number of iterations has reached the maximum. If so, the end criterion is reached. After reaching the end criterion, proceed to the next step S7; otherwise, use the optimal factor of this iteration. Go back to step S3 for the next iteration;

[0014] S7. Decode and determine whether the Doppler compensation algorithm end criterion is met. After the iteration of the current frame ends, output the decoded result and the optimal factor of this frame. Then determine whether it is the last frame. If so, end the Doppler compensation algorithm; otherwise, intercept the data of this frame according to the optimal Doppler factor of this frame, and then go back to step S1 to locate the data of the next frame for demodulation.

[0015] Furthermore, in step S1, record the frame structure of the communication transmission signal as follows: syn a preamble sequence of L gap (>L syn ) guard intervals composed of 0 samples, a data block of L data samples, a guard interval composed of L gap 0 samples, and a postamble sequence of L syn samples; for each frame of received data, first perform coarse synchronization, that is, find the approximate position of the data frame: use a sliding window of length 2L syn points, and slide L syn points each time to intercept the received data. Perform correlation operation on the intercepted received data block and the local synchronization sequence signal S syn , and calculate its normalized correlation coefficient. When the normalized correlation coefficient is greater than the set threshold, it is considered that the coarse synchronization point is found, and the data within the window is used as the preamble sequence data r1. Then find the postamble sequence data r2 by the same method, and record the interval length between the preamble and postamble sequences as L r . Using a sliding window of length 2L syn can better extract the preamble sequence and the postamble sequence from the signal that has experienced the underwater Doppler effect, and the extracted sequences are more complete.

[0016] Furthermore, in step S2, the correlation sequences obtained by performing correlation operations on the two groups of preamble and postamble synchronization data r1 and r2 with the local synchronization sequence data S syn respectively are used to obtain the envelopes of the correlation sequences as C1 and C2 through a Hilbert filter. Then perform cross-correlation on C1 and C2 to obtain the result C3. Find the subscript i0 of the peak point of C3, and calculate α0 as the initial value of the Doppler factor estimated for this frame through the expression , where L frameIndicates the length L of a transmitted frame of signal frame = L syn + 2L gap + L data , L syn is the length of the synchronization sequence, L gap is the length of the zero-sampling point guard interval, L data is the length of the signal data part. Through multiple cross-correlation operations, accurate estimation can still be ensured under the interference of multipath effects.

[0017] Furthermore, in step S2, find the subscript corresponding to the highest peak point in C1 as p max , the subscript corresponding to the second highest peak point is p sec , if p max < p sec , then let p0 = p max and p1 = p sec , otherwise p0 = p sec and p1 = p max ; find the lowest value point in the subscript range [p0, p1] of C1, and mark its subscript as p v ; replace the data segment with a subscript range of [2p0 - p v , p v in C1 with 0 values to obtain C′1, replace the data segment with a subscript range of [p v , 2p1 - p v in C1 with 0 values to obtain C″1, and process C2 in the same way to obtain C′2 and C″2. Perform cross-correlation on C′1 and C′2 to obtain the result C′3, find its highest peak point subscript i m and calculate α through the expression ; perform cross-correlation on C″1 and C″2 to obtain the result C″3, find its highest peak point subscript i m and calculate α through the expression and calculate to obtain α s , and initialize the Turbo equalization iteration count I = 0. In the case of multipath effects, in this way, the Doppler effects experienced by the two arrival paths with relatively large energies can be roughly estimated, and then the difference between the two is used as the width of the possible Doppler factor range, that is, w = |α s - α m |, and this width can reflect the difference in the Doppler effects experienced by the two paths with relatively large energies. s |, and this width can reflect the difference in the Doppler effects experienced by the two paths with relatively large energies

[0018] Furthermore, in step S3, if F = 1 and I = 0, that is, when the first frame of data undergoes the first round of iteration, use the result α0 estimated in the previous step to construct a Doppler factor candidate list If F > 1 and I = 0, that is, when the first-round iteration is performed on non-first-frame data, the optimal factor obtained from the previous frame is used and the estimated factor α0 of this frame to construct a Doppler factor list which can make full use of the prior information of the previous frame and the information of this frame; if F is any value and I > 0, that is, the non-first-round iteration of any data frame, the optimal factor of the previous Turbo iteration of this frame is used to construct a Doppler factor list where is an adjustable step factor. In this way, the range of the Doppler factor can be gradually narrowed, and the more iteration rounds there are, the higher the estimation accuracy of the Doppler factor

[0019] During each round of iteration, all factors in the Doppler factor list are used to perform Doppler compensation and resampling on the data segment part of the received signal of this frame. The Farrow filter is a structure widely used in digital signal processing, mainly for signal interpolation and decimation operations. Here, the Farrow filter is used for interpolation to achieve the purpose of Doppler compensation, and then resampling is performed to finally obtain the baseband signal data y F = [y F,1 , y F,2 , …, y F,N , where N is the number of elements in the factor list in this round of iteration. Through the iterative method, the Doppler factor with the best equalization decoding performance can be selected

[0020] Furthermore, in step S4, the baseband signal data y F = [y F,1 , y F,2 , …, y F,N are respectively input into the Turbo equalizer for equalization and decoding; if I = 0, that is, during the first iteration, the soft symbol of each baseband data is set to for equalization, if I > 0, that is, during non-first iteration, the optimal soft symbol in the previous iteration is used for equalization; after decoding, the extrinsic information matrix L F = [L e,1 , L e,2 , …, L e,k , …, L e,N of each baseband data is calculated, and the 1-norm of each column vector is calculated to obtain l F = [l1, l2, …, l k , …, l N , where l k is the vector L e,kThe sum of the absolute values of each element in. Since the sign of the extrinsic information represents a bit and the value of the extrinsic information represents the reliability, the magnitude of this 1-norm can represent the decoding performance, and the larger the value, the better the performance.

[0021] Further, in the step S5, compare the 1-norm vector l F = [l1, l2, …, l N to obtain the largest element l k , that is, this item represents the best decoding performance among the N items. Select the corresponding factor in the factor list α as the optimal factor for this round of iteration and calculate the soft symbol using l k as the optimal soft symbol for this round of iteration Participating the optimal factor and the optimal soft symbol in the next round of iteration can accelerate the convergence of the iterative process and improve the equalization performance.

[0022] Further, in the step S6, decode and perform CRC check on the optimal result in this round of iteration. If the check is successful, it reaches the end criterion; otherwise, judge whether the iteration round number I reaches the maximum. If so, it reaches the end criterion; after reaching the end criterion, enter the next step S7, and use the optimal factor of this round of iteration as the optimal Doppler factor for this frame Otherwise, let I = I + 1, and use the optimal factor of this round of iteration Go back to step S3 for the (I + 1)-th round of iteration. Transmit the optimal factor in the current iteration as the prior information to the next round of iteration to accelerate the search process of the optimal Doppler factor.

[0023] Further, in the step S7, after the iteration of the current frame ends, output the decoding result and the optimal factor of this frame and judge whether it is the last frame. If so, end the current Doppler compensation algorithm; otherwise, intercept the data of length d F of this frame, let F = F + 1 and go back to step S1 for demodulating the next frame of data, where the corrected length of this frame of data is L frame represents the length of a frame of signal transmitted. When the relative speed between the communication transceiver ends does not change suddenly, the optimal factor of this frame is transmitted as the prior information to the iteration of the next frame, which has good guiding significance.

[0024] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0025] (1) The present invention takes into account the characteristics of the real physical channel in fast - moving underwater acoustic communication, namely, the multipath effect. When the Doppler frequencies of the two paths containing the main energy are inconsistent, by estimating them separately after cutting the correlation peak, the range of the difference between the Doppler factors of the two paths can be estimated. Using this range to construct a Doppler factor list and approaching the true value through a step - by - step iterative method can make the Doppler factor estimation more accurate, with a better compensation effect and a reduction in the overall system bit error rate.

[0026] (2) The present invention uses the optimal Doppler factor output in the previous frame as the prior information for the current frame and takes it as a parameter in the Doppler factor list of the current frame. When sudden factors cause a large deviation in the Doppler estimation of the current frame, this parameter can play a corrective role in the iterative process, making the Doppler factor to be estimated tend towards the true value again, reducing the system bit error rate, and enhancing the adaptability and robustness of the system.

[0027] (3) Based on the limited sampling rate at the receiving end, the present invention adopts a method of combining a Doppler factor list with Turbo equalization iteration. The set step - size factor is continuously refined as the number of iterations increases, approaching the true value from a reasonable Doppler factor range. The resolution of the Doppler factor can be improved by about one order of magnitude. Compared with traditional estimation methods, it can break through the problem of limited Doppler factor resolution caused by the limitation of the actual hardware sampling rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0029] Figure 1 It is a flowchart of the steps of a single - carrier underwater acoustic communication Doppler compensation method based on Turbo equalization disclosed by the present invention;

[0030] Figure 2 It is a schematic diagram of the overall structure of a single frame of the transmitted frame in a single - carrier underwater acoustic communication Doppler compensation method based on Turbo equalization disclosed by the present invention;

[0031] Figure 3 It is a schematic diagram of the data segment structure of a single frame of the transmitted frame in a single - carrier underwater acoustic communication Doppler compensation method based on Turbo equalization disclosed by the present invention;

[0032] Figure 4 It is a schematic diagram of the overall structure of the transmitted frame in a single - carrier underwater acoustic communication Doppler compensation method based on Turbo equalization disclosed by the present invention;

[0033] Figure 5 It is a comparison chart of bit error rates of a simulation communication experiment of a Doppler compensation method for single - carrier underwater acoustic communication based on Turbo equalization disclosed in Embodiment 1 of the present invention.

[0034] Figure 6 It is a comparison chart of bit error rates of a communication experiment of a Doppler compensation method for single - carrier underwater acoustic communication based on Turbo equalization disclosed in Embodiment 2 of the present invention. Detailed implementation manners

[0035] In order to enable those skilled in the art of this technology to better understand the solutions of this application, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of this application. Obviously, the described embodiments are only a part of the embodiments of this application, rather than all the embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative efforts belong to the scope of protection of this application.

[0036] In this application, referring to "embodiment" means that the specific features, structures or characteristics described in conjunction with the embodiment can be included in at least one embodiment of this application. The phrase appears in various positions in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described in this application can be combined with other embodiments.

[0037] Embodiment 1

[0038] For the convenience of understanding the subsequent receiving method, a brief description of the signal model of the communication system will be given first. In this embodiment, the modulation system of the single - frame data segment is as follows: Information bits d k After encoding, interleaving, base - band modulation, 16 - fold up - sampling, and pulse - shaping filtering, a transmission signal is generated:

[0039]

[0040] In the formula, the information bits are bit - encoded and interleaved, and after base - band modulation, the transmitted constellation point x n is obtained. In this embodiment, QPSK modulation is used; g(t) is the shaping filter. In this embodiment, a raised - cosine roll - off filter is used, with a roll - off factor of 0.25 and a symbol period of T; E s is the energy of the symbol. The transmitted symbol is sent out after carrier modulation.

[0041] As Figure 2 shown, it is the single - frame structure of the transmitted frame used in this embodiment, and the length of each frame is L frame, which consists of a preamble sequence, a guard interval, and a data segment. The preamble sequence of the next frame is regarded as the postamble sequence of the current frame, where L syn = L gap That is, the guard interval has the same time length as the synchronization sequence, L frame = L syn + 2L gap + L data . In this example, the sampling rate of the transmitted data is 96 kHz.

[0042] In this embodiment, the synchronization sequence is composed of an HFM signal, and its generation formula is as follows:

[0043]

[0044] In the formula is the instantaneous frequency of the HFM signal at t = 0, is the frequency modulation factor. In this embodiment, the parameters of the HFM signal are: f l = 9 kHz, f h = 15 kHz, T = 21 ms, M = 267.86.

[0045] In this embodiment, the guard interval is composed of L gap zero samples.

[0046] In this embodiment, the data segment is composed in a form that conforms to Turbo equalization iteration. As Figure 3 shown, it is the data segment structure in the transmitted single frame used in this embodiment: Each frame consists of 1 data block, that is, K = 1. Among them, the forward training sequence and the backward training sequence of this data block are both t, x represents the transmitted data of this data block, N t represents the length of the training sequence, N d represents the length of the transmitted data, L data = (2N t + N d ) × 16 represents the total length of the data segment. In this embodiment, the parameter values are as follows: N t = 200, N d = 1936, L syn = 4096, L gap = 4096, L data = 37376, L frame = 47616, F max = 10, I max = 5.

[0047] Such as Figure 4As shown, it is the overall transmission structure of this embodiment. A total of 10 frames are transmitted in this embodiment. The post-synchronization sequence of the current frame is the pre-synchronization sequence of the next frame. A guard interval and a synchronization sequence are added after the last frame to estimate the Doppler of the last frame.

[0048] In this embodiment, the selected underwater acoustic channel is a real underwater acoustic channel measured in Wanlu Lake, Heyuan, Guangdong (H. Zhao, F. Ji, M. Wen, H. Yu and Q. Guan, "Multi-Task Learning Based Underwater Acoustic OFDM Communications," 2021 IEEE International Conference on Signal Processing). This channel is relatively complex. Affected by noise, multipath effects and Doppler effects, it is difficult for traditional methods to obtain accurate Doppler factors, and the effect of direct Doppler compensation is not good. The present invention constructs a Turbo equalization structure for joint Doppler compensation to obtain more accurate Doppler estimation and compensation, and greatly reduces the system bit error rate.

[0049] The algorithm flow of a single-carrier underwater acoustic communication Doppler compensation method based on Turbo equalization is as Figure 1 shown, and specifically includes the following steps:

[0050] S1. Coarse synchronization, that is, find the approximate position of the data frame. Use a sliding window of 8192 points, slide 4096 points each time to intercept the band-pass data received in the simulation channel, perform a correlation operation on the intercepted data and the local synchronization sequence signal, and calculate its normalized correlation coefficient. When the normalized correlation coefficient is greater than the set threshold, it is considered that the coarse synchronization point is found, and the data within the window is used as the pre-synchronization sequence data r1 of the frame. Then, find the post-synchronization sequence data r2 of the frame by the same method, and record the interval length L of the pre- and post-synchronization sequence data of the frame r ;

[0051] S2. Estimate the Doppler factor. Perform a correlation operation on the pre- and post-synchronization data r1 of the frame and the post-synchronization data r2 of the frame with the local synchronization sequence data S syn respectively to obtain correlation sequences. The envelopes of the correlation sequences are obtained through a Hilbert filter to obtain results C1 and C2 respectively. Then, perform a cross-correlation on C1 and C2 to obtain result C3. Find the subscript i0 of the peak point of C3, and calculate α0 as the initial value of the Doppler factor estimated for this frame through the expression ;

[0052] Find the subscript corresponding to the highest peak point in C1 as p max , and the subscript corresponding to the second highest peak point as p sec , if pmax <p sec Then let p0 = p max and p1 = p sec Otherwise, p0 = p sec and p1 = p max ; Find the lowest value point within the C1 subscript range of [p0, p1], and mark its subscript as p v ; Replace the data segment within the C1 subscript range of [2p0 - p v , p v with 0 to obtain C′1, replace the data segment within the C1 subscript range of [p v , 2p1 - p v with 0 to obtain C″1, and process C2 in the same way to obtain C′2 and C″2. Perform cross-correlation on C′1 and C′2 to obtain the result C′3, find the peak point subscript i m and calculate α through the expression Calculate to obtain α m ; Perform cross-correlation on C″1 and C″2 to obtain the result C″3, find the peak point subscript i s and calculate α through the expression And calculate to obtain α s ; Take the difference between the two as the width of the possible Doppler factor range, i.e., w = |α m - α s |, and set I = 0;

[0053] S3. Construct a Doppler factor candidate list α based on α0, w, and the optimal factor of the previous frame to perform Doppler compensation and downsampling.

[0054] If F = 1 and I = 0, i.e., during the first-round iteration of the first-frame data, use the result α0 estimated in the previous step to construct the Doppler factor candidate list

[0055] If F > 1 and I = 0, i.e., during the first-round iteration of non-first-frame data, then use the optimal factor of the previous frame and the estimated factor α0 of the current frame to construct the Doppler factor list

[0056] If F is any value and I > 0, i.e., during the non-first-round iteration of any-frame data, use the optimal factor of the previous Turbo iteration of this frame to construct the Doppler factor list where is an adjustable step factor.

[0057] During each round of iteration, use all the factors in the Doppler factor list to process the partial r of the data segment of this frame FPerform Doppler compensation and resampling, use a Farrow filter for interpolation and resampling, and finally obtain the baseband signal data y corresponding to each factor. F = [y F,1 , y F,2 , …, y F,N , where N is the number of elements in the factor list in this round of iteration;

[0058] S4. Perform Turbo equalization decoding. Input the baseband signal data y F = [y F,1 , y F,2 , …, y F,N into the Turbo equalizer for equalization and decoding respectively. If I = 0, i.e., in the first iteration, the soft symbols of each baseband data are set to for equalization. If I > 0, i.e., in non-first iterations, then use the optimal soft symbols in the previous round of iteration for equalization; after decoding, calculate the extrinsic information matrix L F = [L e,1 , L e,2 , …, L e,k , …, L e,N , and calculate the 1-norm of each column vector to obtain l F = [l1, l2, …, l k , …, l N , where l k is the sum of the absolute values of each element in the vector L e,k , k = 1, …, N;

[0059] S5. Select the optimal value according to the extrinsic information 1-norm. Compare the magnitudes of the elements in the 1-norm vector l F = [l1, l2, …, l N to obtain the largest item l k , k ∈ [1, N], and select the corresponding factor in the factor list α as the optimal factor in this round of iteration and use L e,k to calculate the soft symbols as the optimal soft symbols in this round of iteration

[0060] S6. Determine whether the iteration end criterion is met. Decode and perform CRC check on the optimal result in this round of iteration. If the check is successful, the end criterion is reached; otherwise, determine whether the iteration count I reaches 5. If so, the end criterion is reached;

[0061] After reaching the end criterion, proceed to the next step S7, and use the optimal factor in this round of iteration as the optimal Doppler factor for this frame Otherwise, set I = I + 1 and use the optimal factor in this round of iteration. Return to step S3 for the (I + 1)-th round of iteration;

[0062] S7. Decode and determine whether the algorithm end criterion is met. Output the decoding result and the optimal factor of this frame And determine whether F is equal to 10. If so, end the algorithm; otherwise, after intercepting the data with the length of d for this frame, let F = F + 1 and return to step S1 for demodulating the next frame of data, where the corrected length of this frame of data is F After that, let F = F + 1 and return to step S1 for demodulating the next frame of data, where the corrected length of this frame of data is

[0063] As can be seen from the Figure 5 bit error rate comparison graph given in the embodiments, compared with the traditional algorithm of performing Turbo equalization after direct compensation, the performance of the proposed joint time-domain channel estimation Turbo equalization algorithm with Doppler compensation of the present invention is better. After 5 times of Turbo equalization iteration, the bit error rate of the present invention is reduced by two orders of magnitude compared with the traditional algorithm.

[0064] Embodiment 2

[0065] This embodiment discloses another single-carrier underwater acoustic communication Doppler compensation method based on Turbo equalization. This method uses a real communication system with single transmit and single receive. The experimental location is in Qiandao Lake, Hangzhou, Zhejiang Province, China. The symbol rate is 4.25 kbps, the carrier frequency is 12 kHz, the sampling rate is 96 kHz, the bandwidth is 6 kHz, and the transmitter approaches the receiver at a speed of 3 - 4 m / s. This method is based on each step in the single-carrier underwater acoustic communication Doppler compensation method based on Turbo equalization disclosed in Embodiment 1.

[0066] Among them, the data segment modulation method and the single-frame structure in this embodiment are the same as those in Embodiment 1. Each frame consists of 1 data block, that is, K = 1. Among them, in the data block, there are also forward training sequences, backward training sequences, and transmitted data. N t represents the length of the training sequence, N d represents the length of the transmitted data, L data =(2N t +N d )×16 represents the total length of the data segment. The parameter values in this embodiment are as follows: N t =200, N d =1936, L syn =2048, L gap =2048, L data =37376, L frame =43520, F max =69, I max =7.

[0067] Among them, in the rough synchronization of step S1, a sliding window with a length of 4096 points is used, and each time it slides 2048 points to intercept the band-pass data received from the real underwater acoustic channel. The remaining processing method is the same as that in Example 1;

[0068] Among them, in steps S2 to S5 of this embodiment, the methods of rough estimation of the Doppler factor, construction of the Doppler factor list, Turbo equalization iteration, and selection of the optimal factor are the same as those in Example 1.

[0069] Among them, in step S6 of this embodiment, the optimal result in this round of iteration is decoded and CRC-checked. If the check is successful, the end criterion is reached; otherwise, it is judged whether the iteration round I reaches 69. If so, the end criterion is reached;

[0070] Among them, in step S7 of this embodiment, after outputting the decoding result and the optimal factor of this frame it is judged whether F is equal to 69. If so, the algorithm ends; otherwise, after intercepting the data with a length of d in this frame F let F = F + 1 and return to step S1 to demodulate the next frame of data.

[0071] It can be seen from the bit error rate graphs given in Example 1 and Example 2 that whether it is the bit error result obtained by using the simulation channel to simulate the real underwater acoustic communication environment or the bit error result obtained from the measured communication data, the bit error performance of a single-carrier underwater acoustic communication Doppler compensation method based on Turbo equalization proposed by the present invention is better than that of the traditional direct compensation algorithm. In the simulation results, from the first round of Turbo equalization iteration to the last round of Turbo equalization iteration, the bit error rate of the present invention is approximately two orders of magnitude lower than that of the traditional algorithm; in the bit error results of the experimental data, the bit error rate of the present invention is approximately one order of magnitude lower than that of the traditional algorithm.

[0072] In summary, the present invention proposes a single-carrier underwater acoustic communication Doppler compensation method based on Turbo equalization. Aiming at the Doppler effect problem in the underwater acoustic environment, a Doppler factor list is constructed by roughly estimating the Doppler factor range, and the true value of the Doppler factor is approximated iteratively in combination with Turbo equalization, improving the resolution and accuracy of Doppler estimation and significantly reducing the communication bit error rate.

[0073] The technical features of the above embodiments can be combined arbitrarily. For the sake of brief description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope recorded in this specification.

[0074] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

Claims

1. A single-carrier underwater acoustic communication Doppler compensation method based on Turbo equalization, characterized in that: The single-carrier underwater acoustic communication Doppler compensation method comprises the following steps: S1. The band received from the underwater acoustic channel is located in the Fth frame through the sampling signal, where F = 1, 2, ..., F max , F max is the total number of data frames that need to be demodulated, and the data is intercepted through the sliding window and cross-correlated with the local synchronization sequence to find the pre-frame synchronization sequence r1 and the post-frame synchronization sequence r2; S2, estimating the initial value α0 of the Doppler factor and the width w of the Doppler factor fluctuation range through the pre-frame synchronization sequence data r1 and the post-frame synchronization sequence data r2 of the current frame, and initializing the number of Turbo equalization iteration rounds I=0; S3, if F = 1 and I = 0, then use the result α0 estimated in the previous step to construct the current candidate list α of the Doppler factor; if F>1 and I=0, then use the optimal factor of the previous frame to construct the candidate list α; if I>0, use the optimal Doppler factor of the previous iteration to construct the candidate list α; then use all the factors in the list to perform Doppler compensation, down-conversion and down-sampling on the data segment to obtain N groups of baseband signal data y F =[y F,1 ,y F,2 ,…,y F,k ,…,y F,N ], where N is the number of elements in the factor list in this iteration, y F,k represents the kth group of baseband signal data in the Fth frame, k=1,…,N; S4, the baseband signal data y F =[y F,1 ,y F,2 ,…,y F,N ] is input into the Turbo equalizer for iterative equalization and decoding. If the number of iterations I = 0, the soft symbol of each baseband signal data is set to Perform equalization. If I>0, use the best soft symbol in the previous iteration. Equalize; after each round of iterative decoding, calculate the external information matrix L of each baseband signal data F =[L e,1 ,L e,2 ,…,L e,k ,…,L e,N ], and the 1-norm of each column vector, L e,k Represents the external information column vector of each baseband signal data of the kth group, k=1,…,N; S5. Select the corresponding factor in α as the optimal factor for this round of iteration according to the 1-norm size of the external information. And calculate the corresponding optimal soft symbol S6, decode and CRC check the best result in this iteration, if the check is successful, the end standard is met, otherwise it is determined whether the number of iterations has reached the maximum, if it has reached the end standard, and then proceed to the next step S7, otherwise the best factor of this round of iteration is used Return to step S3 for the next round of iteration; S7: After the current frame iteration is completed, the decoding result and the optimal factor of this frame are output. Then determine whether it is the last frame, if yes, end Doppler compensation, otherwise cut off the data of this frame according to the optimal Doppler factor of this frame, and then return to step S1 to locate the next frame of data for demodulation.

2. The single-carrier underwater acoustic communication Doppler compensation method based on Turbo equalization according to claim 1 is characterized in that: In step S1, the length of the syn A sliding window of points, each sliding L syn The received data is intercepted at a certain point, and the intercepted received data block is correlated with the local synchronization sequence signal, and the normalized correlation coefficient is calculated. When the normalized correlation coefficient is greater than the set threshold, it is considered that the rough synchronization point is found, and the data in the window is used as the pre-frame synchronization sequence data r1, and then the post-frame synchronization sequence data r2 is found by the same method, and the interval length between the pre-frame synchronization sequence and the post-frame synchronization sequence is recorded as L r .

3. The single-carrier underwater acoustic communication Doppler compensation method based on Turbo equalization according to claim 1 is characterized in that: In step S2, the pre-frame synchronization sequence data r1 and the post-frame synchronization sequence data r2 are respectively combined with the local synchronization sequence data S syn The correlation sequence obtained by correlation operation is obtained through Hilbert filter, and the envelope of the correlation sequence is recorded as C1 and C2 respectively. C1 and C2 are cross-correlated to obtain the result C3, and the subscript i0 of the maximum peak point of C3 is found. The expression α0 is calculated as the initial value of the Doppler factor estimated for the current frame, where L frame Indicates the length of a transmitted frame signal L frame =L syn +2L gap +L data , L syn is the synchronization sequence length, L gap is the length of the zero sampling point protection interval, L data The length of the signal data portion.

4. The single-carrier underwater acoustic communication Doppler compensation method based on Turbo equalization according to claim 3 is characterized in that: In step S2, the highest peak point is found in C1, corresponding to the subscript p max The second highest peak point corresponds to the subscript p sec , if p max <p sec , then let p0=p max And p1=p sec , otherwise p0=C sec And p1=p max ; Find the lowest value point in the subscript range of C1 [p0, p1] and mark its subscript as p v ; Change the subscript range in C1 to [2p0-p v ,p v ] is replaced with 0 to get C ′ 1. Change the subscript range in C1 to [p v ,2p1-p v ] is replaced with 0 to get C ′ 1 ′ , and process C2 in the same way to obtain p ′ 2 and p ′ 2 ′ , C ′ 1 and C ′ 2. Do cross-correlation to get result C ′ 3. Find the highest peak point subscript i m And through the expression Calculate α m ; C ′ 1 ′ and C ′ 2 ′ Do the cross-correlation to get the result C ′ 3 ′ , find its highest peak point index i s And through the expression And calculate α s ; The difference between the two is taken as the width of the Doppler factor range, that is, w = |α m -α s |, and initialize the number of Turbo equalization iterations I = 0.

5. The single-carrier underwater acoustic communication Doppler compensation method based on Turbo equalization according to claim 1 is characterized in that: In step S3, if F=1 and I=0, the Doppler factor candidate list is constructed using the result α0 estimated in the previous step. If F>1 and I=0, use the optimal factor obtained in the previous frame Construct the Doppler factor list with the estimated factor α0 of the current frame If F is any value and I>0, use the optimal factor of the previous Turbo iteration of the current frame Constructing a list of Doppler factors in is an adjustable step size factor.

6. The single-carrier underwater acoustic communication Doppler compensation method based on Turbo equalization according to claim 5 is characterized in that: In step S3, in each iteration, all factors in the Doppler factor list are used to perform Doppler compensation and resample the data segment in the received signal of this frame, and the Farrow filter is used for interpolation and resampling, and finally the baseband data y corresponding to each factor is obtained. F =[y F,1 ,y F,2 ,…,y F,N ], where N is the number of elements in the factor list in this iteration.

7. The single-carrier underwater acoustic communication Doppler compensation method based on Turbo equalization according to claim 1 is characterized in that: In step S4, the baseband data y F =[y F,1 ,y F,2 ,…,y F,N ] are input into the Turbo equalizer for equalization and decoding respectively; When I = 0, the soft symbol of each baseband data is set to Perform equalization. When I>0, use the best soft symbol in the previous iteration. Equalize; after decoding, calculate the external information L of each baseband data F =[L e,1 ,L e,2 ,…,L e,k ,…,L e,N ], and calculate the 1-norm of each column vector to get l F =[l1,l2,…,l k ,…,l N ], where l k is the vector L e,k The sum of the absolute values ​​of each element in .

8. The single-carrier underwater acoustic communication Doppler compensation method based on Turbo equalization according to claim 1, characterized in that: In step S5, the 1-norm vector l is compared. F =[l1,l2,…,l N ], and get the largest item l k , select the corresponding factor in the factor list α as the optimal factor for this round of iteration And L e,k Calculate the soft symbol as the optimal soft symbol for this round of iteration 9. The single-carrier underwater acoustic communication Doppler compensation method based on Turbo equalization according to claim 1, characterized in that: In step S6, the optimal result in this round of iteration is decoded and CRC checked. If the check is successful, the end standard is met. Otherwise, it is determined whether the number of iteration rounds I reaches the maximum. If so, the end standard is met. After the end standard is met, the next step S7 is entered, and the optimal factor of this round of iteration is used as the optimal Doppler factor of this frame. Otherwise, let I = I + 1 and use the optimal factor of this iteration Return to step S3 to perform the I+1th iteration.

10. The single-carrier underwater acoustic communication Doppler compensation method based on Turbo equalization according to claim 1, characterized in that: In step S7, after the current frame iteration is completed, the decoding result and the optimal factor of the current frame are output. And determine whether it is the last frame, if it is, end Doppler compensation, otherwise cut off the length d of this frame F After the data is received, let F = F + 1 and return to step S1 to demodulate the next frame of data, where the corrected length of this frame of data is L frame Indicates the length of a transmitted frame signal.

Images