A method for mid-low frequency command level underwater acoustic communication under time-varying doppler and low signal-to-noise ratio channel

Abstract

The application discloses a kind of low frequency instruction level underwater acoustic communication methods under time-varying Doppler and low signal-to-noise ratio channel, comprising: generating the set containing 2 a Modulation symbol, mapping into data frame, joining frame structure to form transmitting signal;After receiving end receives, intercept pilot signal by matched filtering, adopts grid search strategy to carry out Doppler estimation, determines initial Doppler frequency offset factor and first symbol block position;According to Doppler frequency offset factor estimation signal and its position, with nearby K Doppler frequency offset factor to it after resampling, with each modulation symbol carries out matched filtering, according to cumulative peak value determines the accurate signal of n sym Symbol block and its position and Doppler frequency offset factor and corrects sliding window width;Repeat the above process until demodulating all symbol blocks, and restore original bit according to mapping relationship.The application can accurately estimate the Doppler frequency in each period to compensate, and is suitable for instruction level communication between underwater high-speed moving platform.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of underwater acoustic communication, in particular to a kind of low frequency instruction level underwater acoustic communication method under time-varying Doppler and low signal-to-noise ratio channel. BACKGROUND

[0002] In the medium-low frequency band, the propagation loss of sound wave in underwater acoustic channel is small, and it is often used for long-distance underwater communication, but it is also easily affected by environmental changes. When communicating between high-speed moving platforms underwater, the relative speed between platforms changes, which causes the signal frequency to change over time, especially in the medium-low frequency band. In addition, in the scenario of long-distance communication, the attenuation of water body and the interference of noise will make the received signal quality poor, which makes it difficult for traditional modulation and demodulation method to be applicable.

[0003] How to realize reliable communication under time-varying Doppler and strong background noise interference has become a computational problem to be solved. At present, in low signal-to-noise ratio underwater acoustic channel, spread spectrum communication is often used to increase the bandwidth of the signal to enhance the anti-interference ability of the system, but this will cause a significant decrease in spectral efficiency, and the frame length of the signal after spread spectrum is longer. Due to the existence of time-varying Doppler, the Doppler frequency offset factor under a frame of signal will inevitably change. At the same time, the process of spread spectrum signal despreading needs accurate time synchronization with the transmitted signal, and also needs to estimate and match the decoding of different spread spectrum codes, which will lead to too high computational complexity, increasing the cost and maintenance difficulty of the system. SUMMARY

[0004] In view of the problem of communication difficulty under time-varying Doppler and strong background noise interference in the prior art, the present application proposes a kind of low frequency instruction level underwater acoustic communication method under time-varying Doppler and low signal-to-noise ratio channel, which ensures that high-speed moving platforms realize stable and reliable long-distance communication under low signal-to-noise ratio.

[0005] The specific technical solutions are as follows:

[0006] A kind of low frequency instruction level underwater acoustic communication method under time-varying Doppler and low signal-to-noise ratio channel, comprising the following steps:

[0007] S1: generating a set of 2 a modulation symbols based on linear frequency modulation signal at the transmitting end According to the mapping relationship, 2 a frequency modulation is carried out on each modulation symbol to form a data frame, a frame header structure containing a pilot signal is added at the front end, a frame tail signal is added at the tail end, and a transmitting signal is formed;

[0008] S2: The transmitted signal is received by the receiver through the channel. The receiver performs frame synchronization on the received signal, determines the start position of the frame header through matched filtering, and determines the start position of the pilot signal accordingly.

[0009] S3: Perform Doppler estimation on the intercepted pilot signal using a grid search strategy to determine the initial Doppler frequency offset factor. and the starting position of the first symbol block

[0010] S4: Initially set the width of the sliding window used to capture the first symbol block;

[0011] S5: Based on the previous Doppler frequency offset factor Estimate the nth sym The starting position of n symbol blocks sym =2,3,…,N sym N sym The total number of symbol blocks within a data frame is used to extract the signal using a sliding window. by Take K Doppler frequency offset factors at equal intervals around the center, and use the k-th Doppler frequency offset factor to... Perform resampling, k = 1, 2, ..., K;

[0012] S6: Resample the signal The cross-correlation function is obtained by performing matched filtering on each modulation symbol in set S, and the nth peak value is determined based on the cumulative peak value. sym The accurate signal of a symbol block and its starting position and Doppler frequency offset factor

[0013] S7: Determine whether the estimation of all symbol blocks has been completed. If yes, demodulate to obtain the original bits according to the mapping relationship. If not, adjust the width of the sliding window according to the current Doppler frequency offset factor and repeat S5-S7 to estimate the next symbol block.

[0014] Furthermore, in S1, the transmitted signal sequentially includes a frame header structure, a data frame, and a frame tail signal. The frame header structure includes a pilot signal and a guard interval, and the frame tail signal uses a zero guard value.

[0015] Furthermore, S2 is implemented through the following sub-steps:

[0016] S2.1: The receiving end performs frame synchronization on the received signal. The relationship between the received signal y(t) and the transmitted signal x(t) is expressed as follows:

[0017] y(t)=h(t)*x(t)+w(t)

[0018]

[0019] where h(t) is the impulse response of the underwater acoustic channel, w(t) is a Gaussian white noise with mean 0 and power σ 2 , P represents the total number of paths of the underwater acoustic channel, δ represents the impulse response, η p is the signal attenuation of the pth path, α p (t) is the Doppler frequency offset factor of the pth path, which varies with time, τ p is the time delay of the pth path.

[0020] S2.2: Starting from the beginning of the received signal sequence, a signal sequence with the same length as the frame header length is intercepted as the initial preset frame header signal y head (t).

[0021] S2.3: The cross-correlation function of the intercepted preset frame header signal and the frame header signal of the transmitted signal is calculated by using the matched filtering method, and the expression is as follows:

[0022]

[0023] where r head (τ) represents the cross-correlation function of the frame header signal, τ represents the time delay, and x head (t) represents the frame header signal of the transmitted signal.

[0024] S2.4: It is judged whether the correlation peak value of the cross-correlation function is greater than a set threshold value, if yes, the current frame header signal is taken as the accurate frame header signal, and the starting position of the accurate frame header signal is taken as the starting position of the pilot signal; if not, the next signal sequence is intercepted from the received signal sequence with a set step length as a new preset frame header signal, and S2.3-S2.4 are repeated.

[0025] Further, the S3 is realized by the following sub-steps:

[0026] S3.1: A plurality of different Doppler factors α m , m = 1, 2, 3, …, M, M is the total number of preset Doppler factors, the pilot signal y p (t) is resampled under different Doppler factors α m , and the corresponding resampled pilot signal y

[0027] S3.2: The resampled pilot signal y is matched and correlated with the original pilot signal x p (t) of the transmitting end, and the cross-correlation function under the Doppler factor α m is calculated:

[0028]

[0029] where τ represents the time delay;

[0030] S3.3: Selecting the Doppler factor corresponding to the correlation peak of the cross-correlation function as the initial Doppler frequency offset factor S3.3: Selecting the Doppler factor corresponding to the correlation peak of the cross-correlation function as the initial Doppler frequency offset factor The expression is as follows:

[0031]

[0032] According to the initial Doppler factor, the starting position of the first symbol block is estimated

[0033]

[0034] where T represents the starting position of the pilot signal, T head represents the duration of the frame header signal.

[0035] Further, in S3.1, the resampling is performed based on the principle of linear frequency modulation Z transform.

[0036] Further, in S4, the width of the sliding window used for intercepting the first symbol block is initially set as:

[0037]

[0038] where T sym represents the duration of one symbol, T over represents the overlap width between adjacent two sliding windows.

[0039] Further, in S5, according to the last Doppler frequency offset factor , the starting position of the nth sym symbol block is estimated, and the expression is as follows:

[0040]

[0041] where T represents the estimated starting position of the nth sym symbol block, represents the corrected starting position of the nth sym -1 symbol block, T sym represents the duration of one symbol.

[0042] Further, S6 is implemented through the following sub-steps:

[0043] S6.1: Resampling the nth sym symbol block signal The matched filter is matched with each modulation symbol in the set S, and the cross-correlation function between them is calculated, and the expression is as follows:

[0044]

[0045] In the formula, s i represents the i-th modulation symbol, i=1, 2, …, 2 a ; τ represents the time delay;

[0046] S6.2: Calculate the signal The matched filter is matched with each modulation symbol s i The cumulative peak value after matched filtering The expression is as follows:

[0047]

[0048] The modulation symbol corresponding to the maximum cumulative peak value The accurate signal of the n sym th symbol block The expression is as follows:

[0049]

[0050] S6.3: Calculate the modulation symbol Determine the Doppler frequency offset factor when the peak is maximum, as the estimated Doppler frequency offset factor under the n sym th symbol block The expression is as follows:

[0051]

[0052] S6.4: The modulation symbol corresponds to The Doppler frequency offset factor corresponds to Determine the starting position of the n sym th symbol block by correcting τ when the cross-correlation function is maximum, and the expression is as follows:

[0053]

[0054] In the formula, represents the corrected starting position of the n sym th symbol block, represents the starting position correction value of the n sym th symbol block, is the modulation symbol number corresponding to the maximum cumulative peak value obtained in S6.2, is the Doppler frequency offset factor number corresponding to the maximum cumulative peak value obtained in S6.3.

[0055] Further, in the S7, the expression for correcting the width of the sliding window according to the current Doppler frequency offset factor is as follows:

[0056]

[0057] In the formula, represents the corrected sliding window width, represents the uncorrected sliding window width, represents the current Doppler frequency offset factor.

[0058] The beneficial effects of the present application are:

[0059] (1) The present application can accurately obtain the Doppler estimation result of the signal in a symbol block by resampling and matched filtering the signal in each symbol block, so as to ensure reliable medium and low frequency communication under the condition of time-varying Doppler.

[0060] (2) The present application ensures the spectrum utilization efficiency by the modulation method based on the linear frequency modulation signal, and realizes high spectrum utilization rate of underwater medium and low frequency communication.

[0061] (3) The present application improves the symbol duration and increases the cumulative gain of the signal corresponding to a single symbol by the modulation based on the linear frequency modulation signal under the condition of meeting the communication rate, so as to ensure reliable medium and low frequency communication under low signal-to-noise ratio. BRIEF DESCRIPTION OF DRAWINGS

[0062] Figure 1 is a flow chart of the medium and low frequency command level underwater acoustic communication method under the condition of time-varying Doppler and low signal-to-noise ratio in the embodiment of the present application.

[0063] Figure 2 is a time-frequency diagram of 16 modulation symbols generated in the embodiment of the present application.

[0064] Figure 3 is a structural schematic diagram of the transmitted signal in the embodiment of the present application.

[0065] Figure 4 is a schematic diagram of intercepting a symbol block by using a sliding window in the embodiment of the present application.

[0066] Figure 5 is the theory and estimation schematic diagram of the Doppler frequency offset factor estimation in the embodiment of the present application.

[0067] Figure 6 is a performance comparison diagram of the method in the embodiment of the present application and the conventional spread spectrum communication method. DETAILED DESCRIPTION

[0068] The present invention will be described in detail below with reference to the accompanying drawings and preferred embodiments. The objectives and effects of the present invention will become clearer as a result. The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0069] A mid-to-low frequency command-level underwater acoustic communication method for time-varying Doppler and low signal-to-noise ratio (SNR) channels is disclosed. This method employs symbols generated from LFM signals for modulation during the signal modulation process. At the receiving end, the signal within each window of the truncated symbol block is resampled and matched-filtered to achieve Doppler estimation and symbol estimation. Figure 1 As shown, the method includes the following steps:

[0070] S1: At the transmitting end, generate a signal containing 2 based on the Linear Frequency Modulation (LFM) signal. a A set of modulation symbols Based on the mapping relationship, perform 2... a Frequency modulation forms a data frame, which, after being added to the frame structure, forms the transmitted signal. S1 is specifically implemented through the following sub-steps:

[0071] S1.1: Generation based on LFM signal including 2 a A set of modulation symbols In this embodiment, a set S = {s1, s2, ..., s3} containing 16 modulation symbols is generated. 16 The expressions for each modulation symbol are shown in Table 1 below.

[0072] Table 1 Modulation symbol expression

[0073]

[0074]

[0075] Among them, f c Indicates the center frequency of the carrier. B represents bandwidth, T sym Indicates the duration of a symbol. R b This represents the communication bit rate, where A represents the number of modulation symbols, and A = 2. a In this embodiment, A = 16.

[0076] For communication frequency bands in f down ~f up For signals within the range, if the duration of a symbol is T sym Then the time-frequency diagram of the 16 modulation symbols is as follows:Figure 2 as shown.

[0077] S1.2: According to the mapping relationship, each modulation symbol in the set S is mapped to 2 a Modulation forms a data frame, i.e. a is mapped to a symbol in turn, forming a data frame x d (t).

[0078] In this embodiment, when 16 modulation is performed on four-bit, the mapping relationship between the bits and each modulation symbol is shown in Table 2.

[0079] Table 2 Mapping relationship table between four-bit and sixteen modulation symbols

[0080]

[0081] S1.3: A frame header structure x d (t) containing a pilot signal x p (t) and a guard interval is added before the data frame x head (t), the pilot signal is an LFM signal; a frame tail signal x g (t) is added after the data frame, forming a transmission signal x(t). In this embodiment, the frame tail signal is a zero protection value, and the structure of the transmission signal formed is as shown. Figure 3

[0082] S2: The transmitting end sends a transmission signal, which is received at the receiving end after passing through a channel. The receiving end performs frame synchronization on the received signal y(t), performs matched filtering on the signal frame header, and determines the starting position of the frame header according to the correlation peak and intercepts the pilot signal y p (t). S2 is implemented through the following sub-steps:

[0083] S2.1: The relationship expression between the received signal y(t) and the transmission signal x(t) is as follows:

[0084] y(t) = h(t) * x(t) + w(t)

[0085]

[0086] In the formula, h(t) is the impulse response of the underwater acoustic channel, w(t) is a Gaussian white noise with mean 0 and power σ 2 ; P represents the total number of paths of the underwater acoustic channel, δ represents the impulse response, η p is the signal attenuation of the pth path, α p (t) is the Doppler frequency offset factor of the pth path, which changes with time, τ p is the time delay of the pth path.

[0087] ​S2.2: From the beginning of the received signal sequence, a signal sequence with the same length as the frame header length is intercepted as an initial preset frame header signal y head (t).

[0088] S2.3: A matching filter method is used to calculate the preset frame header signal y head (t) and the original frame header signal (i.e., the frame header signal of the transmitted signal) x head (t) cross-correlation function, expressed as follows:

[0089]

[0090] In the formula, r head (τ) represents the cross-correlation function of the frame header signal, and τ represents the time delay.

[0091] S2.4: It is determined whether the correlation peak value of the cross-correlation function is greater than a set threshold value (in this embodiment, the set threshold value is 5 times the correlation average value), and if so, the current preset frame header signal is taken as the accurate frame header signal, and the frame header starting position corresponding to the preset frame header signal is also the starting position of the pilot signal If not, the next block of signal sequence is intercepted from the received signal sequence at a set step as a new preset frame header signal, and the matching filtering and correlation peak value determination are performed again (i.e., S2.3-S2.4 are repeated).

[0092] S3: According to the starting position of the pilot signal , the pilot signal y p (t) is intercepted, and a grid search strategy is used for Doppler estimation on the intercepted pilot signal, that is, the initial Doppler frequency offset factor and the position of the first symbol block are determined through resampling and matching filtering. S3 is implemented through the following sub-steps:

[0093] S3.1: A plurality of different Doppler factors α m , m = 1, 2, 3, …, M, M is the total number of preset Doppler factors, and the pilot signal y p (t) is resampled under different Doppler factors α m to obtain corresponding resampled pilot signals

[0094] In this embodiment, resampling is performed based on the principle of Chirp Z transform (Chirp Z transform, hereinafter referred to as CZT), wherein y p (t) represents the continuous expression of the original pilot signal, y p [n] represents the discrete expression of the original pilot signal, under the Doppler factor αm Continuous representation of the down-resampled pilot signal, represents the Doppler factor α m Discrete representation of the down-resampled pilot signal, for convenience of representation, y p (t) and y p [n], and are equivalent. The detailed process of resampling is as follows:

[0095] (1) Select a suitable frequency point number N FFT , the expression is as follows:

[0096]

[0097] In the formula, N p represents the length of the pilot signal sequence.

[0098] (2) After carrier compensation of the pilot signal sequence y p [n], the generated complex sequence is:

[0099]

[0100] (3) Generate sequences c[n], q[n], z[l] respectively, the expressions are as follows:

[0101]

[0102] (4) Convolve the sequences c[n] and q[n], and discard the first N p -1 items and the last N p -1 items of the convolved sequence c[n]*q[n], and then point multiply with the sequence z[l] to obtain Y[l]:

[0103]

[0104] (5) Exchange the front half and the back half of the Y[k] sequence, that is, perform spectrum shift to obtain Y s [l]:

[0105] Y s [l] = fftshift(Y[l])

[0106] (6) Do point inverse fast Fourier transform on the spectrum shifted sequence to obtain the resampled sequence

[0107]

[0108] S3.2: Resampling the down-resampled pilot signal The original pilot signal x from the transmitting end p (t) Perform matching correlation and calculate the Doppler factor α. m The cross-correlation function is as follows:

[0109]

[0110] S3.3: Compare the correlation peaks of the cross-correlation functions corresponding to different Doppler factors. The magnitude of the Doppler frequency offset factor is chosen by selecting the Doppler factor that maximizes the correlation peak value. Its expression is as follows:

[0111]

[0112] Estimate the starting position of the first symbol block based on the initial Doppler factor.

[0113]

[0114] In the formula, T head Indicates the duration of the frame header signal.

[0115] S4: Initially set the width of the sliding window used to capture the first symbol block. The expression is as follows:

[0116]

[0117] In the formula, T over This indicates the overlap width between two adjacent sliding windows.

[0118] S5: Based on Doppler frequency offset factor Estimate the nth sym The starting position of a symbol block The expression is as follows:

[0119]

[0120] In the formula, Indicates the nth sym The estimated starting position of a symbol block, Indicates the nth sym -1 symbol block correction start position, N sym This indicates the total number of symbol blocks within a data frame.

[0121] like Figure 4 As shown, using a width of Sliding window to the nth sym The starting position of a symbol block Extract the nth sym Signal of a symbol block And in the previous estimated Doppler frequency offset factor Under the influence of K Doppler frequency offset factors Resampling is performed based on the CZT principle, where the Doppler frequency offset factor numbered k is used. The resampled signal is Where k = 1, 2, ..., K. "Nearby" is defined according to a fixed frequency interval, i.e., the Doppler frequency offset factor estimated above. Centered on the center, take K / 2 frequency points on each side, with the interval between adjacent frequency points being the same.

[0122] S6: Resample the signal With each modulation symbol s in set S i The correlation function is obtained by performing matched filtering separately. Where i = 1, 2, ..., 2 a s i This represents the i-th modulation symbol. The n-th symbol is then determined based on the cumulative peak value. sym The accurate signal of a symbol block The starting position and the estimated Doppler frequency offset factor And correct the width parameter of the sliding window. S6 is implemented through the following sub-steps:

[0123] S6.1: Resample the nth... sym One symbol block signal With each modulation symbol s in the modulation symbol set S i Perform matched filtering separately and calculate the cross-correlation function between the two, as shown in the following expression:

[0124]

[0125] S6.2: Calculate the signal With each modulation symbol s i Cumulative peak value after matched filtering The expression is as follows:

[0126]

[0127] The modulation symbol corresponding to the maximum cumulative peak value That is, the nth sym The accurate signal of a symbol block The expression is as follows:

[0128]

[0129] S6.3: Calculate the signal after resampling at K Doppler frequency offset factors. With the determined modulation symbol The related function, its corresponding peak value The Doppler frequency offset factor at its maximum (at this time) ) as the nth estimate sym Doppler frequency offset factor under each symbol block The expression is as follows:

[0130]

[0131] S6.4: Modulation symbol corresponding to The corresponding Doppler frequency offset factor When determined, the nth digit is corrected by τ when the cross-correlation function is maximized. sym The starting position of a symbol block is expressed as follows:

[0132]

[0133] In the formula, Indicates the nth sym The correction start position of each symbol block Indicates the nth sym The starting position correction value for each symbol block. This is the modulation symbol number corresponding to the maximum cumulative peak value obtained from S6.2. This is the Doppler frequency offset factor number corresponding to the maximum cumulative peak value obtained from S6.3.

[0134] S7: Determine whether the estimation of all symbol blocks is complete. If yes, demodulate the original bits according to the mapping relationship specified in Table 2. If not, correct the width of the sliding window according to the current Doppler frequency offset factor, and repeat S5-S7 to estimate the next symbol block. The expression for the corrected sliding window width is as follows:

[0135]

[0136] To verify the performance of the method of the present invention, corresponding performance simulation experiments were conducted, and the following two examples illustrate this in detail.

[0137] Example 1: To verify the communication reliability of the proposed method under time-varying Doppler and low signal-to-noise ratio channel conditions, the parameters of this example are set as follows: communication rate of 40 bps, one modulation symbol of 100 ms, signal transmission bandwidth of 2-3 kHz, a total transmission of 10,000 bits of information, and sampling frequency of 12 kHz. The received signal is affected by marine environmental noise simulated with Gaussian white noise, with a signal-to-noise ratio of -5 dB. Simultaneously, the signal is affected by Doppler frequency shift and multipath interference, with a maximum Doppler rate of 10 knots and a maximum Doppler rate change rate of 1 knot / s.

[0138] The specific parameters of the simulation signal are shown in Table 3.

[0139] Table 3: Specific parameters of simulation signal

[0140] Parameters Settings Communication rate 40 bps Centre frequency 2.5 kHz Signal bandwidth 1 kHz Number of signal bits 10 000 bits Sampling frequency 12 kHz Maximum Doppler rate 10 kont Maximum Doppler change rate 1 kont / s Maximum multipath delay 300 ms Signal-to-noise ratio -5 dB

[0141] Under the simulation condition, based on the modulation characteristics of the LFM signal, the Doppler frequency offset of each symbol of the received signal is estimated by using the method, and the estimation result is shown in Table 4. Figure 5 As shown in the figure, after the Doppler compensation, each symbol can be accurately estimated, and the final bit error rate is 0, which indicates that the method can effectively estimate the Doppler frequency offset of each symbol.

[0142] In order to verify that the communication method proposed in the application has excellent anti-noise capability, the parameters of the embodiment are set as follows: the communication rate is 40 bps, the signal transmission frequency band is 2-3 kHz, 4000 bits of information are transmitted, the sampling frequency is 12 kHz, the signal-to-noise ratio of the received signal is-18-0 dB with an interval of 0.5 dB, and 100 times of Monte Carlo simulation is performed under each signal-to-noise ratio condition without considering the influence of multipath and Doppler by using the method proposed in the application and the traditional direct sequence spread spectrum communication method. The traditional direct sequence spread spectrum communication method uses a sequence with a length of 15, and the modulation mode used is BPSK.

[0143] Figure 6 The simulation performance comparison chart of the method and the direct sequence spread spectrum communication method shows that, within the simulated signal-to-noise ratio range, the bit error rate of the method is significantly lower than that of the direct sequence spread spectrum communication method, which indicates that the method has excellent anti-noise capability.

[0144] In summary, based on the LFM signal modulation, the grid search strategy and the matched filtering method, a low-frequency command-level communication method suitable for time-varying Doppler and low signal-to-noise ratio conditions is proposed. The method can realize reliable low-frequency communication under the condition of extremely low signal-to-noise ratio and time-varying Doppler, so as to ensure that the underwater high-speed moving platform can realize stable and reliable long-distance communication under low signal-to-noise ratio.

[0145] Those skilled in the art can understand that the above description is only a preferred example of the application and is not used to limit the application, although the application has been described in detail with reference to the foregoing examples, and those skilled in the art can still modify the technical solutions recorded in the foregoing examples or replace some technical features with equivalent ones. Any modification, equivalent replacement, etc. within the spirit and principles of the application shall be included in the protection scope of the application.

Claims

1. A method for mid-to-low frequency command-level underwater acoustic communication under time-varying Doppler and low signal-to-noise ratio channels, characterized in that, Includes the following steps: S1: At the transmitting end, generate a signal containing 2 based on a linear frequency modulated signal. a A set of modulation symbols Based on the mapping relationship, perform 2D mapping on each modulation symbol. a Frequency modulation forms a data frame, a frame header structure containing pilot signals is added to the front end, and a frame tail signal is added to the back end to form the transmitted signal; S2: The transmitted signal is received by the receiver through the channel. The receiver performs frame synchronization on the received signal, determines the start position of the frame header through matched filtering, and determines the start position of the pilot signal accordingly. S3: Perform Doppler estimation on the intercepted pilot signal using a grid search strategy to determine the initial Doppler frequency offset factor. and the starting position of the first symbol block ; S4: Initially set the width of the sliding window used to capture the first symbol block; S5: Based on the previous Doppler frequency offset factor Estimate the nth sym The starting position of n symbol blocks sym =2,3,…,N sym N sym The total number of symbol blocks within a data frame is used to extract the signal using a sliding window. ;by Take K Doppler frequency offset factors at equal intervals around the center, and use the k-th Doppler frequency offset factor to... Perform resampling, k=1,2,…,K; S6: Resample the signal The cross-correlation function is obtained by performing matched filtering on each modulation symbol in set S, and the nth peak value is determined based on the cumulative peak value. sym The accurate signal of a symbol block and its starting position and Doppler frequency offset factor ; S7: Determine whether the estimation of all symbol blocks has been completed. If yes, demodulate to obtain the original bits according to the mapping relationship. If no, adjust the width of the sliding window according to the current Doppler frequency offset factor and repeat S5-S7 to estimate the next symbol block. S6 is achieved through the following sub-steps: S6.1: Resample the nth... sym One symbol block signal The cross-correlation function between the modulated symbols in set S and each other is calculated using matched filtering, as shown in the following expression: ； In the formula, s i This represents the i-th modulation symbol, where i = 1, 2, ..., 2 a ; Indicates time delay; S6.2: Calculate the signal With each modulation symbol s i Cumulative peak value after matched filtering The expression is as follows: ； The modulation symbol corresponding to the maximum cumulative peak value For the nth sym The accurate signal of a symbol block The expression is as follows: ； S6.3: Calculate modulation symbols When determined, make the peak value The Doppler frequency offset factor at its maximum is used as the estimated nth frequency. sym Doppler frequency offset factor under each symbol block The expression is as follows: ； S6.4: The modulation symbol corresponding to The corresponding Doppler frequency offset factor When determined, the value obtained through the maximum cross-correlation function. Correct the nth sym The starting position of a symbol block is expressed as follows: ； ； In the formula, Indicates the nth sym The correction start position of each symbol block Indicates the nth sym The starting position correction value for each symbol block. This is the modulation symbol number corresponding to the maximum cumulative peak value obtained from S6.

2. This is the Doppler frequency offset factor number corresponding to the maximum cumulative peak value obtained from S6.

3.

2. The method for mid-to-low frequency command-level underwater acoustic communication under time-varying Doppler and low signal-to-noise ratio channels according to claim 1, characterized in that, In S1, the transmitted signal sequentially includes a frame header structure, a data frame, and a frame tail signal. The frame header structure includes a pilot signal and a guard interval, and the frame tail signal uses a zero guard value.

3. The method for mid-to-low frequency command-level underwater acoustic communication under time-varying Doppler and low signal-to-noise ratio channels according to claim 1, characterized in that, S2 is achieved through the following sub-steps: S2.1: The receiving end performs frame synchronization on the received signal. The relationship between the received signal y(t) and the transmitted signal x(t) is expressed as follows: ； ； In the formula, h(t) is the impulse response of the underwater acoustic channel, and w(t) is the signal with zero mean and power. Gaussian white noise; P represents the total number of paths in the underwater acoustic channel. Indicates the impulse response, η p For the signal attenuation of the p-th path, α p (t) represents the Doppler frequency offset factor of the p-th path, and its value varies with time. Let be the delay of the p-th path; S2.2: Starting from the beginning of the received signal sequence, extract a signal sequence with a length equal to the frame header length as the initial preset frame header signal y. head (t); S2.3: Using the matched filtering method, calculate the cross-correlation function between the truncated preset frame header signal and the transmitted signal frame header signal. The expression is as follows: ； In the formula, The cross-correlation function representing the frame header signal. Indicates time delay, x head (t) represents the frame header signal of the transmitted signal; S2.4: Determine whether the correlation peak of the cross-correlation function is greater than the set threshold. If so, take the current frame header signal as the accurate frame header signal and take the starting position of the accurate frame header signal as the starting position of the pilot signal. If not, take the next signal sequence with a set step size on the received signal sequence as the new preset frame header signal, and repeat S2.3-S2.

4.

4. The method for mid-to-low frequency command-level underwater acoustic communication under time-varying Doppler and low signal-to-noise ratio channels according to claim 1, characterized in that, S3 is achieved through the following sub-steps: S3.1: Preset multiple different Doppler factors α m m=1,2,3,…,M, where M is the preset total number of Doppler factors, and the pilot signal… Different Doppler factors α m Resampling is performed to obtain the corresponding resampled pilot signal. ; S3.2: For the resampled pilot signal The original pilot signal from the transmitting end Perform matching correlation and calculate the Doppler factor α. m The cross-correlation function is as follows: ； In the formula, Indicates time delay; S3.3: Select the option that makes the correlation peak of the cross-correlation function... The Doppler factor corresponding to the maximum value is used as the initial Doppler frequency offset factor. Its expression is as follows: ； Estimate the starting position of the first symbol block based on the initial Doppler factor. : ； In the formula, Indicates the starting position of the pilot signal, T head Indicates the duration of the frame header signal.

5. The method for mid-to-low frequency command-level underwater acoustic communication under time-varying Doppler and low signal-to-noise ratio channels according to claim 4, characterized in that, In S3.1, resampling is performed based on the principle of linear frequency modulation Z-transform.

6. The method for mid-to-low frequency command-level underwater acoustic communication under time-varying Doppler and low signal-to-noise ratio channels according to claim 1, characterized in that, In step S4, the initial width of the sliding window used to capture the first symbol block is set as follows: ； In the formula, Indicates the duration of a symbol. This indicates the overlap width between two adjacent sliding windows.

7. The method for mid-to-low frequency command-level underwater acoustic communication under time-varying Doppler and low signal-to-noise ratio channels according to claim 1, characterized in that, In S5, based on the previous Doppler frequency offset factor Estimate the nth sym The starting position of a symbol block is expressed as follows: ； In the formula, Indicates the nth sym The estimated starting position of a symbol block, Indicates the nth sym -1 symbol block correction start position, Indicates the duration of a symbol.

8. The method for mid-to-low frequency command-level underwater acoustic communication under time-varying Doppler and low signal-to-noise ratio channels according to claim 1, characterized in that, In step S7, the expression for correcting the width of the sliding window based on the current Doppler frequency offset factor is as follows: ； In the formula, This indicates the corrected width of the sliding window. This indicates the width of the sliding window before the correction. This represents the current Doppler frequency offset factor.

Images