A low-complexity time-frequency synchronization method and time-frequency synchronization system in underwater acoustic communication

Abstract

The present invention discloses a low-complexity time-frequency synchronization method and a time-frequency synchronization system in underwater acoustic communication. The method comprises: utilizing the good anti-Doppler performance of a linear frequency modulation sequence to ensure stable time synchronization; utilizing the Doppler sensitivity of an m-sequence to perform a two-dimensional iterative search in the time domain and the frequency domain to achieve time-frequency joint synchronization. The present invention also designs a parallel iterative time-frequency synchronization system, which can implement the proposed method on an FPGA platform with lower resource consumption and computational delay. The advantages of the present invention are: 1. The leading structure of the combined LFM and m-sequence can effectively cope with the delay-Doppler dual expansion characteristics of the underwater acoustic channel. 2. The two-step iterative time-frequency synchronization reduces the number of searches and reduces complexity. 3. The parallel iterative computing structure and the low-complexity synchronization method can be carried on a real-time underwater acoustic communication system with low power consumption and low latency.

Description

Technical Field

[0001] The present invention belongs to the field of underwater acoustic communication, and relates to an underwater acoustic communication synchronization method, and in particular to a low-complexity time-frequency synchronization method and a time-frequency synchronization system in underwater acoustic communication. Background Art

[0002] Underwater acoustic communication channels are often described as time-varying random channels with severe multipath and Doppler effects. The multipath effect causes the phase, amplitude and arrival time of signals arriving from different paths to be different, which ultimately manifests as signal delay expansion, causing a certain degree of inter-symbol interference, while causing fluctuations in the transmitted signal, affecting the detection signal. The Doppler effect causes the frequency of the signal to shift or additional frequency diffusion. The delay-Doppler dual-expansion channel causes large changes in the time and frequency domains of the signal during transmission, increasing the difficulty of system synchronization, which has a restrictive effect on the development of underwater acoustic communications.

[0003] Synchronization is an important part of signal processing. Its purpose is to resist the dynamic characteristics of the channel and make the parameters of the receiving and sending sides consistent. Synchronization can be roughly divided into time domain synchronization and frequency domain synchronization. The function of time domain synchronization is to detect the arrival of the signal and confirm the starting position of the signal, so as to sample and judge the symbols and perform subsequent signal processing operations. In order to achieve long-distance high-speed communication, coherent modulation is generally used to ensure that the coherent carrier and the carrier of the received signal are of the same frequency and phase, so frequency synchronization is also particularly important.

[0004] Considering the data frame structure, there are generally two types of data-assisted synchronization methods, one is the preamble structure, and the other is the data block structure. The data block structure generally uses the block estimation method, which usually adds a Doppler-insensitive signal at the front and back ends of the transmitted data, and estimates the average Doppler factor of the entire frame signal transmission time by obtaining the difference in the data frame length between the received signal and the transmitted signal. However, it needs to cache the entire frame sequence, and then demodulate after the method is calculated and compensated. The storage overhead is extremely large, and it cannot meet the requirement of real-time demodulation of the underwater acoustic communication system.

[0005] In order to implement the synchronization method in the real-time underwater acoustic communication system, in addition to the performance of the method itself, it is also necessary to consider the complexity, hardware resource overhead, redundancy overhead and storage overhead. In order to reduce the processing delay, a parallel architecture design is also required. Therefore, designing a good-performance synchronization method that can be implemented in the underwater acoustic communication system with low resource consumption and computing delay is of great value to improve the quality of real-time underwater acoustic communication. Summary of the invention

[0006] The present invention proposes a low-complexity time-frequency synchronization method in underwater acoustic communication, and implements the method in a programmable logic gate array. The method can be deployed in a real-time underwater acoustic communication system with low power consumption and low latency to improve the quality of underwater acoustic communication.

[0007] The purpose of the present invention is achieved through the following technical solutions:

[0008] A low-complexity time-frequency synchronization method in underwater acoustic communication, the method comprising the following steps:

[0009] S1: Adaptive time coarse synchronization is performed using the front-end LFM signal in the receiving end signal;

[0010] S2: Using the receiving end signal and the local original m-sequence to perform two-dimensional cross-correlation coarse frequency offset synchronization and fine frequency offset iterative synchronization in turn, to obtain a signal with a fully compensated carrier;

[0011] S3: Use the signal after the carrier compensation to perform fine time synchronization with the local original m sequence.

[0012] Furthermore, the S1 is specifically:

[0013] The receiving end uses a sliding window of the same length as the LFM signal for time synchronization: the sliding window signal is represented by r = [r0, r1, ..., R N-1 ], the average energy R(n) of the sliding window signal is calculated as:

[0014]

[0015] Where n represents the moment of the sliding window signal, and N is the length of the LFM signal;

[0016] Calculate the sliding window signal and the local LFM signal t = [t0, t1, ..., t N-1 The average value of the cross-correlation value R r The energy of (n) U(n):

[0017]

[0018] Determine whether the signal detected by the receiving end is synchronized successfully. Once the following formula is satisfied, the synchronization is successful, and the moment n is the moment of successful synchronization. Otherwise, the synchronization fails:

[0019]

[0020] Among them, Q(n) is the energy quotient, Q T (n) is the set energy quotient threshold.

[0021] Furthermore, the S2 is specifically:

[0022] S201: Downsample the received m sequence to single sampling with a sampling rate of f b ; Determine the search range in the time domain [0, τ max ], frequency domain search range [-f max , f max ] and the frequency interval Δf step1 , use the received signal r[n] and the local original m sequence to perform a two-dimensional cross-correlation time-frequency search to obtain the two-dimensional cross-correlation function P[τ, k]:

[0023]

[0024]

[0025] The search position corresponding to the maximum and second maximum values ​​of the two-dimensional cross-correlation function is obtained based on energy.

[0026]

[0027]

[0028] in, represents the time delay corresponding to the maximum value of the two-dimensional cross-correlation function, Indicates removal The set of k.

[0029] Confirm that the starting position of the interval is Calculate the initial frequency of the interval

[0030]

[0031]

[0032] S202: Perform signal carrier compensation by iteration until the maximum number of estimations is reached and then the iteration is stopped to obtain the final estimated carrier frequency offset. Thus, the carrier signal is compensated.

[0033] The delay is The frequency synchronization interval is The maximum number of estimates is The signal u obtained after carrier compensation of signal r[n] (0) [n] is:

[0034]

[0035] Signal u after k-th compensation (k) [n] can be updated iteratively as:

[0036]

[0037] The result of cross-correlation with the m sequence is:

[0038]

[0039] Taking the maximum energy as the standard, determine for:

[0040]

[0041] Furthermore, the S3 is specifically:

[0042] The carrier will be compensated The signal is correlated with the m-sequence:

[0043]

[0044] by Results for time As a threshold, once the following formula is satisfied, the time τ at this time is determined as the result of time domain fine synchronization:

[0045]

[0046] Among them, 0<β<1, which is the coefficient factor;

[0047] Carrier frequency deviation calculated using frequency deviation synchronization The average Doppler factor where f c is the carrier frequency; perform broadband Doppler compensation on the signal, and obtain the received signal y after Doppler compensation through the following formula c (t):

[0048]

[0049] A time-frequency synchronization system based on FPGA, the system comprises a control module, a downsampling and Doppler compensation module, a time coarse synchronization module, a two-dimensional cross-correlation coarse frequency offset synchronization module, a fine frequency offset iterative synchronization module and a time fine synchronization module;

[0050] The control module is used to control the coordinated work of other modules, and change the frequency search interval and the range of time domain search in time-frequency synchronization according to the rate;

[0051] The downsampling and Doppler compensation module is used to reduce the sampling rate of the digital waveform and perform sampling compensation on the data, thereby directly reducing the amount of data and thus reducing the number of calculations;

[0052] The time coarse synchronization module uses the front-end LFM signal in the receiving end signal to perform adaptive time coarse synchronization and provide coarse synchronization information for the two-dimensional cross-correlation coarse frequency offset synchronization module;

[0053] The two-dimensional cross-correlation coarse frequency offset synchronization module is used to perform a two-dimensional cross-correlation time-frequency search on the received signal and the local original m-sequence, send the search value to the comparator to obtain the maximum value, and provide the initial frequency and time domain starting point for the fine frequency offset iterative synchronization module;

[0054] The fine frequency deviation iterative synchronization module uses iteration to perform carrier compensation of the signal until the maximum number of estimations is reached and the iteration is stopped to obtain the final estimated carrier frequency deviation, thereby obtaining a signal with the carrier compensated; each iteration goes through four clock cycles of address acquisition, data reading, calculation, and correlation. In the pipeline structure, the address acquisition operation of the next data is parallel to the data reading operation of the previous data, thereby greatly reducing the delay of real-time synchronization;

[0055] The time fine synchronization module uses the signal of the carrier compensated by the fine frequency deviation iterative synchronization module to perform threshold detection and determine the result of time domain fine synchronization.

[0056] The beneficial effects of the present invention are:

[0057] (1) The preamble structure used in the present invention is a preamble structure that combines an LFM signal and an m-sequence. Compared with the single LFM preamble structure of the traditional method, this structure can synchronize the time domain and the frequency domain at the same time.

[0058] (2) The present invention divides the time-frequency search process into two processes: coarse frequency offset iteration and fine frequency offset iteration. Compared with the traditional method of directly performing time-frequency search, it has lower computational overhead under the same search accuracy.

[0059] (3) The present invention is implemented in a programmable logic gate array with low resource consumption and computational delay, and can be directly deployed in a mobile underwater acoustic communication system. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 It is a structural diagram of the frame of the time-frequency joint synchronization method described in the present invention.

[0061] Figure 2 It is a flow chart of the time-frequency joint synchronization method described in the present invention.

[0062] Figure 3 It is a schematic diagram of the structure of the FPGA synchronization module described in the present invention.

[0063] Figure 4 This is a logical structure diagram of the time coarse synchronization described in the present invention.

[0064] Figure 5This is a graph of the time coarse synchronization rising edge detection result of the present invention.

[0065] Figure 6 It is a logical structure diagram of the two-dimensional cross-correlation coarse frequency offset synchronization module described in the present invention.

[0066] Figure 7 This is the parallel structure of the fine frequency deviation iterative synchronization module described in the present invention. DETAILED DESCRIPTION

[0067] The technical solution of the present invention is described in detail below in conjunction with specific examples and drawings. The described examples are only part of the preferred examples of the present invention. Any modification or equivalent replacement of the technical solution of the patent of the present invention, which does not depart from the spirit and scope of the technical solution of the patent of the present invention, should be included in the protection scope of the present invention.

[0068] Figure 1 The frame structure diagram of the time-frequency joint synchronization method. As shown in the figure, the LFM signal is used as the synchronization header of the data frame to complete the rough time synchronization. A protection interval is set after the LFM signal to reduce the ISI caused by the strong multipath effect of the underwater acoustic channel. An m sequence is added before each data block as a pre-pilot, and the m sequence is used for carrier synchronization and fine time synchronization.

[0069] Figure 2 This is a flow chart of the time-frequency joint synchronization method, which is divided into adaptive time coarse synchronization, two-dimensional cross-correlation coarse frequency offset synchronization, time fine synchronization and Doppler factor estimation, and specifically includes the following steps:

[0070] S1: Use the front-end LFM signal in the receiving end signal to perform adaptive time coarse synchronization.

[0071] The receiving end uses a sliding window of the same length as the LFM signal for time synchronization: the sliding window signal is represented by r = [r0, r1, ..., r N-1 ], the average energy R(n) of the sliding window signal is calculated as:

[0072]

[0073] Where n represents the moment of the sliding window signal, and N is the length of the LFM signal;

[0074] Calculate the sliding window signal and the local LFM signal t = [t0, t1, ..., t N-1 The average value of the cross-correlation value R r The energy of (n) U(n):

[0075]

[0076] Determine whether the signal detected by the receiving end is synchronized successfully. Once the following formula is satisfied, the synchronization is successful, and the moment n is the moment of successful synchronization. Otherwise, the synchronization fails:

[0077]

[0078] Among them, Q(n) is the energy quotient, Q T (n) is the set energy quotient threshold.

[0079] S2: Use the receiving end signal and the local original m-sequence to perform two-dimensional cross-correlation coarse frequency offset synchronization and fine frequency offset iterative synchronization in turn to obtain a signal with a fully compensated carrier.

[0080] S201: Downsample the received m sequence to single sampling with a sampling rate of f b ; Determine the search range in the time domain [0, τ max ], frequency domain search range [-f max , f max ] and the frequency interval Δf step1 , use the received signal r[n] and the local original m sequence to perform a two-dimensional cross-correlation time-frequency search to obtain the two-dimensional cross-correlation function P[τ, k]:

[0081]

[0082]

[0083] The search position corresponding to the maximum and second maximum values ​​of the two-dimensional cross-correlation function is obtained based on energy.

[0084]

[0085]

[0086] in, represents the time delay corresponding to the maximum value of the two-dimensional cross-correlation function, Indicates removal The set of k.

[0087] Confirm that the starting position of the interval is Calculate the initial frequency of the interval

[0088]

[0089]

[0090] S202: Perform signal carrier compensation by iteration until the maximum number of estimations is reached and then the iteration is stopped to obtain the final estimated carrier frequency offset. Thus, the carrier signal is compensated.

[0091] The delay is The frequency synchronization interval is The maximum number of estimates is The signal u obtained after carrier compensation of signal r[n] (0) [n] is:

[0092]

[0093] Signal u after k-th compensation (k) [n] can be updated iteratively as:

[0094]

[0095] The result of cross-correlation with the m sequence is:

[0096]

[0097] Taking the maximum energy as the standard, determine for:

[0098]

[0099] S3: Use the signal after the carrier compensation to perform fine time synchronization with the local original m sequence.

[0100] The carrier will be compensated The signal is correlated with the m-sequence:

[0101]

[0102] by Results for time As a threshold, once the following formula is satisfied, the time τ at this time is determined as the result of time domain fine synchronization:

[0103]

[0104] Among them, 0<β<1, which is the coefficient factor;

[0105] Carrier frequency deviation calculated using frequency deviation synchronization The average Doppler factor where f c is the carrier frequency; perform broadband Doppler compensation on the signal, and obtain the received signal y after Doppler compensation through the following formula c (t):

[0106]

[0107] On the other hand, the FPGA-based time-frequency synchronization system of the present invention is deployed in an FPGA system, which includes a control module, a downsampling and Doppler compensation module, a time coarse synchronization module, a two-dimensional cross-correlation coarse frequency offset synchronization module, a fine frequency offset iterative synchronization module and a time fine synchronization module, such as Figure 3 Among them, the two-dimensional cross-correlation coarse frequency offset synchronization module and the fine frequency offset iterative synchronization module constitute a two-step frequency synchronization module.

[0108] The control module is used to control the coordinated work of other modules and change the frequency search interval and time domain search range in the two-step iterative time-frequency synchronization according to the rate.

[0109] Figure 3 The downsampling and Doppler compensation module in is used to reduce the sampling rate of the digital waveform and perform sampling compensation on the data, thereby directly reducing the amount of data and thus reducing the number of calculations, including:

[0110] 1) Before the data enters the time coarse synchronization module, the symbol sampling rate is changed from the system sampling rate f s Reduce k times, thereby reducing the storage and computational complexity of related operations.

[0111] 2) Before the data enters the two-step frequency synchronization module, the data rate is reduced to single sampling for calculation in subsequent modules.

[0112] 3) According to the information input by the two-step frequency synchronization module, the data is sampled and compensated. The prior information is the result calculated by the previous packet. In the case of small time variability, using the synchronization result of the previous packet can make the method of the current data packet converge faster.

[0113] The time coarse synchronization module uses the front-end LFM signal in the receiving end signal to perform adaptive time coarse synchronization and provide coarse synchronization information for the two-dimensional cross-correlation coarse frequency offset synchronization module. Its logical structure is as follows: Figure 4 The sliding window signal is autocorrelated and locally cross-correlated in parallel, and the energy quotient is obtained by division to detect the rising edge of the signal to obtain the rough synchronization moment. The synchronization result of the coarse synchronization module is as follows: Figure 5 shown.

[0114] The two-dimensional cross-correlation coarse frequency offset synchronization module is used to perform a two-dimensional cross-correlation time-frequency search on the received signal and the local original m sequence, and send the search value to the comparator to obtain the maximum value, providing the initial frequency and time domain starting point for the fine frequency offset iterative synchronization module. Figure 6 shown.

[0115] First, the received single-sampled data r[n] is temporarily stored in the block random access memory (BRAM). Assuming the m sequence of the data is 511, the total length of the data is 511+τ max, and start taking it out from BRAM after the received r[n] arrives.

[0116] Next, r[n] and the carrier e[k]=exp(j2πk·Δf step1 / f b ) is compensated by the complex multiplier IP core.

[0117] The compensated carrier is correlated with the local m-sequence, and the correlation operation only involves 511 accumulation processes. The result P[τ, k] is obtained and sent to the comparator 1.

[0118] Continue to compensate the next carrier e[k+1] until compensation k max =(2f max +1) / Δf step1 The maximum value outputted by comparator 1 is sent to comparator 2 to complete a one-dimensional search in the frequency domain.

[0119] Finally, delay one data to read out r[n+1], and repeat the iteration until the delay is τ max data, comparator 2 determines according to the determination method in S201 Complete a two-dimensional estimate of the coarse frequency offset.

[0120] The fine frequency deviation iterative synchronization module uses iteration to perform carrier compensation of the signal until the maximum number of estimations is reached and the iteration is stopped to obtain the final estimated carrier frequency deviation, thereby obtaining a signal with the carrier compensated. Figure 7 First, the signal after the two-dimensional cross-correlation coarse frequency offset synchronization compensation is stored in a first-in-first-out (FIFO) memory, and the random access memory (RAM) is used to store the frequency Δf in the carrier estimation iteration. step2 The carrier is used for iterative compensation, with a length of 511. Then the signal and carrier in the previous FIFO and RAM are read in turn for compensation operation to complete a compensation iterative update. Correlation is mainly used to perform cross-correlation operation between the compensation signal and the local m sequence. The result of the dot multiplication accumulation P k [n] is sent to the comparator. When the number of comparators reaches k step2，max The fine frequency deviation search is completed, and the module result is determined according to the maximum energy principle.

[0121] To reduce time overhead, the module parallel design is carried out in a pipeline manner. Each iteration goes through four clock cycles: address acquisition, data reading, calculation, and correlation. In the pipeline structure, the address acquisition operation of the next data is parallel to the data reading operation of the previous data, thereby greatly reducing the delay of real-time synchronization.

[0122] The time fine synchronization module uses the signal of the carrier compensated by the fine frequency deviation iterative estimation module to perform threshold detection, determine the signal direct time and output it, and the implementation method reuses the two-dimensional iterative time-frequency synchronization module.

[0123] The above embodiments are used to illustrate the present invention rather than to limit the present invention. Any modification and change made to the present invention within the spirit of the present invention and the protection scope of the claims shall fall within the protection scope of the present invention.

Claims

1. A low-complexity time-frequency synchronization method in underwater acoustic communication, characterized in that: The method comprises the following steps: S1: Adaptive time coarse synchronization is performed using the front-end LFM signal in the receiving end signal; S2: Using the receiving end signal and the local original m-sequence to perform two-dimensional cross-correlation coarse frequency offset synchronization and fine frequency offset iterative synchronization in turn, to obtain a signal with a fully compensated carrier; S3: Use the signal after carrier compensation to perform fine time synchronization with the local original m sequence; The S1 is specifically: The receiving end uses a sliding window of the same length as the LFM signal for time synchronization: the sliding window signal is represented by r = [r0, r1, ..., r N-1 ], the average energy R(n) of the sliding window signal is calculated as: Where n represents the moment of the sliding window signal, and N is the length of the LFM signal; Calculate the sliding window signal and the local LFM signal t = [t0, t1, ..., t N-1 The average value of the cross-correlation value R r The energy of (n) U(n): Determine whether the signal detected by the receiving end is synchronized successfully. Once the following formula is satisfied, the synchronization is successful, and the moment n is the moment of successful synchronization. Otherwise, the synchronization fails: Among them, Q(n) is the energy quotient, Q T (n) is the set energy quotient threshold; The S2 is specifically: S201: Downsample the received m sequence to single sampling with a sampling rate of f b ; Determine the search range in the time domain [0, τ max ], frequency domain search range [-f max , f max ] and the frequency interval Δf step1 , use the received signal r[n] and the local original m sequence to perform a two-dimensional cross-correlation time-frequency search to obtain the two-dimensional cross-correlation function P[τ, k]: The search position corresponding to the maximum and second maximum values ​​of the two-dimensional cross-correlation function is obtained based on energy. in, represents the time delay corresponding to the maximum value of the two-dimensional cross-correlation function, Indicates removal The set of k; Confirm that the starting position of the interval is Calculate the initial frequency of the interval S202: Perform signal carrier compensation by iteration until the maximum number of estimations is reached and then the iteration is stopped to obtain the final estimated carrier frequency offset. Thus, the carrier signal is compensated. The delay is The frequency synchronization interval is The maximum number of estimates is The signal u obtained after carrier compensation of signal r[n] (0) [n] is: Signal u after k-th compensation (k) [n] can be updated iteratively as: The result of cross-correlation with the m sequence is: Taking the maximum energy as the standard, determine for: The S3 is specifically: The carrier will be compensated The signal is correlated with the m-sequence: by Results for time As a threshold, once the following formula is satisfied, the time τ at this time is determined as the result of time domain fine synchronization: Among them, 0<β<1, is the coefficient factor; Carrier frequency deviation calculated using frequency deviation synchronization The average Doppler factor where f c is the carrier frequency; perform broadband Doppler compensation on the signal, and obtain the received signal y after Doppler compensation through the following formula c (t):

2. A time-frequency synchronization system based on FPGA, characterized in that: The system is used to implement the low-complexity time-frequency synchronization method in underwater acoustic communication described in claim 1; the system includes a control module, a downsampling and Doppler compensation module, a time coarse synchronization module, a two-dimensional cross-correlation coarse frequency offset synchronization module, a fine frequency offset iterative synchronization module and a time fine synchronization module; The control module is used to control the coordinated work of other modules, and change the frequency search interval and the range of time domain search in time-frequency synchronization according to the rate; The downsampling and Doppler compensation module is used to reduce the sampling rate of the digital waveform and perform sampling compensation on the data, thereby directly reducing the amount of data and thus reducing the number of calculations; The time coarse synchronization module uses the front-end LFM signal in the receiving end signal to perform adaptive time coarse synchronization and provide coarse synchronization information for the two-dimensional cross-correlation coarse frequency offset synchronization module; The two-dimensional cross-correlation coarse frequency offset synchronization module is used to perform a two-dimensional cross-correlation time-frequency search on the received signal and the local original m-sequence, send the search value to the comparator to obtain the maximum value, and provide the initial frequency and time domain starting point for the fine frequency offset iterative synchronization module; The fine frequency offset iterative synchronization module performs carrier compensation of the signal by iteration until the maximum number of estimations is reached and the iteration is stopped to obtain a final estimated carrier frequency offset, thereby obtaining a signal with a fully compensated carrier. Each iteration goes through four clock cycles: address fetching, data reading, calculation, and correlation. In the pipeline structure, the address fetching operation of the next data is parallel to the data reading operation of the previous data, thereby greatly reducing the delay of real-time synchronization; The time fine synchronization module uses the signal of the carrier compensated by the fine frequency deviation iterative synchronization module to perform threshold detection and determine the result of time domain fine synchronization.

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