CN103888168A - Doppler compensation method for synthetic aperture underwater acoustic communication and system - Google Patents
Doppler compensation method for synthetic aperture underwater acoustic communication and system Download PDFInfo
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Abstract
The invention provides a Doppler compensation method for synthetic aperture underwater acoustic communication and a system. With the method, a receiving end can accurately recover baseband signals sent in a diversity mode by a sending end. The method comprises steps: 101) frame synchronization detection is carried out and each frame length is obtained according to the frame synchronization detection; 102) a Doppler factor of each frame is estimated according to each frame length; 103) resampling is carried out on received signals according to the estimated value of the Doppler factor so as to finish coarse compensation of the Doppler effect; and 104) coherent superposition is carried out on signals of each diversity branch after compensation, despreading and demodulation are carried out, and the baseband signals sent by the sending end are recovered. Fine compensation is further carried out on signals after coarse compensation between the step 103 and the step 104. According to the method provided by the invention, the Doppler effect generated due to relative motion of receiving and sending nodes can be effectively resisted, coherent superposition of signals sent by each virtual subarray can be realized, gain loss of synthetic aperture processing space can be reduced and the quality of underwater acoustic communication can be improved.
Description
The technical field is as follows:
the invention belongs to the field of underwater acoustic communication, and particularly relates to a Doppler compensation method and a Doppler compensation system for synthetic aperture underwater acoustic communication.
Background
The concept of "synthetic aperture" was originally derived from radar, whose basic principle is to use the relative motion of the radar and the target to obtain high spatial resolution. Foreign scholars apply the synthetic aperture technology to underwater acoustic communication, so that the combination with a PSK communication system and a multi-carrier system is realized, and tests prove that better results are obtained. The Synthetic Aperture technology is not used to increase the Aperture size, but is used to obtain spatial diversity, and this Communication method is called Synthetic Aperture Communication (SAC). In the underwater acoustic synthetic aperture communication, a virtual subarray is formed by the relative motion between the transmitting and receiving nodes, so that the SAC system can obtain space diversity by using two array elements, the system equipment is simplified, and the cost is saved.
In shallow sea underwater acoustic communication, the signal-to-noise ratio is low, the multipath interference is strong, and the channel is one of the worst wireless communication channels, so that incoherent detection or spread spectrum technology is generally adopted for transmission. Aiming at the characteristics of a remote underwater acoustic channel and the requirements of a system, the space diversity gain is obtained by combining a synthetic aperture technology and a direct sequence spread spectrum communication system, the output signal-to-noise ratio is improved, and the frequency selective fading caused by multipath propagation is effectively resisted. However, doppler caused by relative motion between transmitting and receiving nodes is unavoidable, and the influence of doppler on system performance is particularly serious, which not only makes multipath signals unable to realize correlation combination, resulting in reduction of space diversity gain, but also makes the spread spectrum gain greatly reduced and the error rate increased due to compression and expansion of spread spectrum codes. Therefore, accurate motion compensation is particularly important in a synthetic aperture spread spectrum communication system, which is a precondition and guarantee for the performance of the system.
Song et al uses the DFPLL phase tracking method to achieve a coarse estimate of doppler, but this method is only applicable to cases where the velocity of the carrier motion is small. While the lemyka et al proposes to perform spectral estimation on the training sequence by using chirp Z transform, the pseudo-random sequence has higher frequency resolution, which means that the detection of the correlation peak cannot be achieved under the condition of large doppler. In radio spread spectrum communication, PLL is usually used to track frequency and phase, however, PLL convergence needs to pass through tens or hundreds of symbols, and cannot be directly applied to underwater acoustic communication at low code rate. Therefore, a high-precision frequency estimation and compensation method suitable for large doppler conditions is urgently needed.
Disclosure of Invention
The present invention is directed to a method and system for doppler compensation in synthetic aperture underwater acoustic communications.
In order to achieve the above object, the present invention provides a doppler compensation method for synthetic aperture underwater acoustic communication, which is used for a receiving end to accurately recover a baseband signal diversity-transmitted by a transmitting end, the method comprising:
step 101) a step for detecting frame synchronization and obtaining the frame length of each frame according to the frame synchronization detection;
step 102) a step for estimating a Doppler factor of each frame according to the frame length of each frame;
step 103) is used for resampling the received signal according to the Doppler factor estimated value to complete the coarse compensation of the Doppler effect;
and 104) coherent superposition is carried out on the signals compensated by each diversity branch, despreading demodulation is carried out, and the baseband signals sent by the sending end are recovered.
The step of performing fine compensation on the coarsely compensated signal between the step 103) and the step 104), which specifically includes:
carrying out down-conversion and despreading processing on a training signal contained in a signal received by a receiving end;
performing frequency and phase estimation on the signals subjected to down-conversion and joint-spreading processing by adopting full-phase FFT;
and accurately compensating the time, the frequency and the phase of the re-sampled signal of each frame according to the estimated values of the frequency and the phase.
The above estimating the frequency and the phase further comprises the steps of:
step 201) determining interception L point data according to the initial position of a training signal obtained by frame synchronization detection and the uncertainty of the maximum time, and taking 2N-1 point data (L >2N-1);
step 202) performing full-phase preprocessing on the data of the 2N-1 point by adopting a double-window adding method;
step 203), performing N-point FFT on the data after full-phase preprocessing to obtain a full-phase spectrum analysis result;
step 204) carrying out peak detection and threshold judgment on the full-phase FFT result, if the peak value exceeds a preset threshold, adjusting the initial position of 2N-1 point data by a certain step length, repeating the steps, and comparing the peak values obtained by R times of full-phase spectrum analysis so as to obtain the maximum peak value, which indicates that time-frequency fine synchronization is successful: the initial point of the information segment obtained by the maximum peak value eliminates the time ambiguity caused by Doppler, the frequency corresponding to the maximum peak value and the phase corresponding to the maximum peak value are accurate estimation results, wherein R is the total number of the L data divided by taking 2N-1 points as units;
step 205) intercepting the phase value phi on the main spectral line of the full-phase spectral analysis obtained in the step 203)ap(k*) Wherein k is*The initial phase of the signal can be accurately obtained for the spectrum serial number; the frequency is corrected by using a full-phase time shift phase difference method, and the correction formula is as follows:
wherein phi (k)*) Is a phase spectrum, phiap(k*) Is a full phase spectrum, both have a time shift difference of N;
step 206) converting the digital domain frequency into a continuous domain frequency, and obtaining frequency and phase estimation values respectively as follows:
wherein,
analyzing the frequency estimation result for the full phase spectrum, phiap(k*) For N points full phase FFT main line phase, fsRepresenting the sampling rate.
The step 101) further includes:
firstly, performing matched filtering on an input signal by using a lead code copy, finishing frame detection through threshold judgment, and determining the initial position of the signal;
the end position of the signal is then determined using the midamble copies.
The step 102) is specifically performed by the following method:
after receiving signals and carrying out frame synchronization detection, the length of a data packet is estimated by using a lead code and a postamble contained in a frame structure
And obtaining Doppler factor estimated value by the following formula
Wherein, TtA known frame length stored for the receiving end.
Based on the above method, the present invention further provides a doppler compensation system for synthetic aperture underwater acoustic communication, the system is used for a receiving end to accurately recover a baseband signal transmitted by a transmitting end in diversity, and the system comprises:
the frame synchronization detection module is used for detecting frame synchronization, obtaining the starting position and the ending position of each frame and obtaining the frame length of each frame;
the Doppler factor estimation module is used for estimating the Doppler factor of each frame according to the frame length of each frame;
the resampling coarse compensation module is used for resampling the received signal according to the Doppler factor estimated value to complete the coarse compensation of the Doppler effect; and
and the baseband signal recovery module is used for performing relevant superposition on the signals compensated by each diversity branch, performing despreading demodulation and recovering the baseband signals sent by the sending end.
A fine compensation module is also included between the resampling coarse compensation module and the baseband signal recovery module, and the fine compensation module is used for performing frequency and phase compensation on the coarsely compensated signal and obtaining an accurate signal starting time;
the fine compensation module further comprises:
the processing submodule is used for carrying out down-conversion and despreading processing on a training signal contained in a signal received by a receiving end;
the frequency and phase estimation submodule is used for carrying out frequency and phase estimation on the signals subjected to down-conversion and spread-spectrum processing by adopting full-phase FFT; and
and the compensation submodule is used for respectively compensating the resampled signals of each frame according to the estimated values of the frequency and the phase.
The frequency and phase estimation sub-module further comprises:
the preprocessing module is used for determining interception L point data according to the initial position of a training signal obtained by frame synchronization detection and the uncertainty of the maximum time, intercepting 2N-1 point data from the initial point in primary preprocessing, otherwise intercepting the 2N-1 point data according to the result given by the decision judging module, and performing full-phase preprocessing on the data by adopting a double-window adding method;
the Fourier transform module is used for performing N-point FFT on the data after the full-phase preprocessing;
the decision-making decision module is used for carrying out threshold decision and peak detection on the full-phase FFT result, carrying out fine search on a time domain according to the full-phase spectrum analysis result, controlling the preprocessing module to adjust the position of the intercepted data, and obtaining a time starting point corresponding to the maximum peak in the R-segment data, wherein the frequency corresponding to the maximum peak and the phase corresponding to the maximum peak are accurate estimation results, so that the system completes time-frequency fine synchronization;
a first processing module for directly intercepting the phase value on the main spectral line, and correcting the frequency by using a full-phase time-shift phase difference method, wherein the correction formula is
Wherein phi (k)*) Is a phase spectrum, phiap(k*) Is a full phase spectrum, both have a time shift difference of N;
a frequency and phase estimation module, configured to convert a digital domain frequency into a continuous domain frequency, where the obtained frequency and phase estimation values are:
wherein,
analyzing the frequency estimation result for the full phase spectrum, phiap(k*) For N points full phase FFT main line phase, fsRepresenting the sampling rate.
The frame synchronization detecting module further comprises:
the frame initial position determining module is used for performing matched filtering on the input signal by using the lead code copy, finishing frame detection through threshold judgment and determining the initial position of the signal;
and the frame end position determining module is used for determining the end position of the signal by using the postamble copy.
The doppler factor estimation module further comprises:
a data packet length estimation module for estimating the length of the data packet by the preamble and the postamble contained in the frame structure obtained after the frame synchronization detection
A Doppler factor estimation module for obtaining a Doppler factor estimation value by the following formula
Wherein, TtA known frame length stored for the receiving end.
Compared with the prior art, the invention has the technical advantages that:
the invention provides an effective Doppler effect estimation and compensation method aiming at the problem of larger synthetic aperture processing space diversity gain loss caused by movement in synthetic aperture underwater acoustic spread spectrum communication.
Drawings
FIG. 1 is a schematic diagram of synthetic aperture underwater acoustic communications;
FIG. 2 is a schematic diagram of a frame structure;
FIG. 3 is a block flow diagram of a method for motion compensation of synthetic aperture communications provided by the present invention;
FIG. 4 is a schematic diagram of a full phase pre-process provided by an embodiment of the present invention;
FIG. 5 is a schematic diagram of a full-phase-difference method for spectrum correction according to an embodiment of the present invention;
FIG. 6 is a sound velocity profile of an experimental sea area provided by an embodiment of the present invention;
fig. 7 is a channel impulse response diagram according to an embodiment of the present invention.
The specific implementation mode is as follows:
the invention is described in detail below with reference to the figures and specific embodiments.
The invention provides an effective high-precision Doppler effect estimation and compensation method. The method firstly eliminates most Doppler by utilizing a resampling technology, then adopts a full-phase FFT method to realize high-precision estimation of frequency and phase, and simultaneously eliminates time ambiguity caused by Doppler.
First, frame sync detection
In the underwater acoustic communication system, a frame synchronization signal is required to determine the start position of each frame, and the frame synchronization signal adopts a sequence with better autocorrelation, such as an M sequence, a chirp signal (LFM signal), and the like. Considering that the ambiguity function of the LFM signal is wider on the doppler axis, which means that the LFM signal has a higher tolerance to doppler, the LFM signal is selected as the preamble and the postamble for burst information frame detection and frame length estimation, thereby avoiding detection failure and data packet loss caused by fast carrier motion speed. And the ambiguity function of the pseudo-random sequence has a sharp related peak on a Doppler axis and has higher frequency resolution, so that the pseudo-random sequence is used as a training sequence to realize high-precision estimation of carrier consistent Doppler frequency offset.
As the system is in a burst working mode, a receiving end firstly uses a lead code copy to carry out matched filtering on an input signal, completes frame detection through threshold judgment and determines the initial position of the signal. The end position of the signal is then determined using the midamble copies. At the receiving end, the chirp signal after passing through the channel can be expressed as:
rLFM(t)=sLFM(t)*h(t)
(1)
=∫h(τ)sLFM(t-τ)dτ
wherein s isLFM(t) is the transmitted chirp, h (t) is the channel impulse response, "+" indicates convolution. The received signal is correlated with a local copy of the LFM signal:
RLFM(ζ)＝∫rLFM(t)sLFM(t-ζ)dt
（2）
=∫∫h(τ)sLFM(t-τ)dτsLFM(t-ζ)dt
exchange integration sequence:
RLEM(ζ)
＝∫h(τ)∫ sLFM(t')sLFM(t′+τ-ζ)d(t′+τ)dτ
（3）
=∫h(τ)R(τ-ζ)dτ,
depending on the nature of the LFM, its autocorrelation function is at TBP = | K | T2The larger waveform may be approximated by a sinc function, where K is the rate of change of the signal frequency with time and T is the total time duration of the signal. And the limit of the sinc function is the shock function δ (t), equation (1) can be approximated as:
RLFM(ζ)≈∫h(τ)δ(ζ-τ)dτ=h(ζ)（4）
equation (4) shows that the channel impulse response can be approximately represented by the correlation result of the frame synchronization signal and the local replica signal at the receiving end, and the result is also the input of the frame synchronization detection.
Second, estimating the Doppler factor based on the frame length measurement
In underwater acoustic communication, because the ratio of the carrier frequency to the system bandwidth is small, the underwater acoustic channel is actually a broadband system, the doppler effect of the underwater acoustic signal cannot be simply represented by the doppler shift at the carrier frequency, but should be modeled as compression or expansion of an analog signal in the time domain, that is, the duration T of the signal should be changed from T to T/(1+ α), where α is a doppler factor, and the expression is α = v/c · cos (Φ), v is the relative motion speed, c is the sound speed, and Φ is the angle between the relative motion of the two parties and the sight line direction. Assuming that each path has the same Doppler factor, and considering the multipath, Doppler and noise effects, the received signal of the mth sub-channel can be expressed as
Wherein L represents a total of L propagation paths, dkBase band information sequence, cnBeing pseudo-random code sequences, symbols
Representing spread spectrum modulation, p (t) being a baseband pulse shaped waveform, fcIs a carrier frequency, almAnd τlmFor each path corresponding signal amplitude and delay, nm(t) is the noise of the mth subchannel, αmIs the corresponding doppler factor of the diversity signal.
The problem of estimation of the wideband doppler is the estimation of the doppler factor. After receiving signals and carrying out frame synchronization detection, the length of a data packet is estimated by using a lead code and a postamble contained in a frame structureBy matching a known frame length TtThe Doppler factor estimated value is obtained by comparison, as shown in formula (6)
Thirdly, resampling the received signal according to the Doppler factor estimated value
Estimation using doppler factor
For received signal rm(t) resampling is carried out, coarse compensation of the Doppler effect is completed, and the resampled signal is expressed as follows:
the resampling method is selected in many ways, and in practical engineering application, the requirements of computation and precision can be considered, and a linear interpolation method based on a polyphase filter is selected.
Fourthly, precise Doppler estimation is completed by adopting full-phase FFT
On the basis of completing the Doppler effect coarse compensation, the full-phase FFT is used for carrying out high-precision frequency and phase estimation on the training signals after down-conversion and de-spread, and meanwhile, time ambiguity is eliminated, and the accurate signal starting time is obtained.
The full-phase FFT spectrum analysis reduces errors caused by signal truncation by an overlapping method, an input signal of the full-phase FFT spectrum analysis is a baseband signal containing residual Doppler information, and the specific processing flow is as follows:
firstly, determining intercepted L point data according to the initial position of a training signal given by frame synchronization and the maximum time uncertainty, wherein each 2N-1 point is a segment, R segments are shared, and c is orderedtnRepresenting a training pseudorandom sequence, χm(t) training signal representing unmodulated baseband information:
second, local carrier and training sequence are used to make X pairsm(t) down-converting and despreading, the baseband expression of the training signal being
Wherein, taueFor residual Doppler induced time synchronization error, R (τ)e) Is an ideal normalized pseudo code correlation function when taue→ 0, R (. tau.),e) And = 1. The purpose of fine synchronization is to order τe→0，
Order to
Representing frequency error and phase error, respectively.
Thirdly, in order to better inhibit the frequency spectrum leakage, each section of 2N-1 point data is subjected to full-phase preprocessing by adopting a double-window adding method in sequence; each segment of data is subjected to double-window full-phase preprocessing in sequence, namely a convolution window omegac(N) the front window f (N) and the rear window b (-N) are both Hanning windows, and the observation interval is N E [ -N +1, N-1)]
ωc(n)=f(n)*b(-n)(10)
To be provided with
To representIn discrete form, the data y after overlap-add is weightedm(n) can be represented as
Wherein R isNAnd (N) is a rectangular window with the length of N.
Fourthly, data y after full phase preprocessingm(N) performing N-point FFT to obtain a full-phase spectrum analysis result, which is marked as Ym(ejω)；
Performing threshold judgment and peak detection on the full-phase FFT result of the R-segment data to obtain a time starting point, frequency and phase corresponding to the maximum peak value in the R-segment data;
directly intercepting phase value phi on main spectral line of full-phase spectrum analysisap(k*) (assume the spectrum number is k*) The initial phase of the signal can be accurately obtained; the frequency is corrected by a full-phase time shift phase difference method, and the correction formula is
Seventhly, converting the digital domain frequency into continuous domain frequency, and finally obtaining frequency and phase estimated values of
Fifthly, carrier consistent Doppler frequency offset and phase compensation
And performing CFO compensation on the received signals of each virtual sub-array according to the full-phase spectrum estimation result, and adjusting the estimation phase corresponding to the M = 2-M virtual sub-arrays according to the estimation result when M =1, thereby obtaining the dynamic compensation weighting factor of each sub-array, and using beta as the dynamic compensation weighting factormTo represent
Sixthly, coherent superposition of diversity signals
Coherent combining can be achieved when the dynamic compensation of time and frequency is completed for the received signals of the sub-channels. If the combining is performed with equal gain, the coherently combined signals can be represented as
Seventh step of despreading and demodulating
Despreading and demodulating the combined signals r (t) to recover baseband information
Examples
Example of implementation: the frame structure designed herein is shown in fig. 2. The system bandwidth is 3kHz, so the LFM signal bandwidth is set to 3kHz, with a start frequency and a cut-off frequency of 1kHz and 4kHz, respectively. For a matched filter to have a sufficiently high detection performance, the time-bandwidth product BT of the LFM signal must be made sufficiently large to ensure that its main ridge of the ambiguity function is very narrow, typically meeting BT > 100. Thus, the LFM signal duration T takes 35 ms.
The overall motion compensation method of the present invention is implemented as shown in fig. 3. The receiving end firstly uses the lead code copy to carry out matched filtering on the input signal, completes frame detection through threshold judgment and determines the initial position of the signal. And then, determining the end position of the signal by using the back pilot code copy to obtain an estimated value of the Doppler factor, and resampling the signal to complete the coarse compensation of the Doppler effect. On the basis, the full-phase FFT is used for carrying out high-precision frequency and phase estimation on the training signals after down-conversion and de-spread, and meanwhile, time ambiguity is eliminated, so that the accurate signal starting time is obtained, wherein the full-phase preprocessing and frequency correction processes are respectively shown in the figures 4 and 5. And after frequency compensation and phase adjustment are carried out on the M diversity branches, in-phase superposition is carried out, and the original baseband information is recovered through de-spread and demodulation.
To better verify the performance of the present invention, the computer simulation results of this embodiment are given below:
and modeling the underwater sound channel according to the real ocean sound velocity distribution, and carrying out simulation research. Different channel impulse responses can be obtained by changing the relative horizontal position between the receiving and transmitting nodes, and the channel models have different fading characteristics and are used for further verifying the performance of the synthetic aperture spread spectrum communication in the time-varying underwater acoustic channel with low signal-to-noise ratio, strong multi-channel and frequency selective fading. Fig. 6 is a sound velocity profile actually measured in a certain sea area. The transceiver nodes are all located at a water depth of 20m, the distance between the transceiver nodes and the water depth is about 30km, and different channel impulse responses are obtained by changing the distance between the transceiver nodes and the water depth, as shown in fig. 7. The multipath structures of all the sub-channels are different, and the space-variant performance of the underwater acoustic channel is reflected. The delay spread of the channel is about 1s, namely the coherence bandwidth is 1Hz, and the sub-channels are all a mixture of minimum phase channels and maximum phase channels, which is extremely unfavorable for frame detection and extremely complex and harsh channel environment. In order to make the simulation more approximate to the real underwater sound environment, the simulation of the underwater sound channel is carried out from the following aspects: respectively convolving the frame structure direct sequence spread spectrum data frame shown in fig. 2 with the 6 channel models, which is equivalent to the transmission of data by a transmitting node at 6 different positions; each transmission is provided with different Doppler, which indicates that the relative movement speed of the transceiving node is changed; and finally, adding Gaussian white noise on the data frame. The specific simulation condition parameters are shown in table 1:
TABLE 1 simulation conditions of the System
| Modulation system | BPSK |
| Carrier frequency fc | 2.5kHz |
| Type of training sequence | m sequence |
| Training sequence length | 255 chips |
| Spreading code type | Gold code |
| Spreading code length | 127 |
| Code rate Rc | 2kcps |
| Information rate Rb | 15.7bps |
Table 2 below summarizes the bit error rate statistics for 3200-bit baseband data based on monte carlo simulations. It can be seen that the error code performance of the system is obviously improved with the increase of M, when SNR = -15dB, the error code rate of 0.8% can be achieved through the synthetic aperture processing system, which fully shows that the motion compensation method provided by the invention effectively obtains the synthetic aperture space diversity gain and improves the communication quality.
TABLE 2 bit error rate statistics for different virtual subarray numbers
In summary, the present invention provides an effective doppler effect estimation and compensation method for the problem of large loss of the synthetic aperture processing space diversity gain caused by motion in the synthetic aperture underwater acoustic spread spectrum communication. The method firstly utilizes the resampling technology to eliminate most Doppler, and further adopts a full-phase FFT method to realize high-precision estimation of frequency and phase on the basis, and meanwhile eliminates time ambiguity caused by Doppler. Theoretical simulation verification shows that the method can effectively resist Doppler effect generated by relative motion of the receiving and transmitting nodes, realize coherent superposition of all virtual subarray transmitting signals, reduce the loss of synthetic aperture processing space gain, and further improve the underwater acoustic communication quality.
Finally, it should be noted that the above embodiments are only used for illustrating the technical solutions of the present invention and are not limited. Although the present invention has been described in detail with reference to the embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the spirit and scope of the invention as defined in the appended claims.
Claims (10)
1. A doppler compensation method for synthetic aperture underwater acoustic communication, which is used for a receiving end to accurately recover a baseband signal diversity-transmitted by a transmitting end, the method comprising:
step 101) a step for detecting frame synchronization and obtaining the frame length of each frame according to the frame synchronization detection;
step 102) a step for estimating a Doppler factor of each frame according to the frame length of each frame;
step 103) is used for resampling the received signal according to the Doppler factor estimated value to complete the coarse compensation of the Doppler effect;
and 104) coherent superposition is carried out on the signals compensated by each diversity branch, despreading demodulation is carried out, and the baseband signals sent by the sending end are recovered.
2. The new doppler compensation method for synthetic aperture underwater acoustic communication according to claim 1, wherein the step of performing fine compensation on the coarsely compensated signal between the step 103) and the step 104) further comprises:
carrying out down-conversion and despreading processing on a training signal contained in a signal received by a receiving end;
estimating the frequency and the phase of the signal subjected to the down-conversion and the joint spread processing by adopting a full-phase FFT;
and accurately compensating the time, the frequency and the phase of the resampled signal of each frame according to the frequency and phase estimation.
3. The new doppler compensation method for synthetic aperture underwater acoustic communication of claim 2 wherein said performing frequency and phase estimation further comprises the steps of:
step 201) determining interception L point data according to the initial position of a training signal obtained by frame synchronization detection and the uncertainty of the maximum time, and taking 2N-1 point data (L >2N-1);
step 202) performing full-phase preprocessing on the data of the 2N-1 point by adopting a double-window adding method;
step 203), performing N-point FFT on the data after full-phase preprocessing to obtain a full-phase spectrum analysis result;
step 204) carrying out peak detection and threshold judgment on the full-phase FFT result, if the peak value exceeds a preset threshold, adjusting the initial position of 2N-1 point data by a certain step length, repeating the steps, and comparing the peak values obtained by R times of full-phase spectrum analysis so as to obtain the maximum peak value, which indicates that time-frequency fine synchronization is successful: the initial point of the information segment obtained by the maximum peak value eliminates the time ambiguity caused by Doppler, the frequency corresponding to the maximum peak value and the phase corresponding to the maximum peak value are accurate estimation results, wherein R is the total number of the L data divided by taking 2N-1 points as units;
step 205) intercepting the phase value phi on the main spectral line of the full-phase spectral analysis obtained in the step 203)ap(k*) Wherein k is*The initial phase of the signal can be accurately obtained for the spectrum serial number; the frequency is corrected by using a full-phase time shift phase difference method, and the correction formula is as follows:
wherein phi (k)*) Is a phase spectrum, phiap(k*) Is a full phase spectrum, both have a time shift difference of N;
step 206) converting the digital domain frequency into a continuous domain frequency, and obtaining frequency and phase estimation values respectively as follows:
wherein,
analyzing the frequency estimation result for the full phase spectrum, phiap(k*) For N points full phase FFT main line phase, fsRepresenting the sampling rate.
4. The new doppler compensation method for synthetic aperture underwater acoustic communication of claim 1 wherein said step 101) further comprises:
firstly, performing matched filtering on an input signal by using a lead code copy, finishing frame detection through threshold judgment, and determining the initial position of the signal;
the end position of the signal is then determined using the midamble copies.
5. The new doppler compensation method for synthetic aperture underwater acoustic communication according to claim 1, wherein said step 102) specifically adopts the following method:
after receiving signals and carrying out frame synchronization detection, the length of a data packet is estimated by using a lead code and a postamble contained in a frame structure
And obtaining Doppler factor estimated value by the following formula
Wherein, TtA known frame length stored for the receiving end.
6. A doppler compensation system for synthetic aperture underwater acoustic communication for a receiving end to accurately recover a baseband signal diversity-transmitted from a transmitting end, the system comprising:
the frame synchronization detection module is used for detecting frame synchronization, obtaining the starting position and the ending position of each frame and obtaining the frame length of each frame;
the Doppler factor estimation module is used for estimating the Doppler factor of each frame according to the frame length of each frame;
the resampling coarse compensation module is used for resampling the received signal according to the Doppler factor estimated value to complete the coarse compensation of the Doppler effect; and
and the baseband signal recovery module is used for performing relevant superposition on the signals compensated by each diversity branch, performing despreading demodulation and recovering the baseband signals sent by the sending end.
7. The doppler compensation system for synthetic aperture underwater acoustic communication of claim 6, wherein a fine compensation module is further included between the resampling coarse compensation module and the baseband signal recovery module, the fine compensation module is configured to perform frequency and phase compensation on the coarsely compensated signal and obtain an accurate signal start time;
the fine compensation module further comprises:
the processing submodule is used for carrying out down-conversion and despreading processing on a training signal contained in a signal received by a receiving end;
the frequency and phase estimation submodule is used for carrying out frequency and phase estimation on the signals subjected to down-conversion and spread-spectrum processing by adopting full-phase FFT; and
and the compensation submodule is used for respectively compensating the resampled signals of each frame according to the estimated values of the frequency and the phase.
8. The doppler compensation system for synthetic aperture underwater acoustic communication of claim 7, wherein the frequency and phase estimation sub-module further comprises:
the preprocessing module is used for determining interception L point data according to the initial position of a training signal obtained by frame synchronization detection and the uncertainty of the maximum time, intercepting 2N-1 point data from the initial point in primary preprocessing, otherwise intercepting the 2N-1 point data according to the result given by the decision judging module, and performing full-phase preprocessing on the data by adopting a double-window adding method;
the Fourier transform module is used for performing N-point FFT on the data after the full-phase preprocessing;
the decision-making decision module is used for carrying out threshold decision and peak detection on the full-phase FFT result, carrying out fine search on a time domain according to the full-phase spectrum analysis result, controlling the preprocessing module to adjust the position of the intercepted data, and obtaining a time starting point corresponding to the maximum peak in the R-segment data, wherein the frequency corresponding to the maximum peak and the phase corresponding to the maximum peak are accurate estimation results, so that the system completes time-frequency fine synchronization;
a first processing module for directly intercepting the phase value on the main spectral line, and correcting the frequency by using a full-phase time-shift phase difference method, wherein the correction formula is
Wherein phi (k)*) Is a phase spectrum, phiap(k*) Is a full phase spectrum, both have a time shift difference of N;
a frequency and phase estimation module, configured to convert a digital domain frequency into a continuous domain frequency, where the obtained frequency and phase estimation values are:
wherein,
analyzing the frequency estimation result for the full phase spectrum, phiap(k*) For N points full phase FFT main line phase, fsRepresenting the sampling rate.
9. The doppler compensation system for synthetic aperture underwater acoustic communication of claim 6 wherein said frame sync detection module further comprises:
the frame initial position determining module is used for performing matched filtering on the input signal by using the lead code copy, finishing frame detection through threshold judgment and determining the initial position of the signal;
and the frame end position determining module is used for determining the end position of the signal by using the postamble copy.
10. The doppler compensation system for synthetic aperture underwater acoustic communication of claim 6, wherein the doppler factor estimation module further comprises:
a data packet length estimation module for estimating the length of the data packet by the preamble and the postamble contained in the frame structure obtained after the frame synchronization detection
A Doppler factor estimation module for obtaining a Doppler factor estimation value by the following formula
Wherein, TtA known frame length stored for the receiving end.
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