CN110266622A - An Orthogonal Multi-Carrier M-element Chaotic Phase Modulation Spread Spectrum Underwater Acoustic Communication Method - Google Patents

Abstract

The invention discloses an orthogonal multi-carrier M-element chaotic phase-modulation spread-spectrum underwater acoustic communication method. The method includes: Step 1) The transmitting end divides the input information sequence sent by the information source into K groups with a length of a+2 The information sequence is sent to K groupers respectively, and each grouper performs M-element chaotic phase modulation spread spectrum mapping on the first a bit to obtain a chaotic phase modulation spread spectrum code, and performs QPSK modulation on the last two bits before using The obtained chaotic phase modulation spread spectrum code is subjected to spread spectrum modulation to obtain baseband signals, and then K groups of baseband signals are respectively mapped to subcarriers of OFDM symbols for OFDM modulation to obtain digital signals to be transmitted and transmit them; Step 2) Receiver Using a section of chaotic phase-modulation spread-spectrum signal in OFDM symbol as pilot signal, time-frequency two-dimensional search is performed on the received digital signal to complete time-frequency synchronization; then OFDM demodulation is performed to obtain K groups of baseband signals, which are respectively input into K combiners to complete Despread and QPSK demodulate to get the binary sequence.

Description

An Orthogonal Multi-Carrier M-element Chaotic Phase Modulation Spread Spectrum Underwater Acoustic Communication Method

technical field

The invention relates to the technical field of underwater acoustic communication, in particular to an orthogonal multi-carrier M-element chaotic phase-modulation spread-spectrum underwater acoustic communication method.

Background technique

The underwater acoustic channel is a time-varying, space-varying and frequency-varying channel with limited bandwidth, complex multi-path interference, and serious background noise. The complexity of the channel severely limits the communication performance of underwater acoustic communication. Due to the existence of spread spectrum gain, spread spectrum communication has good anti-noise and anti-multipath interference characteristics, and is widely used in the field of long-distance underwater acoustic communication. However, due to the limitation of the bandwidth of the underwater acoustic channel, the communication rate of spread spectrum communication is low, and often can only meet the transmission rate of tens of bits or even a few bits per second, so the practicability is greatly limited.

Using the combined spread spectrum communication method, the data rate can be further improved. However, it is difficult to guarantee complete orthogonality between the superimposed spread spectrum codes, so there is mutual interference between channels, which reduces the bit error rate performance of the system; The cyclic shift keying spread spectrum communication method modulates the information on the symbol phase, and uses the size of the symbol phase to represent the information. On the one hand, the bandwidth utilization rate is further improved, and on the other hand, the problem of inter-channel interference is solved through the orthogonal channel. Problem: Using the Orthogonal Multi-carrier Spread Spectrum (OMCSS) method, the spread signal is simultaneously modulated on multiple orthogonal subcarriers, combined with cyclic shift keying and M-ary spread spectrum Modulation further increases the communication rate. However, the above time-domain spread spectrum scheme has limited improvement in the efficiency of spread spectrum communication, and the communication performance is limited under the underwater acoustic conditions with complex channel structures. In addition, the pseudo-random (Pseudo-Noise, PN) sequence used in the traditional spread spectrum modulation has obvious periodicity and binary characteristics. Once the signal is intercepted, the corresponding characteristics such as chip rate and code period are easy to be extracted by the enemy. And use, leading to information leakage, does not have the characteristics of confidentiality.

Contents of the invention

The purpose of the present invention is to solve the above-mentioned deficiencies in the prior art, further improve the communication rate of underwater acoustic spread spectrum communication, improve the secrecy performance of spread spectrum signals at the same time, and provide a kind of communication system with high reliability, good confidentiality and high communication efficiency. Underwater Acoustic Communication Methods.

In order to achieve the above object, the present invention provides a method for orthogonal multi-carrier M-element chaotic phase-modulation spread-spectrum underwater acoustic communication, said method comprising:

Step 1) The transmitting end converts the input information sequence sent by the source into serial-to-parallel conversion according to each (a+2) bit group, and obtains K groups of information sequences with a length of a+2, and sends them into K groups respectively In the device, each grouper performs M-element chaotic phase modulation spread spectrum mapping on the first a bit respectively to obtain a chaotic phase modulation spread spectrum code, performs QPSK modulation on the last two bits, and then uses the obtained chaotic phase modulation spread spectrum code to perform Spread spectrum modulation to obtain baseband signals, and then map K groups of baseband signals to subcarriers of OFDM symbols for OFDM modulation to obtain digital signals to be transmitted, and finally convert digital signals to be transmitted into underwater acoustic signals and transmit them;

Step 2) The receiving end converts the received underwater acoustic signal into a digital signal, adopts a section of chaotic phase-modulation spread spectrum signal in the OFDM symbol as the pilot signal, and uses the spreading gain of the chaotic phase-modulation spread spectrum code to time-shift the digital signal. Frequency two-dimensional search completes time-frequency synchronization; then performs OFDM demodulation on the signals after time-frequency synchronization to obtain K groups of baseband signals, which are respectively input into K combiners to complete despreading and QPSK demodulation to obtain binary sequences.

As an improvement of the above method, the step 1) specifically includes:

Step 1-1) Use Quadratic mapping to obtain the original chaotic sequence, thereby generating M spreading codes for modulating the information sequence and a pilot sequence for time-frequency synchronization, and the spreading lengths are N and D respectively; the generated The M spread spectrum codes form a chaotic phase modulation spread spectrum sequence set {c ₁ ,c ₂ ,...,c _M };

Step 1-2) The transmitting end converts the input information sequence sent by the source into serial-to-parallel conversion according to each group of (a+2) bits, and obtains K groups of information sequences with a length of a+2, and sends them to K in a grouper;

Step 1-3) Each grouper performs segmentation processing on the input (a+2) bit information, performs M-ary mapping on the first a bit, and performs QPSK mapping on the remaining 2-bit information; then the kth grouper obtains The M-ary data x _k after M-element mapping, select the x _kth spreading code in the M spreading sequences in the set of chaotic phase modulation spreading sequences as output, and get the corresponding spreading code After QPSK mapping, d(k) is obtained, and the spreading code is used The baseband signal b(n) corresponding to the kth grouper is obtained after spread spectrum modulation of d(k):

where n=0,1...,KN, q=n-kN, k=1,...K, Spreading code The qth chip of ;

Steps 1-4) Each grouper outputs a baseband signal with a length of N, maps K groups of baseband signals and pilot sequences to L subcarriers of OFDM symbols one by one, obtains the total baseband signal, and performs IFFT on the total baseband signal Transform, the time domain signal S(t) is obtained as:

Wherein, f _n is the frequency of each subcarrier, T is the time length of an OFDM symbol, T _g is the time length of the cyclic prefix, T'=T+T _g ; the complete M element CPM spread spectrum OFDM signal is expressed as:

Among them, b(n,i) is the data modulated on the nth subcarrier in the i-th symbol, and g(t) is the pulse waveform of each symbol, expressed as:

As an improvement of the above method, the step 1-1) specifically includes:

Step 1-1-1) Utilize Quadratic mapping to obtain the original chaotic code, and the mapping equation is expressed as:

y(m+1)=P-Qy ² (m)

Among them, when 3/4<PQ<2, the original chaotic code y(m)∈(-2/Q,2/Q), Q=4, P=1/4, y(0)∈(-0.5, 0.5), y(m)∈(-0.5,0.5), m is a positive integer;

Step 1-1-2) Modulate the y(m) without binary quantization on the complex exponent to generate the chaotic phase modulation spread spectrum code c, c contains N chips, wherein the qth chip is:

c(q)=exp{j2πy(q)},q=0,1...N-1

Step 1-1-3) Select M mutually orthogonal chaotic phase modulation spreading codes to form an M-element chaotic phase modulation spreading code sequence set {c ₁ ,c ₂ ,...,c _M };

Step 1-1-4) Using the original chaotic sequence y(m) to generate a fixed chaotic phase-modulation spreading code, the spreading code length is D, and the spreading code is used as a pilot sequence.

As an improvement of the above method, a in the step 1-2) is:

As an improvement of the above method, the size of K in the step 1-2) is determined by the length N of the chaotic phase modulation spread spectrum code and the subcarrier number L of a multi-carrier M element chaos phase modulation spread spectrum symbol S , whose relationship satisfies:

K×N+D≤L.

As an improvement of the above method, the step 2) specifically includes:

Step 2-1) The receiving end performs time-frequency synchronization on the received OFDM signal;

Step 2-2) Perform Fourier transform on the synchronized signal r(t) within the time period [iT′,iT′+T], and obtain the nth subcarrier in the output i symbol by formula (7) The signal on is:

in, is the phase deviation caused by synchronization error;

Step 2-3) performing M-element correlation despreading in the frequency domain to spread spectrum signals distributed on different subcarriers;

Assuming that the position of the k-th spread spectrum signal to be despread in the i-th OFDM symbol on the subcarrier is γ=[0 1... N-1], then the process of related despreading is:

Among them, r _ni represents the baseband signal after subcarrier extraction of the i-th OFDM symbol according to the position of the k-th spread spectrum signal in the sub-carrier, and c _p represents the p-th spread spectrum in the set of M-ary chaotic phase-modulated spread-spectrum sequences code, Indicates the conjugate operation, is the result obtained after despreading r _ni and c _p ; according to The estimated value of the M-ary data x _k can be obtained:

For M base Perform inverse mapping to obtain corresponding a bit information;

Step 2-4) order right Perform QPSK inverse mapping to demodulate to obtain the last 2 bits of information;

Step 2-5) input the a bit and 2 bit information obtained in step 2-3) and step 2-4) into the kth combiner to combine to obtain the kth group information sequence;

Step 2-6) Repeat step 2-5) to demodulate to obtain all K groups of information sequences, and perform parallel-to-serial conversion on these K groups of information sequences to obtain the final demodulated information sequence.

As an improvement of the above method, the step 2-1) specifically includes:

Step 2-1-1) Intercept OFDM symbols to be synchronized with a time delay of a time-domain search step, and perform high-resolution FFT transformation on the time-domain OFDM symbols according to the frequency-domain variable sampling algorithm to obtain the Doppler effect of each subcarrier Spectrum shifted after interference;

Step 2-1-2) Calculate the subcarrier position of the pilot signal with a Doppler factor of a frequency domain search step, and extract the above-mentioned frequency spectrum according to the obtained position to obtain the baseband signal corresponding to the pilot signal;

Step 2-1-3) Perform frequency-domain spread-spectrum despreading on the above-mentioned baseband signal and the chaotic phase-modulation spreading code used for time-frequency synchronization locally generated by the receiving end, and obtain the energy value of the despreading result as this frequency-domain the results of the search;

Step 2-1-4) Update the Doppler factor of the frequency domain search with the above-mentioned frequency domain search step size, iteratively search according to the process of step 2-1-2) to step 2-1-3) within the search range, and find The maximum value of each frequency domain search result is taken as the result of this time domain search;

Step 2-1-5) Update the time delay with the above-mentioned time-domain search step size, iteratively search within the search range according to the process from step 2-1-1) to step 2-1-4), and obtain the time-domain search The maximum value of the result, and calculate the Doppler factor and time delay corresponding to this search as the final output, and obtain the Doppler estimation value required for signal demodulation and the time domain synchronization point of the OFDM symbol.

The advantages of the present invention are:

1, the method of the present invention uses CPM sequence as frequency domain spread spectrum code, and obtains higher communication rate in conjunction with orthogonal multi-carrier technology, superimposed QPSK modulation on M element spread spectrum has improved frequency band utilization rate further; After removing CP protection The system performance will not decrease significantly, and the communication efficiency is further improved without affecting the reliability; in addition, the CPM sequence makes full use of the excellent correlation characteristics of the chaotic sequence, overcomes the shortcomings of the poor cross-correlation of the conventional PN sequence, and has the ability to generate Simple, huge number of features;

2. The method of the present invention combines frequency-domain M-element spread spectrum and orthogonal multi-carrier technology, and introduces a CPM sequence to propose a new orthogonal multi-carrier M-element chaotic phase-modulation spread-spectrum underwater acoustic communication mode, which ensures communication error codes While improving the performance, the communication rate is greatly improved, and it can be applied to confidential underwater acoustic communication.

Description of drawings

Fig. 1 is the flowchart of the orthogonal multi-carrier M-element chaos phase modulation spread spectrum underwater acoustic communication method of the present invention;

Fig. 2 is a schematic diagram of realizing time-frequency synchronization of orthogonal multi-carrier M-element chaotic phase-modulation spread-spectrum underwater acoustic communication according to the present invention.

Detailed ways

A kind of orthogonal multi-carrier M element chaotic phase modulation spread spectrum underwater acoustic communication method (M-ary Chaotic Phase Modulation Orthogonal Multi-carrier SpreadSpectrum, M-ary CPM-OMCSS) provided by the present invention is described below in conjunction with specific drawings and embodiments Further explanation.

The flow chart of the orthogonal multi-carrier M-element chaotic phase-modulation spread-spectrum underwater acoustic communication method provided by the present invention is shown in Figure 1, and is specifically described as follows:

Step 1: Use Quadratic mapping to obtain the original chaotic code, and the mapping equation is expressed as:

y(m+1)=P-Qy ² (m) (1)

Among them, when 3/4<PQ<2, the original chaotic code y(m)∈(-2/Q,2/Q), Q=4, P=1/4, y(0)∈(-0.5, 0.5), y(m)∈(-0.5,0.5), m is a positive integer;

Modulate the y(m) without binary quantization on the complex exponent to generate the chaotic phase modulation spreading code c, c contains N chips, where the qth chip is:

c(q)=exp{j2πy(q)},q=0,1...N-1 (2)

Select M mutually orthogonal chaotic phase modulation spreading codes to form an M-element chaotic phase modulation spreading code sequence set {c ₁ ,c ₂ ,...,c _M };

Step 2: Use the original chaotic code y(m) to generate a fixed chaotic phase-modulation spreading code with a length of D, and use the spreading code as a pilot sequence.

Step 3: The communication transmitter performs serial-to-parallel conversion of the input information sequence according to each a+2bit group, and obtains K groups of information sequences with a length of a+2, and sends them to K groupers respectively, as shown in Figure 1 shown. The size of K is determined by the length N of the chaotic phase modulation spread spectrum code and the number L of subcarriers of a multi-carrier M-element chaotic phase modulation spread spectrum symbol S, and the relationship satisfies:

K×N+D≤L.

Each grouper performs segmentation processing on the input (a+2)bit information, performs M-element mapping on the first a bit, and performs QPSK mapping on the remaining 2-bit information; then the kth grouper obtains the M-element mapped For the M-ary data x _k , select the x _kth spreading code in the M spreading sequences in the set of chaotic phase modulation spreading sequences as output, and obtain the corresponding spreading code There are M kinds of selection methods for each spreading code, so the corresponding value of a is:

In the formula Represents a round down operation.

After QPSK mapping, d(k) is obtained, and the spreading code is used The baseband signal b(n) corresponding to the kth grouper is obtained after spread spectrum modulation of d(k):

where n=0,1...,KN, q=n-kN, k=1,...K, Spreading code The qth chip of ;

Step 4: Each of the above groupers corresponds to a baseband signal with a length of N, and maps K groups of baseband signals and pilot sequences to L subcarriers of OFDM symbols one by one to obtain a total baseband with a total length of K×N+D Signal, IFFT transformation is performed on the total baseband signal, and the time domain signal S(t) can be obtained as:

Wherein, f _n is the frequency of each subcarrier, T is the time length of an OFDM symbol, T _g is the time length of the cyclic prefix, T'=T+T _g ; thus the complete M-element CPM spread spectrum OFDM signal can be expressed as :

Among them, b(n,i) is the data modulated on the n-th subcarrier in the i-th symbol, g(t) is the pulse waveform of each symbol, defined as:

Step 5: After receiving the signal, the receiving end performs time-frequency synchronization on it, as shown in Figure 2, the time domain synchronization includes:

Step 5-1) Intercept OFDM symbols to be synchronized with a delay of a time-domain search step, perform high-resolution FFT transformation on the time-domain OFDM symbols according to the frequency-domain variable sampling algorithm, and obtain each subcarrier after Doppler interference shifted spectrum;

Step 5-2) Calculate the subcarrier position of the pilot signal with a Doppler factor of a frequency domain search step, and extract the above-mentioned frequency spectrum according to the obtained position to obtain the baseband signal corresponding to the pilot signal;

Step 5-3) Perform frequency-domain spread-spectrum despreading on the above-mentioned baseband signal and the chaotic phase-modulation spreading code used for time-frequency synchronization locally generated by the receiving end, and obtain the energy value of the despreading result as the frequency-domain search. result;

Step 5-4) Update the Doppler factor of the frequency domain search with the above-mentioned frequency domain search step size, iteratively search according to the process of step 5-2) to step 5-3) within the search range, and obtain the frequency domain search The maximum value of the result is taken as the result of this time domain search;

Step 5-5) update the time delay with the above-mentioned time-domain search step size, iteratively search according to the process of step 5-1) to step 5-4) within the search range, find the maximum value of each time-domain search result, and Calculate the Doppler factor and time delay corresponding to this search as the final output, and obtain the Doppler estimation value required for signal demodulation and the time domain synchronization point of the OFDM symbol.

Perform Fourier transform on the received signal within the time period [iT′,iT′+T] of the synchronized signal, and the signal on the nth subcarrier in the output i symbol is obtained from formula (7):

Among them, r(t) is the received signal after synchronization, is the phase deviation caused by synchronization error.

Step 6: performing M-ary correlation despreading in the frequency domain on the spread spectrum signals distributed on different subcarriers;

Assuming that the position of the k-th spread spectrum signal to be despread in the i-th OFDM symbol on the subcarrier is γ=[0 1... N-1], then the process of related despreading is:

Among them, r _ni represents the baseband signal after subcarrier extraction of the i-th OFDM symbol according to the position of the k-th spread-spectrum signal in the sub-carrier, c _p represents the p-th spreading code in the M-ary CPM sequence set, Indicates the conjugate operation, is the result obtained after despreading r _ni and c _p ; according to The estimated value of the M-ary data x _k can be obtained:

For M base Perform inverse mapping to obtain corresponding a bit information;

Step seven: through the frequency domain despreading of step six, the SINR of the baseband signal is greatly improved, which is beneficial to the recovery of the QPSK modulation information d(k),

right After performing QPSK inverse mapping, the last 2 bits of information can be obtained through demodulation.

Step 8: Input the a-bit and 2-bit information obtained in Step 6 and Step 7 into the kth combiner for combination to obtain the kth group of information sequences.

Step 9: Repeat step 8 to demodulate to obtain all K groups of information sequences, and perform parallel-to-serial conversion on these K groups of information sequences to obtain the final demodulated information sequences.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than limit them. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent replacements to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all of them should be included in the scope of the present invention. within the scope of the claims.

Claims (7)

1. an orthogonal multi-carrier M element chaos phase modulation spread spectrum underwater acoustic communication method, said method comprising: Step 1) The transmitting end converts the input information sequence sent by the source into serial-to-parallel conversion according to each (a+2) bit group, and obtains K groups of information sequences with a length of a+2, and sends them into K groups respectively In the device, each grouper performs M-element chaotic phase modulation spread spectrum mapping on the first a bit respectively to obtain a chaotic phase modulation spread spectrum code, performs QPSK modulation on the last two bits, and then uses the obtained chaotic phase modulation spread spectrum code to perform Spread spectrum modulation to obtain baseband signals, and then map K groups of baseband signals to subcarriers of OFDM symbols for OFDM modulation to obtain digital signals to be transmitted, and finally convert digital signals to be transmitted into underwater acoustic signals and transmit them; Step 2) The receiving end converts the received underwater acoustic signal into a digital signal, adopts a section of chaotic phase-modulation spread spectrum signal in the OFDM symbol as the pilot signal, and uses the spreading gain of the chaotic phase-modulation spread spectrum code to time-shift the digital signal. Frequency two-dimensional search completes time-frequency synchronization; then performs OFDM demodulation on the signals after time-frequency synchronization to obtain K groups of baseband signals, which are respectively input into K combiners to complete despreading and QPSK demodulation to obtain binary sequences. 2. the orthogonal multi-carrier M element chaos phase modulation spread spectrum underwater acoustic communication method according to claim 1, is characterized in that, described step 1) specifically comprises: Step 1-1) Use Quadratic mapping to obtain the original chaotic sequence, thereby generating M spreading codes for modulating the information sequence and a pilot sequence for time-frequency synchronization, and the spreading lengths are N and D respectively; the generated The M spread spectrum codes form a chaotic phase modulation spread spectrum sequence set {c ₁ ,c ₂ ,…,c _M }; Step 1-2) The transmitting end converts the input information sequence sent by the source into serial-to-parallel conversion according to each group of (a+2) bits, and obtains K groups of information sequences with a length of a+2, and sends them to K in a grouper; Step 1-3) Each grouper performs segmentation processing on the input (a+2) bit information, performs M-ary mapping on the first a bit, and performs QPSK mapping on the remaining 2-bit information; then the kth grouper obtains The M-ary data x _k after M-element mapping, select the x _kth spreading code in the M spreading sequences in the set of chaotic phase modulation spreading sequences as output, and get the corresponding spreading code After QPSK mapping, d(k) is obtained, and the spreading code is used The baseband signal b(n) corresponding to the kth grouper is obtained after spread spectrum modulation of d(k): where n=0,1...,KN, q=n-kN, k=1,...K, Spreading code The qth chip of ; Steps 1-4) Each grouper outputs a baseband signal with a length of N, maps K groups of baseband signals and pilot sequences to L subcarriers of OFDM symbols one by one, obtains the total baseband signal, and performs IFFT on the total baseband signal Transform, the time domain signal S(t) is obtained as: Wherein, f _n is the frequency of each subcarrier, T is the time length of an OFDM symbol, T _g is the time length of the cyclic prefix, T'=T+T _g ; the complete M element CPM spread spectrum OFDM signal is expressed as: Among them, b(n,i) is the data modulated on the nth subcarrier in the i-th symbol, and g(t) is the pulse waveform of each symbol, expressed as: 3. the orthogonal multi-carrier M element chaos phase modulation spread spectrum underwater acoustic communication method according to claim 2, is characterized in that, described step 1-1) specifically comprises: Step 1-1-1) Utilize Quadratic mapping to obtain the original chaotic code, and the mapping equation is expressed as: y(m+1)=P-Qy ² (m) Among them, when 3/4<PQ<2, the original chaotic code y(m)∈(-2/Q,2/Q), Q=4, P=1/4, y(0)∈(-0.5, 0.5), y(m)∈(-0.5,0.5), m is a positive integer; Step 1-1-2) Modulate the y(m) without binary quantization on the complex exponent to generate the chaotic phase modulation spread spectrum code c, c contains N chips, wherein the qth chip is: c(q)=exp{j2πy(q)},q=0,1...N-1 Step 1-1-3) Select M mutually orthogonal chaotic phase modulation spreading codes to form an M-element chaotic phase modulation spreading code sequence set {c ₁ ,c ₂ ,...,c _M }; Step 1-1-4) Using the original chaotic sequence y(m) to generate a fixed chaotic phase-modulation spreading code, the spreading code length is D, and the spreading code is used as a pilot sequence. 4. the orthogonal multi-carrier M element chaos phase modulation spread spectrum underwater acoustic communication method according to claim 2, is characterized in that, described step 1-2) in a is: 5. the orthogonal multi-carrier M element chaos phase modulation spread spectrum underwater acoustic communication method according to claim 2, is characterized in that, the size of K in described step 1-2) is by the length of chaos phase modulation spread spectrum code N is determined by the number L of subcarriers of a multi-carrier M-element chaotic phase modulation spread spectrum symbol S, and the relationship satisfies: K×N+D≤L. 6. the orthogonal multi-carrier M element chaos phase modulation spread spectrum underwater acoustic communication method according to claim 2, is characterized in that, described step 2) specifically comprises: Step 2-1) The receiving end performs time-frequency synchronization on the received OFDM signal; Step 2-2) Perform Fourier transform on the synchronized signal r(t) within the time period [iT′,iT′+T], and obtain the nth subcarrier in the output i symbol by formula (7) The signal on is: in, is the phase deviation caused by synchronization error; Step 2-3) performing M-element correlation despreading in the frequency domain to spread spectrum signals distributed on different subcarriers; Assuming that the position of the k-th spread spectrum signal to be despread in the i-th OFDM symbol on the subcarrier is γ=[0 1 ... N-1], then the process of related despreading is: Among them, r _ni represents the baseband signal after subcarrier extraction of the i-th OFDM symbol according to the position of the k-th spread spectrum signal in the sub-carrier, and c _p represents the p-th spread spectrum in the set of M-ary chaotic phase-modulated spread-spectrum sequences code, Indicates the conjugate operation, is the result obtained after despreading r _ni and c _p ; according to The estimated value of the M-ary data x _k can be obtained: For M base Perform inverse mapping to obtain corresponding a bit information; Step 2-4) order right Perform QPSK inverse mapping to demodulate to obtain the last 2 bits of information; Step 2-5) input the a bit and 2 bit information obtained in step 2-3) and step 2-4) into the kth combiner to combine to obtain the kth group information sequence; Step 2-6) Repeat step 2-5) to demodulate to obtain all K groups of information sequences, and perform parallel-to-serial conversion on these K groups of information sequences to obtain the final demodulated information sequence. 7. the orthogonal multi-carrier M element chaos phase modulation spread spectrum underwater acoustic communication method according to claim 6, is characterized in that, described step 2-1) specifically comprises: Step 2-1-1) Intercept OFDM symbols to be synchronized with a time delay of a time domain search step, perform high-resolution FFT transformation on the time domain OFDM symbols according to the frequency domain variable sampling algorithm, and obtain the Doppler effect of each subcarrier Spectrum shifted after interference; Step 2-1-2) Calculate the subcarrier position of the pilot signal with a Doppler factor of a frequency domain search step, and extract the above-mentioned frequency spectrum according to the obtained position to obtain the baseband signal corresponding to the pilot signal; Step 2-1-3) Perform frequency-domain spread-spectrum despreading on the above-mentioned baseband signal and the chaotic phase modulation spreading code used for time-frequency synchronization locally generated by the receiving end, and obtain the energy value of the despreading result as this frequency domain the results of the search; Step 2-1-4) update the Doppler factor of the frequency domain search with the above-mentioned frequency domain search step size, iteratively search according to the process of step 2-1-2) to step 2-1-3) within the search range, and find The maximum value of each frequency domain search result is taken as the result of this time domain search; Step 2-1-5) Update the time delay with the above-mentioned time-domain search step size, iteratively search within the search range according to the process from step 2-1-1) to step 2-1-4), and obtain the time-domain search The maximum value of the result, and calculate the Doppler factor and time delay corresponding to this search as the final output, and obtain the Doppler estimation value required for signal demodulation and the time domain synchronization point of the OFDM symbol.