CN119210965A

CN119210965A - A dual-mode demodulation method and system for underwater acoustic communication based on minimum frequency shift keying

Info

Publication number: CN119210965A
Application number: CN202411233108.5A
Authority: CN
Inventors: 王海斌; 林亦宁; 台玉朋; 胡承昊; 汪俊; 张永霖
Original assignee: Institute of Acoustics CAS
Current assignee: Institute of Acoustics CAS
Priority date: 2024-09-04
Filing date: 2024-09-04
Publication date: 2024-12-27
Anticipated expiration: 2044-09-04
Also published as: CN119210965B

Abstract

The application provides a method and a system for demodulating underwater acoustic communication in double modes based on minimum frequency shift keying, wherein the method comprises the steps of receiving an underwater acoustic signal, preprocessing the underwater acoustic signal, and carrying out channel estimation on a training sequence in the underwater acoustic signal; the method comprises the steps of calculating a phase change index according to a channel estimation result, processing a received signal by adopting a noncoherent demodulation mode when the phase change index is higher than a set bit error rate threshold value, and processing the received signal by adopting a coherent demodulation mode when the phase change index is lower than the set bit error rate threshold value. The method has the advantages of solving the problem of insufficient applicability of a single demodulation mode in different underwater acoustic communication environments, ensuring that the system maintains good communication performance under different channel conditions, avoiding the problem of information lag, breaking through the limitation of the traditional single demodulation mode and remarkably improving the reliability and robustness of the system in variable underwater acoustic channels.

Description

Underwater acoustic communication dual-mode demodulation method and system based on minimum frequency shift keying

Technical Field

The application belongs to the technical field of underwater acoustic communication, and particularly relates to an underwater acoustic communication dual-mode demodulation method and system based on minimum frequency shift keying.

Background

An underwater acoustic channel is a channel with complex changes in time, space and frequency, and its main characteristics include absorption attenuation, multi-path effect, doppler shift, time variability, environmental noise, etc. The severe fluctuations in time-frequency in underwater acoustic communications present significant difficulties and challenges to underwater acoustic communications. In underwater acoustic communication, modulation and demodulation are key links for realizing underwater information transmission, and reducing the bit error rate by optimizing a modulation and demodulation process is an effective way for improving communication reliability.

Conventional underwater acoustic communication systems typically employ a single mode for demodulation, mainly including both coherent demodulation and incoherent demodulation. Coherent demodulation is excellent in the case of obvious multipath effects, and particularly can effectively resist strong phase changes after an equalizer with a Phase Locked Loop (PLL) is introduced. Just because coherent demodulation depends on a reference signal that is phase-synchronized with a transmitting end, a receiving end needs to know the phase and frequency of a transmitting signal precisely, and in a severe phase change environment, a phase-locked loop may have difficulty keeping up with rapid changes in phase, resulting in degradation of performance of coherent demodulation. Incoherent demodulation does not rely on signal phase information, but rather demodulates by detecting the amplitude or envelope of the signal, and therefore is advantageous when the phase changes are severe. However, at low signal-to-noise ratios or large channel delay spreads, the performance of incoherent demodulation can be significantly reduced. Therefore, the demodulation mode of the receiving end has important influence on the communication performance, and the existing single-mode demodulation method has limitations in complex underwater acoustic communication environments, and has not been provided with an adaptive selection system model combining coherent demodulation and incoherent demodulation.

In the field of wireless communication, various adaptive modulation methods based on feedback have been developed, and these methods can dynamically adjust the modulation mode according to the real-time change of the channel condition, so as to ensure the effectiveness and stability of communication. However, there are significant limitations to the application of these techniques in an underwater acoustic communication environment. Since the propagation speed of the underwater acoustic channel is far lower than that of the wireless channel, the channel feedback information generally needs a long transmission time, which causes hysteresis of the feedback information, so that the feedback-based adaptive modulation method is difficult to adapt to the underwater acoustic channel environment with rapid change in real time. Therefore, the conventional adaptive modulation method relying on the feedback mechanism has poor effect in underwater acoustic communication, and cannot cope with severe channel fluctuation and changeable environmental noise. Therefore, in the field of underwater acoustic communication, there is a need to develop an adaptive communication system that is capable of dynamically adapting to channel variations, independent of feedback mechanisms, to better cope with challenges in the underwater acoustic environment and achieve more efficient and reliable communication.

Disclosure of Invention

The application aims to overcome the defect that the existing single demodulation mode is difficult to ensure stability and reliability simultaneously under complex and changeable underwater sound channel conditions.

In order to achieve the above object, the present application provides a method for demodulating underwater acoustic communication in dual modes based on minimum frequency shift keying, comprising:

step 1, receiving underwater sound signals, preprocessing the underwater sound signals, and carrying out channel estimation on training sequences in the underwater sound signals;

Step2, calculating a phase change index according to the channel estimation result;

And 3, when the phase change index is higher than the set bit error rate threshold, adopting a noncoherent demodulation mode to process the received signal, and when the phase change index is lower than the set bit error rate threshold, adopting a coherent demodulation mode to process the received signal.

As an improvement of the method, at the underwater acoustic signal transmitting end, the MSK transmitting signal is composed of a training sequence and an information sequence.

As an improvement of the above method, the pretreatment includes:

band-pass filtering is carried out on the received underwater sound signals, and environmental noise and out-of-band interference are restrained;

The frequency offset caused by the acoustic propagation is dynamically corrected by the doppler shift compensation algorithm.

As an improvement of the above method, the calculating the phase change index includes:

the phase change index DI has the following formula:

Wherein α represents an adjustable factor; n represents the length of the equivalent average relative speed sequence;

wherein v _r (n) represents the equivalent relative speed at the nth time:

Wherein, f _c represents the carrier center frequency, v represents the sound velocity; Representing the equivalent Doppler shift, Δθ representing the change in phase, and f _s representing the sampling frequency;

Δν _r (n) represents the rate of change of the equivalent relative speed:

Δν_r(n)＝ν_r(n)-ν_r(n-1)

Mean fluctuation representing equivalent relative velocity:

As an improvement of the above method, the coherent demodulation mode processes the received signal, comprising:

Step 3a-1, performing roll-off filtering on the received signal;

step 3a-2, filtering the filtered signal and equivalent carrier Making correlation in nT _b≤t≤(n+2)T_b time, wherein n is the sequence number of a transmitting symbol, T _b is the inverse of the symbol rate, and obtaining an equivalent baseband signal of MSK after correlation;

Step 3a-3, performing time domain decision feedback equalization based on a phase-locked loop on the equivalent baseband signal to obtain an equalized signal;

step 3 a-4. The equalized signal is compared with Multiplication to obtainIf it isAnd (3) withAnd the same number, the judgment is 1, and vice versa is 0.

As an improvement of the above method, the incoherent demodulation mode processes the received signal, comprising:

Step 3b-1, finishing envelope detection of the preprocessed signal through four basis function correlators f ₁(t)、f₂(t)、f₃(t)、f₄ (t), wherein the four basis function correlators are as follows:

f₁(t)＝cos(2πf₁t)

f₂(t)＝sin(2πf₁t)

f₃(t)＝cos(2πf₂t)

f₄(t)＝sin(2πf₂t)

Wherein f ₁ and f ₂ are two equivalent frequency points of MSK, f ₁＝f_c-1/4T_b,f₂＝f_c+1/4T_b,T_b is the inverse of the code element rate, f _c represents the carrier center frequency, t represents time;

Sampling the outputs of the four correlators at the end of each signal interval to obtain a sample z ₁、z₂、z₃、z₄, and feeding the sample to the detector;

Step 3b-2, the detector makes detection decision according to the size of the envelope, wherein the envelope is

The application also provides a underwater acoustic communication dual-mode demodulation system based on the minimum frequency shift keying, which is realized based on the method, and comprises the following steps:

the channel estimation module is used for receiving the underwater sound signal of the underwater sound communication sea area, preprocessing the underwater sound signal and carrying out channel estimation on the training sequence;

The phase change index calculating module is used for calculating a phase change index according to the channel estimation result;

And the demodulation signal module is used for processing the received signal by adopting a non-coherent demodulation mode when the phase change index is higher than the set bit error rate threshold value, and adopting a coherent demodulation mode when the phase change index is lower than the set bit error rate threshold value.

Compared with the prior art, the application has the advantages that:

1) The two-mode demodulation scheme is innovatively provided, namely, a coherent and incoherent two-mode demodulation method is introduced at a receiving end for the first time by combining the characteristic of continuous phase modulation, the advantages of the two methods are effectively combined, and the problem of insufficient applicability of a single demodulation mode in different underwater acoustic communication environments is solved;

2) The adaptive demodulation based on the phase change index is realized by proposing the phase change index, as shown in figure 4, the intelligent demodulation mode selection is realized, and the system is ensured to maintain good communication performance under different channel conditions;

3) The dual-mode self-adaptive demodulation improves the reliability of the system, namely, the problem of information lag is avoided by implementing dual-mode self-adaptive demodulation at the receiving end, the limitation of the traditional single demodulation mode is broken through, and the reliability and the robustness of the system in a variable underwater sound channel are obviously improved.

Drawings

FIG. 1 is a flow chart showing the selection of a demodulation scheme;

FIG. 2 (a) is a schematic diagram of a coherent demodulation method;

FIG. 2 (b) is a schematic diagram of a non-coherent demodulation method;

FIG. 3 (a) is a diagram showing the index reference of the error rate and phase change of coherent demodulation;

FIG. 3 (b) is a diagram showing the reference of the index of incoherent demodulation error rate and phase change;

FIG. 4 is a graph showing the simulation performance of bit error rate versus signal to noise ratio;

FIG. 5 is a graph of static channel impulse response;

FIG. 6 (a) is a graph showing the relative speed in the range of-0.05 to 0.15;

FIG. 6 (b) shows a graph of the relative velocity in the range of-0.25 to 0.1.

Detailed Description

The technical scheme of the application is described in detail below with reference to the accompanying drawings.

The invention provides a method and a system for demodulating underwater acoustic communication in double modes based on minimum frequency shift keying. According to the method, the channel state is dynamically estimated, and the self-adaptive switching mechanism is adopted to cope with various channel environments, so that the reliability of communication is improved.

In the present invention, minimum shift keying (MSK, minimum SHIFT KEYING) is introduced as a core modulation technique to achieve dual mode demodulation to meet the need for reliable communications in complex underwater acoustic environments. MSK signal has phase modulation signal and frequency modulation signal's characteristics concurrently, makes at the receiving end can support two kinds of demodulation modes of coherence and incoherent.

The object of the invention is achieved by the following technical solution, the general architecture of which is shown in fig. 1. The invention creatively combines MSK modulation with coherent demodulation modes and incoherent demodulation modes by a dual-mode self-adaptive demodulation method, and realizes dynamic evaluation and self-adaptive demodulation selection of channel phase change aiming at a complex underwater acoustic channel. By introducing a phase change measurement index, the phase jitter is modeled and quantized by using a high-order statistical analysis method, so that the phase disturbance degree of the channel is comprehensively represented. Based on the phase change index, a threshold decision system based on the expected Bit Error Rate (BER) is designed at the receiving end, and the system establishes an optimal threshold of the phase change metric value through a nonlinear mapping model between the phase change index and the BER, so that the adaptive adjustment on the demodulation mode selection is realized, and the system can be automatically switched to an optimal demodulation mode in different channel environments.

The underwater acoustic communication dual-mode demodulation method based on the minimum frequency shift keying comprises the following steps:

signal reception and preprocessing

At the transmitting end, the MSK transmit signal x (n) is composed of a training sequence x _p (n) and an information sequence x _m (n). After passing through the underwater acoustic channel, the underwater acoustic MSK signal at the receiving end can be expressed as:

y(n)=x(n)*h(n)+w(n) (1)

Where x (n), y (n) and w (n) represent the transmitted signal, the received signal and the additive white gaussian noise, respectively, and h (n) represents the hydroacoustic channel impulse response.

For the received MSK signal, a multi-stage preprocessing operation is performed. The method comprises the steps of firstly, carrying out band-pass filtering on signals to inhibit environmental noise and out-of-band interference, ensuring the frequency spectrum purity of the signals, and in addition, dynamically correcting frequency offset caused by underwater sound propagation through a Doppler frequency shift compensation algorithm, thereby enhancing the time consistency of the signals. Finally, the preprocessed training sequence y _p (n) is used for channel estimation, and common channel estimation methods such as Least Mean Square (LMS) and Recursive Least Squares (RLS) may be used.

(II) phase Change index calculation

And for the obtained channel estimation result, the phase of the main arrival path is taken, the phase is expressed as theta, and an effective phase change index calculation method is provided for the phase change so as to effectively represent the phase disturbance condition of the channel. The equivalent doppler shift is first defined as the derivative of phase over time:

where Δθ is the change in phase and f _dopp is the equivalent Doppler shift. Then the equivalent relative velocity between the transceivers, considering the effect of equivalent doppler shift, can be expressed as:

Where f _c is the carrier center frequency and v is the speed of sound, typically 1500m/s. Thus, the calculation formula of the equivalent average relative velocity is:

further defining the change rate of the equivalent relative speed, namely the difference between the equivalent relative speed at a certain moment and the equivalent relative speed at the previous moment, wherein the formula is as follows:

Δν_r(n)＝ν_r(n)-ν_r(n-1) (5)

According to the calculation formula of the fluctuation rate, the average fluctuation of the equivalent relative speed is expressed as:

the index for measuring the phase change is defined as:

Where N is the length of the equivalent average relative velocity sequence and α is an adjustable factor. The phase change index consists of two parts, namely a mean value of the equivalent relative speed after Doppler compensation and a standard deviation of the equivalent relative speed fluctuation. The average value of the equivalent relative speed after Doppler compensation represents the average offset of equivalent Doppler translation, which represents the value of equivalent Doppler, and the standard deviation of equivalent relative speed fluctuation represents the speed of phase change. In practical applications, the value of α may be 0.1.

(III) demodulation method selection

Step 1) selecting a plurality of historical channels of the experimental sea area, acquiring associated data between coherent demodulation error rate and phase change indexes under the condition of the experimental sea area, and drawing corresponding scatter diagrams as shown in fig. 3 (a) and 3 (b). Under the condition of fixed signal-to-noise ratio, the coherent demodulation error rate and the phase change index show a highly positive correlation relationship, the correlation coefficient reaches more than 0.9, and the incoherent demodulation error rate and the phase change index are hardly correlated, and the correlation coefficient is less than 0.1.

Step 2) extracting the corresponding phase change index threshold values under different signal-to-noise ratio conditions from the graph (a) and the graph (b) of fig. 3 by setting a specific error rate threshold value. In the actual received signal processing process, the system first acquires the phase change of the main arrival path by using the channel estimation result obtained by the training sequence. And (3) calculating to obtain the phase change index value under the current environment by the method in the step (II) of calculating the phase change index.

And 3) comparing the phase change index value with a threshold value corresponding to a preset expected error rate, wherein when the phase change index exceeds the threshold value, the system automatically switches to a noncoherent demodulation mode to reduce demodulation errors caused by phase fluctuation, otherwise, when the phase change index is lower than the threshold value, a coherent demodulation mode is selected to fully utilize performance advantages of coherent demodulation under the condition of low phase disturbance.

The demodulation mode selection strategy based on the phase change index ensures that the system can adaptively select the optimal demodulation scheme in a complex ocean channel environment, thereby maximally improving the overall performance and stability of the communication system.

(IV) demodulation

When the coherent demodulation mode is selected, the received signal is subjected to the following steps (as shown in fig. 2 (a):

Step 1) matched filtering, namely, the wave form of the signal at the transmitting end is subjected to roll-off filtering, and the signal at the receiving end is subjected to corresponding matched filtering. Processing the received signal by a matched filter to maximize the signal-to-noise ratio and obtain a filtered signal r (t);

step 2) correlation demodulation of the filtered signal r (t) with an equivalent carrier Making correlation in nT _b≤t≤(n+2)T_b time, wherein n is the sequence number of a transmitting symbol, T _b is the inverse of the symbol rate, and obtaining an equivalent baseband signal r _n of MSK after correlation;

Step 3) equalization, namely compensating channel fading and multipath effect, and performing time domain decision feedback equalization based on phase-locked loop on the equivalent baseband signal to obtain an equalized signal

Step 4) detecting the output of the equalizerAnd (3) withMultiplication to obtainIf it isAnd (3) withAnd the same number, the judgment is 1, and vice versa is 0.

When the incoherent demodulation mode is selected, the received signal is subjected to the following steps (as shown in fig. 2 (b):

Step 1) envelope detection, namely envelope detection of the filtered signal r (t) can be completed through four basis function correlators f ₁(t)、f₂(t)、f₃(t)、f₄ (t). The correlator writes:

Wherein f ₁ and f ₂ are two equivalent frequency points of MSK, f ₁＝f_c-1/4T_b,f₂＝f_c+1/4T_b, and t represents time.

The outputs of the four correlators are sampled at the end of each signal interval to obtain a sample z ₁、z₂、z₃、z₄, which is sent to the detector.

Step 2) detecting, defining the envelope asThe detector makes a detection decision based on the size of the envelope.

The invention provides an effective criterion through the phase change index, and is used for adaptively selecting a coherent demodulation mode or a noncoherent demodulation mode under different channel conditions. Through real-time channel evaluation, the demodulation mode can be dynamically optimized without depending on verification information, so that the transmission reliability of the underwater acoustic communication system is remarkably improved. Compared with the traditional single demodulation method, the technical scheme of the invention obviously enhances the robustness and communication performance of the system.

As shown in fig. 1, embodiment 1 of the present invention proposes a receiving end dual-mode demodulation method based on MSK, and in this embodiment, an MSK underwater acoustic communication system is adopted as an application background, and the effectiveness of the present invention is illustrated through simulation verification. The method comprises the following steps:

Step 1, selecting a typical shallow sea static channel as a simulation channel as shown in fig. 5, and simulating the underwater sound MSK communication process by using the obtained channel. Specifically, the simulation parameters are that a Minimum Shift Keying (MSK) signal is adopted as a transmission signal, the code element rate is set to 10bps, the center frequency of a carrier wave is 450Hz, the total number of transmitted code elements is 500, the duration of the MSK signal is 50 seconds, the length of a training sequence is 10 seconds, and the total transmission duration of the whole signal is 60 seconds.

And 2, adding phase change in the simulation, wherein the phase change is added in the form of equivalent Doppler as the derivative of the phase is in direct proportion to the equivalent relative speed.

Specifically, the equivalent relative velocity is decomposed into an equivalent average velocity and an increment relative to the average velocity. Based on the continuity of the speed, the instantaneous speed at any one instant can be expressed as:

Where v (n) represents the speed at the current time, For average speed, deltav _max is the maximum fluctuation amplitude of the relative average speed and μ is a random number ranging between-0.5 and 0.5. The average relative speed is 0-0.15 m/s, and the absolute value of the relative amplitude is 0-3 m/s. In the simulation, the bit signal to noise ratio was set to 10dB, bit signal-to-noise ratio and signal-to-noise ratio the relationship of the noise ratio is:

Wherein, For bit signal to noise ratio, R _b is symbol rate, R _s is symbol rate, and f _s is sampling frequency.

And 3, calculating the value of the phase change index by using the equivalent relative speed obtained by the simulation in the step 2. Two typical relative velocity changes are shown in fig. 6 (a) and 6 (b). In fig. 6 (a), the relative speed ranges from-0.05 to 0.15 node, the fluctuation is weak, the value of the phase change index is 0.0132, the relative speed ranges from-0.25 to 0.1 node, the fluctuation is strong, and the value of the phase change index is 0.1788. According to the implementation flow of the receiving end double-mode self-adaptive demodulation system, the expected demodulation error rate is assumed to be smaller than 0.01, and when the bit signal-to-noise ratio is 10dB, the phase change index threshold is 0.05. The phase change index value of fig. 6 (a) is smaller than the threshold, and coherent demodulation is selected, and the phase change index value of fig. 6 (b) is larger than the threshold, and noncoherent demodulation is selected.

And 4, performing coherent and incoherent demodulation on the MSK signal added with the two equivalent relative speeds shown in fig. 6 (a) and 6 (b), wherein when the bit signal to noise ratio is 10dB, the bit error rate of coherent demodulation is 0.0040 and the bit error rate of incoherent demodulation is 0.0200 after the equivalent relative speed shown in fig. 6 (a) is added, and the bit error rate of coherent demodulation is 0.4500 and the bit error rate of incoherent demodulation is 0.0200 after the equivalent relative speed shown in fig. 6 (a) is added. The demodulation result also verifies from the side that coherent demodulation is greatly affected by phase change, whereas incoherent demodulation is hardly affected by phase change. As can be seen from the demodulation result, the MSK-based underwater acoustic communication receiving end dual-mode demodulation method can obtain better communication performance in different underwater acoustic communication environments.

Finally, it should be noted that the above embodiments are only for illustrating the technical solution of the present application and are not limiting. Although the present application has been described in detail with reference to the embodiments, it should be understood by those skilled in the art that modifications and equivalents may be made thereto without departing from the spirit and scope of the present application, which is intended to be covered by the appended claims.

Claims

1. A dual-mode demodulation method for underwater acoustic communication based on minimum frequency shift keying, comprising:

Step 1: Receive the underwater acoustic signal, perform preprocessing, and perform channel estimation on the training sequence in the underwater acoustic signal;

Step 2: Calculate the phase change index based on the channel estimation result;

Step 3: When the phase change index is higher than the set bit error rate threshold, the non-coherent demodulation mode is used to process the received signal; when the phase change index is lower than the set bit error rate threshold, the coherent demodulation mode is used to process the received signal.

2. According to the dual-mode demodulation method of underwater acoustic communication based on minimum frequency shift keying according to claim 1, it is characterized in that: at the underwater acoustic signal transmitting end, the MSK transmitting signal consists of a training sequence and an information sequence.

3. The dual-mode demodulation method for underwater acoustic communication based on minimum frequency shift keying according to claim 1, characterized in that the preprocessing comprises:

Perform bandpass filtering on the received underwater acoustic signal to suppress environmental noise and out-of-band interference;

The frequency offset caused by underwater acoustic propagation is dynamically corrected through the Doppler shift compensation algorithm.

4. The dual-mode demodulation method for underwater acoustic communication based on minimum frequency shift keying according to claim 1, characterized in that the calculation of the phase change index comprises:

The calculation formula of phase change index DI is:

Among them, α represents the adjustable factor; represents the equivalent average relative velocity: N represents the length of the equivalent average relative velocity sequence;

Wherein, ν _r (n) represents the equivalent relative velocity at the nth moment:

Where, f _c represents the carrier center frequency; v represents the speed of sound; represents the equivalent Doppler frequency shift, Δθ represents the phase change, and _fs represents the sampling frequency;

Δν _r (n) represents the rate of change of the equivalent relative velocity:

Δν _r (n)＝ν _r (n)-ν _r (n-1)

Express the average fluctuation of the equivalent relative velocity:

5. The dual-mode demodulation method for underwater acoustic communication based on minimum frequency shift keying according to claim 1 is characterized in that the coherent demodulation mode processes the received signal, comprising:

Step 3a-1: performing roll-off filtering on the received signal;

Step 3a-2: Compare the filtered signal with the equivalent carrier Correlation is performed within the time period of nT _b ≤t ≤ (n+2)T _b , where n is the serial number of the transmitted symbol and T _b is the inverse of the symbol rate. After correlation, the equivalent baseband signal of MSK is obtained.

Step 3a-3: performing time-domain decision feedback equalization based on a phase-locked loop on the equivalent baseband signal to obtain an equalized signal;

Step 3a-4: Compare the equalized signal with Multiply to get like and If they have the same sign, the judgment is 1, otherwise it is 0.

6. The dual-mode demodulation method for underwater acoustic communication based on minimum frequency shift keying according to claim 1 is characterized in that the incoherent demodulation mode processes the received signal, comprising:

Step 3b-1: Complete envelope detection of the preprocessed signal through four basis function correlators f ₁ (t), f ₂ (t), f ₃ (t), and f ₄ (t); the four basis function correlators are:

f ₁ (t) = cos(2πf ₁ t)

f ₂ (t) = sin(2πf ₁ t)

f ₃ (t) = cos(2πf ₂ t)

f ₄ (t) = sin(2πf ₂ t)

Wherein, _f1 and _f2 are two equivalent frequency points of MSK, _f1 = _fc -1/ _4Tb , _f2 = _fc +1/ _4Tb , _Tb is the inverse of the symbol rate; _fc represents the carrier center frequency; t represents time;

At the end of each signal interval, the outputs of the four correlators are sampled to obtain sample values z ₁ , z ₂ , z ₃ , z ₄ , and the sample values are sent to the detector;

Step 3b-2: The detector makes a detection decision based on the size of the envelope; the envelope is

7. A dual-mode demodulation system for underwater acoustic communication based on minimum frequency shift keying, implemented based on any method described in claims 1-6, characterized in that the system comprises:

A channel estimation module is used to receive the underwater acoustic signal of the underwater acoustic communication sea area, perform preprocessing, and perform channel estimation on the training sequence;

A phase change index calculation module, used to calculate the phase change index according to the channel estimation result; and

The demodulation signal module is used to process the received signal in a non-coherent demodulation mode when the phase change index is higher than the set bit error rate threshold; and to process the received signal in a coherent demodulation mode when the phase change index is lower than the set bit error rate threshold.