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

Abstract

The invention provides an underwater acoustic communication dual-mode demodulation method and system based on minimum frequency shift keying, and the method comprises the steps: receiving an underwater acoustic signal, carrying out the preprocessing, and carrying out the channel estimation of a training sequence in the underwater acoustic signal; calculating a phase change index according to a channel estimation result; when the phase change index is higher than a set bit error rate threshold value, processing the received signal in an incoherent demodulation mode; and when the phase change index is lower than a set bit error rate threshold value, processing the received signal in a coherent demodulation mode. The method has the advantages that the problem of insufficient applicability of a single demodulation mode in different underwater acoustic communication environments is solved; it is ensured that the system maintains good communication performance under different channel conditions; the problem of information lag can be avoided, the limitation of a traditional single demodulation mode is broken through, and the reliability and robustness of the system in a variable underwater acoustic channel are remarkably improved.

Description

Technical Field

[0001] The present application belongs to the technical field of underwater acoustic communication, and specifically relates to a dual-mode demodulation method and system for underwater acoustic communication based on minimum frequency shift keying. Background Art

[0002] The underwater acoustic channel is a channel with complex changes in time, space and frequency. Its main characteristics include absorption attenuation, multipath effect, Doppler frequency shift, time-varying and environmental noise. The dramatic fluctuations in time, space and frequency of underwater acoustic communication bring great difficulties and challenges to underwater acoustic communication. In underwater acoustic communication, modulation and demodulation are the key links to realize underwater information transmission, and reducing the bit error rate by optimizing the modulation and demodulation process is an effective way to improve communication reliability.

[0003] Traditional underwater acoustic communication systems usually use a single mode for demodulation, mainly including coherent demodulation and incoherent demodulation. Coherent demodulation performs well in the case of obvious multipath effects, especially after the introduction of an equalizer with a phase-locked loop (PLL), it can effectively resist strong phase changes. Because coherent demodulation relies on a reference signal that is phase-synchronized with the transmitter, the receiver needs to accurately know the phase and frequency of the transmitted signal. In an environment with drastic phase changes, the phase-locked loop may find it difficult to keep up with the rapid change of phase, resulting in a decrease in the performance of coherent demodulation. Incoherent demodulation does not rely on signal phase information, but demodulates by detecting the amplitude or envelope of the signal. Therefore, in the case of drastic phase changes, incoherent demodulation has more advantages. However, in the case of low signal-to-noise ratio or large channel delay spread, the performance of incoherent demodulation will be significantly reduced. Therefore, the demodulation method at the receiving end has an important impact on the communication performance. The existing single-mode demodulation methods have their own limitations in complex underwater acoustic communication environments, and there is no adaptive selection system model combining coherent and incoherent demodulation.

[0004] In the field of wireless communications, a variety of feedback-based adaptive modulation methods have been developed. These methods can dynamically adjust the modulation mode according to the real-time changes in channel conditions to ensure the effectiveness and stability of communication. However, there are significant limitations in the application of these technologies in underwater acoustic communication environments. Since the propagation speed of the underwater acoustic channel is much lower than that of the wireless channel, channel feedback information usually requires a long transmission time, which leads to the lag of the feedback information, making it difficult for the feedback-based adaptive modulation method to adapt to the rapidly changing underwater acoustic channel environment in real time. Therefore, the traditional adaptive modulation method that relies on the feedback mechanism is not effective in underwater acoustic communication and often cannot cope with drastic channel fluctuations and variable environmental noise. Therefore, in the field of underwater acoustic communication, it is urgent to develop an adaptive communication system that does not rely on the feedback mechanism and can dynamically adapt to channel changes, so as to better cope with the challenges in the underwater acoustic environment and achieve more efficient and reliable communication. Summary of the invention

[0005] The purpose of this application is to overcome the defect that the existing single demodulation mode is difficult to ensure stability and reliability at the same time under complex and changeable underwater acoustic channel conditions.

[0006] In order to achieve the above objectives, the present application proposes a dual-mode demodulation method for underwater acoustic communication based on minimum frequency shift keying, comprising:

[0007] Step 1: Receive the underwater acoustic signal, perform preprocessing, and perform channel estimation on the training sequence in the underwater acoustic signal;

[0008] Step 2: Calculate the phase change index based on the channel estimation result;

[0009] 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.

[0010] As an improvement of the above method, at the underwater acoustic signal transmitting end, the MSK transmitting signal consists of a training sequence and an information sequence.

[0011] As an improvement of the above method, the preprocessing includes:

[0012] Perform bandpass filtering on the received underwater acoustic signal to suppress environmental noise and out-of-band interference;

[0013] The frequency offset caused by underwater acoustic propagation is dynamically corrected through the Doppler shift compensation algorithm.

[0014] As an improvement of the above method, the calculating of the phase change index includes:

[0015] The calculation formula of phase change index DI is:

[0016]

[0017] Among them, α represents the adjustable factor; represents the equivalent average relative velocity: N represents the length of the equivalent average relative velocity sequence;

[0018]

[0019] Among them, ν r (n) represents the equivalent relative speed at the nth moment:

[0020]

[0021] Among them, f c represents the carrier center frequency; v represents the speed of sound; represents the equivalent Doppler frequency shift, Δθ represents the change in phase, and fs Indicates the sampling frequency;

[0022] Δν r (n) represents the rate of change of the equivalent relative velocity:

[0023] Δν r (n) = v r (n)-ν r (n-1)

[0024] Express the average fluctuation of the equivalent relative velocity:

[0025]

[0026] As an improvement of the above method, the coherent demodulation mode processes the received signal, including:

[0027] Step 3a-1: performing roll-off filtering on the received signal;

[0028] Step 3a-2: Compare the filtered signal with the equivalent carrier In nT b ≤t≤(n+2)T b time, where n is the serial number of the transmitted symbol, T b is the inverse of the symbol rate, and the equivalent baseband signal of MSK is obtained after correlation;

[0029] 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;

[0030] 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.

[0031] As an improvement of the above method, the non-coherent demodulation mode processes the received signal, including:

[0032] Step 3b-1: Complete envelope detection of the preprocessed signal through four basis function correlators f1(t), f2(t), f3(t), and f4(t); the four basis function correlators are:

[0033] f1(t)=cos(2πf1t)

[0034] f2(t)=sin(2πf1t)

[0035] f3(t)=cos(2πf2t)

[0036] f4(t)=sin(2πf2t)

[0037] Among them, f1 and f2 are two equivalent frequency points of MSK, f1 = f c -1 / 4T b , f2=f c +1 / 4T b , T b is the inverse of the symbol rate; f c represents the carrier center frequency; t represents time;

[0038] At the end of each signal interval, the outputs of the four correlators are sampled to obtain sample values ​​z1, z2, z3, and z4, and the sample values ​​are sent to the detector;

[0039] Step 3b-2: The detector makes a detection decision based on the size of the envelope; the envelope is

[0040] The present application also provides a dual-mode demodulation system for underwater acoustic communication based on minimum frequency shift keying, which is implemented based on the above method, and the system includes:

[0041] 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;

[0042] A phase change index calculation module is used to calculate the phase change index according to the channel estimation result;

[0043] 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; 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.

[0044] Compared with the prior art, the advantages of this application are:

[0045] 1) An innovative dual-mode demodulation scheme was proposed: combining the characteristics of continuous phase modulation, a coherent and incoherent dual-mode demodulation method was introduced at the receiving end for the first time, effectively combining the advantages of both, and solving the problem of insufficient applicability of a single demodulation mode in different underwater acoustic communication environments;

[0046] 2) Adaptive demodulation based on phase change index: By proposing a phase change index, intelligent demodulation mode selection is achieved, such as Figure 4 As shown, it ensures that the system maintains good communication performance under different channel conditions;

[0047] 3) Dual-mode adaptive demodulation improves system reliability: By implementing dual-mode adaptive demodulation at the receiving end, the problem of information lag is avoided, breaking through the limitations of the traditional single demodulation mode, and significantly improving the reliability and robustness of the system in the variable underwater acoustic channel. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 Shown is a flow chart for selecting a demodulation method;

[0049] FIG2( a ) is a schematic diagram of a coherent demodulation method;

[0050] FIG2( b ) is a schematic diagram of a non-coherent demodulation method;

[0051] FIG3(a) shows a reference diagram of coherent demodulation bit error rate and phase change index;

[0052] FIG3( b ) shows a reference diagram of the incoherent demodulation bit error rate and phase change index;

[0053] Figure 4 Shown is a comparison chart of bit error rate-signal-to-noise ratio simulation performance;

[0054] Figure 5 Shown is a static channel impulse response diagram;

[0055] Figure 6(a) shows the variation of relative speed in the range of -0.05 to 0.15 knots;

[0056] Figure 6(b) shows the variation of relative speed in the range of -0.25 to 0.1 knots. DETAILED DESCRIPTION

[0057] The technical solution of the present application is described in detail below with reference to the accompanying drawings.

[0058] The present invention proposes a dual-mode demodulation method and system for underwater acoustic communication based on minimum frequency shift keying. The method dynamically evaluates the channel state and adopts an adaptive switching mechanism to cope with various channel environments and improve the reliability of communication.

[0059] In this invention, we introduce minimum shift keying (MSK) as the core modulation technology to achieve dual-mode demodulation, so as to meet the demand for reliable communication in complex underwater acoustic environments. MSK signals have the characteristics of both phase modulation signals and frequency modulation signals, so that both coherent and incoherent demodulation modes can be supported at the receiving end.

[0060] The purpose of the present invention is achieved through the following technical solutions, the overall structure of which is as follows: Figure 1As shown. The present invention innovatively combines MSK modulation with coherent and incoherent demodulation modes through a dual-mode adaptive demodulation method, and realizes dynamic evaluation of channel phase changes and adaptive demodulation selection for complex underwater acoustic channels. By introducing a phase change measurement index and using a high-order statistical analysis method, the phase jitter is modeled and quantified to comprehensively characterize the degree of phase disturbance of the channel. Based on the phase change index, a threshold decision system based on the expected bit error rate (BER) is designed at the receiving end. The system establishes the optimal threshold value of the phase change measurement value through a nonlinear mapping model between the phase change index and the BER, and realizes adaptive adjustment in the demodulation mode selection, so that the system can automatically switch to the optimal demodulation mode in different channel environments.

[0061] The dual-mode demodulation method for underwater acoustic communication based on minimum frequency shift keying includes:

[0062] 1. Signal reception and preprocessing

[0063] At the transmitting end, the MSK transmission signal x(n) consists of the training sequence x p (n) and the 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:

[0064] y(n)=x(n)*h(n)+w(n) (1)

[0065] Where * represents convolution, x(n), y(n) and w(n) represent the transmitted signal, received signal and additive white Gaussian noise respectively, and h(n) represents the impulse response of the underwater acoustic channel.

[0066] For the received MSK signal, a multi-stage preprocessing operation is performed. First, the signal is bandpass filtered to suppress environmental noise and out-of-band interference to ensure the spectral purity of the signal; in addition, the Doppler frequency shift compensation algorithm is used to dynamically correct the frequency offset caused by underwater acoustic propagation, thereby enhancing the time consistency of the signal. Finally, the preprocessed training sequence y p (n) Perform channel estimation. Common channel estimation methods such as the least mean square algorithm (LMS) and the recursive least squares algorithm (RLS) can be used.

[0067] (II) Calculation of phase change index

[0068] For the obtained channel estimation result, the phase of the main arrival path is taken and the phase is expressed as θ. An effective phase change index calculation method is proposed for the phase change to effectively characterize the phase disturbance of the channel. First, the equivalent Doppler shift is defined as the derivative of the phase with time:

[0069]

[0070] Where Δθ is the phase change, f dopp is the equivalent Doppler frequency shift. Considering the influence of the equivalent Doppler frequency shift, the equivalent relative speed between the transmitter and receiver can be expressed as:

[0071]

[0072] Among them, f c is the carrier center frequency, v is the speed of sound, generally 1500m / s. Therefore, the calculation formula for the equivalent average relative speed is:

[0073]

[0074] The change rate of the equivalent relative speed is further defined as the difference between the equivalent relative speed at a certain moment and the equivalent relative speed at the previous moment. The formula is:

[0075] Δν r (n) = v r (n)-ν r (n-1) (5)

[0076] According to the calculation formula of volatility, the average fluctuation of equivalent relative speed is expressed as:

[0077]

[0078] The metric to measure phase change is defined as:

[0079]

[0080] Among them, 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: the mean of the equivalent relative velocity after Doppler compensation and the standard deviation of the equivalent relative velocity fluctuation. The first is the mean of the equivalent relative velocity after Doppler compensation, which represents the average offset of the equivalent Doppler shift and reflects the numerical value of the equivalent Doppler; the second is the standard deviation of the equivalent relative velocity fluctuation, which characterizes the speed of phase change. In practical applications, the value of α can be 0.1.

[0081] (III) Demodulation method selection

[0082] Step 1) Select several historical channels of the experimental sea area, obtain the correlation data between the coherent demodulation bit error rate and the phase change index under the experimental sea area conditions, and draw the corresponding scatter plots as shown in Figure 3 (a) and Figure 3 (b). Under the condition of a fixed signal-to-noise ratio, the coherent demodulation bit error rate and the phase change index show a highly positive correlation, with a correlation coefficient of more than 0.9, while the incoherent demodulation bit error rate and the phase change index are almost uncorrelated, with a correlation coefficient of less than 0.1.

[0083] Step 2) By setting a specific bit error rate threshold, the phase change index threshold values ​​corresponding to different signal-to-noise ratio conditions are extracted from Figure 3 (a) and Figure 3 (b). In the actual receiving signal processing process, the system first uses the channel estimation result obtained by the training sequence to obtain the phase change of the main arrival path. The phase change index value under the current environment is calculated by the method in step "(ii) Phase change index calculation".

[0084] Step 3) compare the phase change index value with the threshold value corresponding to the preset expected bit error rate: when the phase change index exceeds the threshold value, the system will automatically switch to the incoherent demodulation mode to reduce the demodulation error caused by phase fluctuation; conversely, when the phase change index is lower than the threshold value, the coherent demodulation mode is selected to fully utilize the performance advantages of coherent demodulation under low phase disturbance conditions.

[0085] This demodulation mode selection strategy based on phase change indicators ensures that the system can adaptively select the optimal demodulation scheme in a complex ocean channel environment, thereby maximizing the overall performance and stability of the communication system.

[0086] (IV) Demodulation

[0087] When the coherent demodulation mode is selected, the following steps are performed on the received signal (as shown in Figure 2(a)):

[0088] Step 1) Matched filtering: Since the signal waveform at the transmitting end is subjected to roll-off filtering, the signal at the receiving end also needs to be matched filtered accordingly. The received signal is processed by a matched filter to maximize the signal-to-noise ratio and obtain the filtered signal r(t);

[0089] Step 2) Correlation demodulation: The filtered signal r(t) is compared with the equivalent carrier In nT b ≤t≤(n+2)T b time, where n is the serial number of the transmitted symbol, T b is the inverse of the symbol rate. After correlation, the equivalent baseband signal r of MSK is obtained. n ;

[0090] Step 3) Equalization: Compensate for channel fading and multipath effects, and perform time-domain decision feedback equalization based on a phase-locked loop on the equivalent baseband signal to obtain the equalized signal

[0091] Step 4) Detection: The output of the equalizer and Multiply to get like and If they have the same sign, the judgment is 1, otherwise it is 0.

[0092] When the non-coherent demodulation mode is selected, the received signal performs the following steps (as shown in Figure 2(b)):

[0093] Step 1) Envelope detection: Envelope detection of the filtered signal r(t) can be completed by four basis function correlators f1(t), f2(t), f3(t), and f4(t). The correlator is written as:

[0094]

[0095] Among them, f1 and f2 are two equivalent frequency points of MSK, f1 = f c -1 / 4T b , f2=f c +1 / 4T b ; t represents time.

[0096] At the end of each signal interval, the outputs of the four correlators are sampled to obtain sample values ​​z1, z2, z3, and z4, which are sent to the detector.

[0097] Step 2) Detection: Define the envelope as The detector makes a detection decision based on the size of the envelope.

[0098] The present invention provides an effective criterion through the phase change index for adaptively selecting coherent or incoherent demodulation mode under different channel conditions. Through real-time channel evaluation, the demodulation mode can be dynamically optimized without relying on verification information, thereby significantly improving the transmission reliability of the underwater acoustic communication system. Compared with the traditional single demodulation method, the technical solution of the present invention significantly enhances the robustness and communication performance of the system.

[0099] like Figure 1 As shown, Embodiment 1 of the present invention proposes a dual-mode demodulation method for a receiving end based on MSK. This embodiment uses an MSK underwater acoustic communication system as the application background, and the effectiveness of the present invention is demonstrated through simulation verification. It includes the following steps:

[0100] Step 1: Select a typical shallow sea static channel as the simulation channel. Figure 5 As shown, the underwater acoustic MSK communication process is simulated using the obtained channel. Specifically, the simulation parameters are: the transmission signal uses the minimum shift keying (MSK) signal, the symbol rate is set to 10bps, the center frequency of the carrier is 450Hz, the total number of transmitted symbols is 500, the duration of the MSK signal is 50 seconds, the training sequence length is 10 seconds, and the total transmission time of the entire signal is 60 seconds.

[0101] Step 2, add phase change in the simulation. Since the derivative of the phase is proportional to the equivalent relative velocity, the phase change is added in the form of equivalent Doppler.

[0102] Specifically, the equivalent relative speed is decomposed into the equivalent average speed and the increment relative to the average speed. Based on the continuity of speed, the instantaneous speed at any moment can be expressed as:

[0103]

[0104] Among them, ν(n) represents the speed at the current moment, is the average velocity, Δv max is the maximum fluctuation amplitude of the relative average speed, and μ is a random number ranging from -0.5 to 0.5. The average relative speed ranges from 0 to 0.15 m / s, and the absolute value of the relative amplitude ranges from 0 to 3 m / s. In the simulation, the bit signal-to-noise ratio is set to 10 dB, and the relationship between the bit signal-to-noise ratio and the signal-to-noise ratio is:

[0105]

[0106] in, is the bit signal-to-noise ratio, R b is the symbol rate, R s is the symbol rate, f s is the sampling frequency.

[0107] Step 3, calculate the value of the phase change index using the equivalent relative speed obtained by simulation in step 2. Two typical relative speed changes are shown in Figure 6(a) and Figure 6(b). In Figure 6(a), the relative speed range is -0.05 to 0.15 knots, the fluctuation is weak, and the value of the phase change index is 0.0132. In Figure 6(b), the relative speed range is -0.25 to 0.1 knots, the fluctuation is strong, and the value of the phase change index is 0.1788. According to the implementation process of the dual-mode adaptive demodulation system at the receiving end, assuming that the expected demodulation bit error rate is less than 0.01, when the bit signal-to-noise ratio is 10dB, the phase change index threshold is 0.05. The phase change index value of Figure 6(a) is less than the threshold, and coherent demodulation is selected; the phase change index value of Figure 6(b) is greater than the threshold, and incoherent demodulation is selected.

[0108] Step 4, coherent and incoherent demodulation are performed on the MSK signals with the two equivalent relative speeds shown in Figure 6(a) and Figure 6(b). When the bit signal-to-noise ratio is 10dB, after adding the equivalent relative speed of Figure 6(a), the coherent demodulation bit error rate is 0.0040, and the incoherent demodulation bit error rate is 0.0200; after adding the equivalent relative speed of Figure 6(a), the coherent demodulation bit error rate is 0.4500, and the incoherent demodulation bit error rate is 0.0200. The demodulation results also verify from the side that coherent demodulation is greatly affected by phase changes, while incoherent demodulation is almost unaffected by phase changes. From the demodulation results, it can be seen that the proposed dual-mode demodulation method for underwater acoustic communication receiver based on MSK can achieve better communication performance in different underwater acoustic communication environments.

[0109] The present application also provides a dual-mode demodulation system for underwater acoustic communication based on minimum frequency shift keying, which is implemented based on the above method, and the system includes:

[0110] 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;

[0111] A phase change index calculation module is used to calculate the phase change index according to the channel estimation result;

[0112] 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; 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.

[0113] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present application and are not intended to limit it. Although the present application is described in detail with reference to the embodiments, a person skilled in the art should understand that any modification or equivalent replacement of the technical solution of the present application does not depart from the spirit and scope of the technical solution of the present application and should be included in the scope of the claims of the present application.

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. The dual-mode demodulation method for underwater acoustic communication based on minimum frequency shift keying according to claim 1 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 is characterized in that: The pre-processing 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 is characterized in that: The calculating 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; Among them, ν r (n) represents the equivalent relative speed at the nth moment: Among them, f c represents the carrier center frequency; v represents the speed of sound; represents the equivalent Doppler frequency shift, Δθ represents the change in phase, and f s Indicates the sampling frequency; Δν r (n) represents the rate of change of the equivalent relative velocity: Dn r (n)=n r (n)-n 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, including: Step 3a-1: performing roll-off filtering on the received signal; Step 3a-2: Compare the filtered signal with the equivalent carrier In nT b ≤t≤(n+2)T b time, where n is the serial number of the transmitted symbol, T b It is the inverse of the symbol rate, and the equivalent baseband signal of MSK is obtained 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 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 non-coherent demodulation mode processes the received signal, including: Step 3b-1: Complete envelope detection of the preprocessed signal through four basis function correlators f1(t), f2(t), f3(t), and f4(t); the four basis function correlators are: f1(t)=cos(2πf1t) f2(t)=sin(2πf1t) f3(t)=cos(2πf2t) f4(t)=sin(2πf2t) Among them, f1 and f2 are two equivalent frequency points of MSK, f1 = f c -1 / 4T b , f2=f c +1 / 4T b , T b is the inverse of the symbol rate; f c 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 ​​z1, z2, z3, and z4, 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; 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.

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