Multi-carrier underwater communication method based on pulse broadening technology

Abstract

The invention discloses a multicarrier underwater communication method based on a pulse broadening technology, which comprises the following steps of: converting information to be transmitted into a binary array with 8 bits as one byte through an ASCII (American Standard Code for Information Interchange) coding unit, changing the binary array into a new array with 10 bits as one byte and with similar occurrence times of 0 and 1 after the binary array passes through an 8 / 10B coding unit, and modulating the new array through multicarrier OOK (On-Off Keying) of a modulation unit, a broadening coefficient k is solved by a Gaussian envelope approximation and parameter solving unit, a modulated signal is broadened according to the broadening coefficient k in a broadening unit, the signal is transmitted in water and received by a receiving end, and transmitted information is restored after demodulation by a demodulation unit, 10B / 8B coding and ASCII decoding. And the obtained signal has a relatively low peak-to-average ratio, so that underwater long-distance transmission is facilitated.

Description

Technical Field

[0001] The invention relates to the field of communications, and in particular to a multi-carrier underwater communication method based on pulse broadening technology. Background Art

[0002] Wireless communication underwater is very difficult. The key reason is that the commonly used communication frequencies are all within the absorption range of water molecules. Water is a polar molecule whose positive and negative charge centers do not overlap, so it can change direction under the action of the electric field in the electromagnetic wave. Once the frequency of the electromagnetic wave is close to the inherent multiple vibration frequencies of the water molecule, the water molecule can absorb most of the energy in the electromagnetic wave and convert it into its own vibration. The commonly used communication electromagnetic wave frequencies are almost all within the absorption frequency range of water molecules, resulting in very fast attenuation of electromagnetic waves in water. In addition, factors such as underwater noise, multipath effects and complex underwater environment will also have an adverse effect on the reliability of underwater wireless communication. Therefore, for the fields of ocean exploration, marine resource development and military reconnaissance, how to solve the problem of underwater communication has always been a difficult problem.

[0003] Acoustic signals are the most suitable signals for underwater transmission. The receiving transducer on the potential is used to convert the signal into sound waves for transmission, and the other party then converts the sound waves into electrical signals using the transducer to complete the communication task. However, even for the acoustic signals that are most suitable for underwater transmission, the attenuation of high-frequency acoustic signals in water is still serious.

[0004] Due to the serious attenuation of high-frequency acoustic signals, the available bandwidth of underwater acoustic communication is extremely narrow. In order to take into account the transmission rate, multi-carrier modulation is often used to make full use of the bandwidth.

[0005] When the power of the underwater acoustic transducer is constant, the peak-to-average ratio is an important indicator that limits the multi-carrier communication distance. When the peak-to-average ratio is too high, the underwater acoustic transducer works in a low-power state for a long time and only reaches high power in the moment before and after the peak appears. In order to ensure that the signal is not distorted, the rated power of the underwater acoustic transducer must be higher than the peak power that appears in a short time, which greatly limits the working state of the underwater acoustic transducer. Summary of the invention

[0006] The purpose of the present invention is to provide a multi-carrier underwater communication method based on pulse broadening technology. The method uses pulse broadening to greatly reduce the signal's average peak-to-peak ratio, and makes the main power of the signal concentrated at the center of the cycle within a single transmission cycle, thereby reducing the impact of underwater multipath transmission.

[0007] In order to achieve the above-mentioned purpose, the technical solution adopted by the present invention is:

[0008] A multi-carrier underwater communication method based on pulse stretching technology comprises a multi-carrier underwater communication system, which comprises an ASCII encoding unit, an 8B / 10B encoding unit, a modulation unit, a Gaussian envelope approximation and parameter solving unit, a stretching unit, a demodulation unit, a 10B / 8B encoding unit, and an ASCII decoding unit group; information to be transmitted is converted into an 8-bit-per-byte binary array by the ASCII encoding unit, and the binary array is converted into a new 10-bit-per-byte array with similar number of occurrences of 0 and 1 after passing through the 8 / 10B encoding unit; the new array is subjected to multi-carrier OOK modulation of the modulation unit, and a stretching coefficient k is solved by the Gaussian envelope approximation and parameter solving unit; the modulated signal is stretched in the stretching unit according to the stretching coefficient k, the signal is transmitted in water and received by a receiving end, and the transmitted information is restored after being demodulated by the demodulation unit and the 10B / 8B encoding unit and the ASCII decoding unit.

[0009] Preferably, a data peak equalization module composed of an ASCII encoding unit, an 8B / 10B encoding unit, and a modulation unit;

[0010] The 8-bit data of the binary array will become a 10-bit array after 8b / 10b encoding. The number of 0s and 1s in the 10-bit data will appear in three situations: 5 0s and 5 1s, 6 0s and 1s, and 4 0s and 6 1s. When the inequality of the previous byte is -2, the inequality of the next byte will be adjusted to +2, thereby ensuring that the number of 0s and 1s is basically equal for a long time.

[0011] Preferably, the signal s(t) of a single transmission cycle after multi-carrier OOK modulation is as shown in formula (1):

[0012]

[0013] Where M is the row matrix composed of 0 and 1 after the data is 8 / 10B encoded, n is the number of carriers, f is the carrier frequency, t is time, and T is the duration of a single transmission cycle;

[0014] When the amplitude and initial phase of each carrier are the same, s(0) in s(t) is the pulse peak value of a single transmission cycle. If 8b / 10b encoding is not performed, the value range of s(0) is 0 to n.

[0015] After 8b / 10b encoding, the value range of s(0) is 2 / n-1 to 2 / n+1.

[0016] Preferably, a peak-to-average ratio optimization module composed of a Gaussian envelope approximation and parameter solving unit and a broadening unit;

[0017] After peak equalization through the ASCII encoding unit, 8B / 10B encoding unit, and modulation unit, the peak-to-average ratio optimization module is used to reduce the peak-to-average ratio of single-cycle transmission. The reduction of the peak-to-average ratio is achieved by adjusting the phase.

[0018] When the Gaussian envelope approximation and the parameter solving unit are adjusted, as shown in equation (2), the signal is dispersed by adding a phase shift term in the frequency domain after Fourier transformation: Here, p(f) is a second-order polynomial with f as the independent variable.

[0019] Preferably, since the signal form of multiple single-frequency signals superimposed is a plurality of impulse functions after Fourier transformation, it is difficult to solve the optimal phase shift parameter after multiplying by the phase shift term. In order to solve the above problem, in the peak-to-average ratio optimization module, the original signal needs to be approximated by Gaussian envelope; since the Gaussian envelope approximation has a higher fitting degree near the peak point of the signal, and the peak-to-average ratio focuses on the peak point, the expression of Gaussian approximation is shown in formula (3), where σ is the Gaussian envelope width, f0 is the signal band center frequency, and after the expression after Gaussian approximation is Fourier transformed and multiplied by the phase shift term, the optimal phase shift parameter and phase shift term expression can be obtained through the expression after inverse Fourier transformation:

[0020]

[0021] Then, the expression after Fourier transform of the Gaussian envelope approximation signal is shown in formula (4):

[0022]

[0023] Then, the expression of the multiplication of the Gaussian envelope and the phase shift term is shown in equation (5);

[0024]

[0025] Since p(f) is a quadratic polynomial, the inverse Fourier transform of equation (5) is expressed as equation (6), where

[0026]

[0027] It can be seen from formula (6) that the expression is still in the form of Gaussian envelope multiplied by oscillation term, and its peak value becomes The axis of symmetry becomes t=p′(f0), and the frequency of the oscillating term becomes f0-p(f0).

[0028] Preferably, after obtaining the inverse Fourier transform expression according to (6), in order to reduce the peak value as much as possible, under the condition that the symmetry axis remains unchanged, the peak value remains at s(0), and the oscillation term frequency remains unchanged, within a single cycle σp <T, according to formula (2) and formula (3), we can get the result shown in formula (7):

[0029] p(f0)=0

[0030] p'(f0)=0 (5)

[0031] Therefore, the expression of p(f) is as shown in formula (8): p(f) = kf 2 -2kf0f(8), where k is the coefficient to be determined; by combining equations (6) and (8), the peak value expression after inverse Fourier transform is shown in equation (9):

[0032]

[0033] Combining equations (4) and (9), we can get the condition for the value of k as shown in equation (10):

[0034]

[0035] When the k value increases, the peak value decreases rapidly, and when the k value increases to the peak value, it slowly decreases.

[0036] Preferably, select σ g = k value of 0.8T, which makes the data concentrated in the middle 80% of a single communication cycle, avoiding inter-code interference when no empty cycle is used;

[0037] The signal after peak-to-average ratio optimization is shown in formula (7):

[0038]

[0039] The beneficial effects of the present invention are:

[0040] The multi-carrier underwater communication method based on pulse broadening technology in the present invention adopts 8 / 10B coding to reduce the signal peak difference caused by different transmission information in a single cycle equalization, and adopts pulse broadening to greatly reduce the signal average-peak ratio, and makes the main power of the signal concentrated at the center of the cycle within a single transmission cycle, thereby reducing the impact of underwater multipath transmission. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 This is a flowchart of a multi-carrier underwater communication method based on pulse broadening technology.

[0042] Figure 2 It is the relationship between the broadening coefficient k value and the peak value.

[0043] Figure 3 This is a flowchart of Example 4 of a multi-carrier underwater communication method based on pulse broadening technology.

[0044] Figure 4 It is the comparison between the signal after 8B / 10B coding and modulation and the stretched signal of "Tianjin university!!". DETAILED DESCRIPTION

[0045] The present invention is described in detail below in conjunction with the accompanying drawings:

[0046] Example 1

[0047] Combination Figures 1 to 4 A multi-carrier underwater communication method based on pulse broadening technology includes a multi-carrier underwater communication system, which includes an ASCII encoding unit, an 8B / 10B encoding unit, a modulation unit, a Gaussian envelope approximation and parameter solving unit, a broadening unit, a demodulation unit, a 10B / 8B encoding unit, and an ASCII decoding unit group; a data peak equalization module composed of the ASCII encoding unit, the 8B / 10B encoding unit, and the modulation unit; a peak-to-average ratio optimization module composed of the Gaussian envelope approximation and parameter solving unit and the broadening unit.

[0048] Example 2

[0049] Based on the above embodiments, this embodiment further discloses the following:

[0050] When the multi-carrier underwater communication system communicates, the communication method is as follows:

[0051] The information to be transmitted is converted into a binary array of 8 bits per byte through an ASCII encoding unit. After the binary array passes through the 8 / 10B encoding unit, it becomes a new array of 10 bits per byte with similar numbers of 0 and 1 appearing. After the new array passes through the multi-carrier OOK modulation of the modulation unit, the Gaussian envelope approximation and parameter solution unit solves the broadening coefficient k. In the broadening unit, the modulated signal is broadened according to the broadening coefficient k. The signal is transmitted in water and received by the receiving end. After demodulation by the demodulation unit and the 10B / 8B encoding unit and the ASCII decoding unit, the transmitted information is restored.

[0052] The 8-bit data of the binary array will become a 10-bit array after 8b / 10b encoding. The number of 0s and 1s in the 10-bit data will appear in three situations: 5 0s and 5 1s, 6 0s and 1s, and 4 0s and 6 1s. When the inequality of the previous byte is -2, the inequality of the next byte will be adjusted to +2, thereby ensuring that the number of 0s and 1s is basically equal for a long time.

[0053] The signal s(t) of a single transmission cycle after multi-carrier OOK modulation is shown in equation (1):

[0054]

[0055] Where M is the row matrix composed of 0 and 1 after the data is 8b / 10b encoded, n is the number of carriers, f is the carrier frequency, t is the time, and T is the duration of a single transmission cycle. When the amplitude and initial phase of each carrier are the same, s(0) in s(t) is the pulse peak value of a single transmission cycle. If 8b / 10b encoding is not performed, the value range of s(0) is 0 to n. After 8b / 10b encoding, the value range of s(0) is 2 / n-1 to 2 / n+1.

[0056] After peak equalization is performed through the ASCII encoding unit, 8B / 10B encoding unit, and modulation unit, the peak-to-average ratio of single-cycle transmission is reduced through the peak-to-average ratio optimization module. The reduction of the peak-to-average ratio is achieved by adjusting the phase.

[0057] Example 3

[0058] Based on the above embodiments, this embodiment further discloses the following:

[0059] When the Gaussian envelope approximation and the parameter solving unit are adjusted, as shown in equation (2), the signal is dispersed by adding a phase shift term in the frequency domain after Fourier transformation: Here, p(f) is a second-order polynomial with f as the independent variable.

[0060] The signal form of multiple single-frequency signals superimposed is multiple impulse functions after Fourier transformation. After multiplying by the phase shift term, it is difficult to solve the optimal phase shift parameter. In order to solve the above problem, in the peak-to-average ratio optimization module, it is also necessary to perform Gaussian envelope approximation on the original signal; since the Gaussian envelope approximation has a higher fitting degree near the peak point of the signal, and the peak-to-average ratio focuses on the peak point, the expression of Gaussian approximation is shown in formula (3), where σ is the Gaussian envelope width, f0 is the signal band center frequency, and after Fourier transformation and multiplication of the Gaussian approximation expression by the phase shift term, the optimal phase shift parameter and phase shift term expression can be obtained through the expression after inverse Fourier transformation:

[0061]

[0062] Then, the expression after Fourier transform of the Gaussian envelope approximation signal is shown in formula (4):

[0063]

[0064] Then, the expression of the multiplication of the Gaussian envelope and the phase shift term is shown in equation (5);

[0065]

[0066] Since p(f) is a quadratic polynomial, the inverse Fourier transform of equation (5) is expressed as equation (6), where

[0067]

[0068] It can be seen from formula (6) that the expression is still in the form of Gaussian envelope multiplied by oscillation term, and its peak value becomes The axis of symmetry becomes t=p′(f0), and the frequency of the oscillating term becomes f0-p(f0).

[0069] After obtaining the inverse Fourier transform expression according to (6), in order to further reduce the peak value as much as possible, under the condition that the symmetry axis remains unchanged, the peak value remains at s(0), and the oscillation term frequency remains unchanged, within a single cycle σ p <T, according to formula (2) and formula (3), the result shown in formula (7) can be obtained:

[0070] p(f0)=0

[0071] p′(f0)=0 (9)

[0072] Therefore, the expression of p(f) is as shown in formula (8): p(f) = kf 2 -2kf0f(8), where k is the coefficient to be determined; by combining equations (6) and (8), the peak value expression after inverse Fourier transform is shown in equation (9):

[0073]

[0074] Combining equations (4) and (9), we can get the condition for the value of k as shown in equation (10):

[0075]

[0076] When the k value increases, the peak value decreases rapidly, and when the k value increases to the peak value, it slowly decreases.

[0077] Select σ g = k value of 0.8T, which makes the data concentrated in the middle 80% of a single communication cycle, avoiding inter-code interference when no empty cycle is used;

[0078] The signal after peak-to-average ratio optimization is shown in formula (11):

[0079]

[0080] Since the optimal operating frequency band of the underwater acoustic transducer used in this study is 500-700Hz, 600Hz is taken as the center frequency, the frequency interval is 1Hz, and the period is T. After the data "Tianjin university!!" is encoded by 8B / 10B, the modulated signal is as follows Figure 4 As shown in the black curve, the signal after broadening by calculating the broadening coefficient k is as follows Figure 4 As shown in the middle grey curve. As can be seen from the figure, the peak-to-peak value of the signal is significantly reduced after broadening, and the part with stronger energy is concentrated in the middle part of the transmission cycle. The peak-to-average ratio is reduced from 23.01dB to 11.75dB, which is conducive to long-distance transmission of the signal when the rated power remains unchanged.

[0081] Example 4

[0082] When performing multi-carrier underwater communication, a multi-carrier underwater communication method based on pulse stretching technology needs to go through unit procedures including ASCII encoding, 8B / 10B encoding, modulation, Gaussian envelope approximation and parameter solution, stretching, demodulation, 10B / 8B encoding, and ASCII decoding. In the communication method, the information to be transmitted is converted into a binary array of 8 bits per byte through ASCII encoding; after the array is 8 / 10B encoded, it becomes a new array of 10 bits per byte with similar number of occurrences of 0 and 1; after OOK modulation of the modulated multi-carrier, the stretching coefficient k is solved by Gaussian envelope approximation and parameter solution; in the stretching, the modulated signal is stretched according to the stretching coefficient k; the signal is transmitted in the water and received by the receiving end, and after demodulation 610B / 8B encoding and ASCII decoding, the transmitted information is restored.

[0083] As shown in the following formula 001, the information to be transmitted is converted into 8-bit one-byte data after being converted into ASCII code.

[0084] x[1], x[2], x[3]...x[8]; 001;

[0085] In the 8B / 10B encoding process, the first 3 bits x[1], x[2], x[3] are converted into 4-bit data as shown in Formula 002 through the mapping relationship in Table 1 and 3B / 4B encoding. The last 5 bits x[4], x[5], x[6], x[7], x[8] are converted into 6-bit data as shown in Formula 003 through the mapping relationship in Table 2 and 5B / 6B encoding. In the mapping process, the mapping mode of RD=-1 with fewer bits having the value of 1 is always selected.

[0086] X[7],X[8],X[9],X

[10] ; 002;

[0087] X[1],X[2],X[3],X[4],X[5],X[6]; 003;

[0088] Table 1 3B / 4B encoding mapping table:

[0089]

[0090] Table 2 5B / 6B encoding mapping table:

[0091]

[0092] The encoded 4B and 6B data are reassembled into 10-bit data with 6B in the front and 4B in the back, completing 8B / 10B encoding. The reassembled data is shown in formula 004.

[0093] X[1], X[2], X[3], X[4], X[5], X[6], X[7], X[8], X[9],

[0094] The signal s(t) of a single transmission cycle after multi-carrier OOK modulation is shown in Formula 005; where M is a row matrix composed of 0 and 1 after the data is 8 / 10B encoded, n is the number of carriers, f is the carrier frequency, t is time, and T is the duration of a single transmission cycle.

[0095]

[0096] When the amplitude and initial phase of each carrier are the same, s(0) is the pulse peak value of a single transmission cycle. If 8b / 10b encoding is not performed, the value range of s(0) is 0 to n, with a large fluctuation. After 8b / 10b encoding, the value range of s(0) is 2 / n-1 to 2 / n+1, with a very small fluctuation. Although this slightly reduces the effective information transmitted in each transmission cycle, the maximum value of different information peaks is reduced from n to n / 2+1, which is more conducive to long-distance transmission.

[0097] Although 8b / 10b coding can effectively reduce the peak value in multi-carrier OOK modulation, the peak-to-average ratio is still large and the transducer still works at low power for a long time. Therefore, other means are still needed to reduce the peak-to-average ratio of single-cycle transmission.

[0098] In multi-carrier OOK modulation, the frequency of the signal is what actually transmits information. The phase of each carrier wave does not affect the orthogonal relationship between the signals nor the strength of each carrier wave. Therefore, it is feasible to reduce the peak-to-average ratio by adjusting the phase.

[0099] Refer to the pulse broadening technology in pulse lasers. As shown in formula 006, the signal can be "dispersed" by adding a phase shift term in the frequency domain after Fourier transformation. Among them, p(f) is a second-order polynomial with f as the independent variable. Since the signal form of multiple single-frequency signals superimposed is multiple impulse functions after Fourier transformation. After multiplying by the phase shift term, it is difficult to solve the optimal phase shift parameter. The following is obtained:

[0100] In order to solve the above problems, the original signal can be approximated by Gaussian envelope. It can be seen that the Gaussian envelope approximation has a higher fitting degree near the peak point of the signal, while the peak-to-average ratio focuses on the peak point. Among them, σ is the Gaussian envelope width, f0 is the signal band center frequency; the expression after Gaussian approximation is Fourier transformed and multiplied by the phase shift term. The optimal phase shift parameter and phase shift term expression can be obtained through the expression after inverse Fourier transformation:

[0101]

[0102] The expression after Fourier transform of Gaussian envelope approximation signal is shown as follows:

[0103]

[0104] The expression for multiplying the Gaussian envelope by the phase shift term is as follows:

[0105]

[0106] Since p(f) is a quadratic polynomial, the expression after inverse Fourier transform of equation 009 is shown in equation 100;

[0107]

[0108] in It can be seen from the formula that the expression is still in the form of Gaussian envelope multiplied by oscillation term; its peak value becomes The axis of symmetry becomes t=p′(f0), and the frequency of the oscillating term becomes f0-p(f0).

[0109] Example 5

[0110] After the transformation, we hope that: 1) the peak value is as low as possible; 2) the axis of symmetry remains unchanged, and the peak value is still at s(0); 3) the frequency of the oscillation term remains unchanged; 4) since the approximations discussed are all within a single cycle, σ p <T.

[0111] Based on the fact that the axis of symmetry remains unchanged, the peak value remains at s(0) and the frequency of the oscillation term remains unchanged, the result shown in Formula 101 can be obtained.

[0112] p(f0)=0

[0113] p'(f0)=0;101;

[0114] Thus, the expression of p(f) is shown in equation 102: p(f) = kf 2 -2kf0f; 102, where k is the coefficient to be determined. By combining them, the peak value expression after inverse Fourier transform is shown in formula 103:

[0115]

[0116] By approximation, they are all within a single period, and σ p <T, we can also know that the condition for k value is as shown in formula 104: 104; when the k value increases, the peak value decreases rapidly; when the k value increases to a certain extent, the peak value decreases slowly.

[0117] The propagation time of underwater acoustic signals will be affected by the multipath effect. In order to prevent the multipath effect from causing inter-code interference, a blank period is often inserted between two transmission cycles to prevent the interference. In this study, due to the use of pulse stretching technology, the parameters can be adjusted to make most of the energy of a single transmission cycle concentrated in the middle of the transmission cycle, thereby avoiding the use of empty cycles, reducing communication complexity, and increasing the transmission rate to a certain extent. Therefore, this study selects σ g = k value of 0.8T. Most of the energy is concentrated in the middle 80% of a single communication cycle, avoiding inter-code interference when no empty cycle is used.

[0118] The peak-to-average ratio optimized signal is shown below:

[0119] The multi-carrier underwater communication method based on pulse broadening technology in the present invention adopts 8 / 10B coding to reduce the signal peak difference caused by different transmission information in a single cycle equalization. The pulse broadening method greatly reduces the average peak ratio of the signal, and makes the main power of the signal concentrated at the center of the cycle in a single transmission cycle, reducing the impact of underwater multipath transmission.

[0120] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A multi-carrier underwater communication method based on pulse broadening technology, characterized in that: A multi-carrier underwater communication system is included, which includes an ASCII encoding unit, an 8B / 10B encoding unit, a modulation unit, a Gaussian envelope approximation and parameter solving unit, a broadening unit, a demodulation unit, a 10B / 8B encoding unit, and an ASCII decoding unit group; The information to be transmitted is converted into a binary array of 8 bits per byte through an ASCII encoding unit. After the binary array passes through the 8 / 10B encoding unit, it becomes a new array of 10 bits per byte with similar numbers of 0 and 1 appearing. After the new array passes through the multi-carrier OOK modulation of the modulation unit, the Gaussian envelope approximation and parameter solution unit solves the broadening coefficient k. In the broadening unit, the modulated signal is broadened according to the broadening coefficient k. The signal is transmitted in water and received by the receiving end. After demodulation by the demodulation unit, 10B / 8B encoding, and ASCII decoding, the transmitted information is restored.

2. The multi-carrier underwater communication method based on pulse stretching technology according to claim 1, characterized in that: The ASCII encoding unit, 8B / 10B encoding unit, and modulation unit constitute the data peak equalization module; The 8-bit data of the binary array will become a 10-bit array after 8b / 10b encoding. The number of 0s and 1s in the 10-bit data will appear in three situations: 5 0s and 5 1s, 6 0s and 1s, and 4 0s and 6 1s. When the inequality of the previous byte is -2, the inequality of the next byte will be adjusted to +2 to ensure that the number of 0s and 1s is equal for a long time.

3. The multi-carrier underwater communication method based on pulse stretching technology according to claim 1, characterized in that: The signal s(t) of a single transmission cycle after multi-carrier OOK modulation is shown in equation (1): Where M is the row matrix composed of 0 and 1 after the data is 8 / 10B encoded, n is the number of carriers, f is the carrier frequency, t is time, and T is the duration of a single transmission cycle; When the amplitude and initial phase of each carrier are the same, the s(0) data in s(t) is the pulse peak value of a single transmission cycle; If 8b / 10b encoding is not performed, the value range of s(0) is 0 to n; If 8b / 10b encoding is performed, the value range of s(0) is 2 / n-1 to 2 / n+1.

4. The multi-carrier underwater communication method based on pulse stretching technology according to claim 1, characterized in that: Gaussian envelope approximation, parameter solving unit and broadening unit constitute the data peak-to-average ratio optimization module; After peak equalization through the ASCII encoding unit, 8B / 10B encoding unit, and modulation unit, the peak-to-average ratio optimization module is used to reduce the peak-to-average ratio of single-cycle transmission. The reduction of the peak-to-average ratio requires adjusting the phase; Among them, when the Gaussian envelope approximation and the parameter solving unit are adjusted, as shown in formula (2), the signal is dispersed by adding a phase shift term in the frequency domain after Fourier transformation: Here, p(f) is a second-order polynomial with f as the independent variable.

5. The multi-carrier underwater communication method based on pulse stretching technology according to claim 4, characterized in that: In the peak-to-average ratio optimization module, the original signal also needs to be approximated by a Gaussian envelope. Since the Gaussian envelope approximation has a higher fitting degree near the peak point of the signal, and the peak-to-average ratio focuses on the peak point, the expression of the Gaussian approximation is shown in equation (3), where σ is the Gaussian envelope width and f0 is the signal band center frequency. After the expression after Gaussian approximation is Fourier transformed and multiplied by the phase shift term, the optimal phase shift parameter and phase shift term expression can be obtained through the expression after inverse Fourier transformation: Then, the expression after Fourier transform of the Gaussian envelope approximation signal is shown in formula (4): Then, the expression of the multiplication of the Gaussian envelope and the phase shift term is shown in equation (5); Since p(f) is a quadratic polynomial, the inverse Fourier transform of equation (5) is expressed as equation (6), where It can be seen from formula (6) that the expression is still in the form of Gaussian envelope multiplied by oscillation term, and its peak value becomes The axis of symmetry becomes t=p′(f0), and the frequency of the oscillating term becomes f0-p(f0).

6. The multi-carrier underwater communication method based on pulse stretching technology according to claim 5, characterized in that: After obtaining the inverse Fourier transform expression according to (6), in order to reduce the peak value as much as possible, under the condition that the symmetry axis remains unchanged, the peak value remains at s(0), and the oscillation term frequency remains unchanged, within a single cycle σ P <T, according to equations (2) and (3), p(f0)=0 The result shown in formula (7) is obtained: p′(f0)=0(1); Therefore, the expression of p(f) is as shown in formula (8): p(f) = kf 2 -2kf0f(8), where k is the coefficient to be determined; by combining equations (6) and (8), the peak value expression after inverse Fourier transform is shown in equation (9): Combining equations (4) and (9), we can get the condition for the value of k as shown in equation (10): When the k value increases, the peak value decreases rapidly, and when the k value increases to the peak value, it slowly decreases.

7. The multi-carrier underwater communication method based on pulse stretching technology according to claim 6, characterized in that: Select σ g = k value of 0.8T, which makes the data concentrated in the middle 80% of a single communication cycle, avoiding inter-code interference when no empty cycle is used; The signal after peak-to-average ratio optimization is shown in formula (3):

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