A multi-carrier modulation and demodulation method for cross-water space communication

Abstract

The application provides a kind of multi-carrier modulation and solution method for cross water acoustic communication. The signal after 8B / 10B encoding string parallel conversion and modulation is sent out through underwater acoustic transducer, which causes the vibration of water surface. After the vibration signal of water surface is extracted and solved by using millimeter wave radar, the transmitted information is obtained. The application can reduce the number of simultaneously existing carriers, increase the power of a single carrier and thus increase the transmission distance. The use of preamble and prefix enables the receiving end to obtain the time of information arrival and compensate for the amplitude attenuation and frequency change generated during the propagation of acoustic signals in water, thereby reducing the bit error rate.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of cross-medium communication, in particular to a multi-carrier modulation and demodulation method for cross-water-air communication. BACKGROUND

[0002] Implementing cross-medium communication between water and air is an important part of developing ocean strategy and a big difficulty. In terms of ocean resource exploration and ocean communication, the ocean information obtained by underwater sensors needs to be transmitted to water surface equipment for processing or recording. For example, in the scenarios of ocean ecology research, ocean oil exploration and submarine network construction, the cross-medium communication technology between water and air can greatly reduce the time and equipment cost, and has very important potential applications.

[0003] Implementing cross-medium direct wireless information transmission between water and air without relay equipment is particularly important for ocean exploration, ocean resource development and ocean communication, and can greatly save time and equipment costs. Among them, the cross-medium method combining sound waves and microwaves has the most application potential and important research value.

[0004] When sound waves propagate underwater to the sea surface, they will cause vibration of the water surface. Using a millimeter wave radar to detect this vibration signal can obtain the information in the underwater acoustic signal, and realize cross-water-air wireless communication.

[0005] The propagation loss of acoustic signals in water is serious, and the water surface vibration caused by underwater acoustic signals is weak and difficult to detect. When a multi-carrier communication method with a faster communication rate is used, due to the limited total power of the underwater acoustic transducer, as the number of carriers increases, the energy allocated to each carrier also decreases. This results in that when the communication rate is increased, the distance of cross-water-air communication is extremely limited.

[0006] When acoustic signals propagate in water, due to the fluctuation of the sea surface, the difference in sound speed and multipath propagation, etc. The frequency and amplitude of the acoustic signal will change during propagation. This leads to that the general demodulation method is prone to error. SUMMARY

[0007] The main purpose of the present application is to provide a multi-carrier modulation and demodulation method for cross-water-air communication, which can reduce the passband bandwidth while ensuring the measurement speed, thereby improving the signal-to-noise ratio of the input signal and improving the measurement accuracy.

[0008] In order to achieve the above purpose, the technical solution adopted by the present application is:

[0009] A kind of multi-carrier modulation method for across water space communication, the method is by 8B / 10B encoding string parallel conversion and the signal after modulation is sent out by underwater acoustic transducer, causes the vibration of water surface, using millimeter wave radar to extract and solve water surface vibration signal, obtain the information of transmission, it specifically includes the following steps:

[0010] Step one, the information to be transmitted is first converted into eight-bit one-byte information by ASCII code unit, then is changed into 10bit one-byte information after the above-mentioned 8B / 10B coding, when carrying out 8B / 10B coding, always use the smaller coding mode of coding 1, the number of coding 1 in each byte is maximum 5;

[0011] Step two, after string / parallel conversion unit, it becomes ten data of first sending data to tenth sending data;This ten data is multiplied by first multiplier to tenth multiplier, first carrier to tenth carrier for modulation respectively;

[0012] Step three, the data after modulation is combined into a signal in adder unit, prefix and preamble are added in prefix adding unit and preamble adding unit respectively;

[0013] Step four, the signal obtained after adding is converted into acoustic signal by underwater acoustic transducer, and after propagating from water bottom to water surface, causes the vibration of water surface;

[0014] Step six, after vibration transmission, millimeter wave radar is used to detect the vibration of water surface, the signal obtained is filtered by band-pass filter to remove excess noise, and preamble detection is carried out;When the preamble does not appear, continuous detection is carried out, and when the preamble appears, the frequency and amplitude information contained in the prefix are solved in prefix solution;

[0015] Step seven, the amplitude and frequency information are used to demodulate the signal in quadrature demodulation unit, to obtain first receiving data to tenth receiving data, after parallel / serial conversion and 10B / 8B coding, the received information is obtained, and the across water space communication is completed.

[0016] Preferably, in 8B / 10B coding, string / parallel conversion, first sending data to tenth sending data, first multiplier to tenth multiplier, first carrier to tenth carrier and adder unit, eight-bit data is divided into high-bit 3B and low-bit 5B data according to high and low, and 3B / 4B coding and 5B / 6B coding are carried out on 3B and 5B respectively according to data lookup table;

[0017] When coding, always select the mapping mode of RD=-1, so that the number of single-byte 1 after coding is maximum 5;The 6B data after coding is high bit, and the 4B data is low bit, and is recombined into 10bit one-byte data, and after coding, modulation is carried out.

[0018] Preferably, the modulation process is calculated as follows in formula (1):

[0019]

[0020] Where s(t) is the modulated time domain signal, N=10 is the number of symbols per byte; X[i] is the i-th bit of the transmitted data, taking values 0 or 1; A i sin(2πf i ) is the expression of the carrier wave i, where A i is the amplitude of the carrier wave, f i is the frequency of the carrier wave; since the 8B / 10B is performed with RD=-1, the value of is 4 or 5, with the maximum value of 5.

[0021] Preferably, in the case of limited total power of the underwater acoustic transducer, the number of carrier waves transmitted simultaneously per single byte after encoding is maximally 5, and the power distribution of the underwater acoustic transducer is shown in formula (2):

[0022]

[0023] Where P 总 is the total power of the underwater acoustic transducer, P i is the power occupied by each sub-carrier. When the power occupied by each sub-carrier is the same, since maximally 5 sub-carriers exist simultaneously, the power occupied by each sub-carrier is maximally P i =0.2P 总 .

[0024] Compared with the data of the ASCII code before encoding, maximally 8 sub-carriers exist simultaneously, and the maximum power occupied by each sub-carrier is only P i =0.125P 总 .

[0025] Preferably, in the prefix adding unit, the preamble adding unit, the preamble detection, and the prefix solving operation, the underwater acoustic propagation has the multipath phenomenon, and thus the process of a piece of information propagating from the water bottom to the water surface has two or even more different paths;

[0026] When the information is continuously transmitted, the slow path of the previous piece of information reaches the water surface at the same time as the fast path of the next piece of information, and the prefix is added to separate the two pieces of information;

[0027] The prefix is divided into two types, LP and HP, and the expression is shown in formula (3):

[0028]

[0029] Where T pThe prefix is the duration of the prefix; the HP prefix is added before an odd number of bytes, and the LP prefix is added before an even number of bytes; the HP prefix contains carrier one to carrier five, and the LP prefix contains carrier six to carrier ten; the number of carriers contained in the prefix and the suffix is 5, and the power allocated to each carrier is not affected.

[0030] Preferably, the arrival of information is marked by adding a preamble that is not sensitive to frequency changes, and the information needs to be calculated according to the frequency and amplitude information contained in the prefix;

[0031] The expression of the added preamble is shown in formula (4):

[0032]

[0033] Where f1 is the starting frequency, f2 is the ending frequency, A is the preamble amplitude T d The duration of the preamble.

[0034] Preferably, in formula (4), the preamble is a signal with linearly changing frequency, and after the frequency changes after underwater propagation, f1 and f2 change, but the characteristic of linearly changing frequency does not change;

[0035] In the preamble detection, the cross-correlation detection is used to detect the water surface vibration before the signal arrives;

[0036] After the preamble appears, the prefix is calculated, the carrier frequency after underwater propagation is calculated, and the frequency is used for orthogonal demodulation of the subsequent transmitted signal to calculate the information contained in the signal.

[0037] Advantages:

[0038] The multi-carrier modulation and calculation method for cross-water space communication proposed in the application uses 8B / 10B encoding to reduce the maximum number of single-byte information existing carriers from 8 to 5. The power occupied by a single carrier exists 总 The power is increased to 0.2P 总 , which greatly improves the transmission distance. The use of HP and LP prefixes contains the frequency and amplitude information of all carriers, provides frequency and amplitude information for orthogonal demodulation, and reduces the bit error rate. BRIEF DESCRIPTION OF DRAWINGS

[0039] Fig. 1 It is a multi-carrier modulation and calculation method flow chart for cross-water space communication.

[0040] Fig. 2 It is the data packet format after encoding, modulation, prefix addition, and preamble addition.

[0041] Fig. 3 is the data packet format after TJU encoding, modulation, prefix addition, and preamble addition. DETAILED DESCRIPTION

[0042] The present disclosure will be further described by way of illustration with reference to the accompanying drawings and examples.

[0043] It should be noted that the following detailed description is merely exemplary and is intended to provide further description of the present disclosure. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. The description is made in such a way that the embodiments of the present disclosure are described in a progressive manner, and the description is merely a preferred embodiment of the present disclosure and is not intended to limit the scope of protection of the present disclosure. Any modification, equivalent replacement and improvement made within the spirit and principle of the present disclosure shall be included in the scope of protection of the present disclosure.

[0044] It should be noted that the terms used herein are merely intended to describe specific embodiments and are not intended to limit exemplary embodiments according to the present disclosure. As used herein, the singular form is intended to include the plural form unless the context clearly indicates otherwise, and it should be understood that when the term "comprising" and / or "including" is used in the specification, it means that the features, steps, operations, devices, components and / or their combinations are present.

[0045] Embodiment 1

[0046] In conjunction with Figs. 1 to 3 A multi-carrier modulation method for underwater acoustic communication, the method flow includes ASCII code unit, 8B / 10B encoding, serial-parallel conversion, first data bit transmission, second data bit transmission, third data bit transmission, fourth data bit transmission, fifth data bit transmission, sixth data bit transmission, seventh data bit transmission, eighth data bit transmission, ninth data bit transmission, tenth data bit transmission, first multiplier to tenth multiplier, first carrier to tenth carrier, adder unit, prefix addition unit, preamble addition unit, underwater acoustic transducer, millimeter wave radar, band-pass filter, preamble detection, prefix solution, quadrature demodulation unit.

[0047] The multi-carrier modulation method further includes: receiving first data bit, receiving second data bit, receiving third data bit, receiving fourth data bit, receiving fifth data bit, receiving sixth data bit, receiving seventh data bit, receiving eighth data bit, receiving ninth data bit, receiving tenth data bit, parallel / serial conversion, and 10B / 8B encoding.

[0048] The method flow steps include the following:

[0049] Step one, the information to be transmitted is first converted into eight-bit one-byte information by ASCII code unit, and then converted into 10-bit one-byte information after 8B / 10B coding. During 8B / 10B coding, the coding mode of 1 is always selected to be smaller, so that the number of coding 1 in each byte is maximized to be 5;

[0050] Step two, after the serial / parallel conversion unit, it becomes ten bits of data from the first bit of the sending data to the tenth bit of the sending data, which are respectively multiplied by the first carrier to the tenth carrier in the first multiplier to the tenth multiplier to perform modulation;

[0051] Step three, it is combined into a signal in the adder unit, and the prefix and the preamble are respectively added in the prefix adding unit and the preamble adding unit;

[0052] Step four, the obtained signal is converted into an acoustic signal by a hydrophone, and the vibration of the water surface is induced after the acoustic signal is transmitted from the water bottom to the water surface;

[0053] Step six, the vibration of the water surface is detected by using a millimeter wave radar, the obtained signal is filtered by a band-pass filter to remove redundant noise, the preamble is detected, and the detection is continuously performed when the preamble does not appear. When the preamble appears, the frequency and amplitude information contained in the prefix are calculated in the prefix calculation.

[0054] Step seven, the amplitude and frequency information are used to demodulate the signal in the quadrature demodulation unit to obtain the first bit of the received data to the tenth bit of the received data. After parallel / serial conversion and 10B / 8B coding, the received information is obtained, and the cross-water space communication is completed.

[0055] Embodiment 2

[0056] On the basis of the above-mentioned embodiment, the present embodiment further discloses the following:

[0057] In the 8B / 10B coding, serial / parallel conversion, first bit of the sending data to the tenth bit of the sending data, first multiplier to the tenth multiplier, first carrier to the tenth carrier, and adder unit, the eight-bit data is divided into 3B high-bit data and 5B low-bit data according to high and low. According to the data lookup table, 3B / 4B coding and 5B / 6B coding are respectively performed on 3B and 5B.

[0058] During coding, the mapping mode of RD=-1 is always selected, so that the number of single-byte 1 after coding is maximized to be 5. The 6B data after coding is high-bit, and the 4B data is low-bit. The 10-bit one-byte data is recombined, and modulation is performed after coding.

[0059] The modulation process is described as follows:

[0060]

[0061] where s(t) is the modulated time domain signal, N=10 is the number of symbols per byte; X[i] is the i-th bit of the transmitted data, taking values 0 or 1; A i sin(2πf i ) is the expression of the carrier i, where A i is the amplitude of the carrier, f i is the frequency of the carrier; since the 8B / 10B is performed with RD=-1, the value of is 4 or 5, with the maximum value of 5.

[0062] Embodiment 3

[0063] On the basis of the disclosure of the above embodiments, the present embodiment further discloses as follows:

[0064] In the case of limited total power of the underwater acoustic transducer, the number of simultaneously transmitted carriers per single byte after encoding is maximally 5, and the power distribution of the underwater acoustic transducer is shown in equation (2):

[0065]

[0066] where P 总 is the total power of the underwater acoustic transducer, P i is the power occupied by each sub-carrier. When the power occupied by each sub-carrier is the same, since there are maximally 5 kinds of sub-carriers simultaneously appearing, the power occupied by each sub-carrier is maximally P i =0.2P 总 .

[0067] Compared with the data of the ASCII code before encoding, there are maximally 8 kinds of sub-carriers simultaneously existing, and the maximum power occupied by each sub-carrier is only P i =0.125P 总 .

[0068] In the operation of the prefix adding unit, the preamble adding unit, the preamble detection and the prefix solving, since the underwater acoustic propagation has the multipath phenomenon, the process of a piece of information propagating from the water bottom to the water surface has two or even more different paths;

[0069] If the information is continuously transmitted, the slow path of the previous piece of information will arrive at the water surface at the same time as the fast path of the next piece of information, causing serious interference.

[0070] Therefore, it is necessary to add a prefix before the information to separate two pieces of information, and the prefix is divided into two kinds, LP and HP, and the expression is shown in equation (3):

[0071]

[0072] where T pThe prefix is the duration of time; wherein, before the odd number of bytes, the HP prefix is added, and before the even number of bytes, the LP prefix is added; the HP prefix contains carrier one to carrier five, and the LP prefix contains carrier six to carrier ten.

[0073] The number of carriers contained in the prefix and the suffix is five, and the power allocated to each carrier is not affected.

[0074] The arrival of information is marked by adding the preamble which is not sensitive to frequency change, and the information needs to be calculated according to the frequency, amplitude and other information contained in the prefix.

[0075] The expression of the added preamble is shown as formula (4):

[0076]

[0077] Wherein f1 is the starting frequency, f2 is the ending frequency, A is the preamble amplitude, and T d is the duration of the preamble.

[0078] In formula (4), the preamble is a signal with linearly changing frequency, and after the frequency change caused by underwater propagation, only the slight changes of f1 and f2 will occur, and the characteristic of linearly changing frequency will not be changed.

[0079] In the preamble detection, the cross-correlation detection is used to detect the water surface vibration continuously before the signal arrives.

[0080] After the preamble appears, the prefix is calculated, the carrier frequency after the underwater propagation is calculated, the frequency is used to perform the orthogonal demodulation on the subsequent transmitted signal, and the information contained in the signal is calculated.

[0081] Embodiment 4

[0082] On the basis of the above embodiment, this embodiment further discloses the mode in the specific implementation:

[0083] In the system, the information to be transmitted is first converted into 8-bit one-byte information through the ASCII code unit, and then converted into 10-bit one-byte information after 8B / 10B encoding. When performing 8B / 10B encoding, a smaller encoding mode is always used, so that the number of 1s in each byte is maximized to five.

[0084] After the series / parallel conversion unit, the data is converted into ten bits of data from the first bit 4 of the transmitted data to the tenth bit 13 of the transmitted data, which are respectively multiplied by the first carrier to the tenth carrier in the first multiplier to the tenth multiplier to perform modulation. In the adder unit, they are combined into the same signal, and the prefix and the preamble are added in the prefix adding unit and the preamble adding unit, respectively.

[0085] The obtained signal is converted into an acoustic signal by a hydrophone, and the acoustic signal is propagated from the water bottom to the water surface to induce vibration of the water surface. The vibration of the water surface is detected by using a millimeter wave radar. The obtained signal is filtered by a band-pass filter to remove excess noise, preamble detection is performed, and when the preamble does not occur, continuous detection is performed, and when the preamble occurs, the frequency and amplitude information contained in the prefix are calculated in a prefix calculation.

[0086] The signal is demodulated in a quadrature demodulation unit using the amplitude and frequency information. After receiving data first bit to receiving data tenth bit are obtained, parallel / serial conversion and 10B / 8B encoding are performed, and the received information is obtained, and the cross-water space communication is completed.

[0087] As shown in formula A.1, the information to be transmitted is converted into ASCII code, and becomes 8-bit data of one byte.

[0088] x[1], x[2], x[3]...x[8] (A.1)

[0089] In the 8B / 10B encoding process, the first three bits x[1], x[2], x[3] are converted into four-bit data shown in formula A.2 by 3B / 4B encoding through the mapping relationship of table 1. The last five bits of data x[4], x[5], x[6], x[7], x[8] are converted into six-bit data shown in formula A.3 by 5B / 6B encoding through the mapping relationship of table 2. In the mapping process, the mapping mode of RD=-1 with fewer bits with a value of 1 is always selected.

[0090] X[7], X[8], X[9], X

[10] (A.2)

[0091] X[1], X[2], X[3], X[4], X[5], X[6] (A.3)

[0092] Table 1 3B / 4B encoding mapping table

[0093]

[0094] Table 2 5B / 6B encoding mapping table

[0095]

[0096]

[0097] The 4B and 6B data after encoding are re-spliced into 10-bit data with the first 4B and the last 6B, and the 8B / 10B encoding is completed. The data after re-splicing is shown in formula A.4.

[0098] X[1], X[2], X[3], X[4], X[5], X[6], X[7], X[8], X[9], X

[10] (A.4)

[0099] After the serial / parallel conversion, the ten data are operated as shown in equation A.5; wherein s(t) is the modulated time domain signal, N=10 is the number of symbols per byte. X[i] is the i-th bit of the transmitted data, taking the value of 0 or 1; A i sin(2πf i ) is the expression of the carrier i, wherein A i is the amplitude of the carrier, f i is the frequency of the carrier.

[0100]

[0101] After the prefix addition and the preamble addition, the obtained data packet is shown in Fig. 2 ; the expression of the first three blocks of the data packet is shown in equation A.6.

[0102]

[0103] After the signal is output as an acoustic signal by the underwater acoustic transducer, it propagates to the water surface from the water bottom, causing vibration of the water surface. The vibration signal is detected by the millimeter wave radar, and after being filtered by a band-pass filter, the signal quality is improved by filtering out the excess noise.

[0104] In the preamble detection, the cross-correlation coefficient C(t) of the signal s'(t) filtered by the band-pass filter and the local signal s(t) is detected as shown in equation A.7; when the value of C(t) is greater than a certain threshold at a time t0, it is considered that the preamble has arrived, and the signal reception is started.

[0105]

[0106] After t0, the prefix is detected; when the prefix is HP, the received signal is subjected to Fourier transform, and the amplitude attenuation and frequency change of the carriers one to five after passing through the underwater acoustic channel can be obtained; when the prefix is LP, the relevant information of the carriers six to ten can be obtained.

[0107] Through the amplitude and frequency information solved in the prefix, the signal with information is subjected to quadrature demodulation, and the ten information X[1]-X

[10] contained in the signal can be solved; after the serial / parallel conversion and 10B / 8B coding of the ten information, the transmitted information can be received.

[0108] Embodiment 5

[0109] On the basis of the disclosure of the above embodiments, the present embodiment further discloses the following:

[0110] Fig. 3 An example of a multi-carrier modulation and demodulation method for underwater acoustic communication is given. The three-byte information "TJU" is converted into three 8-bit binary data after ASCII encoding. The binary data after 8B / 10B encoding is shown in Table 3. As can be seen from the table, the number of 1s in the three characters is 5, i.e. each carrier can occupy a power of 0.2P 总 .

[0111] Table 3 Binary data of "TJU" after 8B / 10B encoding

[0112] T 00101 10101 J 01010 10101 U 10101 00101

[0113] The data packet format after prefix addition and preamble addition is shown in Table 3.

[0114] The signal is converted into an acoustic wave by the underwater acoustic transducer, transmitted from the water bottom to the water surface to induce water surface vibration. After the water surface vibration signal is detected by the millimeter wave radar, the excess noise is filtered out by the band-pass filter. The time t0of the occurrence of the preamble is calculated.

[0115] The signal is demodulated after t0. The amplitude and frequency information contained in the prefix can be used to determine the amplitude and frequency of carriers 1 to 10 after transmission. The known carrier frequency is used to quadrature demodulate the information, and the demodulated 8B / 10B ten-bit binary data corresponding to "TJU" is obtained. After 10B / 8B encoding, the data is converted into 8-bit ASCII code data. The underwater transmitted "TJU" information can be obtained by the ASCII code.

Claims

1. A multi-carrier modulation method for cross-water and air communications, characterized in that: The method uses an 8B / 10B encoded serial-to-parallel conversion and modulated signal to send it through an underwater acoustic transducer, causing vibration of the water surface. The millimeter-wave radar is used to extract and resolve the water surface vibration signal to obtain the transmitted information. The specific steps include: Step 1: The information to be transmitted is first converted into 8-bit 1-byte information through ASCII code units, and then converted into 10-bit 1-byte information after the above-mentioned 8B / 10B encoding. When performing 8B / 10B encoding, the smaller encoding method of code 1 is always used, and the maximum number of code 1s in each byte is 5; Step 2: After passing through the serial / parallel conversion unit, the data is converted into 10 bits of data, from the first bit of the transmitted data to the tenth bit of the transmitted data; the 10 bits of data are modulated by multiplying them by the first multiplier to the tenth multiplier and the first carrier to the tenth carrier respectively; Step 3: The modulated data are combined into a single signal in the adder unit, and a prefix and a preamble are added in the prefix adding unit and the preamble adding unit respectively; Step 4: The signal obtained after the addition is converted into an acoustic signal through an underwater acoustic transducer, which propagates from the bottom of the water to the surface of the water, causing vibration of the water surface; Step 6: After the vibration transmission, the millimeter-wave radar is used to detect the vibration of the water surface. The obtained signal is filtered through a bandpass filter to remove excess noise, and the preamble code is detected. If the preamble code does not appear, the detection is continued. If the preamble code appears, the frequency and amplitude information contained in the prefix is ​​calculated in the prefix solution. Step 7: Use the amplitude and frequency information to demodulate the signal in the orthogonal demodulation unit to obtain the first to tenth bits of the received data, perform parallel / serial conversion and 10B / 8B encoding to obtain the received information, and complete the cross-water-air communication; In the 8B / 10B encoding, serial-to-parallel conversion, transmission of the first to tenth bits of data, the first multiplier to the tenth multiplier, the first carrier to the tenth carrier, and the adder unit, the eight bits of data are divided into high-order 3B data and low-order 5B data according to the high and low values. According to the data lookup table, the 3B and 5B are respectively 3B / 4B encoded and 5B / 6B encoded; When encoding, the mapping mode of RD=-1 is always selected, so that the maximum number of 1s in the encoded single byte is 5; the encoded 6B data is the high bit and the 4B data is the low bit, which are recombined into 10-bit byte data. After encoding, modulation is performed; During the operations of the prefix adding unit, preamble adding unit, preamble detection, and prefix resolution, underwater sound propagation has a multipath phenomenon, so a piece of information may propagate from the bottom of the water to the surface via two or even multiple different paths. When information is sent continuously, the slow path of the first segment of information will reach the surface of the water at the same time as the fast path of the second segment of information. A prefix is ​​added to separate the two segments of information. There are two types of prefixes, LP and HP, and their expressions are shown in formula (3): Among them, T p The duration of the prefix; before odd-numbered bytes, add the HP prefix, and before even-numbered bytes, add the LP prefix; the HP prefix includes carriers one to five, and the LP prefix includes carriers six to ten; the number of carriers included in the prefix and suffix is ​​5, which does not affect the power allocated to each carrier; where A i is the amplitude of the carrier, f i is the frequency of the carrier.

2. A multi-carrier modulation method for cross-water-air communication according to claim 1, characterized in that: The modulation process is calculated as follows: Where s(t) is the modulated time domain signal, N=10 is the number of symbols per byte; X[i] is the i-th bit of the transmitted data, which can be 0 or 1; A i sin(2πf i ) is the expression of carrier i; since 8B / 10B with RD = -1 is performed, The value is 4 or 5, and the maximum value is 5.

3. A multi-carrier modulation method for cross-water-air communication according to claim 1, characterized in that: When the total power of the underwater acoustic transducer is limited, the maximum number of carriers transmitted simultaneously for each single byte after encoding is 5. The power distribution of the underwater acoustic transducer is shown in formula (2): Among them, P 总 is the total power of the underwater acoustic transducer, P i The power occupied by each subcarrier. When each subcarrier occupies the same power, since there are at most 5 subcarriers at the same time, the maximum power occupied by each subcarrier is P i =0.2P 总 ; Compared with the ASCII code data before encoding, there are up to 8 subcarriers at the same time, and the maximum power of each subcarrier is only P i =0.125P 总 .

4. The multi-carrier modulation method for cross-water-air communication according to claim 1, characterized in that: By adding a preamble that is insensitive to frequency changes, the arrival of the information is marked. The information needs to be decoded based on the frequency and amplitude information contained in the prefix; The expression of the added preamble is shown in formula (4): Where f1 is the starting frequency, f2 is the ending frequency, and A is the preamble amplitude T d is the preamble duration.

5. A multi-carrier modulation method for cross-water-air communication according to claim 4, characterized in that: In formula (4), the preamble is a signal with linear frequency variation. After the frequency changes during underwater propagation, it will cause changes in f1 and f2, but will not change its linear frequency variation characteristics. In preamble detection, before the signal arrives, the water surface vibration is continuously detected using cross-correlation detection; After the current pilot code appears, the prefix is ​​solved to calculate the carrier frequency after propagation in water. This frequency is used to perform orthogonal demodulation on the subsequently transmitted signal to solve the information contained in the signal.

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