Multi-carrier modulation and calculation method for cross-water-air communication

Abstract

The invention provides a multi-carrier modulation and calculation method for cross-water-air communication. Signals which are subjected to 8B / 10B coding serial-parallel conversion and modulation are sent out through an underwater acoustic transducer, vibration of the water surface is triggered, and after a millimeter wave radar is used for extracting and resolving water surface vibration signals, transmitted information is obtained. According to the invention, the number of coexisting carriers can be reduced, and the power of a single carrier is improved, thereby increasing the transmission distance. The preamble and the prefix are used, so that a receiving end can obtain the time when information arrives and compensate for amplitude attenuation and frequency change generated in the process that sound signals are propagated in water, and therefore the error rate is reduced.

Description

Technical Field

[0001] The present invention relates to the field of cross-medium communication, and in particular to a multi-carrier modulation and resolution method for cross-water-air communication. Background Art

[0002] Realizing cross-media communication between water and air is an important part of developing marine strategies, but also a major difficulty. In terms of marine resource detection and marine communications, the marine information detected by underwater sensors needs to be transmitted to surface equipment for processing or recording. For example, in scenarios such as marine ecological research, offshore oil exploration, and submarine network construction, water-to-air cross-media communication technology can greatly reduce time and equipment costs, and has very important potential applications.

[0003] Realizing direct wireless information transmission from water to air without relay equipment is particularly important for ocean exploration, marine resource development, marine communications, etc., 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 cause vibrations on the water surface. By using millimeter-wave radar to detect this vibration signal, we can obtain the information in the underwater acoustic signal and achieve wireless communication across water and air.

[0005] The propagation loss of acoustic signals in water is relatively serious, and the water surface vibration caused by underwater acoustic signals is relatively weak and difficult to detect. When using a multi-carrier communication method with a faster communication rate, 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 increases, the distance of cross-water-air communication is extremely limited.

[0006] When acoustic signals propagate in water, the frequency and amplitude of the acoustic signals will change due to the fluctuations of the sea surface, the difference in sound speed, and multipath propagation. This makes it very easy for general demodulation methods to produce bit errors. Summary of the invention

[0007] The main purpose of the present invention is to provide a multi-carrier modulation and solution 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-mentioned purpose, the technical solution adopted by the present invention is:

[0009] A multi-carrier modulation method for cross-water-air communication, which is to send out the signal after 8B / 10B coding serial-to-parallel conversion and modulation through an underwater acoustic transducer to induce vibration of the water surface, and use a millimeter wave radar to extract and resolve the water surface vibration signal to obtain the transmitted information, which specifically includes the following steps:

[0010] Step 1: The information to be transmitted is first converted into eight-bit one-byte information through ASCII code units, and then converted into 10-bit one-byte information after the above-mentioned 8B / 10B encoding. When performing 8B / 10B encoding, a smaller encoding method with code 1 is always used, and the maximum number of code 1s in each byte is 5;

[0011] Step 2: After passing through the serial / parallel conversion unit, the data is converted into a total of ten bits of data, from the first bit of the transmitted data to the tenth bit of the transmitted data; the ten bits of data are modulated by multiplying the first multiplier to the tenth multiplier and the first carrier to the tenth carrier respectively;

[0012] Step 3: The modulated data is combined into a same signal in the adder unit, and a prefix and a preamble are added in the prefix adding unit and the preamble adding unit respectively;

[0013] Step 4: The signal obtained after adding 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 and causes vibration of the water surface;

[0014] 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 out with a bandpass filter to remove excess noise, and the preamble code is detected. When the preamble code does not appear, the detection is continued. After the preamble code appears, the frequency and amplitude information contained in the prefix is ​​solved in the prefix solution.

[0015] 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.

[0016] Preferably, in the 8B / 10B encoding, serial-to-parallel conversion, the first bit of the transmitted data to the tenth bit of the transmitted data, the first multiplier to the tenth multiplier, the first carrier to the tenth carrier and the adder unit, the eight-bit data is divided into high-bit 3B and low-bit 5B data according to the high and low bits, and 3B and 5B are respectively encoded with 3B / 4B and 5B / 6B according to the data lookup table;

[0017] 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 one-byte data and modulated after encoding.

[0018] Preferably, the modulation process is calculated as follows:

[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, which can be 0 or 1; A i sin(2πf i ) is the expression of carrier i, where A i is the amplitude of the carrier, f i is the frequency of the carrier; since 8B / 10B with RD = -1 is performed, The value is 4 or 5, and the maximum value is 5.

[0021] Preferably, 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, 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 The power occupied by each subcarrier. When the power occupied by each subcarrier is the same, since there are at most 5 subcarriers at the same time, the maximum power occupied by each subcarrier is P i =0.2P 总 ;

[0024] 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 .

[0025] Preferably, during the operation of the prefix adding unit, the preamble adding unit, the preamble detection, and the prefix resolution, the propagation of underwater sound has a multipath phenomenon, so there are two or even multiple different paths in the process of a piece of information propagating from the bottom of the water to the surface of the water;

[0026] When information is sent continuously, the slow path of the previous information will reach the surface of the water at the same time as the fast path of the next information. The prefix is ​​added to separate the two information segments.

[0027] There are two types of prefixes, LP and HP, and their expressions are shown in formula (3):

[0028]

[0029] Among them, T pis the duration of the prefix; before odd-numbered bytes, an HP prefix is ​​added, and before even-numbered bytes, an LP prefix is ​​added; 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 will not affect the power allocated to each carrier.

[0030] Preferably, a preamble code that is insensitive to frequency changes is added to mark the arrival of the information, and the information needs to be resolved based on the frequency, amplitude and other 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, and A is the preamble code amplitude T d is the preamble duration.

[0034] Preferably, in formula (4), the preamble is a signal with a linear frequency change, which will cause changes in f1 and f2 after the frequency changes during underwater propagation, but will not change its linear frequency change characteristics;

[0035] In preamble detection, before the signal arrives, the water surface vibration is continuously detected using cross-correlation detection;

[0036] After the current pilot code appears, the prefix is ​​solved to solve the carrier frequency after propagation in the water. This frequency is used to perform orthogonal demodulation on the subsequently transmitted signal to solve the information contained in the signal.

[0037] Beneficial effects:

[0038] The present invention proposes a multi-carrier modulation and resolution method for cross-water and air communications, which uses 8B / 10B coding to reduce the maximum number of carriers that can simultaneously contain single-byte information from 8 to 5. The power occupied by a single carrier is reduced from 0.125P 总 Improved to 0.2P 总 , greatly improving the transmission distance. HP and LP prefixes are used, which contain the frequency and amplitude information of all carriers, provide frequency and amplitude information for orthogonal demodulation, and reduce the bit error rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 The invention is a flowchart of a multi-carrier modulation and solution method for cross-water and air communications.

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

[0041] Figure 3 It is the data packet format after "TJU" is encoded, modulated, prefixed, and preambled. DETAILED DESCRIPTION

[0042] The present disclosure is further described below in conjunction with the accompanying drawings and embodiments.

[0043] It should be noted that the following detailed descriptions are illustrative and are intended to provide further explanation of the present disclosure. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those of ordinary skill in the art to which the present disclosure belongs. The following describes in detail the embodiments of the present invention in a step-by-step manner. The description is only a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

[0044] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present disclosure. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0045] Example 1

[0046] Combination Figures 1 to 3 A multi-carrier modulation method for cross-water and air communication, the method flow includes ASCII code unit, 8B / 10B encoding, serial-to-parallel conversion, sending data first bit, sending data second bit, sending data third bit, sending data fourth bit, sending data fifth bit, sending data sixth bit, sending data seventh bit, sending data eighth bit, sending data ninth bit, sending data tenth bit, first multiplier to tenth multiplier, first carrier to tenth carrier, adder unit, prefix adding unit, preamble adding unit, underwater acoustic transducer, millimeter wave radar, bandpass filter, preamble detection, prefix resolution, orthogonal demodulation unit.

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

[0048] The method steps include the following:

[0049] Step 1: The information to be transmitted is first converted into eight-bit one-byte information through ASCII code units, and then converted into 10-bit one-byte information after 8B / 10B encoding. When performing 8B / 10B encoding, a coding method with a smaller 1 is always used, so that the maximum number of 1s in each byte is 5;

[0050] Step 2: After passing through the serial / parallel conversion unit, the data is converted into a total of ten bits of data, from the first bit of the transmitted data to the tenth bit of the transmitted data, which are respectively multiplied by the first carrier to the tenth carrier in the first multiplier to the tenth multiplier for modulation;

[0051] Step 3: Combine the signals into one signal in the adder unit, and add the prefix and the preamble in the prefix adding unit and the preamble adding unit respectively;

[0052] Step 4: The obtained signal is converted into an acoustic signal by an underwater acoustic transducer, which propagates from the bottom of the water to the surface of the water and causes vibration of the water surface;

[0053] Step 6: Use millimeter-wave radar to detect the vibration of the water surface. The obtained signal is filtered out with a bandpass filter to remove excess noise, and the preamble code is detected. When the preamble code does not appear, the detection is continued. After the preamble code appears, the frequency and amplitude information contained in the prefix is ​​solved in the prefix solution;

[0054] 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.

[0055] Example 2

[0056] Based on the disclosure of the above embodiment, this embodiment further discloses the following:

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

[0058] 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 one-byte data and modulated after encoding.

[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, which can be 0 or 1; A i sin(2πf i ) is the expression of carrier i, where A i is the amplitude of the carrier, f i is the frequency of the carrier; since 8B / 10B with RD = -1 is performed, The value is 4 or 5, and the maximum value is 5.

[0062] Example 3

[0063] Based on the disclosure of the above embodiment, this embodiment further discloses the following:

[0064] 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:

[0065]

[0066] Where P 总 is the total power of the underwater acoustic transducer, P i The power occupied by each subcarrier. When the power occupied by each subcarrier is the same, since there are at most 5 subcarriers at the same time, the maximum power occupied by each subcarrier is P i =0.2P 总 ;

[0067] 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 .

[0068] During the operation of the prefix adding unit, the preamble adding unit, the preamble detection, and the prefix solving, due to the multipath phenomenon of underwater sound propagation, there are two or even multiple different paths for a piece of information to propagate from the bottom of the water to the surface of the water;

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

[0070] Therefore, it is necessary to add a prefix before the information to separate the two pieces of information. There are two types of prefixes, LP and HP, and their expressions are shown in formula (3):

[0071]

[0072] Where T pis the duration of the prefix; wherein, an HP prefix is ​​added before an odd-numbered byte, and an LP prefix is ​​added before an even-numbered byte; the HP prefix includes carriers one to five, and the LP prefix includes carriers six to ten.

[0073] The number of carriers included in the prefix and suffix is ​​5, which does not affect the power allocated to each carrier.

[0074] The arrival of information is marked by adding a preamble that is insensitive to frequency changes. The information needs to be decoded based on the frequency, amplitude and other information contained in the prefix.

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

[0076]

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

[0078] In formula (4), the preamble is a signal with a linear frequency change. After the frequency changes during underwater propagation, it will only cause a slight change in f1 and f2, and will not change its linear frequency change characteristics.

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

[0080] After the current pilot code appears, the prefix is ​​solved to solve the carrier frequency after propagation in the water. This frequency is used to perform orthogonal demodulation on the subsequently transmitted signal to solve the information contained in the signal.

[0081] Example 4

[0082] Based on the disclosure of the above embodiments, this embodiment further discloses the specific implementation method:

[0083] In the system, the information to be transmitted is first converted into 8-bit-one-byte information through ASCII code units, and then becomes 10-bit-one-byte information after 8B / 10B encoding. When performing 8B / 10B encoding, a coding method with a smaller number of 1s is always used, so that the maximum number of 1s in each byte is 5.

[0084] After passing through the serial / parallel conversion unit, it becomes a total of ten bits of data, from the first bit 4 of the transmitted data to the tenth bit 13 of the transmitted data, which are multiplied with the first carrier to the tenth carrier for modulation in the first multiplier to the tenth multiplier, respectively. It is combined into the same signal in the adder unit, and the prefix and 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 an underwater acoustic transducer, which propagates from the bottom of the water to the surface of the water, causing vibration of the water surface. The vibration of the water surface is detected using a millimeter-wave radar. The obtained signal is filtered out of excess noise through a bandpass filter, and the preamble code is detected. When the preamble code does not appear, the detection is continued. After the preamble code appears, the frequency and amplitude information contained in the prefix is ​​solved in the prefix solution.

[0086] The signal is demodulated in the orthogonal demodulation unit using the amplitude and frequency information. The first bit to the tenth bit of the received data are obtained, and after parallel / serial conversion and 10B / 8B encoding, the received information is obtained, completing the cross-water-air communication.

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

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

[0089] 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 A.2 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 A.3 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.

[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 encoded 4B and 6B data are re-spliced ​​into 10-bit data with 6B in front and 4B in the back, completing 8B / 10B encoding. The re-spliced ​​data is shown in formula A.4.

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

[0099] After the serial / parallel conversion, the ten bits of data are operated as shown in formula A.5; where s(t) is the modulated time domain signal, and 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, where A i is the amplitude of the carrier, f i is the frequency of the carrier.

[0100]

[0101] After the prefix is ​​added and the preamble is added, the resulting data packet is as follows Figure 2 As shown; the expressions of the first three blocks of the data packet are shown in formula A.6.

[0102]

[0103] After the signal is output as an acoustic signal by the underwater acoustic transducer, it propagates from the bottom of the water to the surface, causing vibration of the water surface. After the vibration signal is detected by the millimeter-wave radar, it is filtered out through a bandpass filter to remove excess noise and improve signal quality.

[0104] In the preamble detection, as shown in Formula A.7, the mutual correlation coefficient C(t) between the signal s′(t) filtered by the bandpass filter and the local signal s(t) is detected; when the value of C(t) is greater than a certain threshold at a certain time t0, it is considered that the preamble has arrived and the signal reception begins.

[0105]

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

[0107] By using the amplitude and frequency information calculated from the prefix, the signal carrying information is orthogonally demodulated to calculate the ten bits of information X[1] to X

[10] contained in the signal; these ten bits of information are converted from parallel to serial and encoded in 10B / 8B to receive the transmitted information.

[0108] Example 5

[0109] Based on the above embodiments, this embodiment further discloses the following:

[0110] Figure 3 An example of multi-carrier modulation and resolution method for cross-water and air communications is given. After ASCII encoding, the three-byte information "TJU" becomes three 8-bit binary data. The binary numbers after 8B / 10B encoding are shown in Table 3. It can be seen from the table that the number of 1s in the three characters is 5, that is, the power that each carrier can occupy is 0.2P 总 .

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

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

[0113] The format of the data packet after adding the prefix and preamble is as follows Figure 3 shown.

[0114] The signal is converted into sound waves through the underwater acoustic transducer, which is transmitted from the bottom of the water to the surface of the water, causing the surface of the water to vibrate. The surface vibration signal is detected by the millimeter wave radar and then filtered out by the bandpass filter. The time t0 when the preamble appears is calculated.

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

Claims

1. A multi-carrier modulation method for cross-water-air communication, characterized in that: The method is to send out the signal after 8B / 10B coding serial-to-parallel conversion and modulation through an underwater acoustic transducer to induce vibration of the water surface, and use a millimeter wave radar to extract and resolve the water surface vibration signal to obtain the transmitted information, which specifically includes the following steps: Step 1: The information to be transmitted is first converted into eight-bit one-byte information through ASCII code units, and then converted into 10-bit one-byte information after the above-mentioned 8B / 10B encoding. When performing 8B / 10B encoding, a smaller encoding method with 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 a total of ten bits of data, from the first bit of the transmitted data to the tenth bit of the transmitted data; the ten bits of data are modulated by multiplying the first multiplier to the tenth multiplier and the first carrier to the tenth carrier respectively; Step 3: The modulated data is combined into a same 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 adding 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 and causes 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 out with a bandpass filter to remove excess noise, and the preamble code is detected. When the preamble code does not appear, the detection is continued. After the preamble code appears, the frequency and amplitude information contained in the prefix is ​​solved 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.

2. A multi-carrier modulation method for cross-water-air communication according to claim 1, characterized in that: In the 8B / 10B encoding, serial-to-parallel conversion, the first bit of the transmitted data to the tenth bit of the transmitted data, the first multiplier to the tenth multiplier, the first carrier to the tenth carrier and the adder unit, the eight-bit data is divided into high-bit 3B and low-bit 5B data according to the high and low bits, and the 3B and 5B are respectively encoded with 3B / 4B and 5B / 6B according to the data lookup table; 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 one-byte data and modulated after encoding.

3. A multi-carrier modulation method for cross-water-air communication according to claim 2, 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, where A i is the amplitude of the carrier, f i is the frequency of the carrier; since 8B / 10B with RD = -1 is performed, The value is 4 or 5, and the maximum value is 5.

4. 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): Where P 总 is the total power of the underwater acoustic transducer, P i The power occupied by each subcarrier. When the power occupied by each subcarrier is the same, 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 .

5. A multi-carrier modulation method for cross-water-air communication according to claim 1, characterized in that: In the operation of prefix adding unit, preamble adding unit, preamble detection and prefix resolution, the propagation of underwater sound has multipath phenomenon, so there are two or even multiple different paths in the process of a piece of information propagating from the bottom of the water to the surface of the water; When information is sent continuously, the slow path of the previous information will reach the surface of the water at the same time as the fast path of the next information. The prefix is ​​added to separate the two information segments. There are two types of prefixes, LP and HP, and their expressions are shown in formula (3): Among them, T p is the duration of the prefix; before odd-numbered bytes, an HP prefix is ​​added, and before even-numbered bytes, an LP prefix is ​​added; 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 will not affect the power allocated to each carrier.

6. A multi-carrier modulation method for cross-water-air communication according to claim 5, characterized in that: By adding a preamble code that is insensitive to frequency changes, the arrival of information is marked. The information needs to be resolved based on the frequency, amplitude and other 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 code amplitude T d is the preamble duration.

7. A multi-carrier modulation method for cross-water-air communication according to claim 6, characterized in that: In formula (4), the preamble is a signal with a linear frequency change. After the frequency changes during underwater propagation, it will cause changes in f1 and f2, but will not change its linear frequency change 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 solve the carrier frequency after propagation in the 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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