A multi-band cross-layer reliable transmission method based on real-time underwater acoustic communication system

Abstract

The present invention discloses a multi-band cross-layer reliable transmission method based on a real-time underwater acoustic communication system, comprising: a transmitting end divides a complete transmission bit stream equally, encapsulates it separately, and performs LDPC‑CRC encoding and frequency band modulation; a receiving end performs LDPC‑CRC decoding taking into account random phase offset on the equalized received signal, calculates the signal-to-noise ratio of each frequency band to adaptively adjust the decoding code rate; returns the data packet information and code rate of the decoded error to the transmitting end, and each frequency band contains all the data packets of the decoded error and retransmits them; if the data packet of any frequency band is decoded correctly, it is considered that the data packet is retransmitted successfully, otherwise it is retransmitted again until the maximum number of retransmissions is reached; when all data packets are decoded correctly, the transmitting end is instructed to send the next frame of data to be sent. The present invention reduces the number of retransmission requests, improves the probability of successful retransmission, and improves the reliability of underwater acoustic communication.

Description

Technical Field

[0001] The present invention relates to the field of underwater acoustic communication, and in particular to a multi-band cross-layer reliable transmission method based on a real-time underwater acoustic communication system. Background Art

[0002] Reliable data transmission aims to ensure that the receiving end can correctly obtain the original data from the sending end. In the field of terrestrial wireless radio frequency communication and networks, the widely used mechanisms for reliable transmission are increasing redundancy and retransmission. With the development of underwater acoustic communication technology, these two methods are also used in underwater acoustic communication and networks. However, compared with wireless radio frequency networks, reliable data transmission has become more challenging in underwater acoustic communication. The underwater acoustic channel has the characteristics of poor quality, strong time variation, and large propagation delay, which limits the widespread application of redundancy mechanisms and affects the performance of retransmission mechanisms.

[0003] The purpose of the redundancy mechanism is to provide the receiver with error detection and correction capabilities, and the purpose of the retransmission mechanism is to increase the probability of successful reception at the receiver while ensuring reliable transmission. A typical redundancy mechanism is forward error control (FEC), but it cannot always guarantee transmission reliability. If the bit error rate of the channel is too high and the receiver cannot correct all errors, retransmission should be triggered to successfully receive the message. The typical retransmission mechanism is automatic repeat request (ARQ), which uses the receiver's confirmation scheme to update the sender's receiving status and retransmission scheme.

[0004] Although a series of equalization technologies can be used at the physical layer to enhance transmission reliability, packet decoding errors are still unavoidable when the underwater acoustic channel is very bad. FEC and ARQ can alleviate the pressure of equalization technology to a certain extent, but considering the long delay of underwater acoustic propagation, multiple retransmissions will cause a huge waste of time, and the uncertainty of underwater acoustic channel quality will also make it difficult to grasp the redundancy of forward error correction codes.

[0005] In order to realize a reliable transmission method in a real-time underwater acoustic communication system, in addition to the performance of the method itself, it is also necessary to consider issues such as complexity and real-time performance. Therefore, designing a reliable transmission method with good performance and being able to be implemented in an underwater acoustic embedded communication system is of great value to improving the quality of real-time underwater acoustic communication. Summary of the invention

[0006] In view of the deficiencies of the prior art, the present invention proposes a multi-band cross-layer reliable transmission method based on a real-time underwater acoustic communication system, and can implement a system for real-time underwater acoustic communication in embedded chips and programmable logic gate arrays.

[0007] The specific technical solutions are as follows:

[0008] A multi-band cross-layer reliable transmission method based on a real-time underwater acoustic communication system comprises the following steps:

[0009] S1: When the data to be sent in the current frame is sent for the first time, the sender divides the complete send bit stream into k parts at the data link layer, encapsulates each bit stream into a corresponding send data structure, and inputs it into the physical layer; the send data structure includes multiple send data frames;

[0010] S2: After the data in the transmission data structure is subjected to variable rate channel coding in the physical layer, it is modulated into k different frequency bands and sent to the receiving end;

[0011] S3: When data is first received, the receiving end performs variable rate channel decoding of the equalized symbols taking into account the random phase offset at the physical layer, and calculates the signal-to-noise ratio of the k frequency bands respectively, and adaptively adjusts the decoding rate of the k frequency bands; the decoded bit data packet and decoding error information, as well as the decoding rate, are input into the data link layer of the receiving end;

[0012] S4: The receiving end stores the correctly decoded data packets at the data link layer, and encapsulates the original frequency band of the incorrectly decoded data packets, the original position in the transmitted data frame, and the adaptively adjusted decoding code rate into an ACK data packet, and returns it to the sending end;

[0013] S5: When data is retransmitted, the sender parses the ACK data packet at the data link layer, updates the encoding rate of the variable rate channel coding with the adaptively adjusted decoding rate, extracts the data packet in the transmission data structure according to the original frequency band where the data packet with decoding error is located and the original position in the transmission data frame, and copies the decoded data packet to each frequency band to form a retransmission data frame; the retransmission data frame is sent to the physical layer, and variable rate channel coding, frequency band modulation and retransmission are performed at the physical layer;

[0014] S6: When receiving data retransmission, the receiving end performs the decoding operation described in S3. If the data packet of any frequency band is decoded correctly, the data packet is considered to be retransmitted successfully. Otherwise, the retransmission is considered to have failed, and the operations of S4-S6 are performed again until the maximum number of retransmissions is reached. When all data packets are decoded correctly, an ACK data packet is generated at the data link layer to instruct the sending end to send the next frame of data to be sent.

[0015] Furthermore, in S1, the size of the complete transmitted bit stream is , the size of each bit stream is , Indicates rounding up; the transmitted data frame includes a synchronization header, a frame header, and multiple data packets; the number of data packets contained in each transmitted data frame is not greater than the threshold , the bit stream size of a data packet is , the number of data frames allocated in each frequency band sending data structure is expressed as follows:

[0016]

[0017] In the formula, Represents the redundant information in each data packet and is used to indicate the number of valid bits in the data packet;

[0018] Before sending the data structure In the frame sending data frame, the number of data packets in each sending data frame is , the number of data packets in the last frame sent is:

[0019] .

[0020] Furthermore, in S3, the variable code rate channel decoding uses LDPC-CRC decoding, and the LDPC-CRC decoding considering the random phase offset is performed on the equalized symbols, which is specifically implemented by the following sub-steps:

[0021] (3.1) The likelihood ratio considering random phase offset is calculated based on the noise variance and phase variance, and the expression is as follows:

[0022]

[0023]

[0024] Where r represents the scalar of the received signal after equalization, t represents the scalar of the transmitted symbol sequence, represents the variance of Gaussian white noise, represents the variance of the random phase shift; Indicates the imaginary part function, * indicates the conjugate symbol; For each transmitted symbol bits, Indicates the first The set of symbols with bits set to 0, for Elements in Indicates the first The set of symbols with bits set to 1, for Elements in

[0025] (3.2) Calculate the check matrix: The master control controls the cyclic shift and data storage according to the code rate and code length information to complete the confirmation of the check matrix H;

[0026] (3.3) Iterative decoding: Use the LDPC method to perform iterative decoding until convergence, obtain the final bit likelihood ratio, and make soft decisions on it to obtain the decoded bits;

[0027] (3.4) CRC check: The decoded bits are subjected to CRC check. The decoded bits that pass the check are sent to the data link layer of the receiving end for storage. The original frequency band and original position of the data packets in the transmitted data frame that fail the check are sent to the data link layer of the receiving end.

[0028] Furthermore, in S3, respectively calculating the signal-to-noise ratios of the k frequency bands and adaptively adjusting the decoding bit rates of the k frequency bands are specifically implemented by the following operations:

[0029] The receiving signal received by the receiving end is , the signals obtained by passing through the bandpass filters of k frequency bands are , calculate the corresponding in-band signal energy; take a noise signal with the same length as the received signal before the synchronization head of each frequency band , calculate the corresponding noise signal capability, and get the signal-to-noise ratio expression of k frequency bands as follows:

[0030]

[0031] The signal-to-noise ratio is divided into different intervals, each interval corresponds to a different decoding rate, and the decoding rate decreases as the signal-to-noise ratio decreases; according to the intervals where the signal-to-noise ratios of different frequency bands are located, the decoding rates of k frequency bands are adjusted respectively. , The decoding code rate is set corresponding to the interval where the signal-to-noise ratio is located; the decoding code rate of the variable code rate channel decoding is updated with the adjusted decoding code rate, and is simultaneously sent to the data link layer of the receiving end.

[0032] Furthermore, the transmitted data frame includes a synchronization header, a frame header, and multiple data packets; the original information bit vector expression of each data packet is as follows:

[0033]

[0034] In the formula, i represents the frame number of the currently transmitted data frame, p represents the number of data packets contained in the current data frame, b represents the frequency band number of the current data frame; s0 represents the first original information bit, s1 represents the second original information bit, and so on; Indicates the total number of transmission data frames allocated in the transmission data structure of the kth frequency band;

[0035] The expression of the retransmission data frame is as follows:

[0036]

[0037] In the formula, Indicates the original frequency band where the data packet with decoding error is located, Indicates the original position of the data packet with decoding error in the transmitted data frame, n=1,2,…,N.

[0038] An underwater acoustic communication system, used to implement the multi-band cross-layer reliable transmission method based on the real-time underwater acoustic communication system, comprising a physical layer and a data link layer; the physical layer and the data link layer transmit data frames, instructions and bit rates through an advanced extensible interface;

[0039] The data link layer includes: a data frame sending module at the sending end, and a data frame receiving module and a retransmission frame sending module at the receiving end; the output of the data frame sending module is a sending data frame, the output of the retransmission frame sending module is a retransmission data frame, and the data frame receiving module is used to receive and store correctly decoded data packets;

[0040] The physical layer includes a variable code rate channel coding module and a multi-band modulation module at the transmitting end, and an equalization module, a signal-to-noise ratio calculation module, and a variable code rate channel decoding module considering random phase offset at the receiving end;

[0041] During the first transmission, the output of the data frame transmission module is used as the input of the variable code rate channel coding module; during the retransmission transmission, the output of the retransmission frame transmission module is used as the input of the variable code rate channel coding module; the output of the variable code rate channel coding module is used as the input of the multi-band modulation module, and the output of the multi-band modulation module is output to the receiving end through the multi-channel transducer;

[0042] The receiving end receives signals through a multi-channel hydrophone, the inputs of the equalization module and the signal-to-noise ratio calculation module are both received signals, the output of the equalization module is used as the input of the variable code rate channel decoding module considering random phase offset, and the outputs of the variable code rate channel decoding module considering random phase offset and the signal-to-noise ratio calculation module are input into the data link layer through an advanced extensible interface; wherein, the correctly decoded data packet is input into the data frame receiving module, and the ACK data frame is encapsulated and returned to the data frame sending module; the incorrectly decoded data packet, its original frequency band, original position in the sent data frame, and adaptively adjusted decoding code rate are encapsulated into the ACK data frame and input into the retransmission frame sending module.

[0043] Furthermore, the data link layer uses an ARM chip, and the physical layer uses a programmable logic gate array.

[0044] Furthermore, the output of the data frame sending module is divided into multiple frequency bands, each frequency band corresponds to a sending data structure, and multiple sending data frames are established in the sending data structure; each sending data frame includes: a synchronization header, a frame header and at most A protection interval is set between the synchronization header and the frame header, and between the frame header and the data packet.

[0045] Furthermore, the frame header of the transmitted data frame contains identification information, including: data frame type, total number of transmitted data frames in the current frequency band, , Current data frame number being sent , the number of data packets in the current sent data frame , Current frequency band number ;

[0046] The data frame types include: sending data frame, ACK frame, and retransmission data frame.

[0047] Furthermore, the ACK data frame includes, in sequence: a synchronization header, a frame header and an ACK data packet; the ACK data packet includes the frequency band where the decoding error data packet is located, the position in the transmitted data frame, and the decoding code rate;

[0048] The retransmission data frame includes: a synchronization header, a frame header and multiple data packets storing valid data, wherein the identification information encapsulated in the frame header includes: data frame type, total number of data frames sent in the current frequency band , Current data frame number being sent , the number of data packets in the current sent data frame , Current frequency band number , the original frequency band where the decoding error data packet is located, and the original position of the decoding error data packet in the transmitted data frame.

[0049] The beneficial effects of the present invention are:

[0050] (1) The present invention combines the forward error correction code mechanism adapted to the underwater acoustic physical layer with the multi-band retransmission mechanism of the data link layer, greatly improving the reliability of underwater acoustic communication transmission. Considering the different channels of each frequency band of the underwater acoustic physical layer, the cross-layer transmission protocol design uses the channel information carried by the ACK data packet to perform adaptive retransmission of the data link layer and code rate adjustment of the physical layer, thereby improving the communication throughput while ensuring communication reliability.

[0051] (2) The present invention utilizes a multi-band retransmission mechanism to reduce the number of retransmission requests and improve the probability of successful retransmission, which can effectively address the problems of slow propagation of underwater acoustic communication and poor quality of underwater acoustic channels.

[0052] (3) The present invention can implement the proposed method on an embedded platform with lower resource consumption and computing delay, and has high real-time performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 It is a schematic diagram of the framework and data flow of the underwater acoustic communication system proposed in an embodiment of the present invention.

[0054] Figure 2 It is a schematic diagram of the structure of sending a data frame in an embodiment of the present invention.

[0055] Figure 3 It is a schematic diagram of the frame header structure of sending data frames in an embodiment of the present invention.

[0056] Figure 4 It is a schematic diagram of the structure of an ACK data frame in an embodiment of the present invention.

[0057] Figure 5 It is a schematic diagram of the structure of the header of the retransmitted data frame in an embodiment of the present invention.

[0058] Figure 6 It is an internal flow chart of a data frame sending module in a data link layer in an embodiment of the present invention.

[0059] Figure 7 It is an internal flow chart of the LDPC-CRC decoding module considering random phase offset in the physical layer in an embodiment of the present invention.

[0060] Figure 8 It is an internal flow chart of the signal-to-noise ratio calculation module in the physical layer in an embodiment of the present invention.

[0061] Fig. 9 It is an internal flow chart of the retransmission frame sending module in the data link layer in an embodiment of the present invention. DETAILED DESCRIPTION

[0062] The present invention will be described in detail below according to the accompanying drawings and preferred embodiments, and the purpose and effect of the present invention will become more clear. The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0063] like Figure 1As shown, an underwater acoustic communication system includes a cross-layer design of a physical layer and a data link layer. The data link layer is implemented using an ARM chip suitable for data parsing, and the ARM includes a data frame sending module at the sending end, and a data frame receiving module and a retransmission frame sending module at the receiving end; the output of the data frame sending module is a sending data frame, and the output of the retransmission frame sending module is a retransmission data frame. The physical layer is implemented using a field programmable gate array (FPGA) with high speed and high parallel processing capability, and the FPGA includes a variable code rate channel coding module and a multi-band modulation module at the sending end, as well as an equalization module, a signal-to-noise ratio calculation module, and a variable code rate channel decoding module considering random phase offset at the receiving end; in this embodiment, the variable code rate channel coding module selects an LDPC-CRC (low density parity check - cyclic redundancy check) coding module, and correspondingly, the variable code rate channel decoding module selects an LDPC-CRC decoding module. The advanced extensible interface (AXI) is used between ARM and FPGA for high-speed transmission of data frames, instructions and bit rates. The receiving end also includes a multi-channel transducer for outputting the transmission signal and a multi-channel hydrophone for receiving the signal.

[0064] The outputs of the data frame sending module and the retransmission frame sending module of the data link layer of the transmitter are input into the physical layer through AXI and used as the input of the LDPC-CRC encoding module. The output of the LDPC-CRC encoding module is used as the input of the multi-band modulation module. The output of the multi-band modulation module is output to the receiving end through a multi-channel transducer. The receiving end receives the signal through a multi-channel hydrophone, and the received signal is used as the input of the equalization module and the signal-to-noise ratio calculation module in the physical layer. The output of the equalization module is used as the input of the LDPC-CRC decoding module considering random phase offset. The output of the LDPC-CRC decoding module considering random phase offset and the signal-to-noise ratio calculation module are input into the data link layer through AXI. The correctly decoded data is input into the data frame receiving module, and the ACK data frame is encapsulated and returned to the data frame sending module; the incorrectly decoded data, its frequency band, position, code rate and other information are encapsulated into the ACK data frame and returned to the retransmission frame sending module.

[0065] The output of the data frame sending module is divided into multiple frequency bands, each frequency band corresponds to a sending data structure, and multiple sending data frames are established in the sending data structure. Figure 2 As shown, each transmitted data frame includes: a synchronization header, a frame header and at most Packets ( A guard interval is set between the synchronization header and the frame header, and between the frame header and the data packet to reduce the inter-symbol interference (ISI) caused by the strong multipath effect of the underwater acoustic channel. Figure 3 As shown, the frame header of the transmitted data frame contains identification information for the receiving end to determine the receiving logic. The identification information includes: data frame type, total number of transmitted data frames in the current frequency band , Current data frame number being sent , the number of data packets in the current sent data frame , Current frequency band number Among them, the data frame types include: sending data frame, ACK frame, and retransmission data frame.

[0066] like Figure 4 As shown, the ACK data frame includes: a synchronization header, a frame header and an ACK data packet in sequence. The ACK data packet includes the frequency band where the decoding error data packet is located, the position in the transmission data frame, and the code rate.

[0067] like Figure 5 As shown, the retransmission data frame includes: a synchronization header, a frame header and multiple data packets storing valid data. The identification information encapsulated in the frame header includes: data frame type, the total number of data frames sent in the current frequency band , Current data frame number being sent , the number of data packets in the current sent data frame , Current frequency band number , the original frequency band where the decoding error data packet is located, and the original position of the decoding error data packet in the transmitted data frame.

[0068] Based on the above-mentioned underwater acoustic communication system, an embodiment of the present invention further proposes a multi-band cross-layer reliable transmission method based on a real-time underwater acoustic communication system, comprising the following steps:

[0069] S1: Figure 6 As shown, when the current frame of data to be sent is sent for the first time, the data frame sending module at the data link layer of the sender divides the complete sending bit stream corresponding to the current frame of data to be sent into k parts, encapsulates each sending bit stream into the corresponding sending data structure, and inputs the k sending data structures into the physical layer. The details are as follows:

[0070] The sender sends the complete bit stream at the data link layer Divide into k parts and put them into k sending data structures. The size of each bit stream is , In the send data structure, multiple send data frames are established. Each send data frame contains a synchronization header, a frame header and at most data packets, and the bit stream size contained in a data packet is (Specifically set by humans), the number of data frames allocated in the data structure for each frequency band can be expressed as:

[0071]

[0072] In the formula, Represents the redundant information in each data packet and is used to indicate the number of valid bits in the data packet.

[0073] in front In the frame sending data frame, the number of data packets in each sending data frame is , the number of data packets in the last frame sent is:

[0074]

[0075] All identification information is encapsulated into the frame header of the transmitted data frame for the receiving end to make a decision on the receiving logic. The original information bit vector of each data packet in the transmitted data frame is encapsulated into the corresponding data packet, and then sent to the physical layer for encoding and modulation. The original information bit vector of each data packet in the transmitted data frame can be expressed as:

[0076]

[0077] In the formula, i represents the frame number of the currently transmitted data frame, p represents the number of data packets contained in the current data frame, b represents the frequency band number of the current data frame; s0 represents the first original information bit, s1 represents the second original information bit, and so on; Indicates the total number of transmission data frames allocated in the k-th frequency band transmission data structure.

[0078] S2: The LDPC-CRC encoding module at the physical layer of the transmitter converts the original information bit vector After CRC encoding and LDPC encoding in sequence, the transmission data structure is multi-band modulated by the multi-band modulation module, and finally the modulated digital signals of k different frequency bands are respectively sent through k groups of multi-channel transducers.

[0079] S3: When the data is received for the first time, the receiving end uses the LDPC-CRC decoding module that takes random phase offset into consideration at the physical layer to perform LDPC-CRC decoding that takes random phase offset into consideration on the symbols equalized by the equalization module. At the same time, the signal-to-noise ratio calculation module calculates the signal-to-noise ratio of the k frequency bands respectively, and adaptively adjusts the decoding code rate of the k frequency bands.

[0080] like Figure 7 As shown, LDPC-CRC decoding is implemented through the following sub-steps:

[0081] (3.1) Considering that the Doppler effect of the underwater acoustic channel produces residual frequency deviation and random phase, and the symbol equalized by the equalization module has compensated for the residual frequency deviation, the received signal after equalization is It can be expressed as:

[0082]

[0083] In the formula, To send a symbol sequence, is a random phase shift with a mean of 0 and a variance of Gaussian distribution of is Gaussian white noise with a mean of 0 and a variance of The complex Gaussian distribution of .

[0084] The conditional probability density function is:

[0085]

[0086] Considering According to the Taylor expansion property, we can get , then the conditional probability density function can be modified as:

[0087]

[0088]

[0089] In the formula, * is the conjugation symbol.

[0090] Considering For Gaussian distribution, the conditional probability density function can be further simplified as:

[0091]

[0092] In the formula, Represents the imaginary part function.

[0093] The likelihood ratio considering random phase offset is calculated based on the noise variance and phase variance: The bit log-likelihood ratio of the suppressed phase noise can be expressed as:

[0094]

[0095] In the formula, For each transmitted symbol bits, for The observed value of Indicates the first The set of symbols with bits set to 0, for Elements in Indicates the first The set of symbols with bits set to 1, for The elements in .

[0096] (3.2) Calculation of the check matrix: Based on the existing LDPC coding method, the master controller controls the cyclic shift and data storage according to the code rate and code length information to complete the confirmation of the check matrix H.

[0097] (3.3) Iterative decoding: The LDPC method is used to perform iterative decoding until convergence, and the final bit likelihood ratio is obtained, and soft decisions are made on it to obtain the decoded bits.

[0098] (3.4) CRC check: The decoded bits are subjected to CRC check. If the final result is all 0, it means the check is passed and the decoding result is correct. The check result and valid bit information are output, and the decoded bit information is sent to the data link layer of the receiving end. Otherwise, it means the check fails and the decoding result is wrong. The check result and data packet related information are output to the data link layer.

[0099] The signal-to-noise ratio calculation module is used to adaptively adjust the signal-to-noise ratio of the k frequency bands, as follows:

[0100] like Figure 8 As shown, suppose the received signal received by the receiving end is , the signals obtained by passing through the bandpass filters of k frequency bands are , calculate the corresponding in-band signal energy; take a noise signal with the same length as the received signal before the synchronization head of each frequency band , calculate the corresponding noise signal capability, and thus calculate the signal-to-noise ratio of k frequency bands. The expression is as follows:

[0101]

[0102] The signal-to-noise ratio is divided into different intervals, each interval corresponds to a different bit rate, and the bit rate decreases as the signal-to-noise ratio decreases to ensure higher reliability. , respectively adjust the decoding bit rate of k frequency bands , The code rate is set corresponding to the signal-to-noise ratio interval. The adjusted decoding code rate is stored in the LDPC-CRC decoding module considering random phase offset at the physical layer of the receiving end, and is sent to the data link layer of the receiving end at the same time.

[0103] S4: The receiving end encapsulates the decoding error data packet information and the adaptively adjusted decoding code rate into an ACK data frame at the data link layer and returns it to the sending end. Specifically:

[0104] The receiving end obtains the bit data packet and correctness information of each frequency band data frame after physical layer decoding at the data link layer, sends the correctly decoded data packet to the receiving structure of the data frame receiving module for buffering, and sends the original frequency band where the decoded wrong data packet is located. and the original position of the packet in the transmitted data frame is sent to the retransmission frame sending module for buffering, where N is the total number of error packets in the data frame; and , and The decoding bit rate after adaptive updating of the frequency band It is encapsulated in a data packet of an ACK data frame and returned to the sender.

[0105] S5: Fig. 9 As shown in the figure, when data is retransmitted, the sender parses the ACK data packet at the data link layer and adaptively updates the decoding rate of the k frequency bands. Send to the physical layer encoding module to update the encoding rate, according to , Extract the data packets in the transmission data structure, copy the data packets to each frequency band, so that each frequency band contains all the error-corrected data packets, and form a retransmission data frame. At this time, the retransmission data frame can be expressed as:

[0106]

[0107] Finally, the retransmitted data frame is sent to the physical layer for encoding, multi-band modulation and underwater acoustic transmission.

[0108] S6: When receiving data retransmission, the receiving end performs the decoding operation in S3. If the data packet of any frequency band is decoded correctly, the data packet is considered to be retransmitted successfully; otherwise, the retransmission is considered to have failed, and the operations of S4-S6 are performed again to send the original data packets of all decoding errors in the new round to the receiving end. and The data is encapsulated into an ACK data frame, returned to the sender and instructed to continue retransmitting until the maximum number of retransmissions is reached. When all data packets are decoded correctly, an ACK data packet is generated at the data link layer to instruct the sender to send the next frame of data to be sent; if the maximum number of retransmissions is reached and all data packets are still not correctly transmitted, the communication is considered to have failed.

[0109] The present invention encodes and decodes valid information bits through the forward error correction code of the physical layer, thereby increasing the probability of successful reception at the receiving end; through the multi-band retransmission mechanism of the data link layer, when the forward error correction code cannot correct all errors, the error data packet is copied to all frequency bands for retransmission, thereby improving the reliability of transmission; when a frame of data is received, the channel quality of each frequency band is evaluated, and the forward error correction code rate of each frequency band is adaptively adjusted to improve the effective data throughput. On the other hand, since the underwater acoustic channel also exhibits different performance in different frequency bands, the total communication frequency band is divided into multiple sub-frequency bands for transmission, and the receiving end evaluates the channel quality of each frequency band to control the encoding and retransmission strategy of the transmitting end, which can effectively reduce the number of retransmissions and increase the transmission throughput under the premise of ensuring successful transmission.

[0110] Those skilled in the art can understand that the above are only preferred examples of the invention and are not intended to limit the invention. Although the invention is described in detail with reference to the above examples, those skilled in the art can still modify the technical solutions recorded in the above examples or replace some of the technical features therein with equivalents. Any modification, equivalent replacement, etc. made within the spirit and principle of the invention shall be included in the protection scope of the invention.

Claims

1. A multi-band cross-layer reliable transmission method based on a real-time underwater acoustic communication system, characterized in that: The following steps are involved: S1: The transmitting end divides the complete transmission bit stream corresponding to the data to be transmitted in the current frame into k parts, and encapsulates each bit stream into a corresponding transmission data structure, wherein the transmission data structure includes multiple transmission data frames; after the data in the transmission data structure is subjected to variable code rate channel coding, it is modulated into k different frequency bands and sent to the receiving end; S2: After equalizing the received signal, the receiving end performs variable rate channel decoding taking into account the random phase offset, and calculates the signal-to-noise ratio of the k frequency bands respectively, and adaptively adjusts the decoding rate of the k frequency bands; Obtaining decoded bit data packets, decoding correctness and error information, and adaptively adjusted decoding bit rate; S3: Store the correctly decoded data packets, and encapsulate the original frequency band of the incorrectly decoded data packets, the original position in the transmitted data frame, and the adaptively adjusted decoding code rate into an ACK data packet, and return it to the sender; S4: The sender parses the ACK data packet, copies the decoded data packet to each frequency band to form a retransmission data frame, updates the encoding code rate based on the adaptively adjusted decoding code rate, and then performs variable code rate channel coding, frequency band modulation and retransmission on it; S5: The receiving end performs the operation described in S2. If the data packet of any frequency band is decoded correctly, it is considered that the data packet is retransmitted successfully. Otherwise, it is considered that the retransmission fails. The operations of S3-S5 are performed again until the maximum number of retransmissions is reached. When all data packets are decoded correctly, the receiving end generates an ACK data packet to instruct the sending end to send the next frame of data to be sent. In S2, the variable code rate channel decoding uses LDPC-CRC decoding, and the LDPC-CRC decoding considering the random phase offset is performed on the equalized symbols, which is specifically implemented by the following sub-steps: (2.1) The likelihood ratio considering random phase offset is calculated based on the noise variance and phase variance, and the expression is as follows: Where r represents the scalar of the received signal after equalization, t represents the scalar of the transmitted symbol sequence, and σ 2 represents the variance of Gaussian white noise, represents the variance of the random phase shift; represents the imaginary part function, * represents the conjugate symbol; b f For the fth bit of each transmitted symbol, represents the set of symbols whose fth bit is 0 in the modulation symbol set, α f for Elements in represents the set of symbols whose fth bit is 1 in the modulation symbol set, β f for Elements in (2.2) Calculate the check matrix: The master control controls the cyclic shift and data storage according to the code rate and code length information to complete the confirmation of the check matrix H; (2.3) Iterative decoding: Use the LDPC method to perform iterative decoding until convergence, obtain the final bit likelihood ratio, and make soft decisions on it to obtain the decoded bits; (2.4) CRC check: The decoded bits are subjected to CRC check, and the decoded bits that pass the check are stored; The original frequency band of the data packets that fail the check and their original positions in the transmitted data frames are sent to the receiving end.

2. The multi-band cross-layer reliable transmission method based on a real-time underwater acoustic communication system according to claim 1 is characterized in that: In S1, the size of the complete transmitted bit stream is a total , the size of each bit stream is Indicates rounding up; the transmitted data frame includes a synchronization header, a frame header, and multiple data packets; the number of data packets contained in each transmitted data frame is not greater than the threshold value L pack , the bit stream size contained in a data packet is a pack , the number of data frames allocated in each frequency band sending data structure is expressed as follows: In the formula, δ pack Represents the redundant information in each data packet and is used to indicate the number of valid bits in the data packet; Before sending the data structure (L frame -1) In the frame sending data frame, the number of data packets in each sending data frame is L pack , the number of data packets in the last frame sent is:

3. The multi-band cross-layer reliable transmission method based on a real-time underwater acoustic communication system according to claim 1 is characterized in that: In S2, respectively calculating the signal-to-noise ratios of the k frequency bands and adaptively adjusting the decoding bit rates of the k frequency bands are specifically implemented by the following operations: The receiving signal received by the receiving end is r0, and the signals obtained by passing through the bandpass filters of k frequency bands are r1, r2, ..., r k , calculate the corresponding in-band signal energy; take a noise signal n1, n2, ..., n before the synchronization head of each frequency band with the same length as the received signal k , calculate the corresponding noise signal capability, and get the signal-to-noise ratio expression of k frequency bands as follows: The signal-to-noise ratio is divided into different intervals, each interval corresponds to a different decoding rate, and the decoding rate decreases as the signal-to-noise ratio decreases; according to the intervals where the signal-to-noise ratios of different frequency bands are located, the decoding rates of k frequency bands are adjusted respectively. The decoding code rate corresponding to the signal-to-noise ratio interval is set, and the decoding code rate of the variable code rate channel decoding is updated with the adjusted decoding code rate.

4. The multi-band cross-layer reliable transmission method based on a real-time underwater acoustic communication system according to claim 1 is characterized in that: The transmission data frame includes a synchronization header, a frame header, and multiple data packets; the original information bit vector expression of each data packet is as follows: i∈{1,2,…,L frame，k } p∈{L pack ,L plast } b∈{1,2,…,k} Where i represents the frame number of the currently transmitted data frame, p represents the number of data packets contained in the current data frame, and b represents the frequency band number of the current data frame; s0 represents the first original information bit, s1 represents the second original information bit, and so on; L frame，k Indicates the total number of transmission data frames allocated in the transmission data structure of the kth frequency band; The expression of the retransmission data frame is as follows: Where, d n Indicates the original frequency band where the data packet with decoding error is located, h n Indicates the original position of the data packet with decoding error in the transmitted data frame, n=1, 2, ..., N.

5. An underwater acoustic communication system, used to implement the multi-band cross-layer reliable transmission method based on a real-time underwater acoustic communication system according to any one of claims 1 to 4, characterized in that: It includes a physical layer and a data link layer; the physical layer and the data link layer transmit data frames, instructions and code rates through an advanced extensible interface; The data link layer includes: a data frame sending module at the sending end, and a data frame receiving module and a retransmission frame sending module at the receiving end; the output of the data frame sending module is a sending data frame, the output of the retransmission frame sending module is a retransmission data frame, and the data frame receiving module is used to receive and store correctly decoded data packets; The physical layer includes a variable code rate channel coding module and a multi-band modulation module at the transmitting end, and an equalization module, a signal-to-noise ratio calculation module, and a variable code rate channel decoding module considering random phase offset at the receiving end; During the first transmission, the output of the data frame transmission module is used as the input of the variable code rate channel coding module; during the retransmission transmission, the output of the retransmission frame transmission module is used as the input of the variable code rate channel coding module; the output of the variable code rate channel coding module is used as the input of the multi-band modulation module, and the output of the multi-band modulation module is output to the receiving end through the multi-channel transducer; The receiving end receives signals through a multi-channel hydrophone, the inputs of the equalization module and the signal-to-noise ratio calculation module are both received signals, the output of the equalization module is used as the input of the variable code rate channel decoding module considering random phase offset, and the outputs of the variable code rate channel decoding module considering random phase offset and the signal-to-noise ratio calculation module are input into the data link layer through an advanced extensible interface; wherein, the correctly decoded data packet is input into the data frame receiving module, and the ACK data frame is encapsulated and returned to the data frame sending module; the incorrectly decoded data packet, its original frequency band, original position in the sent data frame, and adaptively adjusted decoding code rate are encapsulated into the ACK data frame and input into the retransmission frame sending module.

6. The underwater acoustic communication system according to claim 5, characterized in that: The data link layer uses an ARM chip, and the physical layer uses a programmable logic gate array.

7. The underwater acoustic communication system according to claim 5, characterized in that: The output of the data frame sending module is divided into multiple frequency bands, each frequency band corresponds to a sending data structure, and multiple sending data frames are established in the sending data structure; each sending data frame includes: a synchronization header, a frame header and at most L pack A protection interval is set between the synchronization header and the frame header, and between the frame header and the data packet.

8. The underwater acoustic communication system according to claim 7, characterized in that: The frame header of the transmitted data frame contains identification information, including: data frame type, total number of transmitted data frames L in the current frequency band frame 、The current data frame number L frameindex , the number of data packets in the current transmitted data frame L pack , Current frequency band number L band ; The data frame types include: sending data frame, ACK frame, and retransmission data frame.

9. The underwater acoustic communication system according to claim 5, characterized in that: The ACK data frame includes: a synchronization header, a frame header and an ACK data packet in sequence; the ACK data packet includes the frequency band where the decoding error data packet is located, the position in the transmitted data frame, and the decoding code rate; The retransmission data frame includes: a synchronization header, a frame header and multiple data packets storing valid data, wherein the identification information encapsulated in the frame header includes: data frame type, total number of data frames sent in the current frequency band L frame 、The current data frame number L frameindex , the number of data packets in the current transmitted data frame L pack , Current frequency band number L band , the original frequency band where the decoding error data packet is located, and the original position of the decoding error data packet in the transmitted data frame.

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