High-speed incoherent underwater acoustic communication index spread spectrum system

Abstract

The invention discloses a high-speed incoherent underwater acoustic communication index spread spectrum system, and relates to the technical field of underwater acoustic communication. Comprising a transmitting end and a receiving end. A transmitting end performs channel coding, interleaving and system conversion on a binary signal source, selects a plurality of spreading codes by using an index matrix for modulation and linear superposition, and transmits the binary signal source to an underwater acoustic channel after carrier modulation, digital-to-analog conversion and power amplification. A receiving end performs pre-amplification, band-pass filtering, digital-to-analog conversion and carrier demodulation on a received signal, calculates the maximum likelihood ratio of each bit based on the maximum posterior probability theory by using an index matrix and a probability matrix for demodulation, and restores a binary signal source through de-interleaving and channel decoding. The index modulation technology is applied to the spread spectrum system, so that the utilization rate of spread spectrum codes is improved, and the communication rate is improved; and meanwhile, a soft decision demodulation receiver based on the maximum posterior probability is designed, so that the anti-interference performance of the system is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of underwater acoustic communication, and in particular to a high-speed incoherent underwater acoustic communication index spread spectrum system. Background Art

[0002] In the field of underwater acoustic communication engineering applications, incoherent underwater acoustic communication systems are widely used because they are more robust to Doppler effect, low signal-to-noise ratio and phase noise than coherent underwater acoustic communication systems. They can be demodulated directly through energy detection, avoiding the design and deployment of high-complexity receivers. Traditional incoherent underwater acoustic communication systems include on-off keying, spread spectrum, frequency hopping, multi-frequency shift keying, etc. Although they are highly robust, their spectrum utilization and communication rate are limited.

[0003] As people continue to explore and develop the ocean, the problem of low communication rate of incoherent underwater acoustic communication modulation and demodulation technology has gradually attracted more and more attention. Among them, the spread spectrum system has a strong anti-interference ability because it expands the bandwidth of the information to be transmitted. How to improve the rate of the spread spectrum system has received widespread attention.

[0004] In recent years, index modulation technology has been proposed and applied in orthogonal frequency division multiplexing (OFDM) coherent underwater acoustic communication modulation and demodulation technology. The original binary information is converted into a sequence index of the activated subcarrier through base-to-base conversion, which effectively provides the communication rate and shows its broad prospect of combining with incoherent systems to improve the communication rate.

[0005] Channel encoders and decoders can provide good error correction capabilities for underwater acoustic communication systems. However, the quantization process of the traditional energy detection hard decision method inevitably causes information loss of the demodulated signal, resulting in a decrease in error correction capability. The design of the soft decision receiver can retain more channel quality information and provide sufficient soft information input for the channel decoder.

[0006] Therefore, in order to improve the communication rate of the traditional incoherent underwater acoustic communication spread spectrum system, the present invention proposes a high-speed incoherent underwater acoustic communication index spread spectrum system, wherein a soft decision receiver is designed based on the maximum a posteriori probability theory.

[0007] Chinese patent ZL109302208B discloses a parallel combination spread spectrum underwater acoustic communication method of interleaved Gold mapping sequences, focusing on improving the performance of the spread spectrum system, selecting multiple information sequences from the spread spectrum sequence set for multi-way interleaving to generate an information mapping sequence, selecting a sequence that is unrelated to the sequences in the spread spectrum sequence set as a synchronization sequence, orthogonally modulating the synchronization sequence and the information mapping sequence and transmitting them to the underwater acoustic channel; Chinese patent ZL114337729B discloses a communication method, a transmitting device and a receiving device of a parallel combination spread spectrum system, dividing the communication data input by the system into three parts: sequence selection, polarity selection, and sequence selection to be weighted, and performing information modulation in sequence to form the final combination sequence. At present, there is no incoherent underwater acoustic communication index spread spectrum system based on a maximum a posteriori probability soft decision receiver.

[0008] The traditional incoherent underwater acoustic communication spread spectrum system only selects one spread spectrum code symbol in each symbol information modulation process, which does not fully utilize the spread spectrum code, the utilization rate of the spread spectrum code is low, and the number of spread spectrum codes is limited, which further reduces the communication rate. In addition, the receiver of the traditional incoherent underwater acoustic communication spread spectrum system adopts the energy detection hard decision method, in which the quantization mapping process leads to the loss of demodulation information and reduces the error correction capability of the channel decoder. Summary of the invention

[0009] The purpose of the present invention is to provide a high-speed incoherent underwater acoustic communication index spread spectrum system to solve the problem of low communication rate of traditional incoherent underwater acoustic communication systems and improve the communication rate of traditional incoherent underwater acoustic communication spread spectrum systems. By designing a soft decision receiver based on maximum a posteriori probability, the information loss caused by the quantization process of the traditional energy detection hard decision method is overcome, and the system robustness is improved.

[0010] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions.

[0011] A high-speed incoherent underwater acoustic communication index spread spectrum system includes a transmitting end and a receiving end:

[0012] The transmitting end is used to generate an index spread spectrum signal using an index matrix and a probability matrix after channel coding, interleaving and base conversion of a binary source, and transmit the signal to an underwater acoustic channel after carrier modulation; the transmitting end comprises a channel encoder, an interleaver, a base converter, an index matrix and a probability matrix generator, an index spread spectrum modulator, a carrier modulator, a digital-to-analog converter and a power amplifier;

[0013] The channel encoder is used to encode a binary information source; the interleaver is used to interleave the encoded sequence; the base converter is used to convert the binary sequence into a decimal sequence; the index matrix and probability matrix generator are used to generate the index matrix and the probability matrix; the carrier modulator, the digital-to-analog converter, and the power amplifier are used to convert the digital signal into an analog signal and transmit it; the binary information source is encoded by the channel encoder and interleaved by the interleaver, and then converted into a decimal sequence by the base converter, the index matrix and the probability matrix generator generate an index matrix, and the index spread spectrum modulator selects multiple spread spectrum codes according to the index matrix for modulation and linear superposition, and the generated signal is transmitted to the underwater acoustic channel after carrier modulation, digital-to-analog conversion, and power amplification;

[0014] The receiving end is used to receive the distorted signal, pre-process it through an analog circuit and obtain a digital signal through analog-to-digital conversion, use the index matrix and probability matrix, perform soft decision demodulation based on the maximum a posteriori probability theory, and finally restore the binary source; the receiving end includes a preamplifier, a bandpass filter, a digital-to-analog converter, a carrier demodulator, an index matrix and a probability matrix generator, an index spread spectrum demodulator, a deinterleaver and a channel decoder;

[0015] The preamplifier, bandpass filter, analog-to-digital converter, and carrier demodulator are used to process the received distorted signal; the deinterleaver and channel decoder are used to restore the binary information source; after the received signal is preamplified, bandpass filtered, digital-to-analog converted, and carrier demodulated, the index matrix and probability matrix generator generates an index matrix and a probability matrix, the index spread spectrum demodulator calculates the maximum likelihood ratio of each bit according to the index matrix and the probability matrix using the maximum a posteriori probability theory, and the deinterleaver and decoder process the demodulated signal in turn to restore the binary information source transmitted by the transmitter.

[0016] Furthermore, the index matrix and probability matrix generator generates an index matrix based on combinatorial theory, and calculates the maximum likelihood ratio of each bit using the index matrix and the probability matrix.

[0017] Furthermore, the index spread spectrum demodulator at the receiving end calculates the maximum likelihood ratio of each bit based on the maximum a posteriori probability theory using the index matrix and the probability matrix to replace the traditional energy detection hard decision quantization process, thereby retaining demodulation information for the channel decoder.

[0018] Furthermore, the system selects multiple spreading codes in each symbol, converting the selection of a single spreading code sequence into the selection of multiple spreading code index sequences, so as to improve the utilization of the spreading codes.

[0019] A high-speed incoherent underwater acoustic communication index spread spectrum method comprises the following steps:

[0020] 1) At the transmitting end, consider the length The binary source sequence , through the code rate The sequence is obtained after the channel encoder, the interleaver shuffles the order, and the base converter converts the binary sequence into a decimal sequence. ,in Represents the number of symbols in a frame signal, and Represent the channel decoder output length and modulation parameters respectively; the base converter will Convert consecutive binary data into a decimal number, that is ; Consider the traditional spread spectrum system The length is The spreading code, spreading code set and spreading code are represented as and ; Among them, the spreading codes in the spreading code set satisfy orthogonality;

[0021]

[0022] Among them, the symbol Represents a convolution operation.

[0023] In the Only one of the spreading codes needs to be activated among the symbols, which can be expressed as:

[0024]

[0025] The index spread spectrum system applies the index technology to the traditional spread spectrum system, and it is necessary to select spreading codes, that is, converting the selection of a single spreading code sequence into the selection of multiple spreading code index sequences;

[0026] Consider that the index matrix generator uses combinatorial theory to generate a matrix of size The index matrix ,in Indicates from Select the number The number of all possible combinations obtained by Contains all possible multiple activation spreading code index sequences, Each row in represents a pattern of multiple activated spreading code index sequences; therefore, The modulated index spread spectrum symbol contains The spreading code is expressed as:

[0027]

[0028] in, Represented by the matrix Middle The columns together form a submatrix, Represents the index matrix No. Finally, The modulated index spread spectrum symbol linearly superimposes the selected multiple spread spectrum codes, which can be expressed as:

[0029]

[0030] in, Indicates row summation. Compared with the traditional spread spectrum principle and the index spread spectrum principle, the index spread spectrum system proposed in the present invention can greatly improve the traditional spread spectrum communication rate to achieve high-speed incoherent underwater acoustic communication. The binary information rate contained in each symbol of the two can be expressed as

[0031]

[0032]

[0033] in, It is obvious that the combination of multiple spreading codes can obtain Much larger than that obtained by selecting a single spreading code , so the communication rate is greatly improved. In addition, when the number of spreading codes is limited, the index spread spectrum system can overcome the problem of low communication rate of the traditional spread spectrum system.

[0034] After getting each symbol, connect them in series to get the final transmitted digital signal:

[0035]

[0036] It is then combined with the synchronization header to form a complete frame signal. A complete frame signal includes four parts: coarse synchronization, fine synchronization, protection interval and signal. The synchronization header is composed of a sweep frequency signal, which is divided into two steps: coarse synchronization determines the signal to determine the synchronization header related threshold and fine synchronization determines the signal starting point, so as to achieve high-quality signal synchronization; the protection interval is used to overcome the influence of multipath effect on the synchronization header; the frame signal is transmitted to the underwater acoustic channel through carrier modulation, digital-to-analog converter and power amplifier;

[0037] 2) At the receiving end, the received distorted signal is passed through the preamplifier, bandpass filter, analog-to-digital converter, and carrier demodulator to obtain the baseband signal. A frame of baseband signal is divided into symbols. Assume that The received index spread spectrum symbol is , No. received indexed spreading symbols With local spreading code set The spreading code is correlated and the The output of the correlator is expressed as:

[0038]

[0039] Take the maximum value of the correlation result, then The maximum correlation result of the received indexed spread spectrum symbol and the local spread spectrum code set is expressed as:

[0040]

[0041] in, Represents the infinite norm of a vector; consider the The probability matrices of bits being 1 and 0 are and , defined as:

[0042] ,

[0043] ,

[0044] in, and Represents the index matrix belongs to and All row indices of , defined as:

[0045] ,

[0046] ,

[0047] in, , , represents the domain of integers; and Represents binary numbers Shift Right Bits and Binary Numbers Perform AND operation with 1;

[0048] Based on the maximum a posteriori probability theory, using the Bayes formula, Taylor expansion formula and max-log approximation formula, we get The maximum likelihood ratio of bits is:

[0049] in,

[0050] , ,

[0051] in, represents the noise variance, represents a square operation; the maximum likelihood ratio of each bit calculated after demodulating all symbols is connected in series as the input of the interleaver and channel decoder, and finally the binary source transmitted by the transmitter is restored.

[0052] In order to improve the communication rate and maintain good robustness, the present invention applies index modulation technology to the spread spectrum system, designs a soft decision demodulation receiver based on the maximum a posteriori probability, and proposes a high-speed incoherent underwater acoustic communication index spread spectrum system. The system adopts index technology to convert the selection of a single spread spectrum code in the traditional system into an index sequence for selecting multiple spread spectrum codes, that is, multiple spread spectrum codes are selected for modulation in each symbol information modulation process, thereby improving the utilization rate of the spread spectrum code and thus improving the communication rate. The system designs a soft decision demodulation receiver based on the maximum a posteriori probability, and uses the index matrix and probability matrix to calculate the maximum likelihood ratio of each bit to replace the traditional energy detection hard decision quantization process, so as to maximize the retention of demodulation information for the channel decoder and improve the error correction performance.

[0053] Compared with the prior art, the present invention has the following outstanding technical effects and advantages:

[0054] 1. Based on the traditional incoherent underwater acoustic communication index spread spectrum system, the present invention combines the index modulation technology to convert the selection of a single spread spectrum code in the traditional system into an index sequence for selecting multiple spread spectrum codes, that is, multiple spread spectrum codes are selected for modulation in each symbol information modulation process, thereby improving the utilization rate of the spread spectrum code and thus improving the communication rate.

[0055] 2. The present invention designs a soft-decision demodulation receiver based on maximum a posteriori probability, and uses the index matrix and probability matrix to calculate the maximum likelihood ratio of each bit to replace the traditional energy detection hard decision quantization process, so as to retain the demodulation information to the greatest extent for the channel decoder. In a complex underwater acoustic channel environment, such as when there is multipath effect and strong noise interference, the system of the present invention can effectively reduce the bit error rate and significantly improve the reliability and stability of communication.

[0056] 3. Due to the improvement of the utilization rate of the spread spectrum code, under the same bandwidth conditions, the system of the present invention can transmit more information and improve the spectrum efficiency. It can effectively alleviate the problem of spectrum congestion and provide spectrum support for more underwater communication applications.

[0057] 4. The system of the present invention has better adaptability to different underwater acoustic channel environments. Whether it is shallow sea, deep sea, or sea area with complex topography, the system can optimize signal transmission and reception performance by adjusting index parameters and probability matrix to ensure smooth communication. The high-speed and high-reliability communication performance makes the system of the present invention applicable to more complex underwater communication scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1This is a flowchart of a high-speed incoherent underwater acoustic communication index spread spectrum system proposed by the present invention.

[0059] Figure 2 Modulation diagram of traditional incoherent underwater acoustic communication spread spectrum system.

[0060] Figure 3 This is a modulation diagram of the incoherent underwater acoustic communication index spread spectrum system described in the present invention.

[0061] Figure 4 It is a complete signal structure of one frame.

[0062] Figure 5 This is a schematic diagram of the demodulation of the incoherent underwater acoustic communication index spread spectrum system proposed in the present invention.

[0063] Figure 6 These are the simulation experimental results of the incoherent underwater acoustic communication index spread spectrum system proposed in the present invention and the traditional spread spectrum system under different signal-to-noise ratios. DETAILED DESCRIPTION

[0064] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the following embodiments will further illustrate the present invention in conjunction with the accompanying drawings. 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. On the contrary, the present invention covers any substitution, modification, equivalent method and scheme made on the essence and scope of the present invention as defined by the claims.

[0065] The embodiment of the present invention is about the design of a high-speed incoherent underwater acoustic communication index spread spectrum system, including the design of transmitter modulation and receiver. The overall framework of the system is as follows: Figure 1 As shown, the transmitter converts the binary signal source into an analog signal after digital modulation and transmits it into the underwater acoustic channel, including a channel encoder and interleaver, a base converter, an index matrix and a probability matrix generator, an index spread spectrum modulator, a carrier modulator, a digital-to-analog converter, and a power amplifier. The receiver converts the received distorted channel into a digital signal after analog signal processing and demodulates and restores the binary signal source, including a preamplifier, a bandpass filter, a digital-to-analog converter, a carrier demodulator, an index matrix and a probability matrix generator, an index spread spectrum demodulator, a deinterleaver, and a channel decoder.

[0066] Transmitter:

[0067] Consider the length The binary source sequence , through the code rate The sequence is obtained after the channel encoder, the interleaver shuffles the order, and the base converter converts the binary sequence into a decimal sequence. ,in Represents the number of symbols in a frame signal, and represent the channel decoder output length and modulation parameters respectively. Therefore, the base converter will Convert consecutive binary data into a decimal number, that is Consider the traditional spread spectrum system. The length is The spreading code, spreading code set and spreading code can be expressed as and . Among them, the spreading codes in the spreading code set satisfy orthogonality;

[0068]

[0069] Among them, the symbol Represents a convolution operation.

[0070] Conventional spread spectrum system modulation Figure 2 As shown, it can be seen that in Only one of the spreading codes needs to be activated among the symbols, which can be expressed as:

[0071]

[0072] Since the index spread spectrum system proposed by the present invention applies the index technology to the traditional spread spectrum system, it is necessary to select spreading codes, that is, converting the selection of a single spreading code sequence into the selection of multiple spreading code index sequences, such as Figure 3 As shown, in each symbol there is A spreading code is selected to be activated.

[0073] Consider that the index matrix generator uses combinatorial theory to generate a matrix of size The index matrix ,in Indicates from Select the number The number of all possible combinations obtained by Contains all possible multiple activation spreading code index sequences, Each row in represents one or more activation spreading code index sequence patterns. The modulated index spread spectrum symbol contains The spreading code can be expressed as:

[0074]

[0075] in, Represented by the matrix Middle The columns together form a submatrix, Represents the index matrix No. Finally, The modulated index spread spectrum symbol linearly superimposes the selected multiple spread spectrum codes, which can be expressed as:

[0076]

[0077] in, Indicates row summation. Compared with the traditional spread spectrum principle Figure 2 Sum index spreading principle Figure 3 It can be seen that the index spread spectrum system proposed in the present invention can greatly improve the traditional spread spectrum communication rate to achieve high-speed incoherent underwater acoustic communication. The binary information rate contained in each symbol of the two can be expressed as

[0078]

[0079]

[0080] in, It is obvious that the combination of multiple spreading codes can obtain Much larger than that obtained by selecting a single spreading code , so the communication rate is greatly improved. In addition, when the number of spreading codes is limited, the index spread spectrum system can overcome the problem of low communication rate of the traditional spread spectrum system.

[0081] After getting each symbol, connect them in series to get the final transmitted digital signal:

[0082]

[0083] Then combined with the synchronization header to form a complete frame signal, such as Figure 4 The figure shows a complete signal structure of a frame, including four parts: coarse synchronization, fine synchronization, protection interval and signal. The synchronization head is usually composed of a frequency sweep signal, which is divided into two steps: coarse synchronization determines the signal to determine the synchronization head related threshold and fine synchronization determines the signal starting point to achieve high-quality signal synchronization; the interval is used to overcome the impact of multipath effects on the synchronization head. Finally, the frame signal is transmitted to the underwater acoustic channel through carrier modulation, digital-to-analog converter, and power amplifier.

[0084] Receiver:

[0085] The received distorted signal is passed through a preamplifier, a bandpass filter, an analog-to-digital converter, and a carrier demodulator to obtain a baseband signal. A frame of baseband signal is divided into symbols. Assume that The received index spread spectrum symbol is The demodulation process of the index spread spectrum system for incoherent underwater acoustic communication is as follows: Figure 5 As shown, it includes three parts: local spreading code correlation, maximum value extraction and maximum likelihood estimation based on maximum a posteriori probability. The output maximum likelihood estimation is used for subsequent channel decoder decoding.

[0086] No. received indexed spreading symbols With local spreading code set The spreading code is correlated and the The output of the correlator can be expressed as:

[0087]

[0088] Take the maximum value of the correlation result, then The maximum correlation result of the received indexed spread spectrum symbol and the local spread spectrum code set can be expressed as:

[0089]

[0090] in, represents the infinite norm of a vector. Consider the The probability matrices of bits being 1 and 0 are and , which can be defined as:

[0091] ,

[0092] ,

[0093] in, and Represents the index matrix belongs to and All row indices can be defined as:

[0094] ,

[0095] ,

[0096] in, , , Represents a domain of integers. and Represents binary numbers Shift Right Bits and Binary Numbers Based on the maximum a posteriori probability theory, using the Bayesian formula, Taylor expansion formula and max-log approximation formula, we can finally get The maximum likelihood ratio of bits is:

[0097]

[0098] in, , ,in, represents the noise variance, Represents a square operation. The maximum likelihood ratio of each bit calculated after demodulating all symbols is connected in series as the input of the interleaver and channel decoder, and finally the binary source transmitted by the transmitter is restored.

[0099] In order to verify the performance improvement of the proposed system, a simulation experiment was carried out. In the experiment, the spread spectrum code set contained a total of 257 spread spectrum codes with a length of 2048. Based on the modulation principle of the traditional spread spectrum system, each symbol was obtained to contain 8 bits and 6 bits of signals. Based on the index spread spectrum modulation principle, each symbol was obtained to contain 15 bits of signals. Simulation experiments were carried out under different signal-to-noise ratios to obtain the corresponding bit error rates.

[0100] The simulation experimental results of the proposed incoherent underwater acoustic communication index spread spectrum system and the traditional spread spectrum system under different signal-to-noise ratios are shown in Figure 2. Figure 6 As shown, from Figure 6 It can be seen that indexed spreading 15 bits, traditional spreading 8 bits and traditional spreading 6 bits achieve error-free transmission at signal-to-noise ratios of -9dB, -11dB and -12dB respectively. Comparing the results of indexed spreading 15 bits and traditional spreading 8 bits using all spreading codes, although indexed spreading 15 bits is 2dB lower than traditional spreading 8 bits in terms of bit error rate performance, its rate is nearly doubled. In contrast, the maximum rate of the traditional spread spectrum system under this spreading code group is only 8 bits, and its rate is limited by the number of spreading codes. Further analysis shows that traditional spreading 6 bits is 1dB higher than traditional spreading 8 bits in terms of bit error rate performance, which indicates that under this simulation condition, the traditional spread spectrum system can sacrifice 1dB of bit error rate performance in exchange for a rate increase of 2 bits per symbol; however, indexed spreading 15 bits sacrifices 2dB of bit error rate performance to obtain a communication rate increase of 7 bits per symbol. Obviously, the high-speed incoherent underwater acoustic communication index spread spectrum system proposed in the present invention is significantly superior to the traditional spread spectrum system in terms of communication performance.

[0101] In the incoherent underwater acoustic communication spread spectrum system widely used in the field of underwater acoustic communication engineering, only one spread spectrum code symbol is selected in each symbol information modulation process, and the spread spectrum code is not fully utilized, the spread spectrum code utilization rate is low, and the communication rate is low. Therefore, in order to improve the communication rate, the present invention introduces index technology, which converts the selection of a single spread spectrum code in the traditional spread spectrum system into an index sequence for selecting multiple spread spectrum codes, that is, multiple spread spectrum codes are selected for modulation in each symbol information modulation process, thereby improving the utilization rate of the spread spectrum code and thus improving the communication rate. In view of the demodulation information loss problem caused by the quantization process in the energy detection hard decision method used in the traditional spread spectrum system, a soft decision demodulation receiver based on the maximum a posteriori probability is designed, and the maximum likelihood ratio of each bit is calculated using the index matrix and the probability matrix, so as to maximize the retention of demodulation information for the channel decoder and improve the error correction performance.

[0102] The above embodiments are only preferred embodiments of the present invention and cannot be considered to limit the scope of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of the present invention.

Claims

1. A high-speed incoherent underwater acoustic communication index spread spectrum system, characterized in that: It includes a transmitting end and a receiving end, wherein: The transmitting end is used to generate an index spread spectrum signal using an index matrix and a probability matrix after channel coding, interleaving and base conversion of a binary signal source, and transmit the signal to an underwater acoustic channel after carrier modulation; the transmitting end comprises a channel encoder, an interleaver, a base converter, an index matrix and a probability matrix generator, an index spread spectrum modulator, a carrier modulator, a digital-to-analog converter, and a power amplifier; The receiving end is used to receive the distorted signal, preprocess it through the analog circuit and obtain the digital signal through analog-to-digital conversion, use the index matrix and probability matrix, perform soft decision demodulation based on the maximum a posteriori probability theory, and finally restore the binary source; the receiving end includes a preamplifier, a bandpass filter, an analog-to-digital converter, a carrier demodulator, an index matrix and probability matrix generator, an index spread spectrum demodulator, a deinterleaver and a channel decoder.

2. A high-speed incoherent underwater acoustic communication index spread spectrum system as claimed in claim 1, characterized in that: The index matrix and probability matrix generator generates an index matrix based on a combination theory, and calculates the maximum likelihood ratio of each bit using the index matrix and the probability matrix.

3. A high-speed incoherent underwater acoustic communication index spread spectrum system as claimed in claim 1, characterized in that: The index spread spectrum demodulator at the receiving end calculates the maximum likelihood ratio of each bit based on the maximum a posteriori probability theory using the index matrix and the probability matrix, and reserves demodulation information for the channel decoder.

4. A high-speed incoherent underwater acoustic communication index spread spectrum system as claimed in claim 1, characterized in that: The system selects multiple spreading codes in each symbol and converts the selection of a single spreading code sequence into the selection of multiple spreading code index sequences to improve the utilization rate of the spreading codes.

5. A high-speed incoherent underwater acoustic communication index spread spectrum system as claimed in any one of claims 1 to 4, characterized in that: The binary information source at the transmitting end is encoded by the channel encoder and interleaved by the interleaver, and then converted into a decimal sequence by the base converter. The index matrix and probability matrix generator generates an index matrix. The index spread spectrum modulator selects multiple spread spectrum codes for modulation and linear superposition according to the index matrix. The generated signal is transmitted into the underwater acoustic channel after carrier modulation, digital-to-analog conversion and power amplification.

6. A high-speed incoherent underwater acoustic communication index spread spectrum system as claimed in any one of claims 1 to 4, characterized in that: The signal received by the receiving end is pre-amplified, band-pass filtered, analog-to-digital converted, and carrier demodulated. The index matrix and probability matrix generator generates an index matrix and a probability matrix. The index spread spectrum demodulator calculates the maximum likelihood ratio of each bit based on the index matrix and the probability matrix using the maximum a posteriori probability theory. The deinterleaver and decoder process the demodulated signal in turn to restore the binary source transmitted by the transmitting end.

7. A high-speed incoherent underwater acoustic communication index spread spectrum method, characterized in that: The following steps are involved: 1) At the transmitting end, consider the length The binary source sequence , through the code rate The sequence is obtained after the channel encoder, the order is disrupted by the interleaver, and the binary sequence is converted into a decimal sequence by the base converter. ,in Represents the number of symbols in a frame signal, and Represent the channel decoder output length and modulation parameters respectively; the base converter will Convert consecutive binary data into a decimal number, that is ; Consider the traditional spread spectrum system The length is The spreading code, spreading code set and spreading code are represented as and ; Among them, the spreading codes in the spreading code set satisfy orthogonality; ， Among them, the symbol Represents the convolution operation; In the Only one of the spreading codes needs to be activated among the symbols, which can be expressed as: ， The index spread spectrum system applies the index technology to the traditional spread spectrum system, and it is necessary to select spreading codes, that is, converting the selection of a single spreading code sequence into the selection of multiple spreading code index sequences; Consider that the index matrix generator uses combinatorial theory to generate a matrix of size The index matrix ,in Indicates from Select the number The number of all possible combinations obtained by Contains all possible multiple activation spreading code index sequences, Each row in represents a pattern of multiple activated spreading code index sequences; therefore, The modulated index spread spectrum symbol contains The spreading code is expressed as: ， in, Represented by the matrix Middle The columns together form a submatrix, Represents the index matrix No. All elements of the row; Finally, The modulated index spread spectrum symbol linearly superimposes the selected multiple spread spectrum codes, which can be expressed as: ， in, represents the sum of rows; Compared with the traditional spread spectrum system and the index spread spectrum system, the index spread spectrum system proposed in the present invention can greatly improve the traditional spread spectrum communication rate to achieve high-speed incoherent underwater acoustic communication; the binary information rate contained in each symbol of the two can be expressed as: ， ， in, It means rounding down; obviously, the Much larger than that obtained by selecting a single spreading code , so the communication rate is greatly improved; in addition, when the number of spread spectrum codes is limited, the index spread spectrum system can overcome the problem of low communication rate of the traditional spread spectrum system; After getting each symbol, connect them in series to get the final transmitted digital signal: ， Then it is combined with the synchronization head to form a complete frame signal; a complete frame signal includes four parts: coarse synchronization, fine synchronization, protection interval and signal; the synchronization head is composed of a sweep frequency signal, which is divided into two steps: coarse synchronization determines the signal to determine the synchronization head related threshold and fine synchronization determines the signal starting point, so as to achieve high-quality signal synchronization; the protection interval is used to overcome the influence of multipath effect on the synchronization head; the frame signal is transmitted to the underwater acoustic channel through carrier modulation, digital-to-analog converter and power amplifier; 2) At the receiving end, the received distorted signal is passed through the preamplifier, bandpass filter, analog-to-digital converter, and carrier demodulator to obtain the baseband signal. A frame of baseband signal is divided into symbols. Assume that The received index spread spectrum symbol is , No. received indexed spreading symbols With local spreading code set The spreading code is correlated and the The output of the correlator is expressed as: ， Take the maximum value of the correlation result, then The maximum correlation result of the received indexed spread spectrum symbol and the local spread spectrum code set is expressed as: ， in, Represents the infinite norm of the vector; consider the The probability matrices of bits being 1 and 0 are and , defined as: ， ， in, and Represents the index matrix belongs to and All row indices of , defined as: ， ， in, , , represents the domain of integers; and Represents binary numbers Shift Right Bits and Binary Numbers Perform AND operation with 1; Based on the maximum a posteriori probability theory, using the Bayes formula, Taylor expansion formula and max-log approximation formula, we get The maximum likelihood ratio of bits is: in, ， ； in, represents the noise variance, represents a square operation; the maximum likelihood ratio of each bit calculated after demodulating all symbols is connected in series as the input of the interleaver and channel decoder, and finally the binary source transmitted by the transmitter is restored.

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