Underwater visible light communication system and method capable of adaptively modulating and coding

Abstract

The invention provides an underwater visible light communication system and method capable of adaptively modulating and coding, and belongs to the technical field of underwater optical communication. The system comprises a transmitting end and a receiving end, wherein a channel detection module, an adaptive modulation coding module, a multi-subcarrier IFFT module and a control module are additionally designed at the transmitting end; a signal frame detection module and a channel frequency domain estimation and equalization module are additionally designed at a receiving end; the channel detection module detects the water quality condition in the underwater visible light communication channel and feeds back the detected information in real time, and the underwater visible light communication system can switch a modulation coding scheme through a threshold value, so that the adaptability of the underwater visible light communication system to the underwater visible light communication channel is remarkably improved. Compared with an existing underwater full-duplex visible light communication system, the communication mode has the advantages that judgment of the system on the underwater environment can be completed only through simplex communication, and the response speed of the system is increased.

Description

Technical Field

[0001] The present invention belongs to the technical field of underwater optical communication, and in particular relates to an underwater visible light communication system and method capable of adaptive modulation and coding. Background Art

[0002] The ocean is the cradle of life, and more than two-thirds of the Earth is covered by the ocean. With the introduction of the "Transparent Ocean" plan, the exploration of the ocean and the development of resources have continued to deepen, and the demand for efficient and reliable underwater communication technology has increased.

[0003] Underwater optical communication has a high bandwidth and can achieve high-speed information transmission. At the same time, wireless optical communication transceivers are very small and have relatively low power consumption. In view of this, underwater wireless optical communication has great development potential and plays an important role in building high-speed and flexible communication links in underwater wireless sensor networks.

[0004] The underwater optical communication channel is complex and changeable, which brings great challenges to underwater visible light communication. In different sea areas and different time periods, the types and concentrations of seawater molecules, phytoplankton, yellow substances and non-pigmented suspended particles in the underwater visible light communication channel are quite different, which causes the underwater visible light communication channel to be in dynamic change. It is of great research significance to enable the underwater visible light system to adaptively change the modulation and coding method according to the characteristics of the underwater visible light communication channel.

[0005] At present, there are also some adaptive modulation and coding technologies that enable communication systems to adjust modulation and coding schemes. Traditional adaptive modulation and coding technologies include adaptive modulation and coding technologies based on channel prediction. At present, in order to improve the response speed and adaptability of underwater visible light communication systems, there are also many related technologies under development, such as adaptive modulation and coding methods based on reinforcement learning and adaptive modulation and coding methods based on convolutional neural networks.

[0006] Traditional underwater visible light communication systems usually measure the characteristics of the underwater visible light communication channel first, and then fix the system parameters of the underwater visible light communication system before communicating. When the quality of the channel changes, the communication quality is seriously affected. Summary of the invention

[0007] In view of the above problems, the present invention proposes an underwater visible light communication system and method based on AMC-DCO-OFDM, which significantly improves the adaptability of the underwater visible light communication system to the underwater visible light communication channel and improves the data transmission efficiency of the underwater visible light communication.

[0008] A first aspect of the present invention provides an underwater visible light communication system capable of adaptive modulation and coding, comprising a transmitting end and a receiving end, wherein the transmitting end comprises a channel detection module, an adaptive modulation and coding module, a multi-subcarrier IFFT module, a control module, a training sequence module, a cyclic prefix adding and windowing module, a digital-to-analog conversion module, and an electro-optical conversion module; Among them, the channel detection module is placed in the underwater visible light communication channel, and when the water attenuation coefficient changes, different level values ​​are fed back to the control module of the system; the adaptive modulation and coding module is used to modulate and encode the data signal input to the system, and perform scrambling, convolution puncturing, interleaving, and subcarrier modulation on the input binary data; the multi-subcarrier IFFT module is used to perform Hermitian symmetric processing on the data signal, and then convert the frequency domain signal into a time domain signal; the control module is used to generate the clock required by the system and the control signal of each module; The receiving end includes a photoelectric detection module, an analog-to-digital conversion module, a signal frame detection module, a channel frequency domain estimation and equalization module, an FFT module, and a demodulation module; The signal frame detection module is used to detect whether the signal has arrived and find the starting position of the data stream; the channel frequency domain estimation and equalization module is used to multiply the received signal with the known signal, calculate the frequency response of the underwater channel, and then eliminate the influence of the channel response; Preferably, the main component of the channel detection module is a turbidity sensor. The working process of this module is as follows: first, the turbidity sensor is arranged in the underwater channel. The turbidity sensor converts the turbidity of the water body into a voltage value. The working voltage of the turbidity sensor is 5V. The voltage value output by the sensor is transmitted to the analog-to-digital conversion hardware of the field programmable gate array (FPGA). The measured level value analog signal is converted into a digital signal and received by the control module of the system.

[0009] Preferably, the adaptive modulation and coding module performs scrambling, convolution puncturing, interleaving, and subcarrier modulation on the input binary data. The specific process is: S1. The scrambling is implemented by a 7-bit shift register. After the 7-bit shift register is set to an initial value, when 8-bit binary data is input into the scrambler, the binary data is added to the data output by the scrambler. After each addition, the scrambler performs an XOR operation on its own 7th and 4th bits and adds them to itself. The data after the operation is used as the input of the shift register.

[0010] S2. The scrambled data is subjected to convolution puncturing, and the input data enters a 6-bit shift register. The input data is first divided into two paths according to formula (1), and the data bit quantity is doubled. At this time, the coding efficiency is 1 / 2. Then, 1 / 3 of the bits in the two paths of data generated are replaced with 0 values, and the code rate is changed to 3 / 4. The present invention can be selected from two coding schemes with coding efficiencies of 1 / 2 and 3 / 4.

[0011] (1) in and are two channels of data after convolution processing, Indicates that the binary data is shifted N times in the shift register.

[0012] S3. After the data is processed by convolution and puncturing, it is subjected to two-level interleaving. The first level uses standard block interleaving to map adjacent coded bits to non-adjacent subcarriers. The second level of interleaving maps adjacent coded bits to high-significant bits and low-significant bits of the constellation diagram respectively. Both levels of interleaving are completed within a single OFDM symbol, and the interleaving length is consistent with the symbol length.

[0013] S4. When performing subcarrier modulation on the data, three subcarrier modulation modes, BPSK, QPSK and 16QAM, can be selected according to the threshold of the control module. The number of bits required to represent a symbol corresponding to the three modulation modes is 1, 2, and 4 bits respectively.

[0014] Preferably, in the IFFT module of multiple subcarriers, the input data is first processed with Hermitian symmetry to generate a real-valued signal without an imaginary part, and then two types of inverse fast Fourier transform (IFFT) points of 64 and 128 are selected to carry the high-speed data signal to the low-speed subcarrier for transmission. Each subcarrier transmits part of the signal, and the threshold for switching between the two IFFT transform points is generated by the control module.

[0015] Preferably, in the signal frame detection module, the received unknown data signal is first sent to a 16-bit shift register for storage, and the energy values ​​of the 16 data received at the current moment and the 16 data stored in the shift register are calculated respectively. If the ratio of the energy value at the previous moment to the energy value at the current moment is greater than the detection threshold, a valid value is accumulated. When the number of consecutive valid values ​​is greater than 32, it is determined that a valid signal has been received; Preferably, in the channel frequency domain estimation and equalization module, the received signal is a frequency domain signal before FFT processing, a training sequence signal is extracted from the received signal, the received training sequence signal is operated with the training sequence signal stored at the receiving end to obtain the frequency response of the channel, and the implementation process is represented by the following formula (2), and the received data signal is corrected, and the implementation process is represented by the following formula (3).

[0016] (2) in, It represents the received training sequence value. is the value of the training sequence stored at the receiving end, is the conjugate value of the training sequence stored at the receiving end, represents the estimated channel frequency response.

[0017] (3) in, represents the corrected data signal, Represents the original signal received, represents the conjugate of the channel frequency response. It is the energy value of the received training sequence and is a fixed value in a signal frame.

[0018] A second aspect of the present invention provides an underwater visible light communication method capable of adaptive modulation and coding, comprising the following process: Step 1: First, place the turbidity sensor in the underwater channel, and feed back the detected level value to the control module of the transmitter through FPGA; Step 2: The binary data enters the adaptive modulation and coding module, and performs scrambling, convolution puncturing, interleaving, and subcarrier modulation processing on the input binary data according to the system threshold; Step 3, the modulated signal is sent to the multi-subcarrier IFFT module for processing, Hermitian symmetry is performed, and the frequency domain signal is converted into a time domain signal according to the subcarrier point switching threshold; Step 4, sending the signal to the cyclic prefix adding and windowing module, the digital-to-analog conversion module, and the electro-optical conversion module in sequence, and the signal is transmitted in the underwater channel in the form of an optical signal; Step 5: The optical signal enters the photoelectric detection module and the analog-to-digital conversion module at the receiving end in turn to convert the optical signal into a digital signal; Step 6: The digital signal is sent to the signal frame detection module, and the energy value of the data received at the current moment is compared with the energy value of the data in the shift register. If the judgment condition is met, the received signal is considered valid; Step 7, sending the frequency domain signal to the channel frequency domain estimation and equalization module, calculating the received training sequence signal and the training sequence signal stored at the receiving end to obtain the frequency response of the channel, and correcting the received data signal; Step 8: Send the signal to the FFT module and demodulation module, and output binary data. If the channel characteristics change, repeat steps 1-8.

[0019] Preferably, step 1 is specifically as follows: first, a turbidity sensor is arranged in an underwater channel, the turbidity sensor converts the turbidity of the water body into a voltage value, the working voltage of the turbidity sensor is 5V, the voltage value output by the sensor is transmitted to the analog-to-digital conversion hardware of the FPGA, and the measured level value analog signal is converted into a digital signal and received by the control module of the system.

[0020] Preferably, the step 2 is specifically: S1. The data is first scrambled. The scrambling is implemented by a 7-bit shift register. After the 7-bit shift register sets the initial value, when the 8-bit binary data is input into the scrambler, the binary data is added to the data output by the scrambler. After each addition, the scrambler performs an XOR operation on its own 7th and 4th bits and adds them to itself. The data after the operation is used as the input of the shift register.

[0021] S2. The scrambled data is subjected to convolution puncturing, and the input data enters a 6-bit shift register. The input data is first divided into two paths according to formula (1), and the data bit quantity is doubled. At this time, the coding efficiency is 1 / 2. Then, 1 / 3 of the bits in the two paths of data generated are replaced with 0 values, and the code rate is changed to 3 / 4. The present invention can be selected from two coding schemes with coding efficiencies of 1 / 2 and 3 / 4.

[0022] (1) in and are two channels of data after convolution processing, Indicates that the binary data is shifted N times in the shift register.

[0023] S3. After the data is processed by convolution and puncturing, it is subjected to two-level interleaving. The first level uses standard block interleaving to map adjacent coded bits to non-adjacent subcarriers. The second level of interleaving maps adjacent coded bits to high-significant bits and low-significant bits of the constellation diagram respectively. Both levels of interleaving are completed within a single OFDM symbol, and the interleaving length is consistent with the symbol length.

[0024] S4. When performing subcarrier modulation on the data, three subcarrier modulation modes, BPSK, QPSK and 16QAM, can be selected according to the threshold of the control module. The number of bits required to represent a symbol corresponding to the three modulation modes is 1, 2, and 4 bits respectively.

[0025] Preferably, the step 7 is specifically: The received signal is a frequency domain signal before FFT processing. The training sequence signal is extracted from the received signal, and the received training sequence signal is operated with the training sequence signal stored at the receiving end to obtain the frequency response of the channel. The implementation process is expressed by the following formula (2). The received data signal is corrected. The implementation process is expressed by the following formula (3).

[0026] (2) in, It represents the received training sequence value. is the value of the training sequence stored at the receiving end, is the conjugate value of the training sequence stored at the receiving end, represents the estimated channel frequency response.

[0027] (3) in, represents the corrected data signal, Represents the original signal received, represents the conjugate of the channel frequency response. It is the energy value of the received training sequence and is a fixed value in a signal frame.

[0028] Compared with the prior art, the present invention has the following beneficial effects: The present invention proposes an underwater visible light communication system and method based on AMC-DCO-OFDM, adds a channel detection module to the underwater visible light communication system, detects water quality in the underwater visible light communication channel, and feeds back the detected information in real time. The underwater visible light communication system can switch the modulation coding scheme through the threshold value, which significantly improves the adaptability of the underwater visible light communication system to the underwater visible light communication channel. Compared with the existing underwater full-duplex visible light communication system, this communication method only requires simplex communication to complete the system's judgment of the underwater environment, and the system's response speed is improved by 27.14%. The present invention can improve the communication efficiency by 7.62% through the switching of multi-subcarrier IFFT, which improves the communication efficiency of the underwater visible light communication system. Therefore, the present invention has very important engineering value. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions of the present invention or the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, what is described below is only one embodiment of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0030] Figure 1 This is a block diagram of the transmitting end of the underwater visible light communication system of the present invention.

[0031] Figure 2 This is a block diagram of the receiving end of the underwater visible light communication system of the present invention.

[0032] Figure 3 It is the subcarrier BPSK demodulation constellation diagram received by the receiving end of the present invention.

[0033] Figure 4 It is a subcarrier QPSK demodulation constellation diagram received by the receiving end of the present invention.

[0034] Figure 5 It is a subcarrier 16QAM demodulation constellation diagram received by the receiving end of the present invention.

[0035] Figure 6 It is a line comparison chart of the bit error rate of six MC schemes of the system of the present invention.

[0036] Figure 7 Schematic diagram of the AMC solution for underwater full-duplex visible light communication system.

[0037] Figure 8 It is a schematic diagram of the AMC solution of the underwater visible light communication system of the present invention. DETAILED DESCRIPTION

[0038] The invention will be further described below in conjunction with specific embodiments.

[0039] The invention will be further described below in conjunction with specific embodiments.

[0040] A first aspect of the present invention provides an underwater visible light communication system based on AMC-DCO-OFDM. Figure 1 The block diagram of the transmitting end of the underwater visible light communication system includes a transmitting end and a receiving end. The transmitting end includes a channel detection module, an adaptive modulation and coding module, a multi-subcarrier IFFT module, a control module, a training sequence module, a cyclic prefix adding and windowing module, a digital-to-analog conversion module, and an electro-optical conversion module; The channel detection module is placed in the underwater visible light communication channel. When the water attenuation coefficient changes, different level values ​​are fed back to the control module of the system. The adaptive modulation and coding module is used to modulate and encode the data signal input to the system, and perform scrambling, convolution puncturing, interleaving, and subcarrier modulation on the input binary data. The multi-subcarrier IFFT module is used to perform Hermitian symmetric processing on the data signal, and then convert the frequency domain signal into a time domain signal. The control module is used to generate the clock required by the system and the control signals of each module. Figure 2 The receiving end of the underwater visible light communication system is a block diagram, and the receiving end includes a photoelectric detection module, an analog-to-digital conversion module, a signal frame detection module, a channel frequency domain estimation and equalization module, an FFT module, and a demodulation module; The signal frame detection module is used to detect whether the signal has arrived and find the starting position of the data stream; the channel frequency domain estimation and equalization module is used to multiply the received signal with the known signal, calculate the frequency response of the underwater channel, and then eliminate the influence of the channel response; The present invention also provides an underwater visible light communication method based on AMC-DCO-OFDM, and the process principle is as follows: Step 1: First, place the turbidity sensor in the underwater channel, and feed back the detected level value to the control module of the transmitter through FPGA; Step 2: The binary data enters the adaptive modulation and coding module, and performs scrambling, convolution puncturing, interleaving, and subcarrier modulation processing on the input binary data according to the system threshold; Step 3, the modulated signal is sent to the multi-subcarrier IFFT module for processing, Hermitian symmetry is performed, and the frequency domain signal is converted into a time domain signal according to the subcarrier point switching threshold; Step 4, sending the signal to the cyclic prefix adding and windowing module, the digital-to-analog conversion module, and the electro-optical conversion module in sequence, and the signal is transmitted in the underwater channel in the form of an optical signal; Step 5: The optical signal enters the photoelectric detection module and the analog-to-digital conversion module at the receiving end in turn to convert the optical signal into a digital signal; Step 6: The digital signal is sent to the signal frame detection module, and the energy value of the data received at the current moment is compared with the energy value of the data in the shift register. If the judgment condition is met, the received signal is considered valid; Step 7, sending the frequency domain signal to the channel frequency domain estimation and equalization module, calculating the received training sequence signal and the training sequence signal stored at the receiving end to obtain the frequency response of the channel, and correcting the received data signal; Step 8: Send the signal to the FFT module and demodulation module, and output binary data. If the channel characteristics change, repeat steps 1-8.

[0041] 1. Channel detection module The main component of the channel detection module is the turbidity sensor. The working process of this module is as follows: first, the turbidity sensor is arranged in the underwater channel. The turbidity sensor converts the turbidity of the water into a voltage value. The working voltage of the turbidity sensor is 5V. The voltage value output by the sensor is transmitted to the analog-to-digital conversion hardware of the field programmable gate array (FPGA). The measured level value analog signal is converted into a digital signal and received by the control module of the system.

[0042] 2. Multi-subcarrier IFFT module In the multi-subcarrier IFFT module, the input data is first processed with Hermitian symmetry to generate a real-valued signal without an imaginary part. Then, two types of inverse fast Fourier transform (IFFT) points, 64 and 128, are selected to carry the high-speed data signal to the low-speed subcarrier for transmission. Each subcarrier transmits part of the signal, and the threshold for switching between the two IFFT transform points is generated by the control module.

[0043] 3. Adaptive modulation and coding process S1. The scrambling is implemented by a 7-bit shift register. After the 7-bit shift register is set to an initial value, when 8-bit binary data is input into the scrambler, the binary data is added to the data output by the scrambler. After each addition, the scrambler performs an XOR operation on its own 7th and 4th bits and adds them to itself. The data after the operation is used as the input of the shift register.

[0044] S2. The scrambled data is subjected to convolution puncturing, and the input data enters a 6-bit shift register. The input data is first divided into two paths according to formula (1), and the data bit quantity is doubled. At this time, the coding efficiency is 1 / 2. Then, 1 / 3 of the bits in the two paths of data generated are replaced with 0 values, and the code rate is changed to 3 / 4. The present invention can be selected from two coding schemes with coding efficiencies of 1 / 2 and 3 / 4.

[0045] (1) in and are two channels of data after convolution processing, Indicates that the binary data is shifted N times in the shift register.

[0046] S3. After the data is processed by convolution and puncturing, it is subjected to two-level interleaving. The first level uses standard block interleaving to map adjacent coded bits to non-adjacent subcarriers. The second level of interleaving maps adjacent coded bits to high-significant bits and low-significant bits of the constellation diagram respectively. Both levels of interleaving are completed within a single OFDM symbol, and the interleaving length is consistent with the symbol length.

[0047] S4. When performing subcarrier modulation on the data, according to the threshold of the control module, three subcarrier modulation modes can be selected: BPSK, QPSK and 16QAM. Figure 3 is the subcarrier BPSK demodulation constellation diagram received by the receiving end, Figure 4 It is the subcarrier QPSK demodulation constellation diagram received by the receiving end. Figure 5 This is the 16QAM demodulation constellation diagram of the subcarrier received by the receiving end. The number of bits required to represent a symbol corresponding to the three modulation modes is 1, 2, and 4 bits respectively.

[0048] 4. Specific experimental process This example introduces an experimental demonstration of an AMC-DCO-OFDM system and tests and compares the performance of the system. The optical signal is scattered and absorbed by the underwater material in the underwater channel, resulting in attenuation of the signal energy. The experiment selects aluminum hydroxide powder as a dopant to simulate water quality with different turbidity, and uses an AC-S tester to measure the system's anti-water attenuation parameters.

[0049] This experiment uses a 60cm long transparent glass water tank as an underwater communication channel. The transmitter and receiver of the full-duplex communication system are arranged on both sides of the water tank, and the experimental equipment is calibrated and debugged. Clear tap water is injected into the water tank as a clear water environment. After starting the experimental device, align the laser and photodetector. According to the order of the transmission information bit rate from high to low, manually switch the modulation and coding scheme of the system. After using the oscilloscope to confirm that the system can send and receive normal waveforms, use the ILA tool on the PC to capture and save the data in the signal processing process of the receiving end, and use Matlab to calculate the data to give the system bit error rate under clear water. Then use an electronic scale to measure 20mg of aluminum hydroxide powder each time. After the powder is added to the water tank, stir the water thoroughly so that the aluminum hydroxide powder and water can be fully mixed. Use the AC-S attenuation tester to measure the attenuation coefficient in this water body.

[0050] To avoid random errors, 10 sets of system waveform data were recorded under each system’s adaptive modulation and coding scheme, and the bit error rate calculated by Matlab was averaged. The system’s modulation and coding scheme was manually switched in descending order of the transmission information bit rate. If the system bit error rate under this modulation and coding scheme was detected to be higher than 3.8×10 -3 , the experiment of this modulation and coding scheme will be stopped until the bit error rate of all modulation and coding schemes exceeds the upper limit.

[0051] In the experiment, a total of 440 mg of aluminum hydroxide powder was added to the water tank 22 times. The attenuation coefficient of the water body increased from 0.1222 m -1 Rising to 2.698m with 440mg of powder added -1 The experiment records 150 sets of 16QAM modulation 3 / 4 coding scheme data, 150 sets of 16QAM modulation 1 / 2 coding scheme data, 160 sets of QPSK modulation 3 / 4 coding scheme data, 150 sets of QPSK modulation 1 / 2 coding scheme data, 150 sets of BPSK modulation 3 / 4 coding scheme data, and 150 sets of BPSK modulation 1 / 2 coding scheme data, a total of 900 sets of experimental data were recorded. Based on the above experimental data, the bit error rate line graph of the AMC-DCO-OFDM system is drawn as follows Figure 6 As shown, the thresholds for switching 6 modulation codes of the system are obtained at the same time, as shown in the following Table 1.

[0052] Table 1 AMC-DCO-OFDM system switching threshold table

[0053] This system is compared with the adaptive modulation and coding scheme in the underwater full-duplex communication system to detect the response time of the system to water quality changes. Figure 7As shown in the figure, the steps and time consumption of the AMC method of the underwater full-duplex communication system are roughly as follows: before the terminal A and the terminal B establish a communication link, the transmitting end generates a transmission signal containing MC information, and the required time is T1; the optical signal generated by the terminal A is transmitted underwater, and the required time is T2; after the terminal B receives the signal through the photoelectric detector, it extracts the MC and estimates the channel condition through the known training sequence, and the required time is T3; based on the estimation of the channel condition, the terminal B generates a signal containing the adjusted MC information at the transmitting end, and the required time is T4; the signal containing the adjusted MC information is transmitted in the underwater channel, and the required time is T5; the terminal A receives the signal containing the adjusted MC information transmitted by the terminal B, and adjusts the MC information of the terminal, and the required time is T6.

[0054] like Figure 8 As shown in the figure, the steps and time consumption of the AMC method of the communication system in this paper are as follows: the transmitting end of terminal A generates a transmission signal containing MC information, and the required time is T1; the optical signal generated by terminal A is transmitted underwater, and the required time is T2; after terminal B receives the signal through the photoelectric detector, it extracts MC and estimates the channel situation through a known training sequence, and the required time is T3; a water quality sensor is installed at terminal A to monitor the turbidity changes of the underwater channel in real time, and the situation of the underwater channel is fed back to terminal A in real time for processing. Terminal A switches the MC scheme of the system according to the threshold, and the required time is T4.

[0055] The calculated response speeds of the AMC scheme in common full-duplex communication and the AMC scheme proposed in this system are 87.476μs and 63.738μs respectively, proving that the response speed of this system scheme is 27.14% higher than that of the full-duplex communication scheme.

[0056] The system was applied to underwater channels for testing, using clear water and no additional dopants in the water tank. 10,000 signal frames were sent using 64 subcarriers and 128 subcarriers respectively, and the transmission duration of the system in the two modes was tested. The corresponding durations in the 64 and 128 subcarrier modes were measured to be 0.197s and 0.182s, respectively, and the communication efficiency was calculated to be improved by 7.62%.

[0057] The above description is only the preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

[0058] Although the above describes the specific implementation methods of the present invention, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without creative work are still within the scope of protection of the present invention.

Claims

1. An underwater visible light communication system capable of adaptive modulation and coding, comprising a transmitting end and a receiving end, characterized in that: The transmitting end includes a channel detection module, an adaptive modulation and coding module, a multi-subcarrier IFFT module, a control module, a training sequence module, a cyclic prefix adding and windowing module, a digital-to-analog conversion module, and an electro-optical conversion module; Among them, the channel detection module is placed in the underwater visible light communication channel, and when the water attenuation coefficient changes, different level values ​​are fed back to the control module of the system; the adaptive modulation and coding module is used to modulate and encode the data signal input to the system, and perform scrambling, convolution puncturing, interleaving, and subcarrier modulation on the input binary data; the multi-subcarrier IFFT module is used to perform Hermitian symmetric processing on the data signal, and then convert the frequency domain signal into a time domain signal; the control module is used to generate the clock required by the system and the control signal of each module; The receiving end includes a photoelectric detection module, an analog-to-digital conversion module, a signal frame detection module, a channel frequency domain estimation and equalization module, an FFT module, and a demodulation module; Among them, the signal frame detection module is used to detect whether the signal has arrived and find the starting position of the data stream; the channel frequency domain estimation and equalization module is used to multiply the received signal with the known signal, calculate the frequency response of the underwater channel, and eliminate the influence of the channel response.

2. The underwater visible light communication system capable of adaptive modulation and coding according to claim 1, characterized in that: The channel detection module adopts a turbidity sensor; first, the turbidity sensor is arranged in the underwater channel, and the turbidity sensor converts the turbidity of the water body into a voltage value. The voltage value output by the sensor is transmitted to the analog-to-digital conversion hardware of the field programmable gate array FPGA, and the measured level value analog signal is converted into a digital signal and received by the control module of the system.

3. The underwater visible light communication system capable of adaptive modulation and coding according to claim 1, characterized in that: The adaptive modulation and coding module performs scrambling, convolution puncturing, interleaving, and subcarrier modulation on the input binary data. The specific process is as follows: S1, scrambling is realized by a 7-bit shift register. After the 7-bit shift register is set to an initial value, when 8-bit binary data is input into the scrambler, the binary data is added to the data output by the scrambler. After each addition, the scrambler performs an XOR operation on its own 7th and 4th bit data and adds them to itself. The data after the operation is used as the input of the shift register; S2, the scrambled data is subjected to convolution puncturing, and the input data enters a 6-bit shift register. First, the input data is divided into two paths according to formula (1), and the data bit quantity is doubled. At this time, the coding efficiency is 1 / 2. Then, 1 / 3 of the bits in the generated two paths of data are replaced with 0 values, and the code rate is reduced to 3 / 4. （1） in and are two channels of data after convolution processing, Indicates that the binary data is shifted N times in the shift register; S3, after the data is processed by convolution puncturing, it is subjected to two-level interleaving. The first level uses standard block interleaving to map adjacent coded bits to non-adjacent subcarriers. The second level of interleaving maps adjacent coded bits to high-significant bits and low-significant bits of the constellation diagram respectively. Both levels of interleaving are completed within a single OFDM symbol, and the interleaving length is consistent with the symbol length. S4, when performing subcarrier modulation on the data, according to the threshold of the control module, three subcarrier modulation modes of BPSK, QPSK and 16QAM are selected, and the number of bits required to represent a symbol corresponding to the three modulation modes is 1, 2, and 4 bits respectively.

4. The underwater visible light communication system capable of adaptive modulation and coding according to claim 1, characterized in that: In the multi-subcarrier IFFT module, the input data is first processed with Hermitian symmetry to generate a real-valued signal without an imaginary part. Then, two types of fast inverse Fourier transform points, 64 and 128, are selected to carry the high-speed data signal to the low-speed subcarrier for transmission. Each subcarrier transmits part of the signal, and the threshold for switching between the two IFFT transform points is generated by the control module.

5. The underwater visible light communication system capable of adaptive modulation and coding according to claim 1, characterized in that: In the signal frame detection module, the received unknown data signal is first sent to a 16-bit shift register for storage, and the energy values ​​of the 16 data received at the current moment and the 16 data stored in the shift register are calculated respectively. If the ratio of the energy value at the previous moment to the energy value at the current moment is greater than the detection threshold, a valid value is accumulated. When the number of consecutive valid values ​​is greater than 32, it is judged that a valid signal has been received.

6. The underwater visible light communication system capable of adaptive modulation and coding according to claim 1, characterized in that: In the channel frequency domain estimation and equalization module, the received signal is a frequency domain signal before FFT processing. The training sequence signal is extracted from the received signal, and the received training sequence signal is operated with the training sequence signal stored at the receiving end to obtain the frequency response of the channel. The implementation process is expressed by the following formula (2). The received data signal is corrected. The implementation process is expressed by the following formula (3): （2） in, It represents the received training sequence value. is the value of the training sequence stored at the receiving end, is the conjugate value of the training sequence stored at the receiving end, represents the estimated channel frequency response; （3） in, represents the corrected data signal, Represents the original signal received, represents the conjugate of the channel frequency response, It is the energy value of the received training sequence and is a fixed value in a signal frame.

7. An underwater visible light communication method capable of adaptive modulation and coding, characterized in that: The underwater visible light communication system according to any one of claims 1 to 6 is applied, and includes the following process: Step 1: Place the turbidity sensor in the underwater channel and feed back the detected level value to the control module of the transmitter through FPGA; Step 2: The binary data enters the adaptive modulation and coding module, and performs scrambling, convolution puncturing, interleaving, and subcarrier modulation processing on the input binary data according to the system threshold; Step 3, the modulated signal is sent to the multi-subcarrier IFFT module for processing, Hermitian symmetry is performed, and the frequency domain signal is converted into a time domain signal according to the subcarrier point switching threshold; Step 4, sending the signal to the cyclic prefix adding and windowing module, the digital-to-analog conversion module, and the electro-optical conversion module in sequence, and the signal is transmitted in the underwater channel in the form of an optical signal; Step 5: The optical signal enters the photoelectric detection module and the analog-to-digital conversion module at the receiving end in turn to convert the optical signal into a digital signal; Step 6: The digital signal is sent to the signal frame detection module, and the energy value of the data received at the current moment is compared with the energy value of the data in the shift register. If the judgment condition is met, the received signal is considered valid; Step 7, sending the frequency domain signal to the channel frequency domain estimation and equalization module, calculating the received training sequence signal and the training sequence signal stored at the receiving end to obtain the frequency response of the channel, and correcting the received data signal; Step 8, sending the signal to the FFT module and the demodulation module, and outputting binary data; If the channel characteristics change, repeat steps 1 to 8.

8. The underwater visible light communication method capable of adaptive modulation and coding according to claim 7, characterized in that: The step 1 is specifically as follows: The turbidity sensor is arranged in the underwater channel. The turbidity sensor converts the turbidity of the water into a voltage value. The working voltage of the turbidity sensor is 5V. The voltage value output by the sensor is transmitted to the analog-to-digital conversion hardware of the FPGA. The measured level value analog signal is converted into a digital signal and received by the control module of the system.

9. The underwater visible light communication method capable of adaptive modulation and coding according to claim 7, characterized in that: The step 2 is specifically as follows: S1, the data is first scrambled, and the scrambling is implemented by a 7-bit shift register. After the 7-bit shift register sets the initial value, when the 8-bit binary data is input into the scrambler, the binary data is added to the data output by the scrambler. After each addition, the scrambler performs an XOR operation on its own 7th and 4th bits and adds them to itself. The data after the operation is used as the input of the shift register; S2, the scrambled data is subjected to convolution puncturing, and the input data enters a 6-bit shift register. First, the input data is divided into two paths according to formula (1), and the data bit quantity is doubled. At this time, the coding efficiency is 1 / 2. Then, 1 / 3 of the bits in the generated two paths of data are replaced with 0 values, and the code rate is reduced to 3 / 4. （1） in and are two channels of data after convolution processing, Indicates that the binary data is shifted N times in the shift register; S3, after the data is processed by convolution and puncturing, it is subjected to two-level interleaving. The first level uses standard block interleaving to map adjacent coded bits to non-adjacent subcarriers. The second level of interleaving maps adjacent coded bits to high-significant bits and low-significant bits of the constellation diagram respectively. Both levels of interleaving are completed within a single OFDM symbol, and the interleaving length is consistent with the symbol length.

10. The underwater visible light communication method capable of adaptive modulation and coding according to claim 7, characterized in that: The step 7 is specifically as follows: The received signal is a frequency domain signal before FFT processing. The training sequence signal is extracted from the received signal, and the received training sequence signal is operated with the training sequence signal stored at the receiving end to obtain the frequency response of the channel. The implementation process is expressed as formula (2). The received data signal is corrected. The implementation process is expressed as formula (3): （2） in, It represents the received training sequence value. is the value of the training sequence stored at the receiving end, is the conjugate value of the training sequence stored at the receiving end, represents the estimated channel frequency response; （3） in, represents the corrected data signal, Represents the original signal received, represents the conjugate of the channel frequency response, It is the energy value of the received training sequence and is a fixed value in a signal frame.

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