An underwater visible light communication system and method capable of adaptive modulation and coding

Abstract

The present invention provides an underwater visible light communication system and method with adaptive modulation and coding, belonging to the field of underwater optical communication technology. The system includes a transmitting end and a receiving end. The transmitting end is additionally designed with a channel detection module, an adaptive modulation and coding module, a multi-subcarrier IFFT module, and a control module; the receiving end is additionally designed with a signal frame detection module and a channel frequency domain estimation and equalization module. The channel detection module detects water quality in the underwater visible light communication channel and provides real-time feedback of the detected information. The underwater visible light communication system can switch the modulation and coding scheme based on thresholds, significantly improving the adaptability of the underwater visible light communication system to the underwater visible light communication channel. Compared with existing underwater full-duplex visible light communication systems, 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.

Description

Technical Field

[0001] The present invention belongs to the technical field of underwater optical communications, 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, covering over two-thirds of Earth's surface. With the launch of the "Transparent Ocean" initiative, continued ocean exploration and resource development are driving increasing demand for efficient and reliable underwater communication technologies.

[0003] Underwater optical communications offer high bandwidth, enabling high-speed information transmission. Furthermore, wireless optical communication transceivers are compact and consume relatively low power. Consequently, underwater wireless optical communications have significant development potential and play a crucial role in establishing high-speed and flexible communication links within underwater wireless sensor networks.

[0004] The complexity and variability of underwater optical communication channels poses significant challenges for underwater visible light communication. The types and concentrations of seawater molecules, phytoplankton, yellow matter, and non-pigmented suspended particles in underwater visible light communication channels vary significantly across different sea areas and time periods, resulting in constant dynamic changes in the underwater visible light communication channel. Enabling underwater visible light systems to adaptively change modulation and coding methods based on the characteristics of the underwater visible light communication channel is of great research significance.

[0005] Currently, there are also some adaptive modulation and coding technologies that enable communication systems to adjust modulation and coding. Traditional adaptive modulation and coding technologies include adaptive modulation and coding based on channel prediction. Currently, to improve the response speed and adaptability of underwater visible light communication systems, a number of related technologies are also under development, such as adaptive modulation and coding methods based on reinforcement learning and 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] To address 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 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 addition and windowing module, a digital-to-analog conversion module, and an electro-optical conversion module;

[0009] Among them, 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 system control module; 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;

[0010] 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;

[0011] 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 to calculate the frequency response of the underwater channel and then eliminate the influence of the channel response;

[0012] 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 operating 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 system control module.

[0013] Preferably, the adaptive modulation and coding module performs scrambling, convolution puncturing, interleaving, and subcarrier modulation on the input binary data. The specific process is:

[0014] S1. 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 then used as the input of the shift register.

[0015] S2. The scrambled data is subjected to convolution puncturing. The input data enters a 6-bit shift register. First, the input data is divided into two paths according to formula (1). The amount of data bits is doubled, and the coding efficiency is 1 / 2. Then, 1 / 3 of the bits in the two paths of data are replaced with 0 values, and the code rate is reduced to 3 / 4. The present invention can select between two coding schemes with coding efficiencies of 1 / 2 and 3 / 4.

[0016] (1)

[0017] in and are two channels of data after convolution processing, Indicates that the binary data is shifted N times in the shift register.

[0018] S3. After convolutional puncturing, the data undergoes two-level interleaving. The first level uses standard block interleaving, mapping adjacent coded bits to non-adjacent subcarriers. The second level maps adjacent coded bits to the high-significant bits and low-significant bits of the constellation, respectively. Both levels of interleaving are completed within a single OFDM symbol, and the interleaving length is consistent with the symbol length.

[0019] 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.

[0020] Preferably, in the multi-subcarrier IFFT module, the input data is first Hermitian symmetric processed to generate a real-valued signal without an imaginary part, and then two fast inverse Fourier transform (IFFT) points of 64 points and 128 points 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.

[0021] 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;

[0022] Preferably, 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, 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 expressed by the following formula (2), and the received data signal is corrected, and the implementation process is expressed by the following formula (3).

[0023] (2)

[0024] 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.

[0025] (3)

[0026] 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.

[0027] A second aspect of the present invention provides an underwater visible light communication method capable of adaptive modulation and coding, comprising the following process:

[0028] Step 1: First, place the turbidity sensor in the underwater channel and feed back the detected level value to the control module at the transmitter through the FPGA;

[0029] Step 2: The binary data enters the adaptive modulation and coding module, which performs scrambling, convolution puncturing, interleaving, and subcarrier modulation processing on the input binary data according to the system threshold;

[0030] 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;

[0031] Step 4: The signal is sequentially sent to the cyclic prefix addition and windowing module, the digital-to-analog conversion module, and the electro-optical conversion module, and the signal is transmitted in the underwater channel in the form of an optical signal;

[0032] Step 5: The optical signal enters the receiving end photoelectric detection module and analog-to-digital conversion module in sequence, converting the optical signal into a digital signal;

[0033] Step 6: The digital signal is sent to the signal frame detection module, which compares the energy value of the data received at the current moment with the data in the shift register. If the judgment condition is met, the received signal is considered valid;

[0034] Step 7: Send the frequency domain signal to the channel frequency domain estimation and equalization module, calculate the received training sequence signal and the training sequence signal stored at the receiving end to obtain the frequency response of the channel, and correct the received data signal;

[0035] Step 8: Send the signal to the FFT module and demodulation module to output binary data. If the channel characteristics change, repeat steps 1-8.

[0036] 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 operating 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.

[0037] Preferably, the step 2 is specifically as follows:

[0038] S1. The data is first scrambled using a 7-bit shift register. After the 7-bit shift register is initialized, 8-bit binary data is input to the scrambler. This data is then added to the scrambler output. After each addition, the scrambler performs an XOR operation on the 7th and 4th bits of its own data and adds them to itself. The resulting data is then used as the input to the shift register.

[0039] S2. The scrambled data is subjected to convolution puncturing. The input data enters a 6-bit shift register. First, the input data is divided into two paths according to formula (1). The amount of data bits is doubled, and the coding efficiency is 1 / 2. Then, 1 / 3 of the bits in the two paths of data are replaced with 0 values, and the code rate is reduced to 3 / 4. The present invention can select between two coding schemes with coding efficiencies of 1 / 2 and 3 / 4.

[0040] (1)

[0041] in and are two channels of data after convolution processing, Indicates that the binary data is shifted N times in the shift register.

[0042] S3. After convolutional puncturing, the data undergoes two-level interleaving. The first level uses standard block interleaving, mapping adjacent coded bits to non-adjacent subcarriers. The second level maps adjacent coded bits to the high-significant bits and low-significant bits of the constellation, respectively. Both levels of interleaving are completed within a single OFDM symbol, and the interleaving length is consistent with the symbol length.

[0043] 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.

[0044] Preferably, the step 7 is specifically as follows:

[0045] 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, and the implementation process is expressed by the following formula (3).

[0046] (2)

[0047] 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.

[0048] (3)

[0049] 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.

[0050] Compared with the prior art, the present invention has the following beneficial effects:

[0051] The present invention proposes an underwater visible light communication system and method based on AMC-DCO-OFDM. A channel detection module is added to the underwater visible light communication system to detect water quality in the underwater visible light communication channel and provide real-time feedback of the detected information. The underwater visible light communication system can switch the modulation and 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 increase the communication efficiency by 7.62% through the switching of multi-subcarrier IFFT, thereby improving the communication efficiency of the underwater visible light communication system. Therefore, the present invention has very important engineering value. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In order to more clearly illustrate the technical solutions of the present invention or the prior art, a brief introduction will be given below to 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 any creative work.

[0053] Figure 1 This is a block diagram of the transmitting end of the underwater visible light communication system of the present invention.

[0054] Figure 2 This is a block diagram of the receiving end of the underwater visible light communication system of the present invention.

[0055] Figure 3 This is the subcarrier BPSK demodulation constellation diagram received by the receiving end of the present invention.

[0056] Figure 4 This is the subcarrier QPSK demodulation constellation diagram received by the receiving end of the present invention.

[0057] Figure 5 This is the subcarrier 16QAM demodulation constellation diagram received by the receiving end of the present invention.

[0058] Figure 6 The figure is a line comparison chart of the bit error rates of the six MC schemes of the system of the present invention.

[0059] Figure 7 Schematic diagram of the AMC solution for underwater full-duplex visible light communication system.

[0060] Figure 8 Schematic diagram of the AMC solution of the underwater visible light communication system of the present invention. DETAILED DESCRIPTION

[0061] The invention will be further described below with reference to specific embodiments.

[0062] The invention will be further described below with reference to specific embodiments.

[0063] The first aspect of the present invention provides an underwater visible light communication system based on AMC-DCO-OFDM, Figure 1 This is a block diagram of the transmitter of the underwater visible light communication system, including the transmitter and the receiver. The transmitter 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 addition and windowing module, a digital-to-analog conversion module, and an electro-optical conversion module.

[0064] 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 system control module. 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.

[0065] Figure 2 This is a block diagram of the receiving end of the underwater visible light communication system, which 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;

[0066] 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;

[0067] The present invention also provides an underwater visible light communication method based on AMC-DCO-OFDM, the process principle of which is as follows:

[0068] Step 1: First, place the turbidity sensor in the underwater channel and feed back the detected level value to the control module at the transmitter through the FPGA;

[0069] Step 2: The binary data enters the adaptive modulation and coding module, which performs scrambling, convolution puncturing, interleaving, and subcarrier modulation processing on the input binary data according to the system threshold;

[0070] 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;

[0071] Step 4: The signal is sequentially sent to the cyclic prefix addition and windowing module, the digital-to-analog conversion module, and the electro-optical conversion module, and the signal is transmitted in the underwater channel in the form of an optical signal;

[0072] Step 5: The optical signal enters the receiving end photoelectric detection module and analog-to-digital conversion module in sequence, converting the optical signal into a digital signal;

[0073] Step 6: The digital signal is sent to the signal frame detection module, which compares the energy value of the data received at the current moment with the data in the shift register. If the judgment condition is met, the received signal is considered valid;

[0074] Step 7: Send the frequency domain signal to the channel frequency domain estimation and equalization module, calculate the received training sequence signal and the training sequence signal stored at the receiving end to obtain the frequency response of the channel, and correct the received data signal;

[0075] Step 8: Send the signal to the FFT module and demodulation module to output binary data. If the channel characteristics change, repeat steps 1-8.

[0076] 1. Channel detection module

[0077] 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 placed in the underwater channel. The turbidity sensor converts the turbidity of the water into a voltage value. The operating 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 system control module.

[0078] 2. Multi-subcarrier IFFT module

[0079] In the multi-subcarrier IFFT module, the input data is first Hermitian-symmetric processed to generate a real-valued signal without an imaginary part. Then, two fast inverse Fourier transform (IFFT) points, 64 and 128 points, are selected to carry the high-speed data signal onto 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.

[0080] 3. Adaptive modulation and coding process

[0081] S1. 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 then used as the input of the shift register.

[0082] S2. The scrambled data is subjected to convolution puncturing. The input data enters a 6-bit shift register. First, the input data is divided into two paths according to formula (1). The amount of data bits is doubled, and the coding efficiency is 1 / 2. Then, 1 / 3 of the bits in the two paths of data are replaced with 0 values, and the code rate is reduced to 3 / 4. The present invention can select between two coding schemes with coding efficiencies of 1 / 2 and 3 / 4.

[0083] (1)

[0084] in and are two channels of data after convolution processing, Indicates that the binary data is shifted N times in the shift register.

[0085] S3. After convolutional puncturing, the data undergoes two-level interleaving. The first level uses standard block interleaving, mapping adjacent coded bits to non-adjacent subcarriers. The second level maps adjacent coded bits to the high-significant bits and low-significant bits of the constellation, respectively. Both levels of interleaving are completed within a single OFDM symbol, and the interleaving length is consistent with the symbol length.

[0086] S4. When performing subcarrier modulation on the data, three subcarrier modulation modes can be selected: BPSK, QPSK and 16QAM according to the threshold of the control module. 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 receiver. The number of bits required to represent a symbol corresponding to the three modulation modes is 1, 2, and 4 bits respectively.

[0087] 4. Specific experimental process

[0088] This example demonstrates an AMC-DCO-OFDM system and tests and compares its performance. In underwater channels, optical signals scatter and absorb from water substances, resulting in signal energy attenuation. Aluminum hydroxide powder was used as a dopant to simulate water with varying turbidity levels. The system's water attenuation resistance parameters were measured using an AC-S tester.

[0089] This experiment used a 60cm-long transparent glass water tank as the underwater communication channel. The transmitter and receiver of the full-duplex communication system were placed on either side of the tank. The experimental equipment was calibrated and debugged. Clear tap water was filled into the tank to provide a clear water environment. After starting the experimental setup, the laser and photodetector were aligned. The system's modulation and coding schemes were manually switched in descending order of transmission bit rate. An oscilloscope was used to confirm that the system was transmitting and receiving normal waveforms. The ILA tool on a PC was used to capture and save data from the signal processing at the receiver. Matlab was used to calculate the data and calculate the system's bit error rate in clear water. 20mg of aluminum hydroxide powder was then measured using an electronic scale. After adding the powder to the water tank, the water was stirred thoroughly to ensure thorough mixing. The attenuation coefficient in this water was measured using an AC-S attenuation meter.

[0090] 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.

[0091] 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 was 0.1222 m -1 Rising to 2.698m with the addition of 440mg of powder -1 The experiment records 150 groups of 16QAM modulation 3 / 4 coding scheme data, 150 groups of 16QAM modulation 1 / 2 coding scheme data, 160 groups of QPSK modulation 3 / 4 coding scheme data, 150 groups of QPSK modulation 1 / 2 coding scheme data, 150 groups of BPSK modulation 3 / 4 coding scheme data, and 150 groups of BPSK modulation 1 / 2 coding scheme data, a total of 900 groups 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 the six modulation coding switching of the system are obtained at the same time, as shown in Table 1 below.

[0092] Table 1 AMC-DCO-OFDM system switching threshold

[0093]

[0094] 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 the change of water quality. Figure 7 As 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 terminal A and terminal B establish a communication link, the transmitter generates a transmission signal containing MC information, which takes time T1; the optical signal generated by terminal A is transmitted underwater, which takes time T2; after terminal B receives the signal through the photoelectric detector, it extracts MC and estimates the channel conditions through a known training sequence, which takes time T3; based on the estimation of the channel conditions, terminal B generates a signal containing adjusted MC information at the transmitter, which takes time T4; the signal containing the adjusted MC information is transmitted in the underwater channel, which takes time T5; terminal A receives the signal containing the adjusted MC information transmitted by terminal B and adjusts the terminal's MC information, which takes time T6.

[0095] 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 roughly as follows: Terminal A generates a transmission signal containing MC information at the transmitting end, and the required time is T1; the optical signal generated by Terminal A is transmitted underwater, and the required time is T2; after receiving the signal through the photoelectric detector, Terminal B extracts MC and estimates the channel condition through a known training sequence, and the required time is T3; Terminal A is equipped with a water quality sensor to monitor the turbidity changes of the underwater channel in real time, and the underwater channel condition is fed back to Terminal A in real time for processing. Terminal A switches the system's MC scheme according to the threshold, and the required time is T4.

[0096] 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.

[0097] This system was tested in an underwater channel using clear water, with no dopants added to the tank. 10,000 signal frames were sent using both 64 and 128 subcarrier modes. The system's transmission duration was measured to be 0.197s for 64 and 0.182s for 128 subcarrier modes, respectively. This translates to a 7.62% improvement in communication efficiency.

[0098] The above description is merely a preferred embodiment of the present application and is not intended to limit the present application. Various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

[0099] Although the above describes the specific implementation methods of the present invention, it does not 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 transmitter and a receiver, characterized in that: The transmitter 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 addition 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. When the water attenuation coefficient changes, different level values ​​are fed back to the system control module; 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; The channel detection module uses a turbidity sensor. First, the turbidity sensor is placed in the underwater channel. The turbidity sensor converts the turbidity of the water 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). The measured level value analog signal is converted into a digital signal and received by the system control module. 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 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 then used as the input of the shift register; S2, the scrambled data is subjected to convolution puncturing. The input data enters a 6-bit shift register. First, the input data is divided into two paths according to formula (1). The data bit amount will be doubled, and the coding efficiency at this time 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 convolutional puncturing, the data undergoes two-level interleaving. The first level uses standard block interleaving, mapping adjacent coded bits to non-adjacent subcarriers. The second level maps adjacent coded bits to the high-significant bits and low-significant bits of the constellation, respectively. Both levels of interleaving are completed within a single OFDM symbol, and the interleaving length is the same as the symbol length. S4, when performing subcarrier modulation on the data, selects from three subcarrier modulation modes: BPSK, QPSK, and 16QAM, 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; In the multi-subcarrier IFFT module, the input data is first Hermitian symmetric processed to generate a real-valued signal without an imaginary part. Then, two fast inverse Fourier transform (IFFT) points, 64 and 128, are selected to carry the high-speed data signal onto the low-speed subcarrier for transmission. Each subcarrier transmits part of the signal. The threshold for switching between the two IFFT transform points is generated by the control 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, wherein: 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.

3. The underwater visible light communication system capable of adaptive modulation and coding according to claim 1, wherein: 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, and 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.

4. A method for underwater visible light communication with adaptive modulation and coding, characterized by: The underwater visible light communication system according to any one of claims 1 to 3 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 at the transmitter through the FPGA; Step 2: The binary data enters the adaptive modulation and coding module, which 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: The signal is sequentially sent to the cyclic prefix addition and windowing module, the digital-to-analog conversion module, and the electro-optical conversion module, and the signal is transmitted in the underwater channel in the form of an optical signal; Step 5: The optical signal enters the receiving end photoelectric detection module and analog-to-digital conversion module in sequence, converting the optical signal into a digital signal; Step 6: The digital signal is sent to the signal frame detection module, which compares the energy value of the data received at the current moment with the data in the shift register. If the judgment condition is met, the received signal is considered valid; Step 7: Send the frequency domain signal to the channel frequency domain estimation and equalization module, calculate the received training sequence signal and the training sequence signal stored at the receiving end to obtain the frequency response of the channel, and correct the received data signal; Step 8: Send the signal to the FFT module and demodulation module to output binary data; If the channel characteristics change, repeat steps 1 to 8.

5. The underwater visible light communication method capable of adaptive modulation and coding according to claim 4, characterized in that: The step 1 is specifically as follows: The turbidity sensor is placed in the underwater channel. The turbidity sensor converts the turbidity of the water into a voltage value. The operating 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 system control module.

6. The underwater visible light communication method capable of adaptive modulation and coding according to claim 4, characterized in that: The step 2 is specifically as follows: S1, the data is first scrambled. The scrambling is implemented by a 7-bit shift register. After the 7-bit shift register is set to an 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 the 7th and 4th bits of its own data and adds them to itself. The data after the operation is then used as the input of the shift register; S2, the scrambled data is subjected to convolution puncturing. The input data enters a 6-bit shift register. First, the input data is divided into two paths according to formula (1). The data bit amount will be doubled, and the coding efficiency at this time 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 convolutionally punctured, it undergoes 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 the 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.

7. The underwater visible light communication method capable of adaptive modulation and coding according to claim 4, 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, and 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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