A high-speed light emitting diode underwater wireless optical communication system and method

Abstract

The application provides a high-speed light-emitting diode underwater wireless optical communication system and method, and belongs to the technical field of underwater optical communication; a transmitting end comprises successively connected adaptive coding module, 16QAM mapping module, OFDM modulation module, digital-analog conversion module and LED array light source module; a receiving end comprises successively connected photoelectric detection module, signal processing module of integrated post-equalization circuit, analog-digital conversion module, digital filter module, OFDM demodulation module, phase estimation module, 16QAM demapping module and Viterbi decoding module; the application provides a high-speed LED underwater wireless optical communication system and method based on 16-order quadrature amplitude modulation-quadrature frequency division multiplexing and hardware equalization, adopts hardware equalization to effectively expand the modulation bandwidth of the LED light source, significantly improves the transmission performance of high-frequency signals, and sufficiently utilizes the frequency spectrum resources through high-order modulation, and significantly improves the data transmission rate of the system.

Description

Technical Field

[0001] This invention belongs to the field of underwater optical communication technology, and particularly relates to a high-speed light-emitting diode underwater wireless optical communication system and method. Background Technology

[0002] Underwater wireless optical communication (UWOC), as an emerging short-range underwater communication method, boasts advantages such as high speed, low latency, strong robustness, and resistance to electromagnetic interference, making it an important research direction in fields such as underwater robotics, marine resource exploration, and intelligent sensor networks. Existing typical UWOC systems mostly use laser diodes (LDs) or light-emitting diodes (LEDs) as light sources. While LD-based UWOC systems can achieve long-distance, high-speed data transmission due to their high optical power and modulation bandwidth, with some studies achieving transmission capabilities of hundreds of Mbps or even Gbps in ideal underwater environments, LD beams have limitations. Their small divergence angle requires precise optical path alignment and is susceptible to water scattering, multipath effects, and platform attitude changes, making them unsuitable for the dynamic and complex marine environment. In contrast, LED light sources offer advantages such as low cost, easy packaging, large divergence angle, and lower alignment requirements, making them more suitable for engineering deployment in dynamic and complex underwater environments, thus becoming the mainstream choice for current underwater short-range optical communication.

[0003] Currently, most LED-based UWOC systems employ low-order modulation methods such as on-off keying (OOK) or pulse position modulation (PPM). While these modulation schemes are simple to implement and have strong anti-interference capabilities, they suffer from low spectral efficiency, failing to fully leverage the advantages of optical communication in high-data-rate transmission and thus struggling to meet the high-speed demands of applications such as high-definition video backhaul and large-scale data sensing. To expand the effective bandwidth of LED light sources, some research has attempted to introduce equalization techniques, with most employing software equalization. Although software equalization can compensate for the high-frequency response attenuation of LEDs to some extent, it introduces higher computational latency and energy consumption, resulting in low system integration and high power consumption, making it difficult to meet the miniaturized and low-power engineering application requirements. Summary of the Invention

[0004] To address the aforementioned problems, the first aspect of this invention provides a high-speed light-emitting diode underwater wireless optical communication system, comprising a transmitter and a receiver. The hardware portion of the transmitter includes an LED array light source module and a light source driving module; the software portion includes a convolutional coding module, a 16QAM mapping module, an OFDM modulation module, and a digital-to-analog conversion module.

[0005] The LED array light source module serves as the transmission carrier for optical signals; the light source driving module applies a DC bias to drive the LED array to emit light; the adaptive coding module performs interleaving and convolutional coding on the input data stream; the 16QAM mapping module maps the encoded data stream to 16QAM modulation symbols; the OFDM modulation module divides the high-speed data stream into multiple parallel low-speed subcarriers, each subcarrier independently carrying a portion of data information; and the digital-to-analog converter module converts the modulated digital baseband signal into an analog electrical signal.

[0006] The hardware of the receiver includes a photoelectric detection module and a signal processing module with an integrated post-equalization circuit; the software includes an analog-to-digital conversion module, a digital filtering module, an OFDM demodulation module, a phase estimation module, a 16QAM demapping module, and a Viterbi decoding module.

[0007] The system comprises the following modules: a photoelectric detection module for receiving incident light signals and converting them into photocurrent signals; a signal processing module for the integrated equalization circuit for converting the current signal output by the photoelectric detection module into a voltage signal, and performing filtering, amplification, and frequency response compensation; an analog-to-digital conversion module for converting the analog signal processed by the optical receiver into a digital baseband signal; a digital filtering module for filtering the sampled digital signal to suppress spike interference; an OFDM demodulation module for symbol synchronization and converting the time-domain signal into a frequency-domain signal; a phase estimation module for phase offset compensation of the received frequency-domain signal; a 16QAM demapping module for demodulating the received frequency-domain symbols into the corresponding data bitstream; and a Viterbi decoding module for convolutional code decoding of the demodulated data stream.

[0008] Preferably, the LED array light source module consists of eight 1W CREE-XPEBGR green LEDs connected in series. This series structure effectively reduces the equivalent junction capacitance of the LED array, thereby significantly improving its frequency response performance. Each LED is equipped with a 60° focusing lens, which ensures good beam focusing capability while maintaining a dynamic balance between beam coverage and concentration, providing the system with efficient and uniform light intensity distribution.

[0009] Preferably, the specific working process of the 16QAM mapping module and the 16QAM demapping module is as follows:

[0010] 16QAM mapping transmits 4 bits of information per symbol period. First, the bitstream to be transmitted is grouped into sets of 4 bits each. The first two bits map to the imaginary part of a complex symbol, and the last two bits map to its real part, corresponding to a specific symbol point in the 16QAM constellation. This constellation forms a 4×4 matrix, where each element corresponds to a modulation symbol, represented as a complex number, with its real and imaginary parts representing the amplitude and phase of the modulated signal, respectively. At the receiving end, 16QAM demapping is performed on the received modulated signal, reversing the mapping of symbol points to their corresponding 4 bits of data, thereby recovering the original bitstream and achieving data demodulation.

[0011] Preferably, the OFDM modulation module's processing includes Hermitian symmetry, IFFT, addition of pilot symbols, and a cyclic prefix (CP). The OFDM uses 256 subcarriers to divide the high-speed data stream into multiple parallel-transmitting low-speed subcarriers. Each subcarrier carries a portion of the data and is converted into a time-domain signal using an inverse fast Fourier transform (IFFT). Simultaneously, real-valued OFDM signals are generated using Hermitian symmetry, and pilot symbols and a cyclic prefix (CP) are inserted into the signal to achieve symbol synchronization and effectively suppress inter-symbol interference (ISI), thereby improving system transmission performance and stability.

[0012] Preferably, the signal processing module of the integrated post-equalization circuit includes a transimpedance amplifier circuit, a low-pass filter circuit, a post-amplifier circuit, and a post-equalization circuit, which respectively perform current-to-voltage conversion, low-pass filtering, and equalization on the signal. The designed post-equalization circuit is a first-order π-type equalization circuit structure, which consists of resistors, inductors, and capacitors, and has the function of gain compensation for high-frequency signals. This circuit uses inductors and capacitors to form a frequency-selective network, and achieves spectral equalization by appropriately boosting high-frequency components. Its core part includes an inductor connected in series and two parallel capacitors symmetrically distributed at both ends. The equivalent resistance connected in parallel between the input and output is used to match the impedance, thereby forming a typical π-type filter structure. In order to further optimize the input and output impedance matching of the system, a dedicated matching network is introduced on both sides of the circuit, so that the signal reflection is minimized and the energy loss is reduced during transmission, thereby improving the signal transmission efficiency and equalization effect. Through the above structural design, the post-equalization module can effectively expand the frequency response bandwidth of the system, enhance the transmission capability of high-frequency signals, and thus improve the overall transmission performance and anti-interference capability of the communication link.

[0013] Preferably, the digital filtering module is mainly used to filter out spike pulses in the ADC sampling signal. The program implements an algorithm similar to amplitude limiting filtering. When an abnormal amplitude of a signal is detected, the algorithm selects a suitable value based on the amplitude of adjacent signals to replace it, thereby eliminating interference from spike pulses and ensuring signal purity.

[0014] Preferably, the specific working process of the phase estimation module is as follows:

[0015] The received signal undergoes phase compensation processing via a phase estimation module. First, the complex form of the pilot symbol in the frequency domain is decomposed into imaginary and real components, denoted as follows: and Subsequently, the received pilot signal is multiplied by the locally known pilot signal using a complex number, and the phase offset angle is calculated based on the following formulas (1) and (2). Make an estimate:

[0016] (1)

[0017] (2)

[0018] in, Let be the real part of the phase offset angle. Let be the imaginary part of the phase offset angle. and These are the real and imaginary parts of the received pilot signal, respectively.

[0019] Next, based on the estimated phase offset angle (Depend on and (Construction), phase compensation is performed on all received subcarrier signals. The compensation operation is completed by the following formulas (3) and (4):

[0020] (3)

[0021] (4)

[0022] in, The received signal to be compensated. Let the imaginary part of the signal to be compensated be... Let be the real part of the signal to be compensated. The imaginary unit, The output signal after compensation. To compensate for the imaginary part of the signal, This is the real part of the compensated signal.

[0023] This method can effectively eliminate phase distortion caused by frequency offset, sampling error or channel time-varying characteristics, thereby improving the synchronization accuracy and demodulation performance of the system. It is applicable to pilot-assisted synchronization and compensation processes in various communication scenarios.

[0024] Preferably, the OFDM demodulation module's processing includes symbol synchronization, CP removal, FFT, and Hermitian symmetry removal; the specific process is as follows:

[0025] S1. The signal first undergoes symbol synchronization. This module generates a pilot signal that matches the transmitter, consisting of 16 alternating positive and negative 1s. Subsequently, the most significant bit of the first 40 received symbols is determined and restored to positive and negative 1s. Correlation is compared with the local pilot signal through a sliding window to determine the optimal matching position, and then the CP is skipped, thereby accurately locating the starting point of the valid symbol.

[0026] S2. The synchronized time-domain signal is transformed back to the frequency domain using an FFT;

[0027] S3. The second half of the FFT output is truncated to effectively remove the Hermitian symmetric redundancy introduced at the transmitting end, retaining only the first half of the effective subcarrier data, restoring the original frequency domain information, and providing accurate input for subsequent demodulation processing.

[0028] A second aspect of this invention provides a high-speed underwater wireless optical communication method based on LEDs, comprising the following steps:

[0029] Step 1: The data stream undergoes interleaving and convolutional coding processing by the adaptive coding module to improve the reliability of data transmission;

[0030] Step 2: The encoded bit sequence is converted into the corresponding complex number symbol through the 16QAM mapping module;

[0031] Step 3: Perform OFDM modulation on the data from Step 2, dividing the high-speed data stream into multiple low-speed subcarriers that are transmitted in parallel, with each subcarrier carrying a portion of the data.

[0032] Step 4: The modulated signal is converted from a digital signal to an analog signal by a digital-to-analog converter module. Then, the amplitude is adjusted by an amplifier and an adjustable attenuator to make the output signal level fall within the linear operating range of the LED array light source, and then transmitted in the form of an optical signal.

[0033] Step 5: The photodetector receives the optical signal and converts it into an electrical signal;

[0034] Step 6: The electrical signal is amplified, filtered, and equalized by the signal processing module of the integrated equalization circuit to improve signal quality;

[0035] Step 7: The signal processed in step 6 is converted into a digital signal by an analog-to-digital converter, and OFDM demodulation and 16QAM demapping are performed using an FPGA (Field Programmable Gate Array).

[0036] Step 8: Calculate the phase offset of the received signal through the phase estimation module, and perform phase compensation on all subcarriers based on the estimation results. Finally, the original data information is recovered through decoding.

[0037] Preferably, step 2 specifically includes:

[0038] The input bitstream is segmented into groups of four bits each. The first two bits determine the imaginary part of the mapped symbol, and the last two bits correspond to the real part. In this way, each group of bits uniquely corresponds to a symbol point in the 16QAM constellation diagram. The constellation diagram can be represented as a 4x4 two-dimensional matrix structure, where each matrix element represents a modulation symbol point, specifically determined by a pair of real and imaginary coordinates, reflecting the amplitude and phase information of the complex symbol, respectively. This structure achieves efficient mapping of multi-bit information in the complex plane, improving spectral efficiency.

[0039] Preferably, step 6 specifically includes:

[0040] The transimpedance amplifier circuit converts the current signal output by the APD into a voltage signal. A 20MHz low-pass filter circuit filters out high-frequency out-of-band noise, and the subsequent amplifier circuit further amplifies the filtered signal. The amplified signal is then processed by a post-equalization circuit to compensate for its high-frequency response. The designed post-equalization circuit constructs a frequency-selective network using inductors and capacitors, which can enhance the response to high-frequency components, thereby achieving the purpose of equalizing the signal spectrum. The main structure of the circuit consists of a series inductor and two parallel capacitors at both ends, arranged in a π-shape. Simultaneously, a resistor is connected in parallel between the input and output terminals to achieve impedance matching, ensuring the stability and integrity of the signal during transmission. To further improve the signal coupling efficiency of the system, matching network structures are introduced at both ends of the circuit to adjust the impedance relationship with the preceding and following stages, thereby reducing signal reflection and energy loss. This design not only improves the circuit's response to high-frequency components but also significantly widens the system's bandwidth, effectively enhancing the overall stability and anti-interference capability of the underwater optical communication link under complex transmission conditions.

[0041] Preferably, step 8 specifically includes:

[0042] The received signal undergoes phase compensation processing via a phase estimation module. First, the complex form of the pilot symbol in the frequency domain is decomposed into imaginary and real components, denoted as follows: and Subsequently, the received pilot signal is multiplied by the locally known pilot signal using a complex number, and the phase offset angle is calculated based on the following formulas (1) and (2). Make an estimate:

[0043] (1)

[0044] (2)

[0045] in, Let be the real part of the phase offset angle. Let be the imaginary part of the phase offset angle. and These are the real and imaginary parts of the received pilot signal, respectively.

[0046] Next, based on the estimated phase offset angle (Depend on and (Construction), phase compensation is performed on all received subcarrier signals. The compensation operation is completed by the following formulas (3) and (4):

[0047] (3)

[0048] (4)

[0049] in, The received signal to be compensated. Let the imaginary part of the signal to be compensated be... Let be the real part of the signal to be compensated. The imaginary unit, The output signal after compensation. To compensate for the imaginary part of the signal, This is the real part of the compensated signal.

[0050] This method can effectively eliminate phase distortion caused by frequency offset, sampling error or channel time-varying characteristics, thereby improving the synchronization accuracy and demodulation performance of the system. It is applicable to pilot-assisted synchronization and compensation processes in various communication scenarios.

[0051] Compared with the prior art, the present invention has the following beneficial effects:

[0052] This invention proposes a high-speed underwater wireless optical communication system and method based on LEDs. This system integrates 16QAM-OFDM high-order modulation technology with a channel coding mechanism, effectively optimizing spectrum resource utilization while improving data transmission rate. Compared to traditional underwater optical communication systems using OOK modulation, this invention requires only 5MHz of bandwidth to achieve a transmission rate of 10Mbps, while the traditional OOK scheme requires approximately 10MHz of bandwidth. Therefore, this invention not only significantly improves spectrum utilization but also enhances the system's anti-interference capability in complex underwater environments, possessing greater practicality and promotional value.

[0053] Furthermore, regarding the hardware architecture: a first-order π-type post-equalization circuit was designed. This scheme successfully extended the modulation bandwidth of commercial LEDs from the original 3MHz to 20MHz, effectively alleviating the high-frequency signal attenuation problem caused by the limitation of light source bandwidth. At the same time, compared with traditional software equalization methods, this hardware equalization scheme has a simple structure, low power consumption, and good real-time performance, overcoming the shortcomings of software equalization such as large processing delay, high energy consumption, and complex implementation.

[0054] In terms of software design: a 16QAM-OFDM modulation and demodulation algorithm with a phase estimation module was designed. Through the phase estimation module, the phase distortion caused by frequency offset, sampling error or channel time-varying characteristics is effectively eliminated, thereby improving the synchronization accuracy and demodulation performance of the system. It is suitable for pilot-assisted synchronization and compensation processes in various communication scenarios.

[0055] In terms of miniaturization and integrated design: the system's transceiver ends are compact, both achieving miniaturized packaging, and employing rigorous waterproof sealing technology to adapt to complex underwater environments. Regarding circuit integration, the system internally integrates signal modulation and demodulation, driving, power supply, and equalization processing functions, eliminating the reliance on external large instruments such as AWG signal generators, oscilloscopes, and linear power supplies found in traditional UWOC systems. This results in high integration and portability, significantly improving practicality and engineering sophistication.

[0056] In terms of system performance: This invention constructs a miniaturized underwater wireless optical communication system based on an LED array and an FPGA platform, achieving stable transmission at a transmission rate of 10 Mbps over a 3m underwater and 6m airborne link. Simultaneously, the system supports a maximum receiving angle of 20°, and its transmission rate, link distance, and robustness all meet or exceed the current domestic and international research levels, verifying the advanced nature and practical value of the solution. Therefore, this invention has significant engineering value. Attached Figure Description

[0057] To more clearly illustrate the technical solutions of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the following description is only one embodiment of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0058] Figure 1 This is a block diagram illustrating the principle of the underwater wireless optical communication system of the present invention.

[0059] Figure 2 This is a physical diagram of the underwater wireless optical communication system of the present invention.

[0060] Figure 3 This is a graph showing the current-voltage characteristics of the optical transmitter of this invention.

[0061] Figure 4 The graphs show the distribution curves of irradiance as a function of divergence angle for the optical transmitter of the present invention equipped with lenses of 15°, 60° and 90° respectively.

[0062] Figure 5 This is a circuit diagram of the optical receiver of the present invention.

[0063] Figure 6 This is a circuit diagram of the first-order π-type equalization circuit of the present invention.

[0064] Figure 7 This is a diagram illustrating the equalization effect of the first-order π-type equalization circuit of this invention.

[0065] Figure 8 This is a block diagram of the 16QAM-OFDM modulation and demodulation module of the present invention.

[0066] Figure 9 This is an experimental diagram of the prototype of the underwater optical communication system terminal of the present invention.

[0067] Figure 10 This is a graph showing the change in bit error rate of the 16QAM-OFDM signal of this invention as a function of communication rate.

[0068] Figure 11 This is a spectrum diagram of the 10Mbps 16QAM-OFDM signal of the present invention.

[0069] Figure 12 This is the demodulation constellation diagram for the 10Mbps 16QAM-OFDM signal of this invention.

[0070] Figure 13 The graph shows the bit error rate of the 5Mbps and 10Mbps 16QAM-OFDM signals of this invention as a function of transmission distance.

[0071] Figure 14 This is a graph showing the change in bit error rate of the 10Mbps 16QAM-OFDM signal of this invention as a function of the degree of deviation from the main optical axis. Detailed Implementation

[0072] The invention will be further described below with reference to specific embodiments.

[0073] Example 1:

[0074] This invention proposes a high-speed underwater wireless optical communication method using light-emitting diodes (LEDs), the principle of which is as follows:

[0075] Step 1: After inputting the data to be sent into the network debugging assistant on the host computer, the data is first processed by the parallel-to-serial conversion module inside the FPGA. This module reassembles the multi-bit parallel data transmitted by the host computer into a continuous single-bit serial data stream, thereby meeting the data format requirements of subsequent modulation, encoding, and other processing stages in the communication link.

[0076] Step 2: The serially input data first enters the interleaving module to rearrange the data stream. This module uses a matrix structure for storage and reconstruction. Specifically, the bit data is written into the matrix cells in column-major order and then read out in row-major order, thereby changing the order of the bits on the time axis. This interleaving mechanism can effectively break up the continuously distributed bits in the original data, significantly reducing the impact of burst errors on system performance, thus improving the robustness and error resistance of the communication system in complex interference environments.

[0077] Step 3: The interleaved bit data is fed into a convolutional coding module with a code rate of 1 / 2 for error control coding. This module introduces redundant information to perform structured coding on the original data, thereby improving the data's anti-interference capability and error correction capability in complex underwater channel environments. During the coding process, for every 1 information bit input, 2 coded bits are output, forming a redundant coding structure, which effectively enhances the system's resistance to random and burst errors, providing a reliable redundancy foundation for subsequent demodulation and decoding processes.

[0078] Step 4: The encoded data undergoes mapping processing via a 16QAM mapping module, carrying 4 bits of information per symbol period. Specifically, the bit stream to be transmitted is segmented into groups of 4 bits each. The first two bits form the imaginary part of the modulation symbol, and the last two bits correspond to the real part, thus forming a complex modulation symbol. This modulation symbol corresponds to a specific symbol point in the 16QAM constellation diagram on the complex plane. This constellation diagram can be viewed as a 4×4 two-dimensional matrix structure, where each matrix element represents a unique complex symbol point, and its real and imaginary parts represent the amplitude and phase of the modulation signal, respectively. This constellation mapping mechanism enables efficient bit information carrying and spectrum utilization.

[0079] Step 5: Using a Hermitian symmetry module, Hermitian symmetry construction and bit-width expansion operations are performed on the complex modulated data after 16QAM mapping to ensure that the complex signal input to the IFFT module outputs a real time-domain signal after transformation. This process mainly includes two key steps: first, reversing the symbol order; and second, constructing complex conjugate symmetry. Specifically, the complex symbols obtained from 16QAM modulation are first filled sequentially into the effective subcarrier positions of the first half of the IFFT input vector. Then, these symbols are reversed, and a conjugate operation is performed on each symbol. The result is filled into the second half of the IFFT input vector to satisfy the Hermitian symmetry condition. Furthermore, to ensure that the IFFT input signal matches the bit width or FFT point number set by the system, the beginning of the input vector is aligned and expanded, and zeros are padded at the end, thus ensuring that the input sequence length precisely matches the calculation requirements of the IFFT module. Through these processes, it is ensured that the IFFT output is a real waveform, meeting the system's requirements for real-valued signal transmission.

[0080] Step 6: The complex signal after Hermitian symmetry processing is fed into the IFFT module for spectrum transformation. The IFFT module uses a 256-point subcarrier configuration to achieve efficient frequency-domain to time-domain conversion. In this step, the IFFT module first converts the modulated frequency-domain complex signal into a corresponding time-domain signal for subsequent optoelectronic transmission. To reduce system hardware resource consumption and computational complexity, the IFFT operation is implemented using a fixed-point method. This implementation simplifies the computational logic and hardware architecture, significantly reduces reliance on high-bit-width multipliers and storage resources, and improves the overall system efficiency. Furthermore, to avoid signal amplitude overflow or distortion issues in fixed-point operations, a scaling factor control mechanism is introduced at the IFFT output. In this design, a scaling factor of 1 / 128 is used to control the amplitude of the IFFT result, effectively suppressing peak power fluctuations and ensuring the output signal amplitude remains stable within a reasonable range, thereby improving the overall system stability and signal transmission quality.

[0081] Step 7: Add a cyclic prefix with a length of 1 / 16 of the symbol period to the front of each OFDM data frame. Specifically, copy the 16 time-domain sampling points at the end of the current data frame and insert them into the frame header. The resulting data frame then forms a cyclic prefix structure. This processing method effectively extends the symbol period, alleviates inter-symbol interference caused by multipath effects, and thus enhances the system's anti-interference capability and transmission stability in complex underwater channel environments.

[0082] Step 8: Insert a preset pilot symbol sequence into each data frame to assist the receiver in performing key processing tasks such as symbol synchronization, carrier frequency offset estimation, and channel state estimation. The pilot symbols consist of a three-segment structure: 16 consecutive +1 symbols, 16 alternating ±1 symbols, and 16 consecutive -1 symbols, forming a composite pilot sequence with good autocorrelation and cross-correlation characteristics. Furthermore, to improve the system's synchronization robustness under noise interference or channel fading conditions, an additional 32 redundant fault-tolerant data bits are introduced to further enhance the reliability of pilot identification. This pilot design effectively improves the receiver's accuracy in identifying symbol start points and phase offsets, thereby ensuring the overall synchronization performance and demodulation accuracy of the system.

[0083] Step 9: The processed digital signal is first converted into an analog signal by a DAC, and then sequentially passed through a fixed-gain amplifier and an adjustable attenuator for amplitude conditioning. The fixed-gain amplifier is used to initially boost the signal amplitude, while the adjustable attenuator performs fine amplitude control on the amplified signal to ensure that the output signal level stably falls within the linear operating range of the LED array light source, thereby guaranteeing the stability of the electro-optic modulation process and the linearity of the transmitted signal.

[0084] Step 10: The amplitude-modulated analog signal drives the LED array light source through electro-optic modulation, completing the conversion process from electrical signal to optical signal. The LED array emits a corresponding light intensity signal under the drive of the electrical signal and couples this light signal into the underwater optical channel, enabling wireless data transmission in the water medium and thus constructing a complete underwater optical communication link.

[0085] Step 11: The receiving end uses an APD with a sensitivity of 14 A / W as the optical receiving device to receive optical signals in the underwater channel. The APD generates a current output proportional to the light intensity under light illumination, achieving high-sensitivity photoelectric conversion of the received optical signal and providing a reliable current input basis for subsequent electrical signal processing.

[0086] Step 12: The weak current signal output by the APD is input to the transimpedance amplifier circuit, converting it into a voltage signal with measurable amplitude for subsequent analog processing. This system uses the OPA657 operational amplifier as the core component, which has a high... Gain-bandwidth product ( ) and low input current noise ( It is very suitable for high-speed and high-sensitivity applications.

[0087] Step 13 involves introducing the voltage signal output from the transimpedance amplifier circuit into a low-pass filter module with a defined passband range and flat amplitude-frequency response. This filter module is based on the AD8065 operational amplifier and has a cutoff frequency set to 20MHz. This Butterworth low-pass filter effectively suppresses high-frequency noise components introduced during transimpedance amplification, thereby improving signal purity, enhancing the system's anti-interference capability in complex electromagnetic environments, and ensuring the accuracy and stability of subsequent signal processing.

[0088] Step 14: To further enhance the amplitude of the received optical signal, a post-amplification circuit is added after the low-pass filter module. This amplifier circuit uses the high-bandwidth, low-noise operational amplifier OPA657 as its core component, along with precisely designed feedback resistors and compensation capacitors, to construct a stable and reliable gain control network. This design not only effectively enhances the signal amplitude but also maintains good bandwidth response and low noise characteristics, contributing to improved overall system signal quality and the stability of subsequent processing.

[0089] Step 15: The amplified signal is further input to a first-order π-type post-equalization circuit to expand the system bandwidth. This equalization circuit consists of resistors, inductors, and capacitors, and has the function of gain compensation for high-frequency signal components. Through the frequency-selective network formed by the inductor and capacitor, the circuit can achieve appropriate amplitude boost in the high-frequency band, thereby achieving spectral equalization. Its core structure consists of a series inductor and parallel capacitors symmetrically distributed at both ends, forming a π-type topology. An equivalent matching resistor is connected in parallel between the input and output terminals to achieve impedance matching and avoid energy loss caused by signal reflection. In addition, to further optimize the port impedance matching characteristics of the system, a dedicated impedance matching network is introduced on both sides of the circuit to reduce the reflection coefficient of the signal during transmission, thereby improving transmission efficiency and equalization effect. Based on the above design, this post-equalization module can effectively expand the frequency response range of the system, improve the transmission capability of high-frequency components, and thus enhance the bandwidth utilization and anti-interference performance of the overall communication link.

[0090] Step 16: The signal, after being processed by the receiver at each stage of analog processing, is input to an analog-to-digital converter module with a sampling rate of 125Msps to convert the analog signal into a corresponding digital signal. The digital signal is then sent to the FPGA for subsequent digital processing and analysis, thereby completing the signal transition from analog to digital domain and ensuring the system's real-time processing capability and data processing accuracy.

[0091] Step 17: The converted digital signal first enters the digital filtering module to suppress transient interference components such as spikes, thereby improving the system's robustness in complex environments. This module implements a filtering algorithm with amplitude limiting characteristics. When an abnormal change in the signal amplitude at a certain sampling point is detected, the system dynamically selects a reasonable value to replace it based on the amplitude information of its adjacent sampling points to eliminate the influence of spike interference. Through this filtering strategy, the smoothness and purity of the signal are effectively improved, ensuring that the signal received by subsequent processing modules has good stability and reliability.

[0092] Step 18: After digital filtering is completed, the signal first enters the clock recovery module, which extracts valid clock information from the received data. This module analyzes the timing characteristics in the received signal to generate a local system clock synchronized with the transmitter clock, thereby achieving time-domain alignment of the receiving link and ensuring the timing accuracy and stability of subsequent symbol synchronization and data parsing processes.

[0093] Step 19: After clock synchronization is completed, the system performs frame header identification and detection on the received signal. Specifically, by detecting whether a continuous sequence of 24 high and low level changes appears in the received signal, the system determines whether a data frame has arrived and thus determines the start position of the frame, achieving accurate positioning of the frame structure. This frame header identification mechanism can effectively improve frame synchronization accuracy and ensure the timing consistency and reliability of subsequent data processing.

[0094] Step 20: After detecting the frame header, the system performs symbol synchronization on the received data frame to accurately locate the starting position of valid OFDM symbols. Specifically, the receiver first locally generates a set of pilot sequences consistent with the transmitter. These pilot sequences consist of alternating ±1 values ​​of length 16, serving as reference signals for subsequent correlation matching. Then, the system extracts the first 40 symbol samples from the received data and performs symbol decision based on the most significant bit of each symbol, converting it into a binary form corresponding to ±1. Based on this, the system uses a sliding window algorithm to perform point-by-point correlation calculation between the extracted symbol sequence and the local pilot template, searching for the matching position with the highest correlation. When the optimal matching point is detected, the system locates the starting boundary of the valid data and skips the preceding cyclic prefix, thereby achieving precise synchronization of valid OFDM symbols. This symbol synchronization mechanism based on the sliding correlation algorithm significantly improves the accuracy of symbol boundary detection, ensuring accurate time bases for subsequent demodulation and channel estimation operations, thus enhancing the overall robustness of the receiving link and the stability of data parsing.

[0095] Step 21: After symbol synchronization is completed, the first 16 sampling points of the received signal are discarded to remove the previously added cyclic prefix, thereby restoring the complete structure of the original OFDM symbol. This processing step helps to eliminate inter-symbol interference caused by multipath and lays the foundation for subsequent frequency domain processing.

[0096] Step 22: Input the received signal with the cyclic prefix removed into the FFT module for time-frequency domain transformation. This step converts the time-domain sampled signal into a corresponding frequency-domain signal representation, extracts the complex symbols on each subcarrier, and provides basic data support for subsequent phase compensation and demodulation operations.

[0097] Step 23: Perform phase compensation processing on the received signal using the phase estimation module. First, decompose the complex form of the pilot symbol in the frequency domain into imaginary and real parts, denoted as follows: and Subsequently, the received pilot signal is multiplied by the locally known pilot signal using a complex number, and the phase offset angle is calculated based on the following formulas (1) and (2). Make an estimate:

[0098] (1)

[0099] (2)

[0100] in, Let be the real part of the phase offset angle. Let be the imaginary part of the phase offset angle. and These are the real and imaginary parts of the received pilot signal, respectively.

[0101] Next, based on the estimated phase offset angle (Depend on and (Construction), phase compensation is performed on all received subcarrier signals. The compensation operation is completed by the following formulas (3) and (4):

[0102] (3)

[0103] (4)

[0104] in, The received signal to be compensated. Let the imaginary part of the signal to be compensated be... Let be the real part of the signal to be compensated. The imaginary unit, The output signal after compensation. To compensate for the imaginary part of the signal, This is the real part of the compensated signal.

[0105] This method can effectively eliminate phase distortion caused by frequency offset, sampling error or channel time-varying characteristics, thereby improving the synchronization accuracy and demodulation performance of the system. It is applicable to pilot-assisted synchronization and compensation processes in various communication scenarios.

[0106] Step 24: After completing the phase compensation process, the first half of the subcarrier data is extracted from the frequency domain signal to remove the Hermitian symmetric structure constructed by the transmitter to ensure the real value of the IFFT output, thereby extracting the data subcarriers that actually carry valid information. This step helps to recover the original modulation information and provides accurate input for subsequent demodulation and decoding processes.

[0107] Step 25: Perform 16QAM demapping on the phase-compensated frequency domain signal to restore the original bit data. The demodulation module adopts a hard decision method and dynamically calculates the demodulation decision threshold in conjunction with pilot symbols to improve demodulation accuracy and system reliability. Specifically, the system uses a preset known byte 1B as the basis for the decision threshold. The byte 1B contains four binary combinations: "00", "01", "10", and "11", covering all possible modulation mappings in the 16QAM constellation diagram. For symbols corresponding to "10" and "01", the demodulation threshold can be obtained by calculating the average of the real and imaginary parts of the corresponding complex symbol; while the decision threshold for "11" is derived by averaging the decision thresholds for "10" and "01". Through this threshold decision mechanism based on known bytes, the system can effectively improve the accuracy of the decision boundary during 16QAM demodulation, thereby enhancing the reliability of bit restoration and improving the overall demodulation performance and anti-interference capability of the communication system.

[0108] Step 26: Input the demodulated bitstream into the Viterbi decoder to perform convolutional code decoding. By performing maximum likelihood decision on the encoded path, bit errors introduced by channel noise interference during signal transmission are effectively corrected, thereby improving the accuracy of data recovery and the overall error correction capability of the system.

[0109] Step 27: The bit data after Viterbi decoding enters the deinterleaving module to recover the original bit sequence before interleaving at the transmitting end, thus achieving reverse restoration of the data structure. Specifically, the deinterleaving process is performed using matrix reconstruction: the serial bit stream is written into the matrix cells in row order, and then the output is read in column order, thereby rearranging the scrambled bit order during interleaving. This mechanism can effectively restore the original signal structure, ensuring the correctness and consistency of subsequent data reconstruction and application processing.

[0110] Step 28: Finally, the recovered serial bit data is input to the serial-to-parallel conversion module, converted into the corresponding parallel data format, and sent to the lower-level system for subsequent data processing, analysis, or specific applications.

[0111] Example 2:

[0112] Based on the method of Embodiment 1, the present invention also provides an underwater wireless optical communication system using high-speed light-emitting diodes. Figure 1 This is a block diagram illustrating the principle of an underwater wireless optical communication system. Figure 2 The image shown is a physical diagram of the system. The transmitter of this invention uses an array of eight 1W high-power LEDs as the light source. Each LED is equipped with a 60° focusing lens to enhance the directionality and concentration of the beam, improving energy utilization and coverage distance during underwater transmission. Modulation and channel coding are implemented using an FPGA, supporting high-order modulation schemes including 16QAM, significantly improving the system's data transmission rate and anti-interference capability. The receiver uses an APD with high sensitivity and high gain as the photodetector, enabling efficient reception of weak light signals. Simultaneously, the receiver link integrates an analog signal processing circuit with post-equalization, effectively expanding the system bandwidth and improving overall frequency response performance through gain compensation of high-frequency components, thereby achieving stable underwater optical communication at high data rates.

[0113] Figure 3 This is a graph showing the current-voltage characteristics of an optical transmitter. Figure 4 The graphs show the irradiance distribution as a function of divergence angle for the optical transmitter equipped with 15°, 60°, and 90° lenses. As can be seen from the graphs, while the 90° lens provides a uniform light intensity distribution, its large divergence angle leads to energy diffusion and limited transmission efficiency. The 15° lens offers highly concentrated beams but has a small coverage area, making it suitable for scenarios requiring precise alignment. The 60° lens achieves a good balance between focusing and coverage, significantly improving transmission efficiency and optimizing light intensity distribution. Therefore, the 60° focusing lens offers the best overall performance and is the optimal optical solution for this system, providing excellent anti-misalignment capability and link robustness.

[0114] Figure 5This is the circuit diagram of the optical receiver. The optical receiver module mainly consists of an FPGA development board, an ADC module with a sampling rate of 125Msps, an APD with a sensitivity of 14A / W, an APD driver circuit, and a signal processing circuit with an integrated post-equalization module. The signal processing section includes several key functional modules: First, a transimpedance amplifier circuit is used to convert the weak photocurrent output by the avalanche photodiode into a measurable voltage signal; second, a 20MHz low-pass filter is used to suppress out-of-band high-frequency noise and improve the signal-to-noise ratio; subsequently, the amplifier circuit amplifies the filtered signal a second time to meet the requirements of subsequent processing; finally, the post-equalization circuit further compensates for the attenuation of the system's frequency response, thereby expanding the effective bandwidth of the system.

[0115] Figure 6 This is a circuit diagram of a first-order π-type equalization circuit. In underwater optical communication systems, due to the limited bandwidth of the light source itself, the system's response to high-frequency signals is weak, leading to significant attenuation of high-frequency components in the transmission link, thus affecting overall communication performance. To improve this problem and enhance the system's effective bandwidth and frequency response characteristics, this study designs a post-equalization circuit. This circuit achieves gain compensation for high-frequency signals through the synergistic effect of resistors, inductors, and capacitors. Its core structure consists of inductor L1, capacitor C1, and an equivalent matching resistor. Matching networks are introduced on both sides to optimize the system's input and output impedance matching, thereby enhancing the equalization effect and improving the overall frequency response characteristics of the system. The transfer function of this equalization circuit is... The expression is:

[0116] (5)

[0117] in:

[0118] (6)

[0119] (7)

[0120] For frequency, The imaginary unit, The input impedance of the system. For input matching impedance, To match the output impedance, (i=1,2,3) are resistors. (i=1,2,3) represents the capacitor. (i=1,2,3) represents the inductance. This represents the impedance transformation characteristics of the input matching network. This indicates the impedance transformation characteristics of the output matching network.

[0121] When the frequency response reaches its peak, its bandwidth limit is reached. This can be approximated as:

[0122] (8)

[0123] In this design, to ensure the impedance stability of the equalization network in the actual system while also considering the frequency response characteristics, the component parameters are selected as follows based on the circuit simulation results:

[0124] (9)

[0125] Figure 7 The diagram illustrates the equalization effect of the designed π-type equalization circuit. As can be seen from the diagram, the designed first-order π-type equalization circuit achieves a gain boost of approximately 23.1 dB in the 0–20 MHz range, effectively compensating for the approximately 23.92 dB frequency response attenuation of the original UWOC system in the same frequency band, thus achieving comprehensive compensation for the high-frequency amplitude reduction of the system. With the help of this equalization network, the system's 3 dB bandwidth is extended from the original 3 MHz to 20 MHz, significantly improving the spectral efficiency and high-speed transmission capability of the UWOC system under broadband conditions, providing strong support for high-speed, broadband underwater optical communication based on LEDs.

[0126] Figure 8 This is a block diagram of the 16QAM-OFDM modulation and demodulation module. To achieve higher data transmission rates under the limited modulation bandwidth of the UWOC system, this paper adopts 16QAM-OFDM as the main modulation scheme and introduces channel coding technology to enhance the system's anti-interference capability and link stability. In the designed UWOC system, the time-domain signal generation and original signal recovery of 16QAM-OFDM are both implemented using the Xilinx FPGA development platform. This architecture achieves efficient signal link construction and end-to-end communication flow control while ensuring real-time signal processing capabilities.

[0127] At the transmitting end, parallel multi-bit data from the host computer is first converted into a serial single-bit data stream by a parallel-to-serial conversion module. Subsequently, the data undergoes interleaving and 1 / 2 code rate convolutional coding to enhance its anti-interference capability during transmission. For convolutional coding, the interleaving operation is particularly critical: when continuous burst errors occur during signal propagation in the channel, the error correction capability of convolutional coding combined with Viterbi decoding is severely weakened; however, by rearranging the bit sequence through an interleaver, the originally concentrated errors can be dispersed to different locations, thereby significantly improving the error correction performance of the Viterbi decoder. After completing convolutional coding and interleaving operations, the data sequence is mapped into a complex signal using 16QAM. To ensure that the IFFT output is a real number, the modulation symbols need to be Hermitian symmetrically extended in the frequency domain. The time-domain signal obtained after IFFT operation will be further augmented with a 1 / 16-length cyclic prefix to suppress inter-symbol interference, and pilot symbols will be inserted for symbol synchronization and channel estimation. Finally, the processed time-domain signal is used as the input to the optical transmitter for subsequent data transmission.

[0128] At the receiving end, due to potential interference such as thermal noise and spike pulses in the sampled signal, the system first introduces a digital filtering module to suppress noise. Next, a symbol synchronization module based on correlation detection is used to accurately determine the starting position of the valid signal. This module first generates a set of frequency domain training sequences locally that match the pilot signals at the transmitting end, and performs correlation calculations with the pilot symbols in the received signal. Using a sliding window approach, the maximum correlation coefficient of each symbol is searched to determine the optimal matching point and mark it as the symbol's starting position, thereby improving the accuracy and robustness of symbol synchronization. After synchronization, the received symbols are converted to the frequency domain using FFT. To further eliminate phase offsets generated in the transmission path, this invention designs a phase estimation algorithm based on pilot sequences to compensate for signal phase offsets. First, the complex form of the pilot symbols in the frequency domain is decomposed into imaginary and real components, denoted as […]. and Subsequently, the received pilot signal is multiplied by the locally known pilot signal using a complex number, and the phase offset angle is calculated based on the following formulas (1) and (2). Make an estimate:

[0129] (1)

[0130] (2)

[0131] in, Let be the real part of the phase offset angle. Let be the imaginary part of the phase offset angle. and These are the real and imaginary parts of the received pilot signal, respectively.

[0132] Next, based on the estimated phase offset angle (Depend on and (Construction), phase compensation is performed on all received subcarrier signals. The compensation operation is completed by the following formulas (3) and (4):

[0133] (3)

[0134] (4)

[0135] in, The received signal to be compensated. Let the imaginary part of the signal to be compensated be... Let be the real part of the signal to be compensated. The imaginary unit, The output signal after compensation. To compensate for the imaginary part of the signal, This is the real part of the compensated signal.

[0136] This method can effectively eliminate phase distortion caused by frequency offset, sampling error or channel time-varying characteristics, thereby improving the synchronization accuracy and demodulation performance of the system. It is applicable to pilot-assisted synchronization and compensation processes in various communication scenarios.

[0137] After phase compensation is completed, the received signal will go through subcarrier deinterleaving, Viterbi decoding and 16QAM demapping processes in sequence to finally recover the original data signal.

[0138] Specific experimental procedures

[0139] This embodiment introduces an experimental demonstration of a high-speed underwater wireless optical communication system based on LEDs, and performs performance testing and comparison of the system. The experimental test environment is as follows: Figure 9 As shown, the underwater channel length is 3m, and the system uses 16QAM-OFDM modulation for data transmission. During the experiment, the data transmission rate was gradually increased from 1Mbps to 10Mbps to evaluate the system's bit error rate performance at different rates. The test results are as follows. Figure 10 As shown, the BER of the 16QAM-OFDM signal gradually increases with the increase of the communication rate. When the communication rate reaches 10 Mbps, the system BER is 0.0014, which is lower than the FEC threshold, indicating that the system still has reliable data transmission capability at this rate.

[0140] Figure 11The spectrum diagram shows a 16QAM-OFDM signal at a rate of 10 Mbps. The high-energy segment A in the diagram represents the effective data carrying area, mainly concentrated within a bandwidth of 5 MHz. The segment B on the right side of the spectrum represents redundant components with gradually decaying frequency response; this is a non-data area in the OFDM signal structure and does not affect the accuracy of data transmission. Spectral analysis shows that the 16QAM-OFDM modulation method achieves efficient utilization of spectrum resources, effectively improving the system's data transmission rate and spectral efficiency.

[0141] To further verify the performance of this system under high transmission rate conditions, Figure 12 The 16QAM constellation diagram recovered by the receiver at a data rate of 10 Mbps is shown. It can be observed that the constellation points are evenly distributed around the ideal convergence location, exhibiting good aggregation and symmetry, indicating that the system possesses strong noise immunity and symbol stability. This result further verifies that the designed underwater optical communication system can still achieve high-quality symbol detection and reliable data recovery even under high data rate operating conditions, demonstrating excellent modulation and demodulation performance and system robustness.

[0142] To further evaluate the maximum transmission distance of the designed optical communication terminal, we constructed a water-air cross-medium channel based on the existing 3m water tank environment by introducing air links of different lengths to simulate longer-distance transmission scenarios. The test results are as follows: Figure 13 As shown. Experiments were conducted using 16QAM-OFDM signals with transmission rates of 5Mbps and 10Mbps. Based on a 3m underwater channel, air links of 3m, 6m, and 7m were added sequentially. Under the 10Mbps condition, the system's bit error rate in the above three composite channels was 1.8 × 10⁻⁶. - ³、3.6×10 - ³ and 4.6×10 - ³. Experimental results show that the designed system can achieve stable cross-medium communication at a rate of 10 Mbps, covering a depth of 3 m underwater and 6 m in air, demonstrating good transmission robustness and distance extension capability.

[0143] To evaluate the system's offset resistance, this paper conducts a BER performance test experiment under receiver offset from the main optical axis. Under the conditions of a 3m underwater channel and a fixed transmission rate of 10Mbps, the offset angle of the receiver relative to the main optical axis was gradually adjusted, and the corresponding bit error rate was measured. The test results are as follows: Figure 14 As shown in the figure, it can be observed that the system BER increases with the degree to which the receiving angle deviates from the principal optical axis. Specifically, when the deviation angle is 0°, 10°, 20°, and 30°, the system BER is 1.4 × 10⁻⁶. - ³、2.3×10 -³、3.4×10 - ³ and 6.2×10 - ³. Experimental results show that in a 3m underwater channel, when the receiver deviates from the main optical axis by less than 20°, it can effectively receive the signal emitted by the LED array light source and maintain a low bit error rate, indicating that the system has a certain angle tolerance and anti-offset capability.

[0144] In summary, this invention designs and develops a high-speed underwater wireless optical communication system based on LEDs. The system employs 16QAM-OFDM modulation and integrates channel coding and equalization techniques, effectively improving spectrum utilization and anti-interference capabilities. Experimental results show that the developed system can achieve stable data transmission of 10 Mbps in a 3m underwater channel, with a bit error rate (BER) below the FEC threshold, verifying the system's modulation and demodulation performance and communication reliability. By introducing a π-type post-equalization network, the system's 3dB bandwidth is extended from 3MHz to 20MHz, successfully compensating for the high-frequency response attenuation caused by the limited bandwidth of the LED array light source, laying a bandwidth foundation for high-speed communication. Furthermore, even in complex environments such as cross-medium (water-air) links and receiver offset from the main optical axis, the system maintains a low bit error rate, demonstrating good robustness and engineering practical value.

[0145] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

[0146] While the specific embodiments of the present invention have been described above, they are 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 without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A high-speed light-emitting diode underwater wireless optical communication system, comprising a transmitter and a receiver, characterized in that: The transmitter includes an adaptive coding module, a 16QAM mapping module, an OFDM modulation module, a digital-to-analog converter module, and an LED array light source module connected in sequence. The adaptive coding module is used to interleave and convolutionally code the input data stream; the 16QAM mapping module is used to map the encoded data stream to 16QAM modulation symbols; the OFDM modulation module is used to divide the high-speed data stream into multiple parallel low-speed subcarriers, each subcarrier independently carrying a portion of data information; the LED array light source module is used as the transmission carrier of optical signals. The receiving end includes a photoelectric detection module, a signal processing module with integrated post-equalization circuit, an analog-to-digital conversion module, a digital filtering module, an OFDM demodulation module, a phase estimation module, a 16QAM demapping module, and a Viterbi decoding module connected in sequence. The integrated equalization circuit's signal processing module converts the current signal output by the photoelectric detection module into a voltage signal, and performs filtering, amplification, and frequency response compensation on it. The digital filtering module filters the sampled digital signal. The OFDM demodulation module performs symbol synchronization and converts the time-domain signal into a frequency-domain signal. The phase estimation module compensates for the phase shift of the received frequency-domain signal. The 16QAM demapping module demodulates the received frequency-domain symbols into the corresponding data bitstream. The Viterbi decoding module performs convolutional code decoding on the demodulated data stream. The specific working process of the phase estimation module is as follows: First, the complex form of the pilot symbol in the frequency domain is decomposed into imaginary and real components, denoted as follows: and Subsequently, the received pilot signal is multiplied by the locally known pilot signal using a complex number, and the phase offset angle is calculated based on the following formulas (1) and (2). Make an estimate: (1) (2) in, Let be the real part of the phase offset angle. This represents the imaginary part of the phase offset angle. and These represent the real and imaginary parts of the received pilot signal, respectively. Next, based on the estimated phase offset angle Phase compensation is performed on all received subcarrier signals; Depend on and The construction and compensation operations are completed using the following formulas (3) and (4): (3) (4) in, The received signal to be compensated. Let the imaginary part of the signal to be compensated be... Let be the real part of the signal to be compensated. The imaginary unit, The output signal after compensation. To compensate for the imaginary part of the signal, This is the real part of the compensated signal.

2. The high-speed light-emitting diode underwater wireless optical communication system as described in claim 1, characterized in that: The specific working process of the 16QAM mapping module and the 16QAM demapping module is as follows: Using 16QAM mapping, 4 bits of information are transmitted in each symbol period. First, the bit stream to be transmitted is grouped into groups of 4 bits each. The first two bits are mapped to the imaginary part of the complex symbol, and the last two bits are mapped to the real part, thus corresponding to a specific symbol point in the 16QAM constellation diagram. This constellation diagram forms a 4×4 matrix, where each element corresponds to a modulation symbol, represented as a complex number. The real and imaginary parts represent the amplitude and phase of the modulation signal, respectively. At the receiving end, the received modulation signal is demapped using 16QAM, and the symbol point is reversed and mapped to the corresponding 4 bits of data, thereby recovering the original bit stream.

3. The high-speed light-emitting diode underwater wireless optical communication system as described in claim 1, characterized in that: The OFDM modulation module's processing includes Hermitian symmetry, IFFT, addition of pilot symbols, and cyclic prefix (CP). The OFDM uses 256 subcarriers to divide the high-speed data stream into multiple parallel low-speed subcarriers. Each subcarrier carries a portion of the data and is converted to a time-domain signal via inverse fast Fourier transform. Simultaneously, real-valued OFDM signals are generated using Hermitian symmetry, and pilot symbols and cyclic prefix (CP) are inserted into the signals to achieve symbol synchronization and effectively suppress inter-symbol interference.

4. The high-speed light-emitting diode underwater wireless optical communication system as described in claim 1, characterized in that: The signal processing module of the integrated post-equalization circuit includes a transimpedance amplifier circuit, a low-pass filter circuit, a post-amplifier circuit, and a post-equalization circuit, which respectively perform current-to-voltage conversion, low-pass filtering, and equalization on the signal. The post-equalization circuit is a first-order π-type equalization circuit structure, which has the function of gain compensation for high-frequency signals. Its core part includes an inductor connected in series and two parallel capacitors symmetrically distributed at both ends. The equivalent resistance connected in parallel between the input and output is used to match the impedance, forming a typical π-type filter structure. At the same time, a dedicated impedance matching network is introduced on both sides of the circuit to minimize signal reflection and reduce energy loss during transmission.

5. The high-speed light-emitting diode underwater wireless optical communication system as described in claim 1, characterized in that: The OFDM demodulation module's processing includes symbol synchronization, CP removal, FFT, and Hermitian symmetry removal; the specific process is as follows: The signal is first synchronized with the symbol to generate a pilot signal that matches the transmitter. This signal consists of 16 alternating positive and negative 1s. Then, the most significant bit of the first 40 received symbols is determined and restored to positive and negative 1s. The correlation between the received symbols and the local pilot signal is compared through a sliding window to determine the best matching position. After that, the CP is skipped, thereby accurately locating the starting point of the valid symbol. Synchronized time-domain signals are transformed back to the frequency domain using FFT; The second half of the FFT output is truncated to remove the Hermitian symmetric redundancy introduced at the transmitting end, retaining only the first half of the effective subcarrier data to restore the original frequency domain information.

6. A high-speed light-emitting diode underwater wireless optical communication method, characterized in that, Includes the following processes: Step 1: The data stream undergoes interleaving and convolutional coding processing via an adaptive coding module; Step 2: The encoded bit sequence is converted into the corresponding complex number symbol through the 16QAM mapping module; Step 3: Perform OFDM modulation on the data from Step 2, dividing the high-speed data stream into multiple low-speed subcarriers that are transmitted in parallel, with each subcarrier carrying a portion of the data. Step 4: The modulated signal is converted from a digital signal to an analog signal by a digital-to-analog converter module. Then, the amplitude is adjusted by an amplifier and an adjustable attenuator to make the output signal level fall within the linear operating range of the LED array light source, and then transmitted in the form of an optical signal. Step 5: The photodetector receives the optical signal and converts it into an electrical signal; Step 6: The electrical signal is amplified, filtered, and equalized by the signal processing module of the integrated equalization circuit; Step 7: The signal processed in step 6 is converted into a digital signal by an analog-to-digital converter, and OFDM demodulation and 16QAM demapping are performed using a field-programmable gate array. Step 8: Calculate the phase offset of the received signal through the phase estimation module, and perform phase compensation on all subcarriers based on the estimation results. Finally, the original data information is recovered through decoding. Specifically: First, the complex form of the pilot symbol in the frequency domain is decomposed into imaginary and real components, denoted as follows: and Subsequently, the received pilot signal is multiplied by the locally known pilot signal using a complex number, and the phase offset angle is calculated based on the following formulas (1) and (2). Make an estimate: (1) (2) in, Let be the real part of the phase offset angle. This represents the imaginary part of the phase offset angle. and These represent the real and imaginary parts of the received pilot signal, respectively. Next, based on the estimated phase offset angle Phase compensation is performed on all received subcarrier signals; Depend on and The construction and compensation operations are completed using the following formulas (3) and (4): (3) (4) in, The received signal to be compensated. Let the imaginary part of the signal to be compensated be... Let be the real part of the signal to be compensated. The imaginary unit, The output signal after compensation. To compensate for the imaginary part of the signal, This is the real part of the compensated signal.

7. The high-speed light-emitting diode underwater wireless optical communication method as described in claim 6, characterized in that: Step 2 specifically involves: The input bitstream is segmented into groups of four bits each. The first two bits determine the imaginary part of the mapped symbol, and the last two bits correspond to the real part. Each group of bits uniquely corresponds to a symbol point in the 16QAM constellation diagram. The constellation diagram is represented as a 4x4 two-dimensional matrix structure, where each matrix element represents a modulation symbol point, specifically determined by a pair of real and imaginary coordinates, reflecting the amplitude and phase information of the complex symbol, respectively.

8. The high-speed light-emitting diode underwater wireless optical communication method as described in claim 6, characterized in that: Step 6 specifically involves: The transimpedance amplifier circuit converts the current signal output by the APD into a voltage signal. A 20MHz low-pass filter circuit filters out high-frequency out-of-band noise. The subsequent amplifier circuit further amplifies the filtered signal. The amplified signal is then processed by a post-equalization circuit to compensate for its high-frequency response. The post-equalization circuit is a first-order π-type equalization circuit structure, which has the function of gain compensation for high-frequency signals. Its core part includes an inductor connected in series and two parallel capacitors symmetrically distributed at both ends. An equivalent resistance connected in parallel between the input and output is used to match the impedance, forming a typical π-type filter structure. At the same time, a dedicated impedance matching network is introduced on both sides of the circuit to minimize signal reflection and reduce energy loss during transmission.

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