Underwater wireless optical communication (UWOC) featuring high speed, low delay, and high confidentiality can form a medium and short distance local area network (LAN) with high speed and strong real-time properties or can complement the advantages of underwater acoustic communication hybrid networking. It has become a research hotspot in recent years. The deployment and application of UWOC require high-performance, low-cost, and miniaturized practical optical transceivers. By employing a laser diode as a light source, the transmission distance is long with high speed, but the alignment of the light receiver and light sender is demanding, which is difficult for communication of non-fixed positions and orientations in the dynamic seawater environment. If the light-emitting diode (LED) is the light source, the alignment requirements can be greatly reduced, while the LED array can be adopted to enhance the light signals to extend the transmission distance. Meanwhile, the high-order modulation can be utilized to improve the problem of insufficient bandwidths. The existing UWOC systems based on LED design have a large room for improvement in transmission distance and transmission rate. Additionally, the system design can realize real-time high-order modulation, coding and signal processing, and miniaturized system integration by leveraging field programmable gate array (FPGA) at both receiving and transmitting ends, and further improve the system performance and practicality. Aiming at the application requirements of underwater UWOC with high speed, long distance, low cost, and miniaturization, we design a highly robust integrated miniaturized UWOC system based on high-power LED array and FPGA. The system can realize on-off keying (OOK) modulation, and flexibly realize high-order modulation and channel coding. Meanwhile, it can achieve a longer transmission distance and a higher transmission rate than the LED UWOC systems reported in previous references, with miniaturized integration of optical receivers and transmitters.

The optical transmitter employs 45×1 W high-power LED array as the light source and takes total internal reflection (TIR) lens and parabolic reflector tube structure as the optical antenna to realize high-order modulation and coding by FPGA. In terms of optical receivers, an avalanche photodiode-automatic gain amplifier circuit (APD-AGC) optical receiver based on FPGA is designed. After being converted into the current signal by APD, the optical signal is converted into a voltage signal by transresistance amplifier module, then amplified into a voltage signal with fixed peak amplitude by automatic gain amplifier module, and finally input to the analog-to-digital conversion (ADC) module. The receiving FPGA can synchronously demodulate and decode the received signal in real time. In real-time synchronous signal transmission, the receiving and sending ends of the system adopt FPGA to process signals, which can support the real-time synchronous underwater transmission of commonly applied OOK signals and high-order modulation signals by taking 16QAM encoded by Reed Solomon (RS) channel as an example, which can meet the application requirements under different underwater scenarios. Finally, the optical communication receiver and transmitter are miniaturized and integrated with strict waterproof packaging and practical significance.

Under a 12 m underwater channel, the UWOC system employs OOK and 16QAM modulation for data transmission. Within the error threshold, the transmission rate of the optical terminal can reach 30 Mbps. The 16QAM bit error rate is always higher than that of the OOK system. This is because QAM modulation requires high real-time synchronization of the signal, and the wide beam of the LED array makes underwater channel synchronization difficult due to the multi-path effect. However, at the same transmission rate, 16QAM has a higher bandwidth utilization than OOK modulation, which is because the system is limited by ADC sampling rate and synchronization algorithm, the bandwidth advantage of 16QAM high-order modulation signal is not fully utilized, and there is still great potential in rate improvement. In terms of robustness, the receiver can achieve effective reception of the deviation degree from the main optical axis within 40° at a distance of 12 m, reducing the difficulty of alignment and possessing strong robustness. To test the limit transmission distance of the system, we successfully build a cross-media link communication system with a distance of 12 m water+30 m air (a total of 42 m) and transmit 22 Mbps nonreturn to zero (NRZ)-OOK shaper signal within the error decision threshold. In practical applications, the system can be applied to underwater scenarios. For example, it can be carried on the submarine to achieve underwater link deployment, and underwater node information collection or signal transmission. Additionally, its high-power LED array also supports the deployment of air-water link wireless optical communication links, such as the deployment of end machines on ships and underwater frogman or underwater robot community information interaction.

We design and develop a high-robustness miniaturized UWOC system based on FPGA and high-power LED array light source, which supports traditional OOK modulation and m-QAM modulation. The system is tested experimentally by taking 16QAM modulation as an example, and the results show that under the real-time transmission of a 12 m water channel, the bit error rates for OOK modulation and 16QAM modulation are 2.467×10^-4 and 3.467×10^-3 respectively at 30 Mbps. The 22 Mbps NRZ-OOK shaping signal 12 m water +30 m air is transmitted across media links with a bit error rate of 3.619×10^-4. Additionally, the light source transmitter of a high-power LED array with optical collimation antenna and the optical receiver based on 3 mm large aperture APD automatic gain control can receive signals within 40° of the main optical axis in the 12 m underwater channel. Finally, the stringent requirements for UWOC system alignment and focusing are greatly reduced, and the system robustness for applications in the underwater environment is improved.