In recent years, there has been a strong demand for high-speed and long-range underwater wireless communication technologies owing to an increase in underwater activities, e. g., marine surveys, offshore oil exploration, submarine monitoring, and a series of new underwater monitoring and communication technologies such as an unmanned underwater vehicle(UUV), which has recently been rapidly developed. The traditional underwater acoustic communication frequency is generally between 10 Hz and 1 MHz, with low data transmission speed and high communication delay, which has gradually failed to meet the requirements of underwater activities. With the development of visible-light communication, underwater wireless optical communication (UWOC) has attracted increasing attention. UWOC has higher transmission bandwidth, faster data rate, lower link delay, higher security and lower cost than hydroacoustic communication, making it an attractive and viable alternative. Although underwater optical communication with blue and green light can minimise the transmission attenuation effect, in water, photons inevitably interact with water molecules and other particulate matter and suffer severe absorption and scattering effects, thereby weakening the transmission of optical signals and limiting the communication distance and quality. Therefore, it is essential to improve the sensitivity of underwater optical communication receivers.

The communication system designed in this study uses the on-off keying (OOK) modulation method, which occupies a small bandwidth and has a high transmission rate per code element. It is the preferred optical modulation technique for existing mature underwater laser communication systems. However, the traditional OOK underwater optical communication transmitter and transceiver can cause huge attenuation and power jitter in the transmitted signal owing to the underwater channel as well as pulse spreading and other signal distortion phenomena, which bring a great challenge to the system’s BER capability. Herein, we design a high-sensitivity underwater optical communication transceiver using hardware circuitry and digital signal processing through field-programmable gate array digital devices. Additionally, we improve the signal-to-noise ratio (SNR) of underwater communication using a series of digital algorithms, such as source coding, adaptive judgement threshold, and FIR filtering. Furthermore, we test the BER performance of the transceiver under different water quality conditions to verify the overall BER performance of the system.

We conducted internal and external field experiments using three different types of water. The experimental diagram of the indoor pool of the underwater communication system is shown (Fig. 8). The transceiver communication rate is 5 Mbps, modulation format is OOK, transmitting light source is a band blue light-emitting diode with 470 nm wavelength and the power is 1.2 W, the pass-light aperture is 75 mm; the underwater communication system’s external field test diagram is shown (Fig.9). The wireless optical communication terminal transmitter and receiver are placed in the lake water, 2-m deep from the lake surface. An indoor pool test with the water quality attenuation coefficient of 0.17 m^-1 belongs to class I of water quality with the pseudo-random code sequence rate of 5 Mbps, when the error rate is 10^-6. The communication distance is 20 m, and the system sensitivity reaches -38.28 dBm. The outdoor pool test with the water quality attenuation coefficient of 0.47 m^-1 belongs to class II of water quality with the pseudo-random code sequence rate of 5 Mbps, when the error rate is 10^-5. The communication distance is 10 m, and the system sensitivity reaches -38.46 dBm. At 2 m depth of Qiandao lake shore water, the test water quality attenuation coefficient is 1.33 m^-1, which belongs to class III of water quality with pseudo-random code sequence rate of 5 Mbps. Then, the BER is 10^-6 and the communication distance is 4.5 m. The SNR considerably improves after digital filtering. The reception sensitivity of the system reaches -37.52 dBm (Table 2).

This study describes the underwater OOK channel model and analyses the correspondence between BER and SNR of underwater OOK modulation methods in different water quality conditions. To cope with the impact of an underwater channel on the optical signal transmission, an underwater optical communication transmitter-transceiver based on hardware circuits and field-programmable logic gate devices is designed. The digital signal processing modules such as FIR filtering (to improve the SNR of the system), adaptive judgement threshold and sliding mean filtering are designed to improve the communication BER performance. The communication performance of the underwater communication transmitter and transceiver is verified under different water quality conditions. The experimental results show that the terminal can achieve a sensitivity of -38 dm at a transmission rate of 5 Mbps and BER of 10^-6. The transmission distance can reach 20, 10 and 4.5 m in class I, class II and class III waters, respectively. In the class III water test, the communication distance of 5 m and BER of 10^-5 can meet the demand of voice transmission. The distortion-free image and SD video transmission function can be realised at the communication distance of 4.5 m and BER of 10^-6, verifying the feasibility of underwater optical communication based on digital signal processing.