A Real-Time MPPC-Based Duplex UWOC System

Underwater wireless optical communication (UWOC) has emerged as a critical technology for real-time and medium-distance underwater data transmission. In this paper, a real-time duplex UWOC system is established in a standard 50-m swimming pool based on multiple blue/green light sources and a high-sensitive multi-pixel photon counters (MPPC). Specifically, the 4-quadrature amplitude modulation (4-QAM) orthogonal frequency division multiplexing (OFDM) technique and (2, 1, 7) convolutional codes are deployed and realized using a field programmable gate array (FPGA) to complete real-time signal processing. To emulate the maximum transmission distance, cascaded neutral density (ND) filters are exploited to attenuate the received optical power (ROP) of the MPPC. In the 7-m tank, the maximum accepted optical attenuations of the designed system for the blue and green links are both 68 dB at a 1 × 10⁻³ threshold. In the 50-m swimming pool, the maximum transmission distances for the blue and green links are estimated to be 131 and 120 m with geometry loss, which can be extended to 152 and 138 m without geometry loss. Additionally, the impacts of air bubbles and horizontal offset between transceivers on the bit error rate (BER) are also investigated. Under weak turbulence, the sensitivity of the ROPs for the blue and green links are only reduced by 4 dB and 3 dB at a 64-L/min air bubble density, respectively. The horizontal detection range of the system can reach up to 125 cm caused by the high-sensitivity MPPC. Moreover, to verify the transmission reliability under dynamic interference, a series of field trials are conducted based on a six-degree-of-freedom (6-DOF) platform. These experimental results reveal that the proposed scheme can be beneficial to the real deployment of UWOC.