A High-Speed Photon-Counting UWOC System With Multiple Channels to Suppress the Randomness in Detection

The employment of photon-counting receivers is an important solution for extending the distance of underwater wireless optical communication (UWOC) links, but it also brings new challenges. Apart from the well-studied Poisson randomness, we first investigate the impact of the usually ignored randomness in electron multiplication on the bit error rate of photon-counting UWOC systems, and propose multi-channel detection with a modified log-likelihood ratio to suppress the randomness. Benefitting from this method, the slot time can be even shorter than the width of the counting pulses of the detectors, which effectively improves the data rate. The experimental system achieves a 10.13-Mbps underwater transmission with a sensitivity of -85 dBm with a low-density parity check code, requiring 0.358 photons per bit on average. The link remains stable against the turbulence with a maximum scintillation index of 0.0329. The maximum link loss is 94.09 dB, equivalent to 459 m in Jerlov IA water (c = 0.0284 m(-1)). The method proposed in this work can attain a high energy efficiency with low sacrifice in data rate and is meaningful for the design of high-order pulse position-modulated long-range UWOC systems.