Synchronization scheme of photon-counting underwater optical wireless communication based on PPM

In order to realize underwater optical wireless communication (UOWC) under long-distance, single photon avalanche diode (SPAD) is used as the receiver. Since the SPAD outputs discrete single photon pulse signals, the key issue is how to recover data from the discrete single photon pulses. Traditional photon-counting UOWC uses SPAD array with high gain to recover data directly. The disadvantage of this method is that the number of photons required to transmit 1 bit data is large, and the transmission distance is also limited. In order to reduce the number of photons required to transmit 1 bit data, we proposed a method of extracting the time slot synchronization clock (TSSC) from the received discrete photon-counting pulses and recovering the data. Based on the random Poisson process of the receiver and the attenuation process of the underwater channel, a photon-counting UOWC model was established. Meanwhile, to verify the method proposed, an underwater light emitting diode (LED) communication system based on SPAD was built. The experimental results show that without considering the background light noise, the photon-counting UOWC system can achieve a satisfactory performance of 0.875 photons/bit with a symbol error rate (SER) of 1.8 x 10(-3) by using 256-pulse-position modulation (PPM) and the TSSC extraction method we proposed.