High fidelity underwater wireless optical communication with a phase-conjugated frame structure

In this paper, the influence factors of phase noise are first analyzed in detail and verified by experiments, and its principle is theoretically derived. Besides, we propose a novel, to the best of our knowledge, frame structure for m-quadrature amplitude modulation-orthogonal frequency division multiplexing (m-QAM-OFDM) in underwater wireless optical communication. The frame structure contains OFDM signals and their phase-conjugated signals. At the receiving end, by the simple superposition of phase-conjugated symbols, the noise suppression can be achieved. The feasibility is experimentally demonstrated by transmitting m-QAM-OFDM signal in different modulation formats and scenarios. The results show that bit error rate performance can be significantly improved, and there is also a significant increase in transmission capacity compared to the traditional phase-conjugated method. Moreover, the proposed frame structure can provide a robust and simple compromise between transmission capacity and distance.