Solution for Self-Interference of NOMA-Based Wireless Optical Communication System in Underwater Turbulence Environment

Non-orthogonal multiple access (NOMA) is a multiple access technology of great potential and it has superior performance in underwater wireless optical communications (UWOC). In this paper, we investigate the self-interference problem caused by the superimposed transmission of NOMA-based wireless optical communication system in the underwater turbulent environment, and then propose an alternating composite phase shift (ACPS) coding scheme based on constellation optimization, which can effectively eliminate the decoding error caused by the self-interference within the NOMA paired group. To enhance the optimization efficiency of ACPS in the medium- and strong-turbulence environments, we design a golden-section-approach-based power allocation (GSPA) algorithm that minimizes the bit error rate (BER). The simulation results show that the BER of the low-power user within the NOMA paired group can be reduced by up to 3 orders of magnitude in the ACPS-based UWOC-NOMA system compared to the conventional UWOC-NOMA system. In addition, the proposed GSPA algorithm fully takes into account the light-intensity scintillation caused by the ocean turbulence, which not only significantly reduces the average BER of the NOMA system, but also accelerates the convergence rate.