BER Analysis for PAM-based UWOC-NOMA System in Oceanic Turbulence Environment

Pulse amplitude modulation (PAM) scheme and non-orthogonal multiple access (NOMA) are attractive technologies to achieve high rate transmission and massive connectivity in the future Ocean Internet of Things (OIoT). To investigate the performance of underwater wireless optical communication (UWOC) NOMA systems based on PAM modulation, this paper develops a joint mathematical model with ocean turbulence and underwater solar noise based on the Weibull distribution. Specifically, the resolution framework on the ground of constellation multi-boundary judgments is proposed to derive the closed-form BER expressions for the wireless optical NOMA systems under ocean turbulent channels using the Laguerre approximation, which is also applicable to the other NOMA-enabled systems under higher order modulations. The accuracy of the analytical solution is demonstrated by simulation results. In addition, simulation results show that the higher-order PAM modulation can effectively reduce the bit-error rate (BER) gap between NOMA users and balance the fairness of users within the NOMA group. Furthermore, we observe that turbulence is one of the primary factors influencing the BER of the NOMA system in the marine environment, and the tilt angle of NOMA nodes has varying degrees of influence on the BER performance varying turbulence intensities. Therefore, selecting the appropriate tilt angle for the receiving nodes in a NOMA system based on the marine environment is beneficial to lowering the BER of the system.