Fairness-Driven Optimization for NOMA-UWOC Systems With Energy Harvesting Requirements

Abstract

This article investigates a simultaneous lightwave information and power transfer (SLIPT)-enabled two-user nonorthogonal multiple access setup for underwater wireless optical communication. We consider a turbulence-induced fading channel and apply successive interference cancellation at the near user. Specifically, an explicit expression for the power amplifier gain at the transmitter in terms of bias and power allocation coefficients (PACs) is derived to ensure linear operation of the light source (LS). Furthermore, to choose the bias and PACs values, we formulate an optimization problem to maximize fairness in data rate, while meeting a minimum harvested energy threshold controlled by a system defined factor $\alpha \in (0,1]$. Results indicate constant average data rates and harvested energy until a certain value of $\alpha$, named the cut-in value. We also show that the cut-in value depends on the LS's maximum allowed bias current. Finally, we discuss how the proposed framework can be extended to more than two users. It is also shown that significant battery life improvement can be achieved by employing the SLIPT method. Specifically, for a four user scenario the nearest user positioned at 5 m from the source gets a battery life improvement of 38.5% when the data rate is approximately 1.75 b/s/Hz and an improvement of 62% when the data rate is 1 b/s/Hz.