NOMA Assisted Real-Time Downlink UWOC System Using a Software-Configurable Driver Under Different Channel Conditions

In this paper, we demonstrate a real-time, multi-user downlink underwater wireless optical communication (UWOC) system for practical applications via field programmable gate arrays (FPGAs). The performance of the established UWOC system is experimentally investigated under diverse channel conditions. The established UWOC system utilizes arrayed light emitting diodes (LEDs) as the transmitter and employs optical superimposition-based non-orthogonal multiple access (NOMA). The feasibility of employing the arrayed LEDs as the transmitter under different channel conditions is validated by simulating the optical intensity distributions of LED groups. Both the simulation and experimental results reveal that the bit error rate (BER) of user 1 (higher power user) decreases with increasing power allocation ratio (PAR), while the BER of user 2 (lower power user) increases with the increase in PAR. Uniform post-equalization is employed to extend the bandwidth of each LED group. Besides, a metal-oxide-semiconductor field-effect transistor (MOSFET)-based driver circuit and a remaining carrier sweep-out circuit unit (CSCU) are proposed to facilitate the independent control over different LED groups and enhance the response speed of LEDs, which outperform bias-tee and the single MOSFET-based driver circuit. Experimental results also indicate that NOMA exhibits superior spectral efficiency compared to the conventional time division multiple access (TDMA). A PAR of approximately 2:1 is appropriate to ensure both users operate within the forward error correction (FEC) threshold. Based on the proposed schemes, the real-time UWOC system achieves a data rate of 40 Mbps for both users with BER below FEC limit under different channel conditions. The results highlight the significant potential of the designed UWOC system to effectively meet diverse real-time, multi-user UWOC application requirements.