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In this work, a hybrid multiple access scheme for multi-user underwater wireless optical communication (UWOC) systems is proposed and presented. The scheme combines the respective advantages of orthogonal frequency-division multiple access (OFDMA) and non-orthogonal multiple access (NOMA) scheme to reduce the unfairness among users caused by the unique high-frequency fading characteristics of the underwater optical channel. Multi-verse optimizer (MVO) algorithm is further introduced for solving the nonlinear problem of user power allocation. Experimental results in different water quality environments demonstrate that the hybrid OFDMA/NOMA scheme with MVO effectively improves the communication performance of the established 4-user UWOC system compared to the pure NOMA scheme and the pure OFDMA scheme, and a system sum rate of 3.94 Gbit/s is achieved. • A hybrid OFDMA/NOMA scheme is proposed for UWOC system. • MVO algorithm is introduced for user power allocation. • A sum rate of 3.94 Gbit/s is experimentally achieved in a 4-user UWOC system. [ABSTRACT FROM AUTHOR]

In this paper, the performance of a one-channel PDM-VLC system with different signal power ratios, sample rates and transmission distances are investigated, and for three users' transmission, 150 Mbit/s rate and 1.2 m distance can be realized. Furthermore, a two-channel PDM-VLC system with distance of 1 m is established and analyzed for the multi-user application. The total transmission rate can reach 150 Mbit/s for three users and 250 Mbit/s for two users under the influence of the channel interference. • One-channel and two-channel PDM-VLC systems are presented for three users to simultaneously transmit their own signal. • A 150 Mbit/s transmission for three users at a distance of 1.2 m can be realized in one-channel PDM-VLC system. • Two-user 250 Mbit/s and three-user 150 Mbit/s transmissions at 1 m are realized in two-channel PDM-VLC system. • The impact of signal power ratio, sample rate, and transmission distance on PDM-VLC system performance is demonstrated. • It is revealed that PDM can be successful used for multiple user transmission in VLC system with high spectrum efficiency. [ABSTRACT FROM AUTHOR]