Pre-Emphasis Assisted Mixed Modulation PD-NOMA-OFDM System for UOWC

This paper reports the transmission of mixed modulation formats assisted signals, which use orthogonal frequency division multiplexing (OFDM) combined with power domain nonorthogonal multiple access (PD-NOMA) schemes in a multi-user underwater optical wireless communication (UOWC) system. Experimental investigation of two and three-user PD-NOMAOFDM based signal transmission in UOWC systems is carried out using a blue light emitting diode (LED) as the transmitter. To overcome the modulation bandwidth limitation of LED, a technique called differential pre-emphasis (DPE) is introduced that optimally minimizes the rise and fall times of the optical OFDM-NOMA signal. Simulation and experimental results show that the bit error rate (BER) of each user is below the forward error correction (FEC) threshold of (10−3) at the optimum DPE factor for both mixed-modulation and identical modulation formats in two- and three-user PD-NOMA schemes. For the twouser scenario with the same modulation (QPSK-QPSK) formats, the maximum per-user data rate achieved is 24.1 Mbps, while with mixed modulation (QPSK-16-QAM), the transmission rate is 24.1 Mbps for the far user (FU) and 48.2 Mbps for the near user (NU). In the case of a three-user PD-NOMA scheme, the maximum per-user data rate is 24.1 Mbps with identical (QPSK) modulation format. With mixed modulation formats in three user NOMA, the maximum achieved per-user data rates are 12.04 Mbps, 24.1 Mbps, and 48.2 Mbps with BPSK, QPSK, and 16-QAM modulation, respectively. Experimental studies in the turbulent water channel also establish the efficacy of the DPE-enabled mixed-modulation PD-NOMA-OFDM signal transmission in achieving BER below 10−3 in both two-user and three-user schemes.