Nonlinearity mitigation in bandwidth limited blue LED-based OFDM-UOWC systems using differential pre-emphasis.

Direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM) is widely used in blue light-emitting diode (LED)-based underwater optical wireless communication (UOWC), but the limited LED bandwidth leads to nonlinear distortions at higher frequencies. Using experiments, this paper proposes and validates a differential pre-emphasis (DPE) scheme for OFDM signal transmission in underwater channels to mitigate LED modulation bandwidth-induced nonlinearity, therefore improving transmission system performance. It is shown that the optimum DPE value leads to shorter rise time and fall time of the received signal and also avoids over- and under-shoots. The DPE scheme is validated through numerical simulations for QPSK, 16-QAM, and 64-QAM at symbol rates of 10 MS/s, 20 MS/s, and 25 MS/s, respectively, with an LED bandwidth of 8 MHz. The bit error rate (BER) performances are estimated at different values of the DPE factor for each modulation format and symbol rate. The simulation results indicate an optimized modulation format and symbol rate-dependent DPE factor for achieving minimum BER values. We further validate the DPE scheme on QPSK, 16-QAM, and 64-QAM-based OFDM transmissions in a 2.5 m tap water UOWC experimental setup. Results confirm that the optimized DPE factor depends on the modulation format and symbol rate for achieving the best BER performance. Experimental results show that the DPE scheme improves BER performance significantly, providing over 4 orders of BER improvement for all modulation formats at their respective maximum symbol rates. Successful transmissions of the OFDM signal at data rates of 24.09 Mb/s for QPSK, 48.19 Mb/s for 16-QAM, and 72.28 Mb/s for 64-QAM using an 8 MHz blue-LED-based transmitter in the presence and absence of air-bubble-induced turbulence in the water channel are experimentally demonstrated in this paper.