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Nonlinear Compensation based on Bidirectional Gate Recurrent Unit in Underwater Visible Light Communication System

Visible light communication (VLC) has emerged as a promising communication method in the domain of underwater communication. However, the transmission rate is limited by the modulation bandwidth and the nonlinear effect of the LED. In this paper, we proposed a post-equalization method base on Bidirectional Gate Recurrent Unit (BGRU) and high order modulation format Amplitude phase shift keying (64APSK) is applied in underwater visible light communication (UVLC) for the first time. The excellent performance of nonlinearity compensation has been verified through experimental demonstration. With 64APSK and BGRU post-equalization, the transmission rate of underwater 1.2m distance is up to 3.19 Gbps. Compared to the traditional Least Mean Square (LMS) algorithm, the transmission rate increases 140 Mbps with VPP of 0.7V, current bias of 150mA and Q factor increases 2.4dB at the maximum with VPP of 0.9V, current bias of 150mA and bit rate of 3 Gbps. Compared to the Normal-64QAM, the transmission rate increases 65 Mbps and 64APSK shows a larger working range. All the results are demonstrated with 7% FEC of 3.8E-3 as a BER threshold. To the best of our knowledge, it is the first time 64APSK and BGRU post-equalization are used in UVLC system and the bit rate exceeds 3 Gbps. BGRU is shown effective to deal with the nonlinear distortion and APSK modulation format is suitable for underwater high-power transmission, which are proved as a promising method in future UVLC.