Equiprobable Pre-Coding PAM7 Modulation for Nonlinearity Mitigation in Underwater 2 × 1 MISO Visible Light Communications

Abstract

Visible light communication has attracted increasing attention for its cost effective, energy conservation, and high efficiency. Nowadays, underwater visible light communication (UVLC) has drawn more interest in the research community. In order to increase the transmission distance in an underwater environment, a high-power light-emitting diode must be considered. Therefore, the nonlinearity induced by the electrical-to-optical conversion in the transmitter, the underwater channel, and the optical-to-electrical conversion in the receiver will become a challenge. To release the nonlinear restriction, we propose a novel pre-coding pulse amplitude modulation (PAM) 7 modulation for 2 × 1 multiple-input single-output (MISO) system for UVLC. In this scheme, two transmitters with lower modulation level are utilized to reduce the nonlinearity. In comparison with traditional space-time block-coding (STBC) technique, a novel precoding scheme with unique mapping/demapping value is proposed to achieve a equiprobable PAM7 signal. The theoretical analysis and experimental results show the proposed pre-coding method can outperform the traditional STBC MISO system in terms of the mutual information and system capacity. The 2.1-m air-underwater transmission experiment demonstrates an overall 53% capacity improvement, validating the feasibility of the proposed pre-coding PAM7 modulation can be a promising candidate for underwater communications.