A Zero-Forcing Precoder and Mode Modulation Aided Orbital Angular Momentum Multiplexing Transceiver for Underwater Transmissions

Orbital angular momentum (OAM) multiplexing can improve the transmission efficiency and provide higher bandwidth for underwater wireless optical communication (UWOC). However, the turbulence effect in ocean would induce the crosstalk between OAM modes, thereby leading to the reliability performance degradations. In this paper, we first utilize the mode modulation in the underwater OAM multiplexing system to deliver extra information bits. Then we add a zero-forcing (ZF) based precoder at the transmitter to mitigate the crosstalk effects to enhance the reliability performances. At the receiver, we propose a two-step maximum likelihood (TSML) detector which has lower complexity than that of the ML detection to recover the transmitted symbols. Moreover, we derive the bit error rate (BER) expressions for the proposed systems operating under perfect or imperfect channel state information (CSI) conditions, and the simulation results validate the theoretical analysis. Last but not the least, the simulated performances of the proposed system under different underwater channel parameters, e.g. the dissipation rate of temperature, the relative strength of temperature and salinity fluctuations and turbulence strengths are investigated and analyzed.