Optical OFDM spatial diversity system in lognormal fading UVLC channels

Underwater visible light communication (UVLC) is an attractive solution to achieve high-speed and large-data transmission but challenging due to the impairments induced by absorption, scattering and turbulence. To combat effects of multipath and fading for the UVLC system over turbulence channels, optical orthogonal frequency division multiplexing (O-OFDM) schemes with the transceiver spatial diversity were proposed, which employed equal gain combining (EGC) at the receiver side. Underwater path loss was calculated by a generalized Lambertian formula, and the fading induced by weak turbulence was modelled as a lognormal-distribution random variable. Based on the channel model and Monte Carlo (MC) simulation, the bit error ratio (BER) performance for quadrature-amplitude modulation (QAM) asymmetrically clipped optical OFDM (ACO-OFDM) and DC-biased optical OFDM (DCO-OFDM) systems in the channel with and without turbulence was evaluated. Furthermore, the diversity gain was estimated for different diversity orders and scintillation indexes. The results demonstrate that the diversity scheme with EGC is an effective measure to reduce the effect of turbulence and could be useful for designing, predicting, and evaluating the performance of O-OFDM UVLC system in a weak oceanic turbulence condition.