Performance Analysis of Vertical Underwater Visible Light Communication System with Cross-QAM

In this paper, we consider a vertical Underwater Wireless Visible Light Communication (UWVLC) system, where the source is located at the surface of the sea. The distance between both the source and the destination is divided in certain intervals or layers. We assume turbulence in the underwater channel and the intensity of the turbulence, referred in term of scintillation index, varies between consecutive layers. Further, the turbulence based fading channel is modelled by log-normal distribution and the statistics of the channel vary from one layer to another. Thus, we assume cascaded channel with independent but non-identically distributed log-normal distributions. The variance of the channel in each layer is a function of scintillation index of the layer. We assume 32-cross quadrature amplitude modulation (XQAM) scheme and derive closed form expression of Average Symbol Error Probability (ASEP). We analyze the ASEP performance of the system for different layers with different intensities of turbulence. As the number of layers or depth increases, the performance degrades. We also present simulation results of ASEP and compare them with their analytical counterparts. A close matching between both validates our analytical approach. Furthermore, we present the performance with 32-rectangular quadrature amplitude modulation (RQAM) scheme with 16 × 2 and 8 × 4 constellations. We observe that the XQAM outperforms the RQAM.