Single-Input Multiple-Output Scattering Based Optical Communications Using Statical Combining in Turbulent Channels

Spatial diversity receptions are usually employed to improve the performance of scattering based optical communication (SOC) systems. Existing works on the diversity receptions for SOC systems ignored the detecting properties of the photomultiplier tube (PMT) receivers. In this paper, we focus on single-input multiple-output (SIMO) SOC systems using PMT receivers. We first present the system model of SIMO SOC system considering the link geometries and prove that the traditional maximum-ratio combining is no longer the optimal combining method for PMT receivers. Then we propose the statical combining method and derive its statistical characteristics and the bit-error rate (BER). We further derive some analytical results of the optimal weights for the statical combining in terms of minimizing the BER or maximizing the average signal-to-noise ratio (SNR). Simulation results show that, in non-turbulent channels, the statical combining for maximizing the average SNR can always outperform the equal-gain combining, especially when the difference between signal strengths on different branches is large. Besides, more weights should be allocated to the branches with either larger signal strength or small turbulent variances. Fortunately, in practical SOC systems, the branch with larger signal strength usually enjoys smaller turbulent variance.