Optical wireless communications over fading channels: Spatial diversity or multihop relaying?

It has been well-known in optical wireless communications (OWC) that both spatial and cooperative diversities are effective methods for mitigating the impact of fading channels. Cooperative diversity based on multihop relaying could significantly improve the system performance, especially over long distances, its cost is nevertheless increased proportionally with the increase of the number of hops. On the other hand, spatial diversity is could reduce fading variance, its effectiveness, however, is diminished when the fading among sub-channels are correlated. The purpose of this study is to investigate the fundamental performance limits of spatial diversity suffering from correlated atmospheric fading in multihop relaying OWC systems. The negative effects of channel correlation are comprehensively analyzed by quantifying the correlation coefficient as a function of link range, turbulence strength and aperture separation. By showing that there is a substantial reduction in power penalty caused by fading correlation when relays are employed, we comprehensively analyze the performance trade-offs between spatial diversity and multihop relaying.