Free-Space Optical Communications With Relays

Free space optical (FSO) communications can transfer data at Gbps using unlicensed spectrum. But single FSO link cannot be longer than several kilometers due to turbulence-induced fading or scintillation. One solution to the problem of fading is usage of relays. By using the relays, a long link is disassembled into several short links and the end-to-end performance can be improved.In this work, optimal relay placement is studied for FSO multi-hop communication with link obstacles and infeasible regions. An optimal relay placement scheme is proposed to achieve the lowest outage proba-bility, enable the links to bypass obstacles of various geometric shapes, and place the relay nodes in specified available regions. When the number of relay nodes is large, the searching space can grow exponentially, and the searching time becomes unacceptable. Thus, a grouping optimization technique is proposed to reduce the searching time. We numerically demonstrate that the grouping optimization can provide suboptimal so-lutions close to the optimal solutions, but the average searching time grows linearly with the number of relay nodes. Two useful theorems are presented to reveal insights into how to reduce the outage probability, and to help decide original relay locations. Simulation results show that our proposed optimization framework can effectively provide desirable solution to the problem of optimal relay nodes placement.Prior works studied the performance of radio frequency (RF) cooperative communication protocols over FSO channels without considering the possibility of improvement. We propose a distance-dependent coop-erative scheme for free-space optical communications with a single-relay assisted scenario. The new scheme exploits a fact that the diversity order of a free-space optical channel is related to the link distance. It is shown, both analytically and numerically, that the frame error rate of the new scheme can provide higher diversity order than a benchmark adaptive decode-and-forward scheme.When the weather condition is not satisfactory, backup RF links are activated to guarantee the avail-ability of communications. Diversity receptions are common techniques to improve the system performance in RF communications. RF antennas equipped on FSO receivers suffer correlated channel fading due to the insufficient spacing of antennas. In order to evaluate the impact from the correlation, we study the asymptotic performance of hierarchical combinings (SC-MRC, SC-EGC and EGC-MRC) over arbitrarily correlated Ri-cian channels. A comparison is made among the hierarchical combinings and hybrid-selection maximal-ratio combining and hybrid-selection equal-gain combining. Based on the results for a single link, we also derive the asymptotic performance for multi-hop relaying with hierarchical combining applied on nodes.