Performance analysis of free-space optical communications systems for wireless broadband networking applications

Abstract

Recently, free-space optical (FSO) communications have gained significant attention since they have extremely wider bandwidth than radio frequency (RF) counterparts. Indeed FSO communications are able to support much more broadband networks with a license-free and cost-effective wireless optical system than RF communications. Accordingly, FSO communications have been widely considered for space applications such as inter-satellite communications links and satellite-to-ground communications links. More recently, FSO communications have been actively studied for the applications of unmanned aerial vehicle (UAV) -- based high data rate communications by the several projects of Defense Advanced Research Project Agency (DARPA) and Air Force Research Laboratory (AFRL) such as optical RF communications adjunct (ORCA) program and FSO experimental network experiment (FOENEX) program. The programs aim to test an FSO backbone network up to 50 km and 200 km between air-to-ground nodes and air-to-air nodes, respectively. FSO communications, however, suffer from major performance impairments due to the highly degrading effects of atmospheric turbulence and pointing errors as well as fog and cloud. Atmospheric turbulence is caused by variations in the refractive index due to temperature and pressure changes. Pointing errors are caused by the misalignment between the transmitter and receiver. In this dissertation, we investigate various types of FSO communications systems, such as diversity, relay and two-way relay, to overcome atmospheric turbulence, pointing errors, fog and cloud effects for the purpose of long-distance UAV backbone networks. In case of FSO channels, we consider the combined effects of turbulence-induced phase distortion and amplitude fluctuation for the coherent FSO systems and the Gamma-Gamma fading model for the subcarrier intensity-modulated (SIM) systems. We also consider the combined effects of atmospheric turbulence and misalignment effects on the performance analysis. Firstly, we investigate the performance of a coherent FSO communication system with multiple receive apertures over atmospheric turbulence fading channels. In particular, we derive the average error probability and the ergodic capacity of the coherent FSO system with multiple receive apertures in series representations. To mitigate the atmospheric turbulence-induced amplitude fluctuations and phase aberrations, we assume to use, at the receive apertures, the modal compensation technique. Numerical results confirm the derived average error probability and ergodic capacity expressions. Secondly, we propose and analyze an amplify-and-forward (AF) relaying system for coherent FSO applications. In particular, we derive, in series expressions, the lower bound of the outage probability of the proposed system, assuming to use the modal compensation at the receiver to mitigate the atmospheric turbulence-induced amplitude fluctuation and phase aberration. The performance of the derived lower bound of the outage probability is confirmed by the tight Monte-Carlo simulation result. It is noted that the AF relaying system is comparable in performance to the DF relaying system. Thirdly, we investigate the performance of a coherent FSO communications system considering geometric spread and pointing errors as well as atmospheric turbulence fading and path loss. In particular, we derive the outage probability and average error probability of the coherent FSO system, considering the combined effects of path loss, geometric spread and pointing errors, and amplitude fluctuations and phase aberrations in series representation forms. Numerical and Monte-Carlo simulation results confirm the validity of the derived outage probability and average error probability expressions. Fourthly, we analyze the performance of a two-way subcarrier intensity-modulated AF relaying system over FSO communication channels. The analysis takes into consideration attenuations due to atmospheric turbulence and geometric spread and pointing errors at the same time. We derive, in generalized infinite power series expressions, the tight upper and lower bounds of the overall outage probability and average probability of errors of the system. The study finds that this two-way subcarrier intensity-modulated AF relaying system using a binary phase shift keying (BPSK) modulation could be used for practical applications in case of a weak turbulence regime. It is also noted that the pointing errors clearly degrade the performance of the two-way subcarrier intensity-modulated AF relaying system. Fifthly, we analyze the performance of a dual-hop relaying system composed of asymmetric RF and FSO (RF/FSO) links. We consider an asymmetric AF relay which converts the received RF signal into an optical signal using the subcarrier intensity modulation scheme. The RF and FSO channels are assumed to experience Rayleigh and Gamma-Gamma fading distributions, respectively. Particularly, we derive the average probability of error as well as ergodic capacity upper bound of the asymmetric RF/FSO dual-hop relaying system, in closed-forms. As a result, the asymmetric RF/FSO relaying system shows slightly worse performance in average probability of error and ergodic capacity upper bound than does the RF/RF relaying system in the low SNR. Over the SNR of 20 dB, however, the asymmetric RF/FSO relaying system shows very similar performance to the RF/RF relaying system in average probability of error and ergodic capacity upper bound. The derived mathematical expressions are verified by exactly matching Monte-Carlo simulation results.