Relay-Assisted Free-Space Optical Communications

The atmospheric lightwave propagation is considerably influenced by the random variations in the refractive index of air pockets due to turbulence. This undesired effect significantly degrades the performance of free-space optical (FSO) communication systems. Interestingly, the severity of such random degradations is highly related to the range of atmospheric propagation. In this thesis, we introduce relay-assisted FSO communications as a very promising technique to combat the degradation effects of atmospheric turbulence. Considering different configurations of the relays, we quantify the outage behavior of the relay-assisted system and identify the optimum relaying scheme. We further optimize the performance of the relay-assisted FSO system subject to some power constraints and provide optimal power control strategies for different scenarios under consideration. Moreover, an application of FSO relaying technique in quantum communications is investigated. The results demonstrate impressive performance improvements for the proposed relay-assisted FSO systems with respect to the conventional direct transmission whether applied in a classical or a quantum communication channel

Performance Analysis of the Differential Evolution and Particle Swarm Optimization Algorithms in Cooperative Wireless Communications

In this study, we evaluate the performance of differential evolution (DE) and particle swarm optimization (PSO) algorithms in free-space optical (FSO) and mobile radio communications systems. In particular, we obtain the optimal transmission distances for multiple-relay nodes in FSO communication systems and optimal relay locations in mobile radio communications systems for the cooperative-diversity networks, using both algorithms. We investigate the performance comparison of DE and PSO algorithms for the parallel decode-and-forward (DF) relaying. Then, we analyze the cost functions. Furthermore, we present the execution time and the stability of the DE and PSO algorithms

Experimental Investigation of All-Optical Relay-Assisted 10 Gb/s FSO Link Over the Atmospheric Turbulence Channel

This paper presents novel experimental results for a 10 Gb/s triple-hop relay-based all-optical free space optical (FSO) system by employing the amplify-and-forward relaying scheme. We provide a mathematical framework for the end-end signal-to-noise ratio (SNR) and the bit-error rate (BER) performance and confirm that the derived analytical results reasonably match experimental results especially at relatively high SNR. The evaluated BER performances under different atmospheric turbulence regimes (modeled by the Gamma-Gamma distribution) show that the considered relay-assisted FSO system offers a significant performance improvement for weak-to-strong turbulence regimes, even without knowledge of the channel state information. More precisely, at a target BER of 10-5, the proposed scheme offers ~5 and ~4 dB of SNR gains compared to the direct transmission for turbulence strengths Cn2 of 3.8 × 10-10 m-2/3 and 5.4 × 10-12 m-2/3, respectively