Relay Selection Schemes for FSO Communications over Turbulent Channels

Abstract

Free Space Optics (FSO) Communication has attracted the attention of the researchers in the last decade due to its high data rate, security, and low cost. Relay-assisted techniques are used to divide the distance to shorter hops in order to mitigate the effects of turbulence, weather attenuation, pointing error, and geometric loss. Choosing an active relay per time slot has been proven to enhance the performance of the system and decrease the loading effect on the system when compared to all active relays. This paper investigates the best relay that can be selected according to the source to relay (S-R) channel coefficient, relay to destination (R-D) channel coefficient, and source to destination (S-D) channel coefficient. A comprehensive comparison is applied to the three following cases: (a) Broadcast phase from source to relay to select the best (Proactive-Relay); (b) Broadcast phase from relay to destination after broadcasting to all relays then select (Reactive-relays); and, (c) Direct link from source-to-best relay-to-destination to conclude which method is better for different scenarios, such as turbulence regime, number of relays, different pointing error effect, and severity of S-R as compared to R-D and vice versa. The selection methods regard to four aspects: (1) Number of relays (two or three relays); (2) Distance between Source-Relay and Relay-Destination (D = 400–600 m, 500–500 m, and 600–400 m); (3) The different turbulence of Log-normal channel and Gamma-Gamma channel (with a refractive index coefficient( C n 2 = 0.5 × 10−14, 2 × 10−14 and 5 × 10−14)); and finally, (4) Beam waist ω z (pointing error).

Special Issue on Light Communications: Latest Advances and Prospects

“…The next two papers also report on the impact of atmospheric effects on FSO communications, with the addition of intermediate relays between the transmitter and the receiver. Taher, et al [6] have studied the performance of three relay selection schemes (source to relay, relay to destination, and source to destination). The obtained results show that no optimum technique exists for all channel conditions, and the relay selection algorithm needs to take into account the severity of atmospheric induced fading and the pointing errors.…”

Opto-Electronics Review

“…and 𝜉 is the ratio between the receiver equivalent beam width and the standard deviation of the receiver pointing error displacement, given by [18]…”

“…where 𝑥 is the Gaussian random variable (RV) independent and identically distributed with the mean 𝜇 𝑥 and the variance 𝜎 𝑥 2 . To assure that the average power is not reduced or amplified by the fading channel, the fading coefficients are normalized [18,19].…”

Prediction of Received Optical Power for Switching Hybrid FSO/RF System

“…The mutual interaction of the signal from the FSO heads (optical beam) with aerosols leads to a deterioration of the quality of communication in the atmosphere, a reduction of the transmission speed-and in the worst case-to a complete interruption of communication. From a practical point of view, this means that the amount of received optical signal detected on the photo-sensitive element, e.g., on the avalanche photodiode (APD) the FSO head decreases depending on the number of scattered particles in the atmosphere along the transmission channel [4]. The size and concentration of the fog particles are the biggest problem for the FSO signal, as they cause critical values of the attenuation of the transmitted optical signal [5].…”