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).
Highlights
We regard Free Space Optics (FSO) as a strong candidate to integrate with and complete the future technologies, such as 5G wireless networks
The average bit error rate (ABER) over G-G channels for the source to relay (S-R)-D, S-R, and relay to destination (R-D) schemes can be derived by directly substituting Equation (26) in Equations (23), (24) and (25), respectively
Derived analytical results that were corroborated via Monte Carlo simulation are performed with 107 bits transmitted for each depicted signal‐to‐noise ratio (SNR) and the Gauss-Laguerre quadrature approximation order is S ≤ 50
Summary
We regard Free Space Optics (FSO) as a strong candidate to integrate with and complete the future technologies, such as 5G wireless networks. One of the studies is the outage probability of multi-hop free-space optical communication over strong turbulence channels [4]. The relay selected techniques help FSO communication to overcome atmospheric turbulence and path loss challenges through hops. For FSO communications over atmospheric turbulence while using bit error rate (BER) and outage probability is discussed in [11]. With reference to existing literature, the main contribution of this paper is a case study of the three relay selection schemes, which are S-D, S-R-D, and R-D for the BER performance of OOK of FSO communications using the intensity modulation and direct detection (IM/DD) technique. The obtained results show that there is no optimum technique under different conditions, since the optimum technique depends on several factors, such as the number of relays, different distances, turbulence, and pointing error effects.
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