# Relay Selection Schemes for FSO Communications over Turbulent Channels
## 1 Introduction  
We regard Free Space Optics (FSO) as a strong candidate to integrate with and complete the future technologies, such as 5G wireless networks. FSO technology is used widely in various indoor fields (e.g., data centers), space (e.g., intersatellite and deep space communication), (e.g., underwater sensing), terrestrial (e.g., mobile networks), and underwater systems [1]. FSO is an optical communication innovation, where the transmission of information is through spreading light in free space to permit an optical network. FSO operates as an optical fiber network but with different media. In FSO systems, the optical beams have free space or air medium, while the optical fiber network has cores that pass-through glass fiber. The FSO framework comprises an optical transceiver at the two ends to provide full duplex (bidirectional) ability. The existence of FSO communication goes back to the eighth century, however now it is more applicable. A full duplex(bidirectional) connectivity can help to accomplish more than 10 Gbps of data rate while using FSO technology, which is one of a line of sight (LOS) innovation [2]. On-off Keying (OOK) is a binary level modulation scheme where sending logic \"1\" is a presence of light and sending a logic \"0\" represents the absence of light. OOK has relatively poor energy efficiency, however it is still the most commonly used intensity modulation technique due to its implementation simplicity and low cost, while in a coherent FSO system the information is encoded on the optical carrier amplitude and phase and it still requires high speed digital signal processing integrated complex and expensive circuits [3]. A previous work was carried out on the multiple potential relays of cooperative communication. One of the studies is the outage probability of multi-hop free-space optical communication over strong turbulence channels [4]. Another work has investigated serial and parallel relaying that operates in amplify-and-forward (AF) or decode-and-forward (DF) modes [5]. Besides, some researchers discovered that the worst outage performance was achieved when increasing pointing error and total link distance [6]. The study of FSO has proved to be better for the last-mile solution and cost-effective backup of fiber optics, where it is easy to install. FSO has many drawbacks, such as the degradation of signal quality due to atmospheric turbulence, weather attenuation, pointing error, and geometric losses [3; 7]. In [8], the authors investigate whether using a plenoptic sensor can find the best channel, even in the strong turbulence, and whether the target image can be received correctly. The relay selected techniques help FSO communication to overcome atmospheric turbulence and path loss challenges through hops. [PERSON] et al. derived a novel closed-form outage probability and average bit error rate (ABER) expressions of mixed dual-hop radio frequency (RF)/FSO relaying system with the semi-blind AF relaying [9]. The authors of [10] have derived the ABER of two steps blind detection system while using a relay. The authors have also advised that the middle of the channel is the best location for the relay. The investigation of the performance of the best relay selection of source-relay-destination (S-R-D) 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. However, the concept of best relay selection can also be applied to coherent modulation and demodulation. This paper concentrates on IM/DD due to its simplicity and low cost. A different number of relays and different distances between source-relay and relay-destination are considered.