# BICM-ID with Physical Layer Network Coding in TWR Free Space Optical Communication Links +
## 1 Introduction  
Although network coding has emerged as a new paradigm for communication to provide significantly increased network throughput [1], a valuable expansion technique introduced by [2] known as physical layer network coding (PNC) has an improved throughput for a two-way relay (TWR) network. A free space optical (FSO) communications link is an emerging broadband wireless access candidate, which has attracted significant attention for a variety of applications, such as a terrestrial FSO system in complementing the existing wireless radio frequency (RF) solution [3], for example, and utilizing the RF spectrum with cognitive radio networking enabling users to exploit femto base stations to improve the capacity performance [4]. The superior characteristics of the FSO technology are considered an advantage of the communication owing to the high security, cost-effectiveness, virtually unlimited bandwidth for achieving a very high aggregate capacity, simplicity of system design/setup, no licensing requirements for its utilization and reducedinterference [5]. However, atmospheric turbulence is a natural phenomenon commonly observed in terrestrial FSO systems, where the refractive index varies along the propagation path due to changes in atmospheric temperature, pressure and wind speed [6]. Furthermore, the weather condition was discussed in [7] to emphasize the importance of environmental conditions and atmospheric parameters. Therefore, laser beams propagating through the turbulent channel are highly susceptible to the adverse effects of turbulence-induced scintillation [8]. The concept of relay-assisted communications has recently been introduced in FSO systems to mitigate the degrading effects of the atmospheric turbulence and improve the system performance in the presence of the non-line-of-sight (NLOS) path between two FSO nodes in the network [9]. A number of analytical and simulation studies on the TWR-FSO system have been considered such as the amplify-and-forward (AF) schemes [10], decode-and-forward (DF) [11], linear network-coding at the relay node [12] and a recently-formed concept of buffer-aided relaying in [13]. For PNC demapping/mapping at the relay, the full utilization of TWR-FSO network resources was studied in [14]. Forward error correction (FEC) has been adopted for controlling errors in data transmission over a noisy communication channel to ensure a reliable communication system. The researcher in [15] utilized the packet-layer forward error correction to show its strength in a slow atmospheric fading channel and which affected the number of packet losses. Recently, the performance of end-to-end (E2E) convolutional code combined with TWR-FSO PNC was implemented by [16]. [PERSON] et al. in [17; 18] described the bit interleaved coded modulation with iterative decoding (BICM-ID) code detection in parallel FSO and RF links. The powerful FEC is BICM-ID [19], where BICM-ID is applied as an iterative channel coding method in the TWR-FSO PNC system to minimize the degrading effects of the FSO channels and to increase the robustness of a system against the noise and the interference. The effects of turbulence-induced channel fading under a clear weather condition is also considered. The result reveals that the TWR-FSO PNC scheme can employ an iterative decoding means to achieve improved performance in terms of bit error rate (BER). However, extrinsic information transfer (ExIT) charts are given as an advanced technique in a precise manner for describing the convergence behaviour of the iterative decoding method [20]. Therefore, ExIT charts are involved to investigate the achievable convergence for the E2E BICM-ID TWR-FSO PNC scheme. The rest of this paper is organized as follows: Section 2 presents the system description. Section 3 introduces the ExIT charts analysis, and Section 4 illustrates the simulation results. Finally, Section 5 contains the conclusion.