Performance analysis of dual-hop optical wireless communication systems over k-distribution turbulence channel with pointing error

In this paper, we investigate the performance of the dual-hop free space optical (FSO) communication systems under the effect of strong atmospheric turbulence together with misalignment effects (pointing error). We consider a relay assisted link using decode and forward (DF) relaying protocol between source and destination with the assumption that Channel State Information is available at both transmitting and receiving terminals. The atmospheric turbulence channels are modeled by k-distribution with pointing error impairment. The exact closed form expression is derived for outage probability and bit error rate and illustrated through numerical plots. Further BER results are compared for the different modulation schemes. INTRODUCTION Transmission of optical signals through the atmosphere in a line of sight manner is known as optical wireless communication (OWC). It has gained significant research interest in recent years, however its performance is highly ill-protected to unfavorable atmospheric conditions which occur mainly due to turbulence and pointing errors. The turbulence arises due to the inhomogeneities in temperature and pressure of the atmosphere. This causes a change in refractive index and leads to the fluctuation of amplitude and intensity in the transmitted optical signals. The pointing error occur due to the sway of high rise buildings as a result of weak earthquakes, dynamic wind loads and thermal expansions. This causes misalignment between source and destination transceivers. To evaluate the impact of atmospheric turbulence various channel model are developed such as gamma-gamma, negative exponential and log normal model. The log normal distribution is found suitable for weak turbulence regimes were as gammagamma distribution is suitable for moderate to strong turbulence regimes. Another channel model is a k distribution channel model which is very popularly used for strong turbulence regimes. Further in literature, we study two types of relays i.e. amplify and forward (AF) and decode and forward relay (DF) relay. In this paper, we consider Decode and Forward relay assisted FSO system over k-distribution channel model considering the combine effect of atmospheric turbulence and misalignment error. In [2] authors have analyzed the behavior of pointing errors and derived the probability density function (pdf) for normalized irradiance due to boresight and jitter. The effect of pointing errors on the performance of the FSO communication system over log normal and gamma-gamma channel models have been studied in [3]-[5]. In [6]-[8] authors, did outage and BER analysis for relay assisted FSO system over strong atmospheric turbulence channel with spatial diversity and misalignment errors for different modulation scheme. [9] Analyzed the performance of free space optical (FSO) system employing subcarrier intensity modulation with differential phase shift keying described by the lognormal atmospheric turbulence and the pointing error effects. In [10] authors, investigate the outage capacity under the influence of atmospheric turbulence and pointing error over gamma distribution model. The rest of the paper is arranged as follows: Section second discusses the system model and channel model used for proposed work. In section third, the outage probability of relay assisted OWC system is discussed. Section fourth, Let There be Light AIP Conf. Proc. 1849, 020011-1–020011-8; doi: 10.1063/1.4984158 Published by AIP Publishing. 978-0-7354-1521-8/$30.00 020011-1 discusses about bit error rate performance of the system. Section fifth, describes the numerical results and graphical analysis for those results. Finally in section sixth, concluding remarks are highlighted. SYSTEM MODEL AND CHANNEL MODEL System Model In this paper we consider a dual-hop FSO system as shown in figure 1. The system model consists of a source (s) and the destination (d) which is communicating with the help of the relay (r). To enable line of sight communication the transceivers are located on the high rise buildings. We assume that due to practical reasons direct communication between source and destination is not feasible which may be either greater distance between source and destination or non-LOS condition. The system employs a full duplex DF relaying protocol. We assume that channel state information (CSI) is present at source, destination and the relay. The laser beam propagates through k-distributed turbulence channel which is corrupted by additive white Guassian noise (AWGN). Further, we consider that channel model is aggregated which takes into account the combined effects of atmospheric turbulence induced fading and misalignment fading (pointing error). FIGURE 1. System Model. Channel Model The k distribution is a widely accepted model for strong atmospheric turbulence. It is considered as a product of gamma distribution and exponential distribution which are two independent models. The probability density function (pdf) of the normalized irradiance ‘I’ for K distribution is given by,