Polarization shift keying based relay-assisted free space optical communication over strong turbulence with misalignment K. Prabu a,n , D. Sriram Kumar b a School of Electronics Engineering, Vellore Institute of Technology, Vellore 632014, India b Department of Electronics and Communication Engineering, National Institute of Technology, Tiruchirappalli 620015, India article info Article history: Received 6 November 2014 Received in revised form 14 July 2015 Accepted 21 July 2015 Keywords: Free space optics Polarization shift keying Strong atmospheric turbulence Parallel relay Outage probability abstract This paper investigates the outage performance of polarization shift keying (PolSK) based multihop parallel relay assisted free space optics (FSO) system over a strong atmospheric turbulence channel with misalignment fading. An exact closed form expression is derived for the end-to-end outage probability of the system. The results are compared with the direct transmission and on-off keying (OOK) based FSO systems. The results indicate that the performance of the PolSK based relay-assisted FSO system is much better than the direct transmission and OOK systems. The outage performance is enhanced by increasing the number of relay path between the transmitter and receiver. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction Free-space optics (FSO) is a high data rate, economic, secure, interference resistant and high-bandwidth wireless technique for transmitting broadband services via point to point channels [1]. Due to its license-free operation, infinite spectrum and its ease of deployment, it is emerging as an appealing alternative to con- ventional radio frequency (RF)/microwave links. The operation of FSO depends on the environmental conditions affecting the per- formance parameters viz. channel capacity, phase-front distortion, beam broadening, beam wander and redistribution of the intensity within the beam [2,3]. The major challenges in FSO systems are atmospheric turbu- lences and misalignment fading. The reasons for fading due to misalignment are varying refractive index, thermal expansion, wind load or due to quakes which causes throbbing of the trans- mitter beam, resulting in pointing error between the transmitter and receiver [4]. Relay assisted FSO system and multiple-input multiple-output (MIMO) techniques are effective methods to extend coverage and lessen the effects of fading [5]. Since, the amount of fading depends upon the distance covered in the tur- bulent environment, the signal transmission efficiency increases with the number of short hops taken. Polarization shift keying (PolSK) is an effective alternative modulation technique used for long distance wireless optical communications. In PolSK, states of polarizations (SOPs) are used to modulate the binary information. The advantages of PolSK over the conventional modulations schemes are high immunity to the laser phase noise and do not suffer from frequency chirp [6,7]. The heterodyne binary polarization shift keying (BPOLSK)-FSO system by using MIMO technique over the gamma–gamma turbulence channel is studied in [8]. A coherent multilevel polarization shift keying (MPOLSK) based FSO system using the spatial diversity detection is proposed and its symbol error rate performance, diversity gain are studied under various turbulence regimes in [9]. In [10], the performance of the BPOLSK-FSO system over a weak atmospheric turbulence channel has been investigated. In a recent work, the performance analysis of repetition code (RC) and orthogonal space time block code (OSTBC) in an FSO system is investigated over correlated lognormal channels [11]. Average BER and outage performance of the polarization shift keying (PolSK) modulated FSO system with time or wavelength diversity over gamma–gamma channel with pointing errors is analyzed in [12]. The average error rate performance of serial relay assisted FSO system over gamma-gamma channel is analyzed in [13]. The outage probability for on-off keying (OOK) based FSO system with decode-and-forward (DF) parallel relays over strong turbulence fading channel is investigated and the closed-form expression of the outage probability is derived in [14]. In [15], the author analyzed the performance of a dual-hop relay system composed of RF and FSO links, operating in asymmetric Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/optlastec Optics & Laser Technology http://dx.doi.org/10.1016/j.optlastec.2015.07.012 0030-3992/& 2015 Elsevier Ltd. All rights reserved. n Corresponding author. Tel.: þ919884888408. E-mail address: nitprabu@gmail.com (K. Prabu). Optics & Laser Technology 76 (2016) 58–63