Performance Analysis of FSO Communication Using Different Coding Schemes Nidhi Gupta 1 , Siddi Jai Prakash 1 , Hemani Kaushal 2 , V.K. Jain 2 and Subrat Kar 2 1 Electronics and Communication Department, Netaji Subhas Institute of Technology, Delhi, India 2 Department of Electrical Engineering, Indian Institute of Technology, Delhi, India Abstract. A major impairment in Free Space Optical (FSO) links is the turbulence induced fading which severely degrades the link performance. To mitigate turbulence induced fading and, therefore, to improve the error rate performance, error control coding schemes can be used. In this paper, we investigate the bit error performance of FSO links with different coding techniques over log normal atmospheric turbulence fading channels. The modulation scheme considered is BPSK. On the basis of computed results using Monte Carlo simulation, a comparative study of uncoded and coded systems is made. Keywords: FSO, Lognormal fading channels, Convolutional, Trellis Coded Modulation, BPSK PACS: 40, 42, 42.68-w, 42.68.Bz INTRODUCTION Free-Space Optical (FSO) communication, which has attracted a lot of attention recently, provides the essential combination of qualities required to bring the traffic to the optical fiber backbone [1]-[4]. Virtually unlimited bandwidth, low cost, ease and speed of deployment, and excellent security are among the most attractive features of FSO communication. It is becoming a good solution for point to point communications between fixed locations on land, and is also used for communications between moving platforms on land, the surface on sea, and in air. The advantages of FSO do not come without some costs. Aerosol scattering caused by rain, snow, and fog, results in performance degradation[2]. The major problem is the need to combat link failure due to scattering and scintillation. The signal may experience fading due to inhomogenities of the index of refraction in the atmospheric channel. This phenomenon which leads to stochastic amplitude (and power) fluctuations in the signal, degrades the link performance particularly for distances of 1 km and above. To overcome the limitations of FSO communication, error control coding schemes for transmission are used [3]. In this paper, the coding schemes considered are Convolutional and Trellis Coded Modulation (TCM) codes. The modulation scheme used is BPSK. We have considered a lognormal fading channel and additive white Gaussian noise (AWGN) [5]. Monte Carlo simulations are carried out to compute the performance with convolution and TCM codes. Subsequently, a comparative study of their performance in terms of coding gain is made. The organization of the paper is as follows. In section 2, system model is given. Monte Carlo Simulation results are presented in Section 3. In the last and final section 4, conclusions of the study are given. SYSTEM MODEL In a lognormal fading channel, the channel variance depends on atmospheric factors such as wind velocity, zenith angle and optical wavelength [6]. The parameters which are considered in the simulation are as given in Table 1. TABLE 1. Channel Specifications. Parameter Value Wind Velocity 15 m/s Wavelength 1.5 μm Variance 0.0169 The received signal i d (t) at the photodetector can be expressed as i d (t) = h(t) s(t) + i n (t) (1) Optics: Phenomena, Materials, Devices, and Characterization AIP Conf. Proc. 1391, 389-391 (2011); doi: 10.1063/1.3643558 © 2011 American Institute of Physics 978-0-7354-0960-6/$30.00 389 Downloaded 01 Nov 2011 to 180.149.52.43. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/about/rights_permissions