This paper demonstrates an analytical approach on the bit error rate (BER) performance evaluation of a multi- carrier code division multiple access (MC-CDMA) communication link operating over terrestrial free space optical (FSO) channel considering effect of atmospheric turbulence The turbulence induced intensity fading is statistically modeled by Gamma-Gamma PDF (probability density function). Bit error rate performance improvement is proposed using photo detector spatial diversity with Equal Gain Combining (EGC) and Turbo Coding. Analysis is carried out with different bandwidth efficient phase shift keying (PSK) based sub-carrier intensity modulation (SIM) with direct detection. Numerical simulation results of proposed analytical model indicate that, sub-carrier intensity modulation scheme; number of receiver photo detectors, turbo coding parameter and link length should be optimally engineered for ensuring system reliability. It can be inferred from the simulation that, a reliable communication link ( 1 0 ？ 1 0 ？BER) can be established over a link length of 4？Km in strong turbulence fading condition using an array of 4 PIN photo detectors, 8-ary PSK based sub-carrier intensity modulation scheme and appropriate turbo coding parameter with an average 10.2？dB CINR (Carrier to Interference and Noise Ratio) requirement per photo detector. Besides, more than 130？dB average CINR gain is also confirmed from BPSK modulated, un-coded SISO (single input-single output) system for maintaining targeted BER ( 1 0 ？ 1 0 ) in presence of strong turbulence fading. 1. Introduction Free space optical communication (FSO) also known as wireless optical communication technique is a point to point to communication link in which two optical transceivers communicate via infrared laser light propagated through atmospheric channel. FSO recently has become a promising technology both for commercial as well as military application that can replete the gap between end user and backbone fiber optic infrastructure addressing the “last-mile” or “bottleneck” problem [1]. In metropolitan area network or local area network, end user is connected to the optical fiber network by means of existing DSL (digital subscriber line) having data speed in the range of Mbit/sec while FSO provides data rate in the rage of giga？bit/sec. Besides addressing “bottle-neck” problem, FSO is also promising for transmitting RF signal over free space by means of subcarrier multiplexing (SCM). The present technology of transmission of radio signal over optical fiber link is multiplexing of several carriers in
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