Performance Analysis of Space Time Block Codes for Free-Space Optical Channels

Abstract

Free Space Optic(FSO) Communication systems are ideal for setting up high speed short distance (a few kilometres) communication links. They have been in existence for more than two decades but have not gained widespread acceptance and popularity. This is primarily because they suffer from a serious handicap. Changing environmental conditions (atmospheric turbulence and fog) can drastically affect their throughput and reliability. Atmospheric turbulence can affect the FSO link in a manner similar to the impairments introduced by multi-path propagation (signal fading) in a wireless channel. It is well known that the random variation of signal strength due to fading in a communication channel can result in severe bit-error-rate (BER) performance degradation and an increase in the outage probability. This is a major challenge in all wireless communication systems and FSO Communication is not an exception. A number of receiver combining schemes and Space Time Block Codes (STBCs) have been designed to improve the performance of wireless communication systems over fading channels. In this paper, we have conducted numerical studies to determine the performance of the most widely used STBC (namely Alamouti code) over FSO channels perturbed by atmospheric turbulence. We have also employed a STBC derived from a non-binary cyclic code in this application. It is seen that the use of these codes can increase the transmission range for a given link margin and/ or result in substantial improvement in reliability of information transfer for a fixed link margin. These inferences have been arrived at after conducting large scale numerical simulations to determine channel fading losses and bit error rates for different channel conditions.