FPGA implementation of a BCH Codec for free space optical communication system

Future Free Space Optical (FSO) communication systems have the potential of communicating data at very high rates with very high levels of integrity over distances of up to a few kilometers (for terrestrial links). This technology has also been a candidate for setting up very high speed (~3 Gbps) and highly reliable (BER ~10^-9) communication links between satellites in geo-synchronous orbits and ground stations. Since the free space optical medium can induce many forms of distortion (atmospheric turbulence effects, optical beam wander etc), the use of a channel code to detect and correct errors during the process of information transfer over the channel is essential. A correctly designed channel code can reduce the raw BER from unacceptable values to values that can be tolerated in many applications. In this paper, we have designed a Codec (encoder/decoder) pair for a (31, 16, 3) Bose, Ray-Chaudhuri, and Hocquenghem (BCH) code on the Nexys-4 FPGA platform. The performance of this BCH Codec has been tested over an indoor FSO channel and the improvement in terms of BER has been quantified. An improved syndrome computation circuit, parallel Chien search implementation and an improved method for calculating inverses in a finite field are the new features incorporated in this paper. We have been able to design and implement circuits which use these optimized approach and deliver real time encoding and decoding with an information transfer rate of 2 Mbps and can be extended upto a speed of 418Mbps.