Performance study of optical transmission system with forward error correction coding

Abstract

Direct detection has become recent attraction for the optical communication designers. The obvious reasons are low cost, simplicity and a lot more which is yet to bc evaluated. However, the direct detection technique has the limitation in data rate for application in power limited free-space optical channels due to relatively low optical power output of semiconductor laser diode. The work undertaken here is confined to direct detection scheme which though have a few shortcomings posses a significant immunity to noise interference compared to the heterodyne detection concept. This is because the direct detection technique does not use the phase infol1nation. Our attempt is to explore this particular phenomena and judge the overall perfol1nance. A theoretical analysis for direct detection optical .frequency shift keying (FSK) transmission system is provided employing forvvard error correction (FEC) coding to combat the effect of phase noise of transmitting laser due to non-zero Iinewidth. Two types of coding viz. Convolutional coding (CC) and' Reed-Solomon's (RS) coding are considered with hard. decision Viterbi decoding to investigate their relative effectiveness in overcoming the degrading effect oflaser phase noise. The performance results at a bit rate of 2.5 Gb/s are also evaluated for ditlerent receiver and system parameters. The penalty suffered by the coded and. uncoded systems at a bit error rate (BER) of 10.9 are determined in the presence/absence of laser phase noise. The improvements in the receiver sensitivity due to coding i.e. the coding gains are also evaluated at BER = 10.9 for rate 1/2 CC with constraint length K = 4,7 and (15,9), (15,7) RS coding techniques. Furtlher the reductions in the power penalty for the coded system over uncoded system are also detel1nined for a specified bit error rate. The current effort proves beneficial to the context of relaxing laser . specification and cost optimization.