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.
