Minimising intersymbol interference in optical-fibre dicode PPM systems

Dicode pulse-position modulation (PPM) has been proposed as an alternative modulation format to conventional digital PPM, since it offers comparable receiver sensitivities while operating at significantly lower slot-clock rates. A recent numeric analysis considered the performance of dicode PPM, using slope detection, in a dispersive environment. In this paper, consideration is given to an alternative detection strategy, and new results are presented that show that intersymbol interference (ISI) can be eliminated through the use of central decision detection combined with raised cosine filtering. This leads to a more simplified receiver design and results in equivalent sensitivity performance at high fibre bandwidths and significantly improved performance at lower fibre bandwidths. Practical approximations to this approach are considered and design rules presented for determining the bandwidth of both the preamplifier and a simple 3rd order Butterworth equalisation filter. It is shown that setting the bandwidth of both the preamplifier and equaliser to 0.6 times the dicode slot rate gives sensitivities within 0.2 dB of ideal dicode PPM raised cosine filtering, and that such a receiver can operate over a wide range of fibre bandwidths, with minimal degradation in sensitivity.