Multiple pulse position modulation employing raised cosine filtering

Recent work has considered a 1Gbit/s multiple pulse position modulation system (MPPM) operating over plastic optical fibre (POF) while utilising maximum-likelihood detection (MLD) to minimise intersymbol interference (ISI). Here, an original analysis is presented, which significantly simplifies the receiver design by employing raised cosine filtering to eliminate ISI. Practical approximations to the ideal raised cosine filter are examined and it is shown that a very good performance is achieved by using a simple first-order pre-amplifier in cascade with a third-order Butterworth pre-detection filter, both with their bandwidths set at 0.6 times the MPPM slot rate. When operating with a POF bandwidth of 0.7 times the data rate, it is shown that both the ideal raised cosine scheme and the simple Butterworth pre-detection filter system offer improvements in sensitivity of 7.6 and 8dB respectively, over the more complex MPPM MLD system. A simplified analysis for MPPM that predicts sensitivities to within 0.5dB of those determined through the more rigorous treatment is presented.