Suboptimal filtering in a zero-guard, dicode PPM system operating over dispersive optical channels

Optimal filters for use in digital pulse position modulation (PPM) systems consist of a noise-whitened matched filter followed by a proportional-derivative-delay (PDD) network. Although the PDD network can be removed, with a small loss in sensitivity, the practical implementation of the remaining matched filter is complex. An alternative is to use suboptimal filtering and the use of a third-order Butterworth filter in a zero-guard interval dicode PPM system operating over a dispersive optical channel is examined. Gaussian-shape received pulses are assumed, and a bandwidth-limited PIN-bipolar receiver with both frequency invariant and variant noise is considered. This original analysis shows that the bandwidth of the Butterworth filter is relatively independent of the channel. It is also shown that the performance of such a filter is superior to that of a noise-whitened matched filter. It is concluded that a dicode PPM system can use a simple third-order Butterworth filter. The best predicted sensitivities when coding 1 Gbit/s PCM data are -37.48 dBm in a high-bandwidth link and -32.24 dBm with a link bandwidth equal to 1.2 times the data rate.