Optimal and suboptimal detection of optical fibre digital PPM

The discrete time modulation format, known as digital PPM, has been used to exchange channel bandwidth for improved signal-to-noise ratio in optical communication systems. However, the filter that optimises signal detection is complex in that it comprises a prewhitened matched filter in cascade with a proportional-derivative-delay network. In this form, the receiver would be unrealistic for a commercial system. This paper examines the use of both optimum and suboptimum detection. Three suboptimum predetection filters are considered: a matched filter, an optimised three-pole filter and a third-order Butterworth filter. Original receiver sensitivity calculations, carried out at a bit rate of 140 Mbit/s and a wavelength of 1.3 pm, show that the optimum digital PPM system offers an 8.6 dB improvement over a typical PCM system. The suboptimum predetection filters result in sensitivity degradations of 0.4, 0.9 and 1.1 dB, respectively. This clearly illustrates that receiver complexity can be simplified without large reductions in sensitivity. In particular, the well known and simple Butterworth filter can be employed with only 1.1 dB degradation in sensitivity