Design Implications of High-Speed Digital PPM

Work in the area of digital pulse position modulation (digital PPM) has shown that this type of modulation can yield sensitivities that are typically 4-5dB better than an equivalent PCM system. This gain in sensitivity is achieved at the cost of increased transmission speed, which means that the coder/decoder has to run at a very high clock rate. In addition, the detection of short duration digital PPM pulses, implies a very wide bandwidth receiver, and this further complicates the system design.

Recent experimental work has shown that the receiver in a digital PPM system does not need to have a wide bandwidth. Instead, the bandwidth can be very low so that the receiver is effectively impulsed by the digital PPM signal. The advent of very high speed Si digital ICs, and fast lasers, means that digital PPM can now be used to code gigabit PCM signals. This paper presents original theoretical results for a digital PPM system coding 1 Gbit/s PCM signals into 8 Gbit/s digital PPM signals. The paper also addresses the difficulties that the system designer is likely to encounter, and discusses some possible solutions.