Dicode pulse-position modulation: a novel coding scheme for optical-fibre communications

Pulse position modulation (PPM) schemes have been proposed as a method of utilising the bandwidth available in optical fibres, with a 5-11 dB improvement in sensitivity being achieved compared to an equivalent pulse code modulation (PCM) system. However, this improvement comes at a cost. If digital PPM is used, the final data rate can be almost 23 times that of the original PCM, and this makes implementation difficult. The author describes a novel coding technique that combines dicode, a tertiary code sometimes used in magnetic recording, and digital PPM to form dicode PPM. It is shown that dicode PPM gives a receiver sensitivity greater than digital PPM while operating at only four times the original data rate. Original results presented predict that a high fibre bandwidth dicode PPM system can give sensitivities of -50.44 dBm and -44.27 dBm when operating with 155.52 Mbit/s and 1 Gbit/s PCM data, respectively. This should be compared to typical PCM sensitivities of -38 dBm and -28 dBm. It is also shown that dicode PPM outperforms digital PPM at low fibre bandwidths by 3.02 dB.