Duobinary pulse position modulation – a novel coding scheme for the dispersive optical channel

Pulse position modulation (PPM) coding schemes have been proposed and investigated widely as a technique of utilising the very high bandwidth available in optical fibres, achieving a significant improvement in sensitivity of 5-11 dB compared with an equivalent non-return-to-zero on-off keying scheme. However, this improvement has some tradeoffs. When using eight-level digital PPM, the final data rate can be as high as 32 times that of the original data, thus implementation becomes extremely difficult to the point where commercial viability becomes doubtful. In this study, the authors describe a novel coding scheme that combines the duobinary scheme with PPM to form DuoPPM. It is shown that DuoPPM gives a sensitivity greater than digital PPM while operating at two times the original data rate. Original results presented in this study predict that a high fibre bandwidth DuoPPM system can give a sensitivity of -42.2 dBm when operating with 1 Gbit/s on-off keying (OOK) data and a fibre bandwidth of 100 GHz. In addition, it is shown that DuoPPM outperforms optimised digital PPM at low fibre bandwidths (1 GHz) by 8.7 dB.