REED-SOLOMON CODED HOMODYNE DIGITAL PULSE POSITION MODULATION

Digital pulse-position modulation (PPM) is a scheme whereby the abundance of bandwidth available in monomode fibres may be exchanged for improved receiver sensitivity. As this format relies on the position of the pulse to convey information, it is subject to a positional error as well as to the familiar false-alarm and erasure errors. A performance and optimisation analysis for both uncoded homodyne digital PPM and digital PPM employing Reed-Solomon error-correction codes is presented. System performance for a range of fibre bandwidths and PPM symbol sizes is analysed, and it is shown how the predetection filter may be configured in order to minimise the three error sources and achieve maximum transmission efficiency (nats/photon). Results are presented at a bit rate of 565 Mbit/s and a wavelength of 1.5-mu-m, comparing both uncoded and coded homodyne digital PPM with shot-noise-limited coherent PCM. It is shown that there are optimum PPM symbol sizes, fibre bandwidths and Reed-Solomon code rates at which to operate. The conclusion is that uncoded digital PPM offers an improvement of 5 dB over homodyne PSK PCM, and that the Reed-Solomon error-correction coded system offers a 4 dB improvement over uncoded PPM, when operating at the optimum 3/4 code rate.