Overlapping optical fibre PPM for cable TV: synchronisation issues

Utilising overlapping pulse position modulation (OPPM) for cable TV over an optical fibre link offers sensitivities better than those of PCM at clock rates lower than those of orthogonal PPM. The authors report sensitivity and synchronisation results evaluated on an optically preamplified OPPM system operating at 622 Mbit/s and a wavelength of 1.53 μm. The sensitivity calculations indicate that the OPPM system offers a 6.4 dB sensitivity improvement over a comparable PCM system while being only 1.5 dB short of the orthogonal PPM system sensitivity. Chip synchronisation is investigated and an original expression for the power spectral density (PSD) in the presence of jitter is presented. Self synchronisation is examined and the timing variance as well as the wrong chip error probability due to jitter are evaluated. The results enable the selection of the phase lock loop (PLL) bandwidth and the other system parameters for a required performance