Jitter in the square synchroniser on direct detection optical communications. III. Numerical results concerning white circuit noise

Theoretical assessment of the square synchroniser performance, developed in a previous paper, is used to present and discuss numerical results in the presence of white circuit noise. This fully analytical method permits the optimal design (i.e. minimising timing jitter) of square synchronisers. Taking into account all the relevant parameters, for direct detection optical communication systems, a thorough physical explanation of the presented results and their relevance for practical receiver design is provided. The work is increasingly important nowadays because multigigabit per second transmission rates are being used on direct detection optical systems. It is shown that, for low signal-to-noise ratios and under quite general conditions, the performance of the square synchroniser is better in a signal shot-noise-dominant situation (typical of long-distance communications with APD receivers), followed by the white-circuit-noise-dominant situation (typical of long-distance communications with PIN receivers), and is worst for the dark current-noise-dominant situation (typical of germanium APD receivers). The noise contribution to square synchroniser performance can be very high. The timing jitter properties respecting the circuit noise dominant situation usually hold also on the signal shot-noise dominant situation, except for a very narrow (compared with the bit period) optical pulse shape incident on the photodetector