Impact of Laser Diode Performance on Data Rate Capability of PPM Optical Communication

A model is developed for an APD receiver which is shown to have dominant noise contributions from signal-dependent excess noise and thermal noise at high data rate (HDR). For the HDR APD receiver with pulse position modulation (PPM), the data rate (RD) can be written RD-¿¿a where ¿ is the modulation and coding efficiency and ¿a is the average photon rate. It is shown that for an average-power-limited source the data rate capability is optimized by maximizing the coding efficiency ¿, but for a peak-power-limited source both ¿ and ¿a influence the data rate capability. Thus the optimum choice of PPM word size (M) is dependent on the source peak-to-average power ratio. Several commercially available single transverse-mode laser diodes were found to have peak-to-average power ratios of about 8-16. Finally, a method of coding for MY was devised to allow PPM to modulate the laser diode at M-8-16 while simultaneously allowing a Reed-Solomon (N,K) error correcting code to operate with high coding gain.