Capacity of PPM on APD-detected optical channels

This report defines the fundamental parameters affecting the capacity of a soft-decision optical channel, and relates them to corresponding parameters for the well-understood AWGN channel. For example, just as the performance on a standard additive white Gaussian noise (AWGN) channel is fully characterized by its SNR, a corresponding Webb channel is fully characterized by its SNR, and a single additional skewness parameter δ² which depends on the photon detector. In fact, this Webb channel reduces to the standard AWGN channel when δ²→∞. Numerical results show that the capacity of M-ary orthogonal signaling on the Webb channel exhibits the same brick-wall Shannon limit (M ln 2)/(M-1) as on the AWGN channel (≈-1.59 dB for large M), and that soft output channels offer a 3 dB advantage over hard output channels