On the Capacity of the 2-User IM-DD Optical Broadcast Channel

The capacity of the intensity-modulation directdetection (IM-DD) optical broadcast channel (OBC) is investigated. The OBC is modeled as a Gaussian channel with input-independent noise and both average and peak input constraints. Outer and inner bounds on the capacity region are derived. The outer bounds are based on Bergmans' approach. The inner bounds are based on superposition coding with either truncated-Gaussian or discrete input distributions. By comparing the bounds, we observe that the truncated- Gaussian distribution is nearly optimal at high signal-to-noise ratio (SNR). At low SNR on the other hand, on-off keying (OOK) combined with time-division multiple-access (TDMA) is optimal; it achieves any point on the boundary of the developed outer bound. This is interesting in practice since both OOK and TDMA have low complexity. At moderate SNR (typically [0; 8] dB), a discrete input distribution with a small alphabet size achieves a fairly good performance.