Fundamental Limits of Parallel Optical Wireless Channels: Capacity Results and Outage Formulation
Abstract: Multi-channel channel (MC) optical wireless communication (OWC) systems employing wave-division division multiplexing for outdoors free free-space space optical communications, or multi-user time-division division multiple access for indoors visible visible-light light communications, e.g., can be modeled as parallel channels. Multi-input Multi multi-output output OWC systems can also be transformed, possibly with some performance loss, to parallel channels using pre-/post /post-coding. coding. Studying the performance of such MC-OWC MC systems requires characterizing the capacity of th thee underlying parallel channels. In this paper, upper and lower bounds on the capacity of constant parallel OWC channels with a total average intensity constraint are derived. Then, this paper focuses on finding intensity allocations that maximize the lower bounds given channel-state state information at the transmitter (CSIT). Due to its nonconvexity, the Karush-Kuhn-Tucker Tucker conditions are used to describe a list of candidate allocations. Instead searching exhaustively for the best solution, low low-complexity near-optimal timal algorithms are proposed. The resulting optimized lower bound nearly coincides with capacity at high signal signal-to-noise noise ratio (SNR). Under a quasiquasi static channel model and in the absence of CSIT, outage probability upper and lower bounds are derived. Tho Those se bounds also meet at high SNR, thus characterizing the outage capacity in this regime. Finally, the results are extended to a system with both average and peak intensity constraints.