On the Capacity of Free-Space Optical Intensity Channels

Abstract

Abstract-New upper and lower bounds are presented on the capacity of the free-space optical intensity channel. This channel is characterized by inputs that are nonnegative (representing the transmitted optical intensity) and by outputs that are corrupted by additive white Gaussian noise (because in free space the disturbances arise from many independent sources). Due to battery and safety reasons the inputs are simultaneously constrained in both their average and peak power. For a fixed ratio of the average power to the peak power the difference between the upper and the lower bounds tends to zero as the average power tends to infinity, and the ratio of the upper and lower bounds tends to one as the average power tends to zero.The case where only an average-power constraint is imposed on the input is treated separately. In this case, the difference of the upper and lower bound tends to 0 as the average power tends to infinity, and their ratio tends to a constant as the power tends to zero.

Low-SNR Asymptotic Capacity of MIMO Optical Intensity Channels with Peak and Average Constraints

“…The Gaussian noise combines thermal noise which is wellmodeled as Gaussian, and shot-noise which has impact when its intensity is large in which case it is well-approximated as Gaussian. The resulting Gaussian channel model is common in the OWC context [4], [36].…”

“…At the receiver side, the signal is disturbed by several sources of noise, such as ambient light and thermal noise. The resulting channel can be modeled as a Poisson channel [3] or an additive Gaussian noise channel [4], [5] depending on the regime of operation, the latter being more common in regimes with high received signal power. In this A. Chaaban is with the School of Engineering, University of British Columbia, Kelowna, BC Canada V1V 1V7.…”

“…paper, we focus on the IM-DD channel with input-independent Gaussian noise [4], which is a suitable model for OWC with strong ambient light and/or thermal noise [6]. Due to the above constraints, the capacity of the Gaussian IM-DD channel is not the same as the classical Gaussian channel used for modeling radio-frequency (RF) communications.…”

“…Studying the capacity of this channel is important for understanding the fundamental limits of OWC, which has witnessed increasing attention recently, see [2], [7]- [10] and references therein. In this context, the capacity of the singleinput single-output (SISO) IM-DD channel has been studied in [4], [11], [12]. The SISO IM-DD channel with input-dependent (relative intensity) Gaussian noise has been studied in [5].…”

“…To prove its asymptotic tightness at low SNR, we inspire from earlier works asserting the optimality of binary inputs at low SNR, such as [4], [32], [33]. We show that the asymptotically low-SNR optimal input in the MIMO IM-DD context is a maximally correlated vector-binary distributed input.…”