Revisiting the Capacity of Noncoherent Fading Channels in mmWave System

Millimeter wave communications will use large bandwidth and experience severe propagation due to pathloss and blockages. The two above conditions may force the communication system to enter the so-called wideband regime, where signals must be increasingly “peaky” in order to attain a large fraction of the wideband capacity with no peak constraint, especially in the presence of non-line-of-sight propagation. This paper investigates the noncoherent capacity of millimeter wave channels with average and peak power constrained inputs as a function of bandwidth. Capacity upper and lower bounds are provided, and the impact of features peculiar of millimeter wave channels is investigated. Numerical results for a practical scenario based on recent experimental campaigns are provided. It is shown that the achievable rate of a large fraction of users in a typical millimeter wave cell may be bounded far from the wideband capacity without peak constraint because signaling is dense, hence suggesting to reconsider the role of signaling sparsity in future millimeter wave communications.