Capacity of space-time wireless channels: a physical perspective

Existing results on MIMO channel capacity assume a rich scattering environment in which the channel power scales quadratically with the number of antennas, resulting in linear capacity scaling with the number of antennas. However, such scaling in channel power is physically impossible indefinitely. We thus address the following fundamental question: for a given channel power scaling law, what is the best achievable capacity scaling law? For a channel power scaling, ρ_c(N) = O(N^γ), γ ∈ (0,2], we argue that the channel capacity cannot scale faster than C(N) = O(√(ρ_c(N))) = O(N^γ2/). Our approach is based on a family of space-time channels corresponding to different distributions of channel power in the spatial signal space dimensions. We develop the concept of an ideal MIMO channel that achieves the optimal scaling law for a given ρ_c(N). For a given number of antennas, unlike existing results that either emphasize the low or high SNR regimes, we propose a methodology for capacity-optimal signaling at any SNR. The methodology is based on creating the ideal channel from any given physical scattering environment via adaptive-resolution array configurations.