Achieving coherent capacity of correlated MIMO channels in the low-power regime with non-flashy signaling schemes

Existing works on MIMO channel capacity assume that the channel power scales quadratically with antenna dimensions. In a recent work, we introduced a framework for sub-quadratic channel power scaling to model physical MIMO channels, and studied a family of channels with different spatial distributions of channel power. In particular, we introduced the notion of a beamforming channel, realizable in practice via the use of reconfigurable antenna arrays, that yields the highest capacity and maximal spectral efficiency in the low SNR regime. In this paper, we show that the MIMO beamforming channel requires a significantly smaller coherence requirement to achieve the wideband capacity limit at finite bandwidth than has been known in the literature. Contrasting the recent claims that training-based signaling schemes are strictly sub-optimal over communication schemes that do not perform explicit training, we show that training offers significant advantages in achieving wideband capacity. The price of explicitly training the channel over a non-explicit training scheme is a bandwidth penalty, albeit at a significantly less stringent coherence requirement on the channel