Low-Rank Channel and Interference Estimation in mm-Wave Massive Antenna Arrays

Millimeter wave (mm-Wave) communications are characterized by wideband channels with few directional paths, mostly in line-of-sight. Antenna arrays are mandatory to cope with severe path-loss, and the resulting channel response is sparse in the space-time (ST) domain. This paper addresses the sparsity by proposing a channel estimation method that exploits the algebraic structure of channel and interference, without requiring complex antenna-array calibration procedures. The method relies on the recognition that the ST channel is low rank and exhibits slowly and fast-varying features (angles/delays of arrival and fading amplitudes, respectively) and, accordingly, that the interference has a slowly-varying spatial covariance with fast-varying amplitudes. The accuracy of the estimation of quasi-stationary components is increased by introducing averaging mechanisms over multiple sequences. Numerical results show that: i) rank-1 is an effective channel-interference representation in mm-Wave setting with severe interference; ii) fundamental limits (derived in closed form) prove the remarkable performance gains in terms of signal-to interference ratio; iii) circular array arrangement with directive elements is preferable compared to square or triangular configurations.