Compressive tracking with 1000-element arrays: A framework for multi-Gbps mm wave cellular downlinks

We propose and demonstrate the feasibility of multi-Gbps cellular downlinks using the mm-wave band. The small wavelengths allows deployment of compact base station antenna arrays with a very large number (32×32) of elements, while a compressive approach to channel acquisition and tracking reduces overhead while simplifying hardware design (RF beamforming with four phases per antenna element). The base station array transmits multiple compressive (≪ 32 × 32) training beacons by choosing different sets of phases from 0°, 90°, 180°, 270° at random at each of the elements. Each mobile, equipped with a single antenna, reports the observations corresponding to the different beacons (e.g., on an existing LTE link at a lower frequency), allowing the array to estimate the angles of departure. We observe that tracking overhead can be reduced by exploiting the sparsity of the spatial channel to a given mobile (which allows parametric estimation of departure angles for the different paths), and the continuity in the user´s mobility at microsecond timescales (for tracking the evolution of departure angles). We first illustrate the basic feasibility of such a system for realistic values of system parameters, including range of operation, user mobility and hardware constraints. We then propose a compressive channel tracking algorithm that exploits prior channel estimates to drastically reduce the number of beacons and demonstrate the efficacy of the system using simulations.