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.