DoA Estimation and Capacity Analysis for 2D Active Massive MIMO Systems

Mobile data traffic is expected to have an exponential growth in the future. In order to meet the challenge as well as the form factor limitation on the base station, twodimensional (2D) " massive MIMO" has been proposed as one of the enabling technologies for future wireless systems. In 2D " massive MIMO" systems, a base station will rely on the uplink sounding signals to figure out the downlink spatial channel information to perform MIMO precoding. Accordingly, directionof- arrival (DoA) estimation of the underlying three-dimensional (3D) channel at the base station becomes essential for 2D " massive MIMO" systems to realize the predicted capacity gains. In this paper, we will analyze the performance of DoA estimation based on ESPRIT methods and study its impact on the capacity of 2D " massive MIMO" systems. To be specific, for ESPRIT-type algorithms, we will derive the closed-form expressions for the mean square errors of the elevation and azimuth angle estimations. These results will be used to obtain design intuitions for 2D antenna arrays at the base station as well as the capacity of the underlying 2D " massive MIMO" systems.