I. Introduction
The USE of antenna arrays at base stations has recently gained much interest due to their ability to combat fading, increase system capacity and coverage, and mitigate interference [1]–[5]. In the uplink communication mode, signals from different users can be separated at the base station antenna array based on the knowledge of their spatial signatures [5]–[8]. In particular, known spatial signatures can be used for beamforming to separate each user of interest from the other (interfering) users. However, user spatial signatures are usually unknown at the base station and, therefore, have to be estimated.