I. Introduction
The demand for high-quality wireless communication services is increasing at an explosive rate. However, the inherent limited supply of bandwidth and the unpredictability of the propagation channel lead to the investigation of more spectrally efficient and reliable designs. The use of antenna arrays is expected to play an important role in fulfilling these requirements. In the past, antenna arrays were used to increase receive or transmit diversity against multipath fading [1]–[3] or spatially separate mobile devices [4]. Recently, it was shown that in a richly scattering channel, signals even from closely spaced transmit antennas can be separated using adaptive array combining techniques at the receiver. The number of transmit signals that can be separated grows with the number of receive antennas [5], [6]. Consequently, the use of multiple antennas at both transmitter and receiver could increase the degrees of freedom (DOFs) in information transmission, with capacity scaling linearly with the number of transmit or receive antennas, whichever is smaller.