I. Introduction
Multiple antenna signaling techniques have proven to be very effective in improving the performance of wireless communication systems [1]–[3]. Transmit beamforming is one of the main approaches to exploiting the spatial characteristics of the channel, as it is capable of providing spatially matched transmission and reducing interference. The power gain that is achieved by transmit beamforming is proportional to the number of transmit antennas. Unfortunately, it is not usually practical to mount a large number of antennas on a mobile terminal due to various limitations such as size and cost. Collaborative beamforming techniques have recently been introduced to overcome this limitation and achieve multiple antenna gain in wireless communication networks [4].