Capacity Limits of Multiple Antenna Multicasting Using Antenna Subset Selection

Both military and commercial wireless cellular networks often require the same data to be conveyed to multiple users simultaneously. This technique is usually referred to as physical layer multicasting. Unfortunately, the capacity of this multicasting setup is dictated by the smallest maximum achievable rate among all the users. In this paper, we consider the use of antenna subset selection in multicast channels where the transmitter is equipped with multiple antennas and sends a common message to multiple users. In this set-up, the antenna subset selection scheme selects some subset of the basestation transmit antennas to maximize the minimum signal-to-noise ratio (SNR) among the associated users. Using an asymptotic analysis, the average capacity of the antenna subset selection scheme is compared with that provided by max-min transmit beamforming, which requires channel state information at the transmitter to maximize the minimum user SNR, and an open-loop scheme where independent data is transmitted on each antenna. Then, we identify the minimum required antenna configuration condition for multicasting in which the antenna subset selection scheme universally performs better than the open-loop scheme and outperforms max-min transmit beamforming when the number of users is large. In addition, for a fixed number of radio frequency chains and users, it is shown that the antenna subset selection scheme can exploit the antenna selection diversity and its average capacity grows double logarithmically with the number of transmit antennas without bound.