A M-element antenna array (the base station) transmits K bit streams
selectively and simultaneously to K single-antenna terminals. TDD
operation permits the base station to acquire forward-link CSI from
reverse-link pilots. Additional base station antennas are always
advantageous. Even at low SINRs and for short coherence intevals it is
typically feasible and profitable to transmit bit streams simultaneously
to multiple terminals. Our results suggest that the base station of the
future should comprise a hundred or so antennas, each equipped with a
low-power tower-top amplifier, working in conjunction with cheap
single-antenna terminals.