Broadcast space-time coding

In this paper, we study transmission techniques for broadcast channels with a single transmitter and multiple receivers. The transmitter is assumed to be equipped with multiple antennas. Further, each receiver is interested only in part of the transmitted information. Using Gaussian code based information theoretic bounds, we analyze two transmit beamforming techniques. The first technique, zero-forcing beamformer, uses the channel information for all users to send spatially orthogonal signals to different users, i.e., no user receives interference from the other user signals. The second method uses the single-user optimal beamformer with no effort to reduce interference at the receivers. It is shown that the above transmit beamforming techniques are similar to multiuser receivers used in non-orthogonal CDMA systems. In particular, the zero-forcing beamformer is similar to a decorrelating detector and the single-user beamformer is identical to a matched filter. The comparison with CDMA multiuser receivers is strengthened by results on spectral efficiency of proposed beamformers, which follow the behaviour of decorrelating and matched-filter receiver.