SDMA is space-time division multiple access (SDMA) in which for the downlink case, the base station transmits different information signals to multiple users simultaneously using spatial pre-processing at the transmitter (base station) and for the uplink case, one or more than one users transmit information simultaneously to the base station and aggregated signals are decoded at the receiver (base station) using spatial post-processing. Depending on an antenna configuration, we apply beamforming, precoding or hybrid of two processing to the antenna array as effective spatial preprocessing. In a wide sense, beamforming represents both original beamforming used for correlated channels and precoding used for uncorrelated channels.
The beamformed signal is mathematically described in the downlink case as
where and is the beamforming vector and the input signal for user , respectively.
Recently, several multiuser MIMO techniques such as ZFBF and PU2RC are proposed as effective SDMA solutions in terms of the achievable performance and implementation complexity.