citly signaled to the transmitter by the receiver or channel sounding reference signals. The direct channel feedback methods preclude the channel reciprocity requirement, whereas channel sounding methods rely on channel reciprocity. Therefore, explicit control signaling is required for PMI-based (feedback) schemes. However, in reciprocity-based schemes, the sounding signals in the uplink and precoded pilots in the downlink are used to assist the transmitter and receiver to appropriately select the precoding matrix. Reciprocity-based schemes have the additional advantage of not being constrained to a finite set of codebooks. Beamforming relies on long-term statistics of the radio channel and, unlike reciprocity-based techniques, does not require short-term correlation between the uplink and downlink in order to function properly. 10.4.5 Single-User and Multi-User MIMO Single-user MIMO (SU-MIMO) techniques are point-to-point schemes that improve channel capacity and reliability through the use of space-time/space-frequency codes (transmit/receive diversity) in conjunction with spatial multiplexing transmission. In a SU-MIMO transmission, the advantage of MIMO processing is obtained from the coordination of processing among all the transmitters or receivers. In the multi-user channel, on the other hand, it is usually assumed that there is no coordination among the users. As a result of the lack of coordination among users, uplink and downlink multi-user MIMO channels are different. In the uplink scenario, users transmit to the base station over the same channel. The challenge for the base station is to separate the signals transmitted by the users, using array processing or multi-user detection methods. Since the users are not able to coordinate with each other, there is not much that can be done to optimize the transmitted signals with respect to each other. If some channel feedback is allowed from the transmitter back to the users, some coordination may be possible, but it may require that each user knows all the 