 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 chan- nel. 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 transmit- ter back to the users, some coordination may be possible, but it may require that each user know all the other users' channels rather than only its own. Otherwise, the challenge in the uplink is mainly in the processing done by the base station to separate the users. In the downlink channel, where the base station simultaneously transmits to a group of users over the same channel, there is some inter-user interference for each user which is gener- ated by the signals transmitted to other users. Using multi-user detection techniques, it may be possible for a given user to overcome the multiple access interference, but such techniques are often extremely complicated for use at the receivers. Ideally, one would like to mitigate the interference at the transmitter by carefully designing the transmit sig- nal. If CSI is available at the transmitter, it is aware of what interference is caused for each user by the signal it is transmitted to other users. The inter-user interference can be mitigated by beamforming or the use of dirty paper codes. In general, single-user and multi-user MIMO schemes are compared as follows [15]: New Radio Access Physical Layer Aspects (Part 559 SU-MIMO is a point-to-point link with predictable link capacity, whereas MU-MIMO chan