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 inter- ference for each user which is generated by the signals transmitted to other users. Using multi-user detection techniques, it may be possible for a given user to overcome 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 transmitted signal. If CSI is available at the transmitter, it is aware of what interference is being created for user i by the signal it is transmitting to user j and vice versa. The inter-user interference can be mitigated by beamforming CHAPTER 10 The IEEE 802. 16m Physical Layer (Part II) or the use of dirty paper codes. In general, the MU-MIMO and SU-MIMO comparison can be summarized as follows [15]: SU-MIMO is a point-to-point link with predictable link capacity, whereas MU-MIMO channel is a broadcast channel (BC) in the downlink direction and a multiple access channel (MAC) in the uplink direction whose link-level data rates are characterized in terms of capacity regions. Multi-stream SU-MIMO schemes offer stream diversity in the sense that if one stream has a poor SNR, the system will not necessarily experience an outage, whereas in the same situation, a MU-MIMO system be in outage. This is due to the fact that in MU-MIMO schemes, users typically have an equal target data rate and symbol error rate on their respective links, while in SU-MIMO systems, only the sum rate of the overall link is considered since all streams are delivered to the same user. The MU-MIMO schemes suffer from a near-far problem due to the significant difference between the path losses experienced by each user, resulting in large deviations in the SINR of t