In Release 8 of 3GPP, the user equipment (UE) supports multiple receive antennas but only a single transmit antenna. As such, single-user MIMO (SU-MIMO) can only be supported in the downlink, but MU-MIMO can be supported in the uplink.
MU-MIMO allows for uplink transmissions from multiple UEs to a radio base station, e.g., an eNode-B, using common time-frequency resources, i.e., schedule blocks. To separate the transmissions, MU-MIMO relies on the availability of multiple receive antennas at the radio base station and on significantly different spatial radio channels between the UEs and the radio base station.
An important benefit of uplink MU-MIMO is that a similar gain in system throughput as in SU-MIMO may be achieved, without need for multiple transmit antennas at the UE. This allows for a less complex implementation of UEs, i.e., handsets. However, the potential system gain of uplink MU-MIMO relies on more than one UE being available for transmission using a common time-frequency resource. The process of “pairing” UEs for MU-MIMO transmissions is non-trivial and requires favorable radio-channel conditions.
Ideally, paired, i.e., co-scheduled, UEs should have orthogonal channels in order to minimize mutual interference. If the transmissions from two UEs can be perfectly separated at the receiver, and both signals are transmitted with the same power as in single-user mode, there is a potential 100% gain in cell or user throughput due to the increased transmission power. However, co-scheduled UEs are seldom ideally orthogonal, giving rise to mutual interference. The more orthogonal the channels are the less intra-cell interference will be created by MU-MIMO. At the same time, the increase of total transmission power on the schedule blocks used for MU-MIMO transmissions will increase inter-cell interference, thereby reducing the potential gain.