Multi-user MIMO for achieving IMT-Advanced requirements

IMT-advanced wireless systems will require very high spectrum efficiencies in order to achieve the required data rates of 1 Gbit/s and more. In 3GPP, there is broad agreement that enhanced multicarrier based MIMO will be the key to achieve those targets. In particular, higher numbers of antennas than standardized for 3GPP long term evolution (LTE) will be applied. At the node B, we have more freedom to apply multiple antennas than at the UE. Up to 8 or 12 antennas at node B are not considered to be out of scope while the terminal may be equipped with up to 4 antennas. Using significantly more antennas at node B than at the terminals calls for the application of multiuser (MU) MIMO, where the receive antennas are distributed over multiple users. We describe a non-linear MU-MIMO scheme called SESAM which virtually achieves the theoretical limit for the sum capacity. On the other hand, we also discuss significantly less complex linear MU-MIMO methods with limited feedback. A simple variant of linear MU-MIMO is already part of 3GPPLTE. We will show that the limiting factor for MU-MIMO gains in 3GPP-LTE is the small number of feedback bits and that improved feedback or uplink signal based channel estimation at the node B are a prerequisite for successful usage of MU-MIMO in IMT-advanced.