Multiuser MIMO Systems
In Multiple-Input Multiple-Output (MIMO) communications, multiple antenna elements are employed both in the transmitter and the receiver, in order to obtain increased data rates or improved reliability compared to single-antenna transmission. In cellular systems, the Base Station (BS) may operate in Single-User (SU)-MIMO mode (see Figure 5.1(a)), that is, to employ point-to-point transmission of data just for one multi-antenna User Equipment (UE) at a time. Alternatively, the BS can perform Multi-User (MU)-MIMO transmission (see Figure 5.1(b)), that is, spatially multiplex data streams intended for different UEs that may be equipped with arbitrary numbers of antenna elements. The use of MU-MIMO mode offers potential system capacity gains especially when the BS employs a large antenna array.
The theory and practice of point-to-point MIMO communications are well-known, and the SU-MIMO mode has been adopted into wireless system standards such as Long Term Evolution (LTE), as one of the core methods to increase spectral efficiency. On the other hand, while the theory of the MU-MIMO transmission has also been relatively well investigated, the corresponding practical methods are less mature. Thus, the main research challenges reside ...