Correlated UE impairments in ZF MU-MIMO transmissions

Applying Multiple Input Multiple Output (MIMO) techniques in wireless communication allows the parallel transmission of several data streams, thus increasing the throughput while leaving the transmit power unchanged. However, the maximum number of parallel streams is defined by the minimum of transmit- or receive-antennas, where in the downlink the User Equipment (UE) is most likely the limiting factor. To overcome this situation, multiple UEs can be served in parallel by Multi User (MU)-MIMO. Zero Forcing MU-MIMO is a low complex technique that tries to cancel the interference between the streams already at the Base Transceiver Station and that requires only one receive antenna per UE. However, due to various transmission impairments, there exists some residual interference degrading the performance. In this paper we illustrate the negative effects of correlated UEs by providing measurement and simulation results. The measurements are carried out with the Vienna MIMO Testbed and not only is the channel estimation performed by measurements, but also the data transmission is performed over physical channels.