Degrees of Freedom of Downlink Single- and Multi-Cell Multi-User MIMO Systems with Location Based CSIT

Multiple antennas facilitate the coexistence of multiple users in wireless communications, leading to spatial multiplexing and spatial division and to significant system capacity increase. However, this comes at the cost of very precise channel state information at the transmitters (CSIT). We advocate the use of channel propagation models to transform location information into (possibly incomplete) CSIT. We investigate the resulting multi-user sum rate from a DoF (Degree of Freedom, high SNR rate prelog, spatial multiplexing factor) point of view. For single-cell multi-user communications, we argue for a revival of SDMA (Spatial Division Multiple Access). In the MIMO case, the receive antennas can suppress the Non Line of Sight (NLoS) channel components to transform the MIMO channel into a MISO LoS channel, allowing the CSIT to be limited to LoS information. For the multi-cell problem, we consider the feasibility of interference aligment in the case of reduced rank MIMO channels. We then focus on the LoS components. Whereas in general MIMO multi-cell coordinated beamforming, the transmitters require global CSIT due to the coupling between transmit and receive filters, in the LoS case decoupling arises, permitting location based transmit beamforming. Location aided techniques may furthermore exploit location prediction through mobility trajectory information. This would allow slow fading (and even connectivity) predictibility, something that is difficult to achieve without location information.