A Generalized Approach to Block Diagonalization for Multiuser MIMO Downlink

In multiple input multiple output (MIMO) systems, multiuser enhancement is in the implementation phase. Block diagonalization (BD) is known as the safest technique for the multiuser MIMO downlink since it suppresses the inter-user interference (IUI) perfectly. However, BD consumes most of the transmit antenna resource to form many nulls. Thus, it is very difficult to obtain the transmit diversity gain. In the paper, the adaptive selection of the number of layers is considered. Such approaches have been limited to iterative beamforming. Here, we formulate deterministic transmit beamforming as a generalized approach to the BD (GBD), based on a concept of projecting the IUI to a limited vector space. Our proposal can efficiently utilize the extra degrees of freedom, which are obtained in exchange for reducing the number of layers, to provide the diversity gain in the transmit beamforming. The high capability of the GBD is confirmed by computer simulations.