Block lower multi-diagonalization for multiuser MIMO downlink

For multiuser-MIMO (MU-MIMO) downlink, the authors propose a novel precoding concept of block lower multi-diagonalization. Extending the conventional block diagonalization method, the proposed precoder for the desired user is computed to incorporate a predetermined number of interfering users. Consequently, the concept enables us to ensure extra degrees of freedom at the transmit array even after null steering in a linear MU-MIMO strategy, and also to reduce computational load at transmit precanceling in a nonlinear MU-MIMO strategy. From an implementational viewpoint, we here examine two approaches: block bi-diagonalization, allowing one interfering user, and block lower tri-diagonalization, allowing up to two interfering users. In the paper, prior to a study on practical management schemes for residual interference, we reveal the significant potential of the proposed concept through the discussion and numerical evaluation using sum-rate capacity.