A dynamic adaptive modulation scheme for MU-MIMO downlink zero-forcing transmissions in presence of imperfect channel information

The effects on the received signal to interference plus noise ratio (SINR) of multi-user multi-input and multioutput (MU-MIMO) downlink systems with zero-forcing beamforming (ZFBF) in presence of the channel estimation error and feedback delay are investigated. A dynamic adaptive modulation scheme is proposed to reduce the negative effect of channel estimation error. Based on the automatic repeat request (ARQ) acknowledgements (ACK/NACK), the proposed scheduling scheme dynamically adjusts the SINR thresholds for different modulation orders until the data are received correctly. It is demonstrated in numerical results that the proposed algorithm can obtain a majority of the efficient system throughput of the optimal scheme with acceptable implementation complexity.