Downlink Precoding with Correlation-Based User Grouping for Multiuser MISO Systems

Correlation across transmit antennas in multiple- input single-output (MISO) broadcast channel has been studied to be detrimental generally. The current downlink beamforming schemes have not considered correlation enough such that they are not suitable for the broadcast systems with high correlation among transmit antennas. We propose two novel user grouping precoders to improve the sum rate performance for the case of ill-conditioned channel and the case of small transmit antennas (i.e., the number of transmit antennas is less than that of users). The first proposed precoder partitions all the users into small groups step by step based on the condition number. Downlink beamforming is then done in a small size for the optimal channel submatrix with smaller condition number. Inter-group interference from the subsequent allocated groups is nulled out in the null spaces of the previous allocated groups. The second precoder regularizes the number of users in the first group to improve the sum rate further. Simulation results verify that our schemes outperform the current beamforming schemes for the case of high transmit correlation. Moreover, the proposed precoders allow to be applied for the case of small antennas.