User group selection scheme for lattice-reduction-aided precoder in MIMO broadcast systems

In multiple-input multiple-output (MIMO) broadcast channel, the lattice-reduction-aided (LRA) precoder is the modified zero-forcing beamforming (ZFBF) scheme which perturbs data symbols to reduce the transmit power. In this paper, we propose the optimal user group selection (UGS) scheme for the LRA-precoder to maximize the sum-rate in MIMO broadcast systems. We derive the approximated sum-rate of the LRA-precoder and the optimal UGS scheme. We also propose a suboptimal UGS scheme which requires low-complexity. From the simulation results, we show that the sum-rate of the proposed LRA-precoder with the optimal UGS scheme outperforms that of the conventional ZFBF with its optimal UGS scheme, especially when the number of users is small. In addition, we also show that the proposed LRA-precoder with the suboptimal UGS scheme has a considerably larger sum-rate than the conventional ZFBF with its suboptimal scheme, especially when the transmit correlation exists.