On the performance of semi-orthogonal user selection with limited feedback

The sum rates of MIMO broadcast channels degrade because of channel quantization errors (CQE), when semi-orthogonal user selection and zero-forcing beamforming (SUS-ZFBF) are used. To study this degradation, impractically many users have been assumed in the state-of-the-art. We derive two new bounds (i.e., upper and lower) to quantify the degradation for a practical number of users (i.e., K ≤ 100). Validated by simulations, the new bounds show that the degradation is marginal (<3%) from using non-optimal quantization methods. Our bounds also reveal that, for a given overall overhead, increasing quantization bits and reducing users per SUS-ZFBF process is able to substantially improve the sum rate.