Quantization-Error Threshold-Based User Admission for Limited-Feedback MU-MIMO Downlink

Future wireless systems such as 5G-Advanced and 6G are expected to rely increasingly on multi-user MIMO and distributed multi-antenna transmission, where accurate channel direction information (CDI) is essential for interference management. In limited-feedback downlink systems, however, finite-rate CDI feedback introduces quantization error, resulting in residual interference and rate loss in zero-forcing beamforming. This paper proposes a quantization-error-threshold-based user admission scheme for limited-feedback MU-MIMO downlink systems. In the proposed scheme, each user feeds back its quantized CDI and channel quality information only when its CDI quantization error is below a predefined threshold, and the base station performs semi-orthogonal user selection and zero-forcing beamforming over the admitted users. The proposed threshold controls the tradeoff between feedback-overhead reduction and candidate-user availability while improving the reliability of the CDI used for precoding. An analytical framework is developed to characterize the threshold-dependent scheduled-user count, ergodic sum-rate, and feedback overhead. Simulation results show that the proposed scheme improves the sum-rate compared with conventional SUS and substantially reduces the feedback overhead, especially as the number of users increases.