Adaptive Modulation in PU2RC Systems with

This paper addresses a per user unitary precoding and rate control (PU
²
RC) system with adaptive multi-level quadrature amplitude modulation (QAM) modulation, where both the PU
²
RC operation and adaptive modulation are based on finite rate feedback. The system supports two transmission modes (single user transmit beamforming and multiuser transmission with orthogonal beamforming), and switches between the two modes to maximize the average throughput. We first study single user transmit beamforming, and derive closed-form expressions for the average throughput and bit-error rate (BER) with outdated channel information. Then, multiuser transmission with orthogonal beamforming is investigated. We analyze the throughput and BER performance without feedback delay in the normal signal noise ratio (SNR) regime, and the performance with feedback delay in the interference-limited regime. Simulation results demonstrate the effect of time delay, and the preferred regions of the single user transmission mode and multiuser transmission mode.