Active Calibration Approach Addressing Antenna Mutual Coupling and Power Amplifier Output Mismatch in Fully Digital MIMO Transmitters

This paper presents an active calibration scheme designed for fully digital multiple-input multiple-output (MIMO) transmitter systems, a crucial component for achieving channel reciprocity. The theoretical analysis investigates the influence of antenna mutual coupling and power amplifier (PA) output impedances on the MIMO transmitter and channel reciprocity, emphasizing the substantial impact of poor output matching in high-efficiency PAs. The proposed active calibration scheme is formulated to simultaneously characterize and compensate for non-flat responses across all RF chains in the fully digital MIMO transmitter. A proof-of-concept experiment is conducted using a custom-built 16-chain MIMO transmitter, demonstrating the scheme's effectiveness in mitigating the impact of antenna coupling and PA output mismatch on channel reciprocity. Measurement results reveal a reduction in the root normalized mean square error (RNMSE) from 22% to 1% for 16 uncorrelated 200 MHz OFDM signals after calibration, validating the scheme's efficacy.