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

This article introduces an active calibration scheme tailored for fully digital multiple-input multiple-output (MIMO) transmitters, a key step toward ensuring channel reciprocity. The theoretical analysis starts by examining the influence of antenna mutual coupling and power amplifier (PA) output impedances on the MIMO transmitter and channel reciprocity. This analysis highlights the significant impact of the inherently poor output matching, exhibited in high-efficiency PAs, exacerbating the effects of antenna mutual coupling on channel reciprocity. Consequently, an active calibration scheme is formulated to concurrently characterize and compensate for the nonflat responses of all radio frequency (RF) chains in the fully digital MIMO transmitter. To validate the proposed scheme, a proof-of-concept experiment is conducted using a custom-built 16-chain fully digital MIMO system, driven with 200-MHz orthogonal frequency-division multiplexing (OFDM) signals at 3.5-GHz center frequency. Measurement results demonstrate the efficacy of the calibration scheme in mitigating the impact of antenna coupling and PA output mismatch on channel reciprocity. The root normalized mean square error (RNMSE) after calibration is reduced from 22% to 1%.