This paper presents a calibration technique for massive MIMO base stations, where the frequency responses of the transmit and/or receive analog front-ends are individually estimated and compensated for. Calibration is achieved by a first-round of channel sounding between base station antennas, followed by post-processing and a compensation stage. The proposed technique is general in the sense that is does not use external sources, nor internal dedicated circuits for calibration purposes. The only requirement of the technique is that mutual coupling between all pairs of sounded base station antennas exists and is known. Our analysis suggests that mutual coupling can be conveniently used for calibration purposes, and that multipath propagation during calibration is the most prominent source for calibration inaccuracies.
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