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Downlink coherent multiuser transmission is an essential technique forcell-free massive multiple-input multiple output (MIMO) systems, and theavailability of channel state information (CSI) at the transmitter is a basicrequirement. To avoid CSI feedback in a time-division duplex system, the uplinkchannel parameters should be calibrated to obtain the downlink CSI due to theradio frequency circuit mismatch of the transceiver. In this paper, a design ofa reference signal for over-the-air reciprocity calibration is proposed. Thefrequency domain generated reference signals can make full use of the flexibleframe structure of the fifth generation (5G) new radio, which can be completelytransparent to commercial off-the-shelf (COTS) remote radio units (RRUs) andcommercial user equipments. To further obtain the calibration of multiple RRUs,an interleaved RRU grouping with a genetic algorithm is proposed, and anaveraged Argos calibration algorithm is also presented. We develop a cell-freemassive MIMO prototype system with COTS RRUs, demonstrate the statisticalcharacteristics of the calibration error and the effectiveness of thecalibration algorithm, and evaluate the impact of the calibration delay on thedifferent cooperative transmission schemes.

Experimental Performance Evaluation of Cell-free Massive MIMO Systems Using COTS RRU with OTA Reciprocity Calibration and Phase Synchronization