To eliminate phase drift, a synchronization cable between the transmitter and receiver is required, limiting deployment range and flexibility indoors, and precluding most outdoor and mobile scenarios. The figure below demonstrates our blind technique (it requires no reference signal) based on real measurements collected with our 60 GHz virtual array channel sounder, over a period of one hour without a synchronization cable. The phase drift was so bad over the collection period that the beamformed antenna pattern is intelligible. After applying our blind calibration technique in post-processing, the characteristic antenna pattern of the array is clearly recognizable, from which the angle is estimated within 1° of the ground-truth value.
More details about our virtual phased-array channel sounder and blind calibration technique can be found in the reference below:
 J. Chuang, C. Liu, J. Senic, C. Gentile, S. Y. Jun, and D. Caudill, “Blind Calibration of Phase Drift in Millimeter-Wave Channel Sounders,” IEEE Access, vol. 8, pp. 109557 – 109567, June 2020. https://ieeexplore.ieee.org/document/9115009