On stochastically emulating continuous scattering structures by discrete sources for OTA testing of DuT with highly directive antennas

Considered is OTA testing of future generation communications systems, especially of Devices-under-Test of large electrical size and with highly directive antenna patterns. The channel is assumed to be dominated by a single continuous cluster. Two methods of stochastic cluster emulation by discrete incident fields, modelled as plane waves, are investigated. With both methods, a potential problem with calibrating OTA set-ups was noticed. Random variations in received power of more than 15 dB were observed in cases of angular symmetry around the DuT heading. With omnidirectional antennas, these variations can be reduced to less than 0.1 dB by selecting the initial phases. On DuT antennas with different angular phase responses, this has limited effect, still up to 6 dB variations were seen. Only for the MPAC emulation method, introducing angular asymmetry in the array reduced random variations to smaller than 0.15 dB. Additional deterministic power variations, depending on the method either ripple or bias, were observed too. On the highly directive pattern of a car, the ripple could be stronger than 10 dB (peak-to-peak) for a 12-antenna OTA ring, the bias up to 5 dB. The more OTA antennas, the smaller these deterministic power variations. Recommendations for calibration of MPAC set-ups and designing OTA antenna arrays are given.