Real-Time Millimeter-Wave MIMO Channel Sounder for Dynamic Directional Measurements

In this paper, we present a novel real-time multiple-input multiple-output (MIMO) channel sounder for the 28 GHz band. Until now, most investigations of the directional characteristics of millimeter-wave channels have used mechanically rotating horn antennas. In contrast, the sounder presented here is based on a phased array structure that performs fast electronic steering of the beams in the horizontal and vertical domains. This approach drastically shortens the measurement times for measurements that are directionally resolved both at the transmitter (TX) and the receiver (RX), and the measurement time per location is reduced from minutes or hours to milliseconds. This not only enables measurement of more TX-RX locations for a better statistical validity but also allows to perform directional analysis in dynamic environments. The sounder also has high phase stability, which, in conjunction with the short measurement time, leads to a low phase drift between TX and RX. This in turn enables phase-coherent sounding of all beam pairs even when TX and RX have no cabled connection for synchronization, and thus avoids any delay ambiguity. Furthermore, the phase stability over time enables complex RX waveform averaging to improve the signal-to-noise ratio during high path loss measurements. The paper discusses the system design as well as the measurements performed for verification of the sounder performance. Furthermore, we present sample results from double directional measurements in dynamic environments.