  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
capable of performing horizontal and vertical beam steering with the help of
phased arrays. Due to its fast beam-switching capability, the proposed sounder
can perform measurements that are directionally resolved both at the
transmitter(TX) and receiver (RX) in 1.44 milliseconds compared to the minutes
or even hours required for rotating horn antenna sounders. 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
short measurement time combined with the high phase stability limits the phase
drift between TX and RX, enabling phase-coherent sounding of all beam pairs
even when TX and RX have no cabled connection for synchronization without 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 both 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.
