Measuring the Impact of Beamwidth and Bandwidth on the Wide-Sense Stationarity of Millimeter-Wave Channels

If a wireless channel is said wide-sense stationary (WSS), the mean and autocorrelation of its small-scale fading are position invariant. This occurs when the channel is rich in propagation paths – their complex summation averages out any position-specific characteristics. Extensive measurement campaigns have validated the microwave channel to be WSS – it is inherently rich in diffracted paths, the 1G to 4G systems that operate there employ omnidirectional antennas which detect paths from all directions, and the systems feature narrow bandwidths which sum paths over long sample periods. Popular millimeter-wave (mmWave) channel models assume the channel is WSS without measurement-based validation even though the channel is inherently sparse due to weak diffraction, and the 5G systems that operate there employ pencilbeam antennas and feature ultrawide bandwidths. In fact, a recent measurement campaign showed the 60 GHz channel is non-WSS at narrow beamwidths and wide bandwidths, however the campaign considered only one measured channel and only a discrete set of beamwidths and bandwidths. For comprehensive analysis, in this paper we measured 88 channels over three indoor and two outdoor environments with our 60 GHz channel sounder and varied the beamwidth and bandwidth continuously to find the crossover points when the channel transitions from WSS to non-WSS.