Beamforming Impact on Time Dispersion Assessed on Measured Channels

Time dispersion is an important characteristic for radio interface design and parametrization. For example, optimal cyclic prefix length and reference signal density is dependent on delay spread which vary in different environments. A common view is that massive MIMO and beamforming will reduce time dispersion. This paper presents time dispersion results from measurements in different environments and different frequency bands from 5 GHz to 60 GHz. The impact of beamforming on delay spread and OFDM inter-symbol interference is analyzed both with a full scale 5G NR test-bed utilizing analog beamforming as well as channel sounding virtual array data. It is shown that beamforming reduces delay spread and inter-symbol interference in outdoor macro and micro environments. With Gaussian beamforming applied on the channel sounding data the reduction becomes noticeable when the beam width is less than 2 3 times the rms angular spread of the measured channel. With eigen beamforming the reduction is significant except when several paths have equal strength. In indoor environment, the delay spread is already low and the impact from beamforming is very limited.