Out-of-Band Information Aided mmWave/THz Beam Search: A Spatial Channel Similarity Perspective

The transition to higher frequency bands, e.g.,
millimeter-wave (mmWave) and terahertz (THz), will be capitalized
on the long term for future wireless communications.
One of challenges relates to rapid establishment of mmWave/THz
links with low beam training overhead due to highly directional
transmission. A promising solution is to take advantage of the
coexistence of sub-6 GHz, mmWave, and THz wireless networks
and to use out-of-band spatial information for enabling fast
beam search. The success depends on the spatial similarity
of radio channels across different frequency bands. In this
article we promote a feasibility study of low-frequency spatial
channel information assisted high-frequency beam search from
a radio channel point of view. We develop multi-band channel
similarity measure of desired beam directions extracted from
radio channels, which are obtained via filtering propagation paths
by different beampatterns at different frequencies. Measurementand
ray-tracing-based evaluations across multiple frequencies
and environments are performed, which prove the usability of
out-of-band information aided beam search strategy in line-ofsight
(LOS) dominated scenario and even in non-LOS scenario.
Finally, we discuss the challenges associated with exploiting
spatial channel similarity.