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

The transition to higher frequency bands, such as millimeter-wave (mmWave) and terahertz (THz), will be capitalized in the long term for future wireless communications. One of the 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. 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 a multi-band channel similarity measure of desired beam directions extracted from radio channels, which is obtained via filtering propagation paths by different beam patterns at different frequencies. Measurement-and 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 a line-of-sight (LoS)-dominated scenario and even in a non-LoS scenario. Finally, we discuss the challenges associated with exploiting spatial channel similarity.