On the Feasibility of Out-of-Band Spatial Channel Information for Millimeter-Wave Beam Search

Prolonged beam alignment is the main source of overhead in mobile wireless communications at millimeter-wave (mm-wave) frequencies due to narrow beams following the requirement of high antenna gains. Out-of-band spatial information may be used in initial beam search when lower-frequency-band radios are operating in conjunction with mm-wave radios. The feasibility of using low-band channel information for coarse estimation of high-band beam directions strongly depends on the spatial congruence between the two frequency bands. In this work, we try to answer two related questions. First, how similar is the power angular spectrum (PAS) of propagation channels between two widely separated frequency bands? Then, what is the impact of practical antenna configurations on spatial channel similarity? We propose a beam directions-based metric to assess the power loss and the number of false directions if out-of-band spatial information is used instead of in- band information. This metric is more practical and useful than comparing the PASs directly. Point cloud ray-tracing and propagation measurement results across multiple frequency bands and environments are used to show that the degree of spatial similarity of beamformed channels is related to antenna beam widths, frequency gap, and radio link conditions.