  Millimeter wave (mmWave) communication is one feasible solution for high
data-rate applications like vehicular-to-everything communication and next
generation cellular communication. Configuring mmWave links, which can be done
through channel estimation or beam-selection, however, is a source of
significant overhead. In this paper, we propose to use spatial information
extracted at sub-6 GHz to help establish the mmWave link. First, we review the
prior work on frequency dependent channel behavior and outline a simulation
strategy to generate multi-band frequency dependent channels. Second, assuming:
(i) narrowband channels and a fully digital architecture at sub-6 GHz; and (ii)
wideband frequency selective channels, OFDM signaling, and an analog
architecture at mmWave, we outline strategies to incorporate sub-6 GHz spatial
information in mmWave compressed beam selection. We formulate compressed
beam-selection as a weighted sparse signal recovery problem, and obtain the
weighting information from sub-6 GHz channels. In addition, we outline a
structured precoder/combiner design to tailor the training to out-of-band
information. We also extend the proposed out-of-band aided compressed
beam-selection approach to leverage information from all active OFDM
subcarriers. The simulation results for achievable rate show that out-of-band
aided beam-selection can reduce the training overhead of in-band only
beam-selection by 4x.
