  Future wireless multiple-input multiple-output (MIMO) communication systems
will employ sub-6 GHz and millimeter wave (mmWave) frequency bands working
cooperatively. Establishing a MIMO communication link usually relies on
estimating channel state information (CSI) which is difficult to acquire at
mmWave frequencies due to a low signal-to-noise ratio (SNR). In this paper, we
propose three novel methods to estimate mmWave MIMO channels using out-of-band
information obtained from the sub-6GHz band. We compare the proposed channel
estimation methods with a conventional one utilizing only in-band information.
Simulation results show that the proposed methods outperform the conventional
mmWave channel estimation method in terms of achievable spectral efficiency,
especially at low SNR and high K-factor.
