  We propose a hybrid architecture that integrates RF (i.e., sub-6 GHz) and
millimeter wave (mmWave) technologies for 5G cellular systems. In particular,
communications in the mmWave band faces significant challenges due to variable
channels, intermittent connectivity, and high energy usage. On the other hand,
speeds for electronic processing of data is of the same order as typical rates
for mmWave interfaces which makes the use of complex algorithms for tracking
channel variations and adjusting resources accordingly impractical. Our
proposed architecture integrates the RF and mmWave interfaces for beamforming
and data transfer, and exploits the spatio-temporal correlations between the
interfaces. Based on extensive experimentation in indoor and outdoor settings,
we demonstrate that an integrated RF/mmWave signaling and channel estimation
scheme can remedy the problem of high energy usage and delay associated with
mmWave beamforming. In addition, cooperation between two interfaces at the
higher layers effectively addresses the high delays caused by highly
intermittent mmWave connectivity. We design a scheduler that fully exploits the
mmWave bandwidth, while the RF link acts as a fallback mechanism to prevent
high delay. To this end, we formulate an optimal scheduling problem over the RF
and mmWave interfaces where the goal is to maximize the delay-constrained
throughput of the mmWave interface. We prove using subadditivity analysis that
the optimal scheduling policy is based on a single threshold that can be easily
adopted despite high link variations.
