  Using out-of-band (OOB) side-information has recently been shown to
accelerate beam selection in single-user millimeter wave (mmWave) massive MIMO
communications. In this paper, we propose a novel OOB-aided beam selection
framework for a mmWave uplink multi-user system. In particular, we exploit
spatial information extracted from lower (sub-6 GHz) bands in order to assist
with an inter-user coordination scheme at mmWave bands. To enforce
coordination, we propose an exchange protocol exploiting device-to-device
communications, where low-rate beam-related information is exchanged between
the mobile terminals. The decentralized coordination mechanism allows the
suppression of the so-called co-beam interference which would otherwise lead to
irreducible interference at the base station side, thereby triggering
substantial spectral efficiency gains.
