  Relay cooperation and integrated microwave and millimeter-wave (mm-wave)
dual-band communication are likely to play key roles in 5G. In this paper, we
study a two-user uplink scenario in such dual-bands, modeled as a
multiple-access relay channel (MARC), where two sources communicate to a
destination assisted by a relay. However, unlike the microwave band,
transmitters in the mm-wave band must employ highly directional antenna arrays
to combat the ill effects of severe path-loss and small wavelength. The
resulting mm-wave links are point-to-point and highly directional, and are thus
used to complement the microwave band by transmitting to a specific receiver.
For such MARCs, the capacity is partially characterized for sources that are
near the relay in a joint sense over both bands. We then study the impact of
the mm-wave spectrum on the performance of such MARCs by characterizing the
transmit power allocation scheme for phase faded mm-wave links that maximizes
the sum-rate under a total power budget. The resulting scheme adapts the link
transmission powers to channel conditions by transmitting in different modes,
and all such modes and corresponding conditions are characterized. Finally, we
study the properties of the optimal link powers and derive practical insights.
