  Millimetre-wave communication (licensed or unlicensed) is envisaged to be an
important part of the fifth generation (5G) multi-RAT ecosystem. In this paper,
we consider the spectrum bands shared by 5G cellular base stations and some
existing networks, such as WiGig. Sharing the same band among such multiple
radio access technologies (RATs) is very challenging due to the lack of
centralized coordination and demands novel and efficient interference
mitigation and coexistence mechanisms to reduce the mutual interference. To
address this important challenge, we propose in this paper a novel multi-RAT
coexistence mechanism where neighbouring 5G and WiGig base stations, each
serving their own associated UEs, schedule their beam configurations in a
distributed manner such that their own utility function, e.g. spectral
efficiency, is maximized. We formulate the problem as a combinatorial
optimization problem and show via simulations that our proposed distributed
algorithms yield a comparable spectral efficiency for the entire networks as
that using an exhaustive search, which requires global coordination among
coexisting RATs and also has a much higher algorithmic complexity.
