  Heterogeneous cellular networks with small cells densely deployed underlying
the conventional homogeneous macrocells are emerging as a promising candidate
for the fifth generation (5G) mobile network. When a large number of base
stations are deployed, the cost-effective, flexible, and green backhaul
solution becomes one of the most urgent and critical challenges. With vast
amounts of spectrum available, wireless backhaul in the millimeter wave
(mmWave) band is able to provide several-Gbps transmission rates. To overcome
high propagation loss at higher frequencies, mmWave backhaul utilize
beamforming to achieve directional transmission, and concurrent transmissions
(spatial reuse) under low inter-link interference can be enabled to
significantly improve network capacity. To achieve an energy efficient solution
for the mmWave backhauling of small cells, we first formulate the problem of
minimizing the energy consumption via concurrent transmission scheduling and
power control into a mixed integer nonlinear programming problem. Then we
develop an energy efficient and practical mmWave backhauling scheme, where the
maximum independent set based scheduling algorithm and the power control
algorithm are proposed to exploit the spatial reuse for low energy consumption
and high energy efficiency. We also theoretically analyze the conditions that
our scheme reduces energy consumption, and the choice of the interference
threshold for energy reduction. Through extensive simulations under various
traffic patterns and system parameters, we demonstrate the superior performance
of our scheme in terms of energy consumption and energy efficiency, and also
analyze the choice of the interference threshold under different traffic loads,
BS distributions, and the maximum transmission power.
