  As future networks aim to meet the ever-increasing requirements of high data
rate applications, dense and heterogeneous networks (HetNets) will be deployed
to provide better coverage and throughput. Besides the important implications
for energy consumption, the trend towards densification calls for more and more
wireless links to forward a massive backhaul traffic into the core network. It
is critically important to take into account the presence of a wireless
backhaul for the energy-efficient design of HetNets. In this paper, we provide
a general framework to analyze the energy efficiency of a two-tier MIMO
heterogeneous network with wireless backhaul in the presence of both uplink and
downlink transmissions. We find that under spatial multiplexing the energy
efficiency of a HetNet is sensitive to the network load, and it should be taken
into account when controlling the number of users served by each base station.
We show that a two-tier HetNet with wireless backhaul can be significantly more
energy efficient than a one-tier cellular network. However, this requires the
bandwidth division between radio access links and wireless backhaul to be
optimally designed according to the load conditions.
