  Heterogeneous networks (HetNets) are proposed in order to meet the increasing
demand for next generation cellular wireless networks, but they also increase
the energy consumption of the base stations. In this paper, an activity
management algorithm for improving the energy efficiency of HetNets is
proposed. A smart sleep strategy is employed for the operator deployed pico
base stations to enter sleep and active modes. According to that strategy, when
the number of users exceeds the turn on threshold, the pico node becomes active
and when the number of users drop below the turn off threshold, it goes into
sleep mode. Mobile users dynamically enter and leave the cells, triggering the
activation and deactivation of pico base stations. The performance of the
system is examined for three different cellular network architectures: cell on
edge (COE), uniformly distributed cells (UDC) and macro cell only network
(MoNet). Two different user distributions are considered: uniform and hotspot.
The effects of number of hotspot users and sleep energies of pico nodes on the
energy efficiency are also investigated. The proposed activity management
algorithm increases the energy efficiency, measured in bits/J, by $20\%$. The
average bit rates achieved by HetNet users increase by $29\%$ compared with the
MoNet architecture. Thus, the proposed activity control algorithm increases the
spectral efficiency of the network while consuming the energy more efficiently.
