In ultra-densely deployed heterogeneous cellular networks

energy efficiency of the network has become prominent due to the increasing number of small cell base stations.In order to reduce the energy consumption and achieve the goal of "green mobile network"

an improved user association algorithm based on cell sleeping is proposed to optimize the energy efficiency in this paper.At the same time

some light-loaded base stations which satisfy the cell sleeping criterion according to the interference graph will also switch to the sleeping mode.Finally

the performances of the proposed and conventional algorithms are compared in the simulation.The results show that compared with the conventional alrogithm

the proposed algorithm has much better performance in energy consumption and energy efficiency

achieving the goal of optimizing system energy efficiency.