One of the main purposes for heterogeneous wireless networks is to promote the network energy efficiency. Cell Range Expansion (CRE), as a promising solution to offload macro BS traffic and improve the network capacity, will help to improve the network energy efficiency. Users that offloaded to micro Base Stations (BSs) will always have much lower downlink (DL) signal-to-interference-and-noise ratio (SINR) but higher uplink (UL) SINR. Therefore, introducing CRE can improve UL transmission performance while decrease DL performance slightly. In this paper, exploiting stochastic geometry, we focus on the performance of downlink and uplink joint transmission in two-tier heterogeneous network where micro BS employs CRE. We derive the expression of whole network transmission success probability (TSP) and energy efficiency (EE) which considering joint transmission of DL and UL. Both theoretical and simulation results show that there is an optimal CRE bias and an optimal transmit power of macro BS to maximize TSP and EE. When the optimal value of CRE bias is selected, the increase of micro BS density has little influence on EE but can improve TSP considerably.
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