Analysis of energy efficiency on the cell range expansion for cellular-WLAN heterogeneous network

In this paper, we analyse the total network en- ergy efficiency (EE) of cellular-WLAN heterogeneous network (HetNet) that employs cell range expansion (CRE) technique, in order to control the user association to either WLAN or cellular network. To this end, we model the system with OFDM based cellular macro-cells and WiFi access points for a saturated (i.e., full-buffer) downlink scenario, considering practical aspects of each type of access technology. Then we evaluate the EE of network by considering realistic power consumption models for each access network type. We compare the performance of the CRE scheme with two benchmark user association schemes; namely, WLAN-first and Max-RSRP (Reference Signal Receive Power). The results demonstrate that CRE with negative biasing performs best in terms of network EE, while the WLAN-first scheme demonstrates the worst performance. However, the CRE with negative biasing lacks fairness in terms of user throughput, while the WLAN-first scheme shows better fairness. Hence, there is a trade-off between the user fairness and the system EE. We show that by optimising the bias factor of each APs individually, with appropriate utility function, a better balance of this trade-off can be achieved.