To meet the explosive demands of wireless access, existing cellular networks are under a revolutionary change from macro-only homogeneous networks to heterogeneous cellular networks (HCNs) with small cells added into current infrastructure. Since small-cell base stations (SBSs) transmit with a much small power than the macro base stations (MBSs), the cell association scheme based on received signal strength will underuse SBSs and thus the notion of cell range expansion (CRE) has been proposed as a de facto technique for future HCNs. In CRE, a constant range expansion bias (REB) is added to the received signal strength of SBSs to increase the footprint of small cells and in turn offload the MBS onto the SBSs overlaid in the same geographical area. Without taking the cell load into consideration, cell offloading may not be effectively performed because an overloaded MBS needs to offload users more urgently than an underloaded MBS. In this work, we propose a load-based cell association scheme, where a bias adjustment function is introduced to determine the timing and the magnitude of offloading based on the load of individual MBS. Using 3GPP long term evolution advanced (LTE-A) compliant parameters, extensive simulation results are presented to show that the proposed load-based cell association scheme outperforms the existing one in terms of both average user rates and association successful probability.
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