In two-tier macro-femto cellular networks, cross-tier interference is a long-standing problem and may greatly degrade the performance of networks. Since most femtocell base stations (FBSs) can be accessed only by a certain closed subscriber group (CSG), mobile users may suffer from heavy interference and poor signal reception. Hybrid access control is one of methods to improve the communication quality by allowing mobile users to access nearby FBSs when these FBSs have surplus resource. In this paper, we propose a new hybrid access policy that mobile users may access nearby FBSs when they are outside the coverage regions of macrocell base stations (MBSs) and suffer from low signal power. We then derive the coverage probability by assuming that the locations of the macrocell base stations and the femtocell base stations-follow independent Poisson point processes. The proposed hybrid policy can be viewed as a generalization of both the open and the closed access policies, hence the analytical results can also be applied to either the open access policy or the closed access policy.
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