Downlink coverage issues for users (UEs) in a macrocellular network with femtocellular overlay have so far been investigated almost exclusively via simulation~cite{key-2}. In this paper, we employ general models for macro Node B and femto access point (AP) locations to get closed-form expressions for the probability that an arbitrary UE can camp on a macro Node B or an open access (OA) femto AP. These expressions depend only on the relative densities and transmit powers of the macro Node Bs and femto APs, the fraction of femto APs operating in OA vs.~Closed Subscriber Group (CSG) mode, and on the parameters of the wireless channel model. For a macrocellular operator considering a femtocellular overlay on a macrocellular network, we show how a set of universal curves could be used to choose the parameters of the femtocellular deployment.
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