In this paper, we present a Cognitive Radio (CR) based statistical framework for a two-tier heterogeneous cellular network (macro-femto network) to model the outage probability at any arbitrary secondary user (femto user) and primary user (macro user). A system model based on stochastic geometry (utilizing the theory of a Poisson point process (PPP)) is introduced to model the random locations and topology of both primary and secondary networks (macro-femto networks). We provide an overview of how CR idea facilitates interference mitigation in two-tier heterogeneous networks in the presented model. We also study the effect of several important design factors which play vital roles and are usually ignored in determination of outage and interference. We conduct simulations to evaluate the performance of our proposed schemes in terms of outage probability for different values of signal-to-interference-plus- noise-ratio (SINR) target.