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Deploying femtocell networks achieves great spatial reuse at the price of severe interference from concurrent transmissions. To mitigate the downlink interference from femtocell base stations (FBSs) to nearby macrocell users (MUEs), power control is employed to enable FBSs to dynamically reconfigure their power allocation based on information obtained from information exchange between macrocell and femtocells. However, lack of direct coordination between macrocell and femtocell networks makes cognitive radio (CR) a promising solution to interference management of two-tier networks. In this paper we address the problem of power control in the two-tier network and propose an optimal CR-based power control scheme. Equipped with the ability of spectrum sensing, FBSs can obtain necessary information such as the downlink radio resource usage of macrocell and information about nearby MUEs. Then FBSs are able to allocate transmit power on each channel efficiently to maximize system capacity with a per-tier outage constraint satisfied. Finally the effectiveness of the proposed scheme is verified by system level simulation.