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Deploying cognitive radio femtocell network (CRFN) inside a macrocell network can significantly increase the utilization of the available macrocell bandwidth and increase the capacity of the macrocell. However, the success of this deployment in terms of performance degradation of the macrocell and the acceptable throughput for the CRFN is not well defined. In this paper, we propose a time division duplex (TDD) operation of a CRFN and investigate its performance inside a macrocell operating in frequency division duplex (FDD) mode. It is shown that with a proper sensing and transmission scheme the capacity of the CRFN can be increased by simultaneous transmissions on multiple channels, water-filling further improves the result when interference from the macrocell basestation is large. The proposed scheme is applicable to full duplex networks, such as LTE and GSM.

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