In this paper we consider the problem of detecting of primary users (PU) including transmitter and receiver in cognitive radio (CR) networks. In ordinary situation, it is difficult to detect whether the primary receiver exists even where the receiver is. In this work we propose a novel system design combined with CR mobile station and base station to cooperatively detect the location of primary transmitter and to estimate the protection region of primary receiver. The path loss model then is applied to calculate the protection region. Finally, simulation results are presented for the given path loss parameter and noise fluctuate.