A cognitive radio should avoid interference from secondary users to primary users. However, the unknown location of primary receiver makes the problem hard to solve. This paper proposes a novel primary receiver detection to change the passive state of primary receivers. Primary receiver informs the secondary user it's licensed receive frequency according to semaphore architecture via mapping method. The secondary user determines the spectrum holes by the semaphore. The algorithm of the mapping method and the semaphore transmitting power control are proposed. The analysis on the simulation results shows that the scheme can realize the detection of primary receiver, avoid interference and improve the spectrum utilization effectively.