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In cognitive radio (CR), spectrum sensing plays a fundamental and critical role. This paper deals with the issue of spectrum sensing in a distributed CR network, and proposes a novel relay based cooperative spectrum sensing method, which makes one secondary user (SU) with higher signal-to-noise ratio spend part of its sensing time acting as the relay node. Different from conventional relay based methods, this method does not require idle SUs, and thereby is able to work well even when all SUs in the network sense and access the licensed spectrum band simultaneously. Moreover, via maximizing the agility of the entire CR network, its relay policy is optimized. Simulation results are provided to verify the optimal relay policy as well as its agility again.