Cognitive radio technique is proposed as a means of improving the efficiency of current spectrum allocation policy by allowing the unlicensed secondary users (SU) to access the unused spectrum resources (white band) of the licensed primary users (PU). This technique largely relies on the secondary users' capability to detect the unused spectrum band, thus avoiding interference to the primary users. Instead of viewing the spectrum sensing as merely a detection process, in this paper, we analyze the spectrum sensing strategies from the communication perspective, with system capacity and interference as two major system performance metrics. As a result, we then discover the dilemma between PU presence detection and the interference from the SU to the PU caused by the reciprocality of the wireless channel in large-scale fading sense. To solve this dilemma, we propose a new performance criterion called utility loss function to set the threshold in spectrum sensing. Additionally, the cooperation among spectrum sensing users is shown to effectively solve the dilemma.