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In cognitive radio networks, secondary users are allowed to access the under-utilized spectrum and opportunistically transmit whenever no primary signals are detected. So these unlicensed users need to monitor spectrum accurately to detect the presence of the licensed users. In this paper, we exploit a cooperative spectrum sensing between two secondary users to improve the detection performance when one of the users is standing in the decodability boundary of the primary user, which is difficult to detect the presence of this licensed user. We develop this cooperative spectrum sensing with decode-and-forward (DF) diversity protocol, and make a comparison with amplify-and-forward (AF) protocol in terms of detection probability and outage probability. We illustrate that DF protocol has better performance than AF in detection probability, contrarily, AF is better than DF in outage probability. Then, under DF diversity protocol, we extend the number of cooperative secondary users, which is illustrated that as the number is increased, we can get a better detection performance.