Abstract: In cognitive networks, there are many factors which can make an impact on the system performance, such as the probability of detecting primary signals, the probability of detecting idle channels and the beginning or end of primary transmission between two sensing periods. This paper studies this problem by analyzing the relationship among cognitive capacity, sensing time and the rate-of-change of primary signals. It shows that when the probability of primary user is active, the optimal sensing time depends only on the length of cognitive frame and the Signal to Noise Ratio（SNR） of primary radio, and does not depend on the rate-of-change of primary signals；otherwise, cognitive capacity becomes smaller when primary signals change faster.

Key words: cognitive radio, capacity, primary activity, sensing error, sensing efficiency, optimization

摘要： 认知用户使用信道时，主用户信号的检测率、空闲信道的检测率以及两次感知之间主用户信号的离开或到来都对系统性能有着重要的影响。针对这一问题，分析了认知用户容量、感知时间、主用户信号变化率之间的关系。研究发现：在主用户对信道利用率一定的条件下，认知用户的最佳感知时间只取决于认知信号的帧长和主用户信号的信噪比，而与主用户信号的变化率无关；此外认知用户的容量随着主用户信号变化的增快而呈现出减少的趋势。

关键词: 认知无线电, 容量, 主用户活跃度, 感知误差, 感知效率, 最优化