Abstract: In order to improve the link maintenance probability and spectral efficiency while decreasing delay during spectrum handover in cognitive radio systems，two models based on the diversity of spectrum quality and the requirements of secondary services are constructed for the cognitive user，when they want to use the currently available frequency resources，and factors such as the probability of primary user appears，channel selection and conflict avoiding algorithms are also taken into consideration.Finally，an efficient mechanism based on grading is presented.Numerical results show that the propositional mechanisms can implement discriminative operation to kinds of secondary services and guarantee the QoS of primary user and secondary user.

Key words: spectrum handoff, spectrum grading, channel selection algorithm, conflict avoiding?algorithm

摘要： 为解决认知无线电网络中频谱切换机制问题，以提高连接保持概率和频谱利用率，降低频谱切换延迟，综合考虑了频谱质量和次用户的业务差异性，将可用频谱资源进行分类并建立起两种使用模型;结合主用户出现概率、信道选择机制和冲突避免算法等因素，提出了一个完整的基于分级的频谱切换机制。仿真结果表明，该机制使得用户在切换时能够快速且无冲突地找到目标信道，有效地保证了主用户和次用户的服务质量。

关键词: 频谱切换, 频谱分级, 信道选择算法, 冲突避免算法