中文摘要：

针对认知无线电系统中次级用户队列稳定性问题，通过建立发送状态马尔科夫（Markov）模型，提出了一种基于CSMA的自适应分布式频谱接入算法。次级用户根据感知结果自适应地调整退避时长参数，使稳态服务速率逐渐趋近到达速率，最终达到队列稳定。此外，还在满足对主用户碰撞限制的条件下，推导了次级用户的吞吐量上界，并证明当次级用户的数据到达速率小于此上界时，能够通过所提算法保证队列稳定。仿真结果证明了算法的有效性。

英文摘要：

The queue stability of the secondary users in cognitive network was considered. A Markov chain model for the state of users in the cognitive network was constructed and a distributed CSMA algorithm was proposed. The secondary users adjust the parameters of back-off time due to the arrival and service rate to ensure the queue stability. Upper bound of the capacity of secondary users under the collision constrainting to primary user was derived in closed form. If the ar-rival rate of secondary users is smaller than the upper bound of capacity, queue stability can be ensured by the proposed algorithm. Simulations verified the effectiveness of the algorithm.