In cognitive radio networks, the spectrum utilization can be improved by cognitive users opportunistically using the idle channels licensed to the primary users. However, the new arrived cognitive users may not be abl...In cognitive radio networks, the spectrum utilization can be improved by cognitive users opportunistically using the idle channels licensed to the primary users. However, the new arrived cognitive users may not be able to use the channel immediately since the channel usage state is random. This will impose additional time delay for the cognitive users. Excessive waiting delay can make cognitive users miss the spectrum access chances. In this paper, a discrete-time Markov queuing model from a macro point of view is provided. Through the matrix-geometric solution theory, the average sojourn time for cognitive users in the steady state before accessing the spectrum is obtained. Given the tolerant delay of cognitive users, the macro-based throughput is derived and an access control mechanism is proposed. The numerical results show the effects of service completion probability on average sojourn time and throughput. It is confirmed that the throughput can be obviously improved by using the proposed access control mechanism. Finally, the performance evaluations based on users are compared to that based on data packets.展开更多
文摘目的探讨匹那地尔对高钾停搏液处理后大鼠离体心肌细胞内游离钙与舒缩功能的影响。方法SD大鼠心室肌细胞酶解分离静息1~2 h后随机分为对照组,高钾停搏液组,匹那地尔强化组,格列苯脲拮抗组。四组细胞均在24℃下保存2 h,此后对细胞进行复氧、复灌20 m in并测定细胞收缩、[Ca2+]i瞬态、肌浆网内贮钙释放功能。结果匹那地尔使心肌细胞在缺血再灌注中收缩功能,[Ca2+]i瞬态,肌浆网内贮钙释放能力明显优于高钾停搏液组(P<0.01〉及细胞内钙释放后的钙峰回落时间明显短于高钾停搏液组(P<0.01〉。结论匹那地尔通过维持良好的肌浆网和细胞膜Na+/Ca2+交换体功能来调节钙瞬态变化,减少细胞内钙超载从而改善心肌细胞缺血后收缩功能,同时避免了高钾停搏液的负面效应。
文摘In cognitive radio networks, the spectrum utilization can be improved by cognitive users opportunistically using the idle channels licensed to the primary users. However, the new arrived cognitive users may not be able to use the channel immediately since the channel usage state is random. This will impose additional time delay for the cognitive users. Excessive waiting delay can make cognitive users miss the spectrum access chances. In this paper, a discrete-time Markov queuing model from a macro point of view is provided. Through the matrix-geometric solution theory, the average sojourn time for cognitive users in the steady state before accessing the spectrum is obtained. Given the tolerant delay of cognitive users, the macro-based throughput is derived and an access control mechanism is proposed. The numerical results show the effects of service completion probability on average sojourn time and throughput. It is confirmed that the throughput can be obviously improved by using the proposed access control mechanism. Finally, the performance evaluations based on users are compared to that based on data packets.
