色散补偿光纤不同补偿方式的仿真研究

从非线性薛定谔方程入手,分析光纤色散和非线性效应对光纤传输系统的影响,提出利用色散补偿光纤(DCF)对色散进行补偿。比较不同的色散补偿方案(即DCF放置在系统中的不同位置进行色散补偿)补偿性能的优劣;分析占空比、啁啾系数、入射功率等参量对于方案的影响。仿真结果表明混合补偿为最佳补偿方案,同时选取合适的参量,能获取更佳补偿效果。     

Virtual reservation scheme for supporting CBR multimedia services with strict QoS performance over WLAN and wireless mesh

Carrier sense multiple access and its variants have been widely used in mobile ad hoc networks. However, most existing access mechanisms cannot guarantee quality of service for real‐time traffic. In this paper, we present a novel approach to guarantee the performance of real‐time applications such as voice and video that require periodic access to the wireless local area network (WLAN) channel. Our approach can work either with distributed coordination function or enhanced distributed coordination function, and includes distributed reservations of time slots between the stations sharing the WLAN without the need to send any signaling or control messages. Stations can access the wireless medium only if they are able to complete the frame transmission without interruption to the reserved slots of existing real‐time sessions. Simulation results indicate that the proposed mechanism provides high channel utilization, and bounded delays and jitter for real‐time traffic. Copyright © 2009 John Wiley & Sons, Ltd.     

The performances study of IEEE 802.11e to support QoS in channel error environment

One of the most important functions in IEEE 802.11e is enhanced distributed coordination function (EDCF). EDCF is an enhanced version of IEEE 802.11 distributed coordination function (DCF) which provides a priority scheme by differentiating the inter‐frame space and the initial window size. We proposed a modified Markov chain model to study the EDCF priority scheme. We have theoretically analyzed the saturation throughput and delay by using the modified Markov chain model in non‐ideal channel scenario. We also have compared it with the Markov chain model that exists under ideal channel scenario. Simulations are also conducted to validate analytical results. Our study shows that the modified model under ideal channel scenario is better than that in channel error scenario. Copyright © 2011 John Wiley & Sons, Ltd.     

石油开发项目实物期权评价方法

石油开发投资具有可延迟性，这一特点使得石油开发项目具有灵活性价值，科学评估石油开发中延迟期权的价值，可使项目评估结果更为客观可靠。项目评估过程中，对开发方案所设计的未来各年产量进行套期保值，并假设无技术不确定性，采用原油价格服从几何布朗运动、原油便利收益服从均值回复运动的原油价格波动模型，在此基础上，建立石油开发项目无风险折现现金流模型，并采用LSM算法确定石油开发延迟期权的价值。实例分析表明：净现值小于零的开发项目可能具有开发价值，但立即实施一个净现值大于零的项目可能不是最优决策；项目开发价值与可延迟期限正相关，但并不随可延迟期限的增加而一直增加。图2参17     

A delay based backoff scheme for WLAN with QoS guarantee and its performance

Based on the Distributed Coordination Function (DCF) and the Enhanced DCF (EDCF) mechanisms in IEEE 802.11 and 802.11e, a novel backoff scheme for Wireless Local Area Network (WLAN) is proposed. The scheme is to solve the problem of the packet loss and the decrease of performance due to the increasing Contention Window (CW) when there are continuous collisions. In the proposed scheme, the CW of the packet will change dynamically with different delay for the different traffics. Mathematic formulas are presented to indicate the relationship between the CW and the delay character. The performance of the new scheme is also discussed with simulation results. The results show that it helps WLAN system handle multimedia simultaneously.     

一种基于IEEE802.11EDCF的自适应能量调整算法

简述了IEEE802.11及IEEE802.11e的两种访问控制机制DCF和EDCF,提出了一种基于IEEE802.11 EDCF的自适应能量调整算法,并在NS2上进行了仿真实现.仿真实验结果表明,该算法能够根据分组的优先权水平和节点当前能量值动态调度信道接入,延长了全网的生存期.     

一种适用于Ad hoc网络的DCF协议自适应功率控制机制

该文对载波检测无线网络中的冲突干扰问题进行了分析,在此基础上提出了一种适用于Ad hoc网络的DCF协议自适应功率控制机制。该机制能根据各个接收节点的通信状况,自适应的调整接收节点的CTS帧发送功率和收方决定的数据帧发送功率。仿真证明,在DCF协议中引入该机制后,不仅能有效节省节点能耗,延长网络的生存时间,还能同时增大网络的频率空间复用度,提高网络的平均吞吐量。     

IEEE 802.11协议中分布式协调机制的性能模型

IEEE 802.11采用异步传输方式作为媒体层的主要技术,而基于载波检测碰撞避免的分布式接入机制则是其最大的特点.关于分布式接入机制的研究,目前已经有了许多的模型,但是,大部分的模型都是研究终端所产生的数据包是固定长度,很少有模型来研究终端数据包是可变长度的情况.这种情况下的难点就是不易求得碰撞发生时信道所消耗的时间长度.本文则研究在终端数据包长度的分布函数为f(x)下协议的吞吐量和延迟性能模型.首先本文将原标准协议的退避算法看成是有固定大小的竞争窗口,用以求得站点的发送概率;然后,分析信道的工作状态,给出了性能模型,重点在求解碰撞消耗的信道时间,在文章的最后,我们通过仿真试验来验证了模型的正确性.     

IEEE 802.11 DCF的一种优化退避算法

为了提升无线网络的整体性能,通过IEEE 802.11 MAC(媒体访问控制)层DCF(分布式协调功能)在基本工作模式下所使用的二进制退避算法和CW(竞争窗口)的大小对网络整体性能的影响进行了分析。采用了增大初始 CW、取消信道从忙碌变为空闲后继续退避需要等待的DIFS (分布式帧间间隔)和设置一个中间值作为阀值使得节点发送失败或成功后采用不同的退避算法进行退避这3种措施来优化DCF方式下的退避机制。经 OPNET仿真验证,改进算法能够有效地降低网络接入时延并能提高网络吞吐量。     

一种抑制OCDMA系统色散和非线性效应的方法

色散和非线性效应对光CDMA系统性能会产生严重影响。针对此类问题,该文通过对采用严格最佳(,,1) F k光正交码的单极性光CDMA系统进行理论分析,得到了色散和非线性效应对光CDMA系统性能的影响的量化表达式。数值结果表明:光纤信道的色散和非线性效应会导致系统性能明显下降,因此,色散补偿技术必不可少。不同于传统的色散补偿方案,该文提出采用非零色散位移光纤(NZ-DSF)搭配色散补偿光纤(DCF)的新方案,在进行色散补偿的同时,尽可能弱化非线性效应的影响,从而有效地改善系统性能。