IEEE 802.11无线网络的两步指数退避算法

IEEE 802.11标准引入二进制指数退避(Binary Exponential Backoff, BEB) 算法以降低节点发送数据包碰撞的概率。然而,BEB存在着不足之处,当数据包碰撞概率较大时,节点的竞争窗口长度会出现振荡,即节点每次发送数据包之前,需要多次加倍扩大竞争窗口长度,而在发送成功之后又把竞争窗口长度缩小到最小值,这个过程反复出现。为了克服竞争窗口振荡问题以增加吞吐量,该文提出两步指数退避(Two-step BEB, TBEB)算法,利用2维马尔可夫链进行建模,导出TBEB算法中节点的退避状态概率分布、平均竞争窗口长度、平均退避次数、每发送一个数据帧所耗时间以及吞吐量等指标,并通过仿真进行验证。通过求解一个简单的优化问题,TBEB可以获得最优竞争窗口长度复位值,使吞吐量达到最优。     

MP-Start：基于带宽测量的分阶段TCP慢启动机制

针对TCP慢启动机制门限值参数难于设置以及突发流量引起网络抖动的问题，提出了一种基于带宽测量及分阶段实施的TCP慢启动改进算法。该方法利用在线网络带宽测量技术，探测出网络带宽，从而根据网络状态的动态变化实现慢启动门限值的动态更新，同时，采用分阶段方法调节拥塞窗口的增加，在拥塞窗口大于门限值一半时，拥塞窗口增幅为门限值和拥塞窗口差值的一半，逐步迭代逼近门限值，使拥塞窗口在连接启动和过渡到拥塞避免阶段增加幅度比较小，而在中间阶段窗口增加幅度比较大。有效避免了多个分组丢失现象的发生，实现了连接的平滑接入和过渡到拥塞避免阶段；为提高改进算法的性能，设计了一种灵活的参数化的模型和自适应参数设置方案。大量仿真实验结果表明MP-Start能在多种网络环境下有效改善网络性能，如降低分组丢弃数、减少数据传输时间、降低共享链路分组队列长度、保持连接的传输平滑性。     

一种IEEE 802.16中快速有效的冲突解决算法

IEEE 802.16宽带无线接入系统在其上行媒体接入控制层采用时,分复用和资源竞争与预留的方式进行接入,各用户站(SS)之间存在着竞争与冲突,这将导致系统性能下降。该文针对SS在碰撞以后的冲突解决过程提出了一种快速有效的冲突解决算法动态退避控制(Dynamical Backoff Control,DBC)算法。在该算法中,基站(BS)通过预测下一个上行帧中将会出现的带宽请求报文数,动态地控制SS的退避范围,以增大每帧中成功发送的带宽请求报文数,从而提高MAC层的数据吞吐,降低SS的平均接入延迟。利用OPNET对DBC和二进制指数退避两种算法进行了仿真。仿真结果显示DBC算法对系统性能有一定的改善和提高。     

基于小型化卫星导航终端的干扰检测方法研究

为了提高干扰检测技术的准确度,减小算法实现功耗及复杂度,采用基于FFT的频域干扰检测技术,在频域计算检测统计量,并设定干扰门限值为θ*a,由判决器对检测统计量和干扰门限值进行比较,输出干扰存在性信息。通过仿真实验,验证了算法的可行性。该算法易于芯片化实现,提升了小型化卫星导航终端的干扰检测性能。     

正交多载波调制中峰值因子的一种减小方案

在多载波调制系统中，通常调制信号的平均功率恒定，且和实际发送的信息符号向量无关，此时，由N个子载波组成的多载波信号的峰值因子(PF)等于10lgN(dB)。提出了减小多载波调制系统中PF的一种方案，对于瞬时包络功率的最大值超过所给门限值的信息符号向量，所有子载波的幅度被降低至门限值，而对于瞬时包络功率的最大值低于门限值的信息符号向量，所有子载波的幅度被增大至门限值。采用这种方法，整个调制系统的PF被减小，而净比特速率保持不变，但系统的误比特率(BER)略为增加。     

机会网络中一种混合的Epidemic路由算法

机会网络采用存储-携带-转发的路由模式,利用节点移动所形成的相遇机会实现节点间通信。由于网络中节点之间的接触时间周期较短以及节点能量和缓存空间有限,容易导致时延增加和数据分组丢失。为此,提出一种混合的Epidemic路由算法(H–EP算法),通过优先发送目的地址为对方节点的数据分组;同时,根据数据分组的生存时间进行有效的缓存管理;此外,为数据分组设置合理的跳数门限值,达到门限值时,转为直接投递。理论分析和仿真结果表明,所提算法能够在保证分组投递率的同时,有效缩短投递时延,增加缓存利用效率等。     

车联网下基于网络编码的高吞吐量多径路由算法

在车联网中,由于车辆节点间无线链路的频繁中断易导致数据重传,使整个车辆组网性能急剧下降。为此,面向车联网提出了一种增强吞吐量的多径路由算法,其核心是使车辆节点分簇算法支持网络编码,从而达到对乱序和丢失报文恢复的目的。该算法首先采用协同编码通信模型来实现分簇后同簇车辆节点互相协同的多路径传输,然后采用网络编码对源端车辆和中间车辆节点发送的数据进行线性编码操作,最后在目的端节点予以解码。在基于QualNet仿真平台上的实验结果表明,该算法能够有效地提升车联网中多径路由的吞吐量。     

分布式无线广播的设计

本文提出了CORAFA算法,在无线广播网络条件下,当发送信息时产生冲突时,衡量当前节点网络负载与信道占用率情况,动态调整竞争窗口值,合理规范冲突解决时间的选择范围,成功解决了发送信息产生冲突时自动调整重新发送时间段选择的问题,同时使网络的整体通信效率尽可能的高,使用基于最小公倍周期与最大公约时间片的任务优化规划的方法,在了高低负载不同的情况下,依照网络的吞吐量和公平性,仿真得出了CORAFA算法的优良性。     

基于自适应算法的NoC路由单元的系统级设计

针对片上网络(NoC)中采用确定性路由算法的路由器的缺点,提出了一种NoC路由单元的系统级模型.该模型采用了自适应路由算法,取代了2D Mesh拓扑结构中常用的XY维序路由算法,可以根据当前的通道使用状态动态地分配传输路径,并优先保证GS包的发送.在SystemC平台完成了模型的设计与验证,结果表明,所设计的基于自适应算法的NoC路由单元能够在多包并发时避免路径冲突,提高了网络的并行发送能力,并支持GS服务.     

系统极化码的低时延CA-SCL算法研究

文章为了降低极化码的串行抵消列表(Successive Cancellation List,SCL)算法的译码时延,利用简化串行抵消(Simplified SC,SSC)算法思想,设计实现了系统极化码(system polar code,SPC)低复杂度(low complexity)的循环冗余校验辅助串行抵消列表(CRC-Aided SCL,CA-SCL)译码,简称为SPC-LC-CA-SCL算法。仿真结果表明:极化码(1024,512)中"Rate-1"节点并行处理的门限值(Threshold Value)设为64时,SPC-LC-CA-SCL和SPC-CA-SCL算法性能一致,时延减少了6.35%。"Rate-1"节点并行处理的门限值设为32,16时,时延分别减少了17.78%和24.13%,性能则降低了0.4dB和0.5dB。     

A smart exponential‐threshold‐linear backoff mechanism for IEEE 802.11 WLANs

Based on the standardized IEEE 802.11 Distributed Coordination Function (DCF) protocol, this paper proposes a new backoff mechanism, called Smart Exponential‐Threshold‐Linear (SETL) Backoff Mechanism, to enhance the system performance of contention‐based wireless networks. In the IEEE 802.11 DCF scheme, the smaller contention window (CW) will increase the collision probability, but the larger CW will delay the transmission. Hence, in the proposed SETL scheme, a threshold is set to determine the behavior of CW after each transmission. When the CW is smaller than the threshold, the CW of a competing station is exponentially adjusted to lower collision probability. Conversely, if the CW is larger than the threshold, the CW size is tuned linearly to prevent large transmission delay. Through extensive simulations, the results show that the proposed SETL scheme provides a better system throughput and lower collision rate in both light and heavy network loads than the related backoff algorithm schemes, including Binary Exponential Backoff (BEB), Exponential Increase Exponential Decrease (EIED) and Linear Increase Linear Decrease (LILD). Copyright © 2011 John Wiley & Sons, Ltd.     

IEEE 802.11移动自组织网络节点竞争窗口长度的概率分布

 在基于IEEE 802.11的移动自组织网络中,MAC(Medium Access Control)层提供了DCF(Distributed Coordinate Function)以控制节点对无线信道的争用.DCF包括了BEB (Binary Exponential Backoff)算法.该文对BEB的重要参数——竞争窗口CW(Contention Window)进行研究,通过随机建模,导出了竞争窗口长度的概率分布,并进行数值分析.研究结果可应用于IEEE 802.11移动自组织网络.     

一种基于实时优化思想的多址接入协议性能分析

该文提出了一种基于慢退避和实时优化思想的碰撞减少多址接入CRMA (Collision Reduced Multiple Access)协议。CRMA协议将实时优化的思想同慢退避的思想相结合,有效地解决了IEEE 802.11协议二进制指数退避算法成功发送数据帧后没有记录网络当前繁忙程度的缺点,能够更准确地记录数据帧成功发送后高负荷网络的退避阶数,降低数据帧接入信道的碰撞概率,提高无线信道的利用率。新的多址接入协议能够与现有的IEEE 802.11协议完全兼容,具有简单、无开销、完全分布性和自适应性的特点。分析和仿真结果表明,CRMA协议较之改进前的IEEE 802.11协议和其它最新的多址接入算法能够更有效地利用网络中已有的信息,更准确地估计网络当前的竞争状态,表现出更好的网络性能。     

A backoff algorithm based on self-adaptive contention window update factor for IEEE 802.11 DCF

The binary exponential backoff (BEB) mechanism is applied to the packet retransmission in lots of wireless network protocols including IEEE 802.11 and 802.15.4. In distributed dynamic network environments, the fixed contention window (CW) updating factor of BEB mechanism can’t adapt to the variety of network size properly, resulting in serious collisions. To solve this problem, this paper proposes a backoff algorithm based on self-adaptive contention window update factor for IEEE 802.11 DCF. In WLANs, this proposed backoff algorithm can greatly enhance the throughput by setting the optimal CW updating factor according to the theoretical analysis. When the number of active nodes varies, an intelligent scheme can adaptively adjust the CW updating factor to achieve the maximal throughput during run time. As a result, it effectively reduces the number of collisions, improves the channel utilization and retains the advantages of the binary exponential back-off algorithm, such as simplicity and zero cost. In IEEE 802.11 distributed coordination function (DCF) protocol, the numerical analysis of physical layer parameters show that the new backoff algorithm performance is much better than BEB, MIMD and MMS algorithm.     

Optimal contention window size for IEEE 802.15.3c mmWave WPANs

The millimeter-wave (mmWave) band offers the potential for multi-gigabit indoor Wireless Personal Area Networks (WPANs). However, it has problems such as short communication coverage due to high propagation losses. In order to compensate for this drawback, utilization of directional antennas at the physical layer is highly recommended. In this paper, we consider the adequate contention window (CW) size for directional carrier sense multiple access with collision avoidance (CSMA/CA). To find the optimal CW size that enhances the performance of conventional directional CSMA/CA, we propose an enhanced directional CSMA/CA algorithm. The algorithm is considered in IEEE 802.15.3c, a standard for mmWave WPANs, under saturation environments. For the algorithm, we present a Markov chain model and analyze it for the no-ACK mode. The effects of directional antennas and the features of IEEE 802.15.3c Medium Access Control (MAC) such as backoff counter freezing are considered in the model. The optimal CW sizes for the two different objective functions are derived from the numerical results. The numerical results also show that the system throughput and average transmission delay of the proposed algorithm outperform those of conventional one and the overall analysis is verified by simulation. The obtained results provide the criterion for selecting the optimal parameters and developing a MAC protocol that enhances the performance of mmWave WPANs.     

Distributed Optimal Contention Window Control for Elastic Traffic in Single-Cell Wireless LANs

This paper presents a theoretical study on distributed contention window control algorithms for achieving arbitrary bandwidth allocation policies and efficient channel utilization. By modeling different bandwidth allocation policies as an optimal contention window assignment problem, the authors design a general and fully distributed contention window control algorithm, called general contention window adaptation (GCA), and prove that it converges to the solution of the contention window assignment problem. By examining the stability of GCA, we identify the optimal stable point that maximizes channel utilization and provide solutions to control the stable point near the optimal point. Due to the generality of GCA, this work provides a theoretical foundation to analyze existing and design new contention window control algorithms.     

Improving throughput through dynamically tuning contention window size in dense wireless network

With the boom of wireless devices, the number of wireless users under wireless local area networks (WLANs) has increased dramatically. However, the standard backoff mechanism in IEEE 802.11 adopts fixed initial contention window (CW) size without considering changes of network load, which leads to a high collision probability and low channel utilization in bursty arrivals. In this paper, a novel CW dynamic adjustment scheme is proposed to achieve high throughput performance in dense user environment. In the proposed scheme, the initial CW size is dynamically adjusted to optimum according to the measured packet collision probability. Simulation results show that the proposed scheme can significantly improve the throughput performance.     

紫外Ad hoc网络中基于历史的动态公平性避退算法

本文把UV通信与Ad hoc网络相结合,利用Ad hoc网络的多跳特性来克服UV通信距离有限的弱点。为了使UV Ad hoc网络中的各个节点能够公平有效的接入信道,本文在二进制指数退避（Binary Exponential Backoff,BEB）算法的基础上,以提高公平性为目的,提出了一种基于历史的动态公平性避退算法（Uistory-Based Dynamic Fairness Backoff,HDFB）来改善节点接入信道的公平性。仿真结果表明：新算法能有效地改善信道接入的不公平性。     

A Cross-Layer Approach to Reduce Channel Access Delay Jitter in IEEE 802.11 WLANs

In this paper, we propose a consistent random backoff (CRB) scheme to reduce the channel access delay jitter in voice over wireless local area networks. In the CRB scheme, a contention window (CW) size at each backoff stage is determined by hashing the session identifier and the talk spurt index. Therefore, all packets in the same talk spurt of a session have the same CW sizes if they are transmitted at the same backoff stage. Since a modulo-division operation with the identical maximum CW value is applied, fairness with the legacy backoff scheme (i.e., binary exponential backoff) is also provided. Extensive simulation results demonstrate that the CRB scheme can reduce the channel access delay jitter by 54 %.