联合动态聚合帧长机制的速率自适应算法

在IEEE 802.11n协议中引入帧聚合机制能够提高无线局域网MAC层的效率,但会增加速率自适应算法对信道变化的反应时间,降低网络吞吐量。针对该问题,提出一种改进的速率自适应算法JodaRA。引入权重思想统计聚合帧的发送情况,设计基于子帧丢失加权和的速率选择方案。采取混合型速率选择策略,利用发送端的RSSI与接收端的传感器信息进行链路状态感知,根据链路状态选取合适的速率选择方案并动态设定聚合帧的最大长度,以提高算法的实时性与准确性。仿真结果表明,在不同的节点移动状态下,与RRAA、MinstrelHT和SampleLite 3种算法相比,JodaRA算法的平均UDP吞吐量分别提高90%、57%和22%。     

IEEE 802.11n中速率、模式及信道的联合自适应算法

针对IEEE 802.11n无线网络中的速率、MIMO模式与信道宽度的联合自适应问题,提出了一种基于非静态Multi-Armed Bandit学习方法的联合自适应算法,并设计了一种新颖的报酬函数.为解决该算法收敛时间较慢的问题,基于分类回归树设计了MCS、MIMO模式以及信道宽度预测算法,其能够有效利用无线网卡驱动程序采集的相关统计数据预测不同MCS、MIMO模式以及信道宽度组合的报酬函数,大幅度缩小联合自适应算法的搜索空间.该算法具有易实现、近似最优及计算复杂度低的特点.真实实验结果表明:在无干扰和不同干扰环境下,联合自适应算法都能够有效地提高UDP吞吐量.     

非理想信道下无线局域网的性能分析

提出了一种在非理想信道下分析IEEE 802.11无线局域网的饱和吞吐量的方法。分析的过程中考虑到信道“捕获”和退避“冻结”的问题，采用条件概率建立了数学模型。在IEEE 802.11b无线局域网定义的不同数据速率下，利用该方法对基本访问机制和RTS/CTS访问机制进行了分析。仿真表明，分析结果与DCF模式下的饱和吞吐量是吻合的。     

WLAN跨层链路自适应机制

针对IEEE 802.11无线局域网（WLAN）标准并没有给出速率自适应机制的问题,提出了联合物理层与MAC层的跨层链路自适应机制CLLA。该机制在考虑信道干扰、区分碰撞丢失和噪声干扰丢失的前提下,分别对物理层和MAC层进行了数学描述,建立了以吞吐量为性能指标的两层参数之间的函数关系,自适应地选择发送速率以提高系统的吞吐量。仿真实验及与现有链路自适应机制的比较表明,提出的跨层链路自适应机制CLLA不仅能适应噪声干扰与信道的变化,而且明显地提高了系统的吞吐量。     

蓝牙微微网对WLAN网络同频干扰的建模

由于同在2.4 GHz的ISM频段下工作,IEEE 802.11b局域网和蓝牙微微网不可避免地产生相互干扰,严重影响各自的网络性能。通过对基于CSMA/CA和RTS/CTS机制的IEEE 802.11b协议进行数学分析,剖析了其吞吐量降低与发送不同蓝牙数据包之间的内在关系,建立了一个蓝牙微微网对IEEE 802.11b局域网干扰影响的数学模型。仿真结果表明：该模型能较好地逼近IEEE 802.11b局域网在蓝牙微微网干扰下的吞吐量仿真曲线,在蓝牙DH1、DH3、DH5数据包干扰的情况下,平均相对误差分别为17.86%、10.39%和6.64%。根据该数学模型,提出了一个降低蓝牙微微网干扰的有效方案。该方案能快速、低成本、自适应地调整IEEE 802.11b发送帧长,有效提高其在蓝牙微微网干扰下的吞吐量。     

一种具有自适应传输门限的变速率MAC协议研究

IEEE802.11a是已得到广泛应用的无线局域网标准,可支持多种传输模式。然而,标准中并未给出如何在多种传输模式之间自适应切换的方法。基于IEEE802.11a,提出了一种Ad Hoc网络应用中具有自适应传输门限的变速率MAC,即AV-MAC(Variable rate Media Access Control protocol with Adaptive transmission-threshold)。该算法通过计算发送端所发送的RTS控制帧的接收信噪比,感知信道状态,并依据计算所得的自适应传输门限来判断是否可以发送其后的数据分组。如可以发送该数据分组,发送端将进一步根据接收信噪比选择最佳传输模式,从而优化网络的通过率。利用仿真工具OMNET++4.0对算法进行性能仿真分析,结果表明所提出的AV-MAC可显著提升网络的通过率。     

无线信道自适应传输技术综述

文章对基于地面无线通信的无线信道自适应传输技术的最新研究进展进行了总结,按发送端ACM自适应传输技术、接收端自适应传输技术进行分类,介绍了发送端的可变速率调制、自适应编码、混合自动重传请求和接收端的无速率码、速率自适应解调等相关技术的基本思想、技术实现及特点,并分析比较了发送端802.11b ACM、接收端速率自适应等典型技术的吞吐量性能。指出了无线信道自适应传输技术的研究趋势,最后介绍了地面无线信道自适应传输技术在卫星通信中的扩展应用,对于无线自适应技术的研究具有参考意义。     

无线局域网的速率自适应算法设计与仿真分析

目前的IEEE802.11在物理层提供了不同的传输速率,因此根据不同的信道条件可以选择一个合适的传输速率,以使系统吞吐量达到最大。在WLANs中,当有多个用户向同一个接入点传输数据时,碰撞就会产生。Collision-Aware Rate Adaptation（CARA）算法能够有选择地启用Request-To-Send/Clear-To-Send（RTS/CTS）识别出这种碰撞,很大程度上避免了由于碰撞引起的吞吐量下降。但随着碰撞的增加,CARA吞吐量还是有明显下降。为了提高吞吐量,提出了一种新的速率自适应算法Adaptive Avoid Collision Rate Adaptation（Adaptive-ACRA）,该算法根据信道条件不同改变门限值,有效地提高了大量碰撞存在情况下的吞吐量。最后,通过大量NS2仿真证明了新算法提高系统吞吐量的有效性。     

面向编码机会路由的无线Mesh 网络广播信道接入

编码机会路由是有损无线Mesh网络中提供高吞吐量和高可靠性传输的理想方案.该路由机制建立在无线广播的多用户分集优势和随机网络编码的纠删特性之上,为广播MAC的设计引入了新的机会和挑战.基于最优停止理论,研究面向编码机会路由的机会广播信道接入问题,提出一种在接入延迟和信道交付能力之间加以折衷,以获得最优的平均有效速率的方法,并在IEEE 802.11 DCF协议基础上设计实现面向NCOR的广播MAC协议O-BCast.仿真结果表明,该协议显著提高了编码机会路由的端到端吞吐量,具有网络负载自适应的良好特性.     

基于无线信道的冲突分解算法仿真研究

目前,无线局域网(WLAN)中的媒体接入控制层(MAC)广泛采用基于IEEE802.11的二进制指数回退算法(BEB)的DCF协议.当WLAN中的节点数目迅速增加时,该协议存在系统吞吐量会急剧降低,网络性能变差的缺点.详细分析了无线局域网中IEEE 802.11MAC层的二进制退避协议,指出原协议在进行冲突处理上的不足,并结合树型分解算法(TSA)和快速分解算法(FCR),提出一种改进的树型冲突分解算法(ITSA).并用MATLAB仿真了CSMA/CA协议和ITSA算法.分析和仿真结果证明,与IEEE 802.11等协议采用的二进制指数退避算法相比,该算法能较大的提高系统吞吐量.     

Throughput analysis of IEEE 802.11 multirate WLANs with collision aware rate adaptation algorithm

This paper presents a mathematical model that analyzes the throughput of the IEEE 802.11b distributed coordination function (DCF) with the collision aware rate adaptation (CARA) algorithm. IEEE 802.11 WLANs provide multiple transmission rates to improve system throughput by adapting the transmission rate to the current channel conditions. The system throughput is determined by some stations using low transmission rates due to bad channel conditions. CARA algorithm does not disturb the existing IEEE 802.11b formats and it can be easily incorporated into the commercial wireless local area networks (WLAN) devices. Finally, we verify our findings with simulation.     

噪声环境下IEEE 802.11 DCF的性能仿真及算法改进

在对噪声环境下IEEE 802．11 DCF性能进行仿真分析的基础上，提出了一种适应于无线噪声环境的DCF改进算法，该算法通过接收端反馈消息来区分因冲突和误码产生的数据包传输失败，并在发送端针对不同的传输失败原因分别采取不同的退避算法。仿真结果表明改进算法的性能优于DCF，饱和吞吐量比DCF最多能够提高103％。     

基于加权丢帧率的可调门限速率自适应算法

为了在IEEE 802．11无线网络中根据信道的动态变化恰当地选择传输速率,提出基于加权丢帧率的可调门限速率自适应算法。该机制加权计算丢帧率,自适应调整门限,能克服随机丢帧造成的误判,避免速率频繁波动,使速率选择更准确。仿真结果表明,该机制在Rayleigh信道和Ricean信道中均表现稳定,与其他速率自适应算法相比,吞吐量较优。     

A performance analysis of block ACK scheme for IEEE 802.11e networks

The demand for the IEEE 802.11 wireless local-area networks (WLANs) has been drastically increasing along with many emerging applications and services over WLAN. However, the IEEE 802.11 medium access control (MAC) is known to be limited in terms of its throughput performance due to the high MAC overhead, such as interframe spaces (IFS) or per-frame based acknowledgement (ACK) frame transmissions. The IEEE 802.11e MAC introduces the block ACK scheme for improving the system efficiency of the WLAN. Using the block ACK scheme can reduce the ACK transmission overhead by integrating multiple ACKs for a number of data frames into a bitmap that is contained in a block ACK frame, thus increasing the MAC efficiency.In this paper, we mathematically analyze the throughput and delay performance of the IEEE 802.11e block ACK scheme in an erroneous channel environment. Our extensive ns-2 simulation results validate the accuracy of our analytical model and they further demonstrate that the block ACK scheme enhances the MAC throughput performance at the cost of the resequencing delay at the receiving buffer.     

Channel measurement-based access point selection in IEEE 802.11 WLANs

With the wide deployment of IEEE 802.11 Wireless Local Area Networks, it has become common for mobile nodes (MNs) to have multiple access points (APs) to associate with. With the Received Signal Strength Indicator (RSSI)-based AP selection algorithm, which is implemented in most commercial IEEE 802.11 clients, the AP with the best signal strength is selected regardless of the candidate AP’s available throughput, resulting in unbalanced distribution of clients among the APs in the network. Several studies have shown performance improvement in not just the new MN (nMN), but also the network as a whole when the selection process considers the current load status of candidate APs. However, the proposed algorithms in these studies assume that there are no hidden terminal problems that severely affect the performance of the network. Hidden terminal problems frequently occur in wireless networks with unlicensed frequencies, like IEEE 802.11 in the 2.4 GHz band. Moreover, none of the previous studies have considered frame aggregation, a major improvement in transmission efficiency introduced and widely deployed with the IEEE 802.11n standard. In this paper, we propose a new AP selection algorithm based on the estimation of available throughput calculated with a model based on the IEEE 802.11 distributed coordination function in consideration of hidden terminal problems and frame aggregation. The proposed algorithm is evaluated through extensive simulation, and the results show that the nMN with the proposed AP selection algorithm can achieve up to 55.84% and 22.31% higher throughput compared to the traditional RSSI-based approach and the selection algorithm solely based on the network load, respectively.     

802.11 DCF中的自适应RTS门限调整算法*

通过分析IEEE 802．11 DCF的分组发送过程,获得了RTS和基本方式下分组传输的时间开销。给出了以最小化传输时间开销代价为优化目标的最优RTS门限的计算公式。通过对当前信道分组发送的成功概率P,的预测,实现了最优RTS门限的计算,并给出了一个具体的自适应RTS门限调整算法,使终端能自动调整其RTS门限以达到或接近最优值。仿真表明自适应RTS门限调整算法有效减小了MAC分组传送的时间开销,提高了信道的传输效率。     

Ad Hoc网络中基于双向多信道预留的媒体接入算法

结合多信道和双向信道预留的策略,本文提出了一种新的应用于Ad Hoc网络的多信道媒体接入算法-NBMC算法,并对其吞吐量进行了性能分析。仿真实验结果表明,与IEEE802.11DCF相比,NBMC算法使Ad Hoc网络上的TCP吞吐量和各TCP流信道占用的公平性都得到了有效的改善。     

Distributed channel assignment algorithms for 802.11n WLANs with heterogeneous clients

As the latest IEEE 802.11 standard, 802.11n applies several new technologies, such as multiple input multiple output (MIMO), channel bonding, and frame aggregation to greatly improve the rate, range and reliability of wireless local area networks (WLANs). In 802.11n WLANs, access points (APs) are often densely deployed to provide satisfactory coverage. Thus nearby APs should operate at non-overlapping channels to avoid mutual interference. It is challenging to assign channels in legacy 802.11a/b/g WLANs due to the limited number of channels. Channel assignment becomes more complex in 802.11n WLANs, as the channel bonding in 802.11n allows WLAN stations (APs and clients) to combine two adjacent, non-overlapping 20MHz channels together for transmission. On the other hand, IEEE 802.11n is backward compatible, such that 802.11n clients will coexist with legacy clients in 802.11n WLANs. Legacy clients may affect the performance of nearby 802.11n clients, and reduce the effectiveness of channel bonding. Based on these observations, in this paper, we study channel assignment in 802.11n WLANs with heterogeneous clients. We first present the network model, interference model, and throughput estimation model to estimate the throughput of each client. We then formulate the channel assignment problem into an optimization problem, with the objective of maximizing overall network throughput. Since the problem is NP-hard, we give a distributed channel assignment algorithm based on the throughput estimation model. We then present another channel assignment algorithm with lower complexity, and aim at minimizing interference experienced by high-rate, 802.11n clients. We have carried out extensive simulations to evaluate the proposed algorithms. Simulation results show that our algorithms can significantly improve the network throughput of 802.11n WLANs, compared with other channel assignment algorithms.