ADCF:IEEE 802.11 DCF协议的自适应简便算法

本文提出了一种自适应型IEEE802．11 DCF协议：ADCF。该协议根据网络规模的变化，动态调节RTS／CTS机制的门限值，优化系统性能。在分析DCF协议的基础上，本文给出了一种估算网络规模的简便算法。仿真结果表明，该算法计算准确快捷。     

IEEE802．11无线局域网络性能分析

无线局域网是目前无线通信领域中的主要通信网络之一，如何有效的分析IEE802．11无线局域网中的饱和吞吐率和接入时延一直是无线局域网中的研究重点．本文引入了一种新型的分析模型，分析了IEEE802．11DCF协议下Ad Hoc网络的饱和吞吐量，该模型不仅简单而且准确，有效地降低了分析方法的复杂度；同时本文还建立了一种有效的模型，分析了IEEE802．11DCF协议下Ad Hoc网络的接入时延．仿真结果表明所有的分析结果和仿真结果都很吻合，这说明了我们的分析模型是可行的．     

两种MAC层协议对提高无线自组网QoS的分析比较

介绍了无线自组网络中的分布式资源分配协议和IEEE 802．11e协议，并对两种协议在提高网络的吞吐量和端到端的时延性能等方面做了分析比较，仿真结果表明在轻负载情况下，由于802．11e没有采用节点之间的握手联络信号，具有较低的时延；但由于竞争接入信道时缺乏协调控制，丢包率较高。在重负载情况下，分布式资源分配协议的性能指标均优于802．11e。     

WLAN上的多媒体业务质量研究

随着无线网络的迅速发展，网络上的业务流量与日俱增，如何保证无线局城网实时业务服务质量的问题也越来越突出．文中分别介绍了IEEE802．11中的MAC层协议和802．11e标准中所采纳的改进方式，通过对比分析DCF和EDCF的性能特性，具体仿真时延和丢包率两个性能参数，指出了802．11e标准对MAC层所做的改进．     

IEEE802．11无线局域网EDOF接入研究与仿真

分析了现有IEEE 802．11无线局域网的基本接入机制DCF(Distributed Coordination Function)对QoS(Quality of Service)技术支持的局限性，介绍了未来具有QoS保证的IEEE 802．11e标准采用的一种新的MAC层接入机制EDCF(Enhanced DCF)。对EDCF的性能进行了仿真，并对比原有DCF的性能对仿真结果进行了分析。     

Ad Hoc网络中一种具有优先控制的自适应协同MAC协议

ARQ(Auto Repeat reQuest)机制通常用来在传输失败后重传数据,但是在深衰落信道下,源节点多次重传有可能导致系统性能持续变坏,作为未来无线通信的重要技术,协同通信能够大大提高链路的可靠性,可以有效解决上述问题。本文针对Ad Hoc网络,提出了一种基于优先控制的自适应协同MAC(Media Access Control)协议。此协议采用ARQ机制,根据瞬时信道情况,自适应地选择信道质量更好的中继节点来重传数据,并且在优化的退避算法和中继选择算法的控制下,赋予中继节点相对源节点更高的优先级,协议中还在传统RTS/CTS分组交互基础上加入了新的握手交互机制,使得协议的性能进一步提升。本文还对经典二维Markov模型进行了改进,加入了协同机制并提出中继节点冲突模型,对所提协议进行了建模并推导出了吞吐量性能表达式。仿真结果表明,本文所提协议相对IEEE 80211 DCF协议和传统采用ARQ机制的协同MAC协议有着更好的性能。
      

IEEE 802_11e EDCF饱和状态下性能分析

IEEE 802．11e协议草案以增强型分布式协调功能（EDCF）为基础。用于加强对无线局域网标准IEEE 802.11的QoS支持力度。文章简要介绍了802．11中的分布式协调功能（DCF）和点怫调功能（PCF）。重点阐述了802.11e EDCF的基本原理和组成结构。使用网络模拟工具NS-2对比了DCF和EDCF的吞吐量，并对EDCF在饱和状态下以及站点慢速移动情况下的性能进行了详细的分析和比较。     

IEEE802．11DCF／PCF机制仿真与分析

首先对IEEE802．11的关键技术进行了简要的阐述,包括载波监听碰撞避免机制、隐藏节点和暴露节点问题、RTS／CTS握手机制以及MAC的分布式协调功能和点协调功能。实现了PCF机制的代码并用NS-2对DCF和PCF机制进行了仿真,验证了协议的实现,从时延和吞吐量方面对两个机制的性能进行比较和分析。     

移动Ad Hoc网络中的一种功率控制MAC协议

本文提出了一种应用于移动Ad Hoc网络中的功率控制MAC（medium access contr01）协议，通过两个通信节点之间的控制包中的信息交换来决定数据包的发送功率以及其它邻节点下一次发送RTS控制包的功率。通过仿真与IEEE 802．11 MAC协议进行比较，由仿真结果可以看出，该协议能大大减少移动节点消耗的功率，提高节点的能量利用效率，并保持系统的吞吐量性能。     

IEEE802．11网络VoIP容量估算方法

传统的IEEE802．11无线局域网环境下承栽VolP（Voice over IP）语音业务时,QoS和接入容量都严重受损,成为制约VoWLAN发展的瓶颈。文章以XG729编码的语音包在IEEE802．11b长PPDU模式传输的帧为例,通过分析VoIP包成帧效率、IEEE802．11b成帧效率和DCF协议效率,得到饱和条件下归一化系统吞吐率;同时计算出IEEE802．11a／b协议下VoIP语音理论传输容量,为进一步提升IEEE802．11无线局域网环境下语音传输能力研究提供基础。     

Performance of Energy-Efficient Cooperative MAC Protocol with Power Backoff in MANETs

This paper presents an energy-efficient cooperative MAC (EECO-MAC) protocol using power control in mobile ad hoc networks. Cooperative communications improve network performance by taking full advantage of the broadcast nature of wireless channels. The power control technique improves the network lifetime by adjusting the transmission power dynamically. We propose the best partnership selection algorithm, which takes energy consumption into consideration for selection of the optimal cooperative helper to join in the transmission. Through exchanging control packets, the optimal transmission power is allocated for senders to transmit data packets to receivers. In order to enhance energy saving, space–time backoff and time–space backoff algorithms are proposed. Simulation results show that EECO-MAC consumes less energy and prolongs the network lifetime compared to IEEE 802.11 DCF and CoopMAC at the cost of delay. Performance improvement offered by our proposed protocol is apparent in congested networks where nodes have low and limited energy.     

IEEE 802.11在无线自组网中的应用研究

深入分析了IEEE 802.11 DCF机制应用于无线自组网存在的固有缺陷及潜在原因,并阐明了数据流竞争、物理层机制对MAC协议性能的影响。IEEE 802.11 DCF是针对全连通adhoc网络结构设计的,分析表明,要在实际的多跳无线自组网中应用还存在很多问题需要解决。在此基础上提出了将IEEE 802.11有效应用于多跳无线自组网的进一步研究方向。     

Analysis and determination of cooperative MAC strategies from throughput perspectives

In recent years, cooperative communication has been developed as a new communication strategy that incorporates a relay node to assist direct point-to-point transmission. By exploiting cooperative diversity, different types of techniques have been proposed to improve transmission reliability from the physical layer perspective. However, owing to the longer transmission time resulting from the cooperative schemes, there is no guarantee to enhance network throughput in view of the medium access control (MAC) performance. In this paper, system throughput of combined direct/cooperative communication is evaluated by exploiting the proposed analytical model based on the IEEE 802.11 MAC protocol. The feasibility of adopting either cooperative or direct communication is also studied in the analytical model. In terms of network throughput, whether to adopt cooperative schemes depends on the tradeoff between cooperative transmission delay and channel quality of direct communication. Moreover, two cooperative MAC protocols are proposed to determine the circumstances to activate cooperative communication according to the channel quality. The full-channel quality indicator based cooperative (FCC) MAC protocol is introduced to choose both the transmission scheme and the relay node according to the full channel quality information. However, the overhead caused by the FCC scheme can degrade the throughput performance as the number of available relays is significantly increased. Therefore, the bitwise competition based cooperative (BCC) MAC protocol is utilized to efficiently determine a feasible relay node for data transmission. Simulations are performed to validate the effectiveness of proposed analytical models and cooperative MAC protocols. It is observed that the proposed BCC scheme can outperform both the FCC protocol and conventional direct transmission with enhanced system throughput.     

BTAC: A Busy Tone Based Cooperative MAC Protocol for Wireless Local Area Networks

Cooperative communications has been actively studied as an effective approach to achieve multi-user/spatial diversity gains and better overall system performance by coordinating multiple users in a dynamic wireless network to share their resources and capabilities. Based on the concept of cooperative communications, this paper proposes and analyzes a Busy Tone based cooperative Medium Access Control (MAC) protocol, namely BTAC, for multi-rate Wireless Local Area Networks (WLANs). A cross-layer Markov chain model is then developed to evaluate the performance of BTAC under dynamic wireless channel conditions. Analytical and simulation results show our BTAC protocol is simple, robust, fully compatible with the IEEE 802.11b standard and can achieve better throughput and delay performance than the standard Distributed Coordination Function (DCF) protocol and the recently-proposed CoopMAC protocol.     

IEEE 802．11网络分析模型

IEEE 802.11在MAC层采用DCF作为主要的信道接入方式。本文分析了现有的几种802.11网络分析模型,其中B ianch i模型很好的描述了饱和状态下802.11 DCF的性能;X iao模型针对802.11e进行了多优先级的扩展,实现了EDCF的性能分析。最后介绍了一种新的分析模型,新模型同时考虑了业务优先级和内部调度算法,够较准确地描述网络性能。     

Batch and Sequential Bayesian Estimators of the Number of Active Terminals in an IEEE 802.11 Network

The performance of the IEEE 802.11 protocol based on the distributed coordination function (DCF) has been shown to be dependent on the number of competing terminals and the backoff parameters. Better performance can be expected if the parameters are adapted to the number of active users. In this paper we develop both off-line and online Bayesian signal processing algorithms to estimate the number of competing terminals. The estimation is based on the observed use of the channel and the number of competing terminals is modeled as a Markov chain with unknown transition matrix. The off-line estimator makes use of the Gibbs sampler whereas the first online estimator is based on the sequential Monte Carlo (SMC) technique. A deterministic variant of the SMC estimator is then developed, which is simpler to implement and offers superior performance. Finally a novel approximate maximum a posteriori (MAP) algorithm for hidden Markov models (HMM) with unknown transition matrix is proposed. Realistic IEEE 802.11 simulations using the ns-2 network simulator are provided to demonstrate the excellent performance of the proposed estimators     

基于网络编码的协作HARQ协议

该文基于网络编码技术提出了一种新的协作HARQ协议。在中继节点的协助下两个用户与基站进行通信。中继节点响应基站的重传请求,对两用户的增加冗余帧采用物理层网络编码即信号重叠调制技术进行处理,然后发送至基站。基站对接收信号采用多用户联合检测以及联合软判决译码。在平坦瑞利衰落信道下,该网络编码协作HARQ协议较传统的非协作HARQ协议可获得较大的性能改善,其性能接近没有采用网络编码的协作HARQ协议,且需要较少的硬件和带宽资源。     

Space and network diversity combination for masked node collision resolution in wireless ad hoc network

In wireless ad hoc networks, the traditional carrier sensing multiple access/collision avoidance protocol cannot solve the masked node problem, which affects the network performance greatly. Our proposed collision separation technique overcomes the shortcoming of the IEEE 802.11 request-to-send-clear-to-send handshake by combining the space diversity provided by the antenna array and network diversity provided by the medium access control layer. In this work, the colliding packets caused by masked nodes are not discarded but stored and combined with the selected retransmission packets to separate the data from different nodes. The steady states of the nodes in the network are analyzed via a Markov chain model. The network throughput and delay performance are also investigated. Compared to network assisted diversity multiple access, our proposed method can provide significantly higher throughput and lower delay     

Relay-based cooperative MAC protocol with rate adaptation

The existing wireless cooperative MAC protocols rarely consider the impact of channel condition and node energy consumption.To this issue,a new wireless cooperative MAC protocol was proposed,which used the random linear network coding,and introduced a mechanism of request relay frame to solve the hidden terminal problem.Specifically,the proposed method adaptively adjusted the node rate according to RSSI,and determined the relay node based on the requirements of cooperative transmission.The simulation results show that the network performance can be improved through deploying the proposed protocol.     

Incomplete cooperation‐based service differentiation in WLANs

In the IEEE 802.11 wireless LAN (WLAN), the fundamental medium access control (MAC) mechanism—distributed coordination function (DCF), only supports best‐effort service, and is unaware of the quality‐of‐service (QoS). IEEE 802.11e enhanced distributed channel access (EDCA) supports service differentiation by differentiating contention parameters. This may introduce the problem of non‐cooperative service differentiation. Hence, an incompletely cooperative EDCA (IC‐EDCA) is proposed in this paper to solve the problem. In IC‐EDCA, each node that is cooperative a priori adjusts its contention parameters (e.g., the contention window (CW)) adaptively to the estimated system state (e.g., the number of competing nodes of each service priority). To implement IC‐EDCA in current WLAN nodes, a frame‐analytic estimation algorithm is presented. Moreover, an analytical model is proposed to analyze the performance of IC‐EDCA under saturation cases. Extensive simulations are also carried out to compare the performances of DCF, EDCA, incompletely cooperative game, and IC‐EDCA, and to evaluate the accuracy of the proposed performance model. The simulation results show that IC‐EDCA performs better than DCF, EDCA, and incompletely cooperative game in terms of system throughput or QoS, and that the proposed analytical model is valid. Copyright © 2010 John Wiley & Sons, Ltd.