UWB和802.11a WLAN的兼容性分析

研究了超宽带(UWB)通信系统与基于IEEE802.11a标准的WLAN设备之间的兼容问题。介绍了UWB和WLAN的理论基础,给出了干扰信号对系统SIR(信号与干扰的比率)影响的仿真结果。结果显示,在视距(LOS)环境中,UWB干扰对802.11a系统的性能影响不大;在非视距(NLOS)环境中,UWB信号对802.11aWLAN系统的性能有很大影响,而802.11aWLAN干扰也会严重影响UWB接收机的性能。对研究UWB系统与其它通信系统的共存有一定的参考价值。     

露天煤矿临近车辆检测ad hoc协议测试与选择

通过分析目前临近车辆检测技术,即超声波传感技术、脉冲雷达和激光探测技术、计算机视觉技术、RFID和电子标签技术、GPS技术,指出GPS集成无线通信技术最适合应用在露天煤矿临近车辆检测中;提出一种GPS集成ad hoc的临近车辆检测无线通信系统:只要相邻车辆处于通信距离范围内,车辆间便可采用IEEE 802.11协议,以单跳或多跳的方式交换GPS定位数据。以该系统为研究对象,模拟真实的露天煤矿场景,测试分析了IEEE 802.11a/b/p三种版本的通信协议在视距传输、非视距传输、接收机在斜坡底部和接收机在斜坡顶部4种不同场景下的信号接收强度和有效通信距离。测试结果表明,在以上4种不同场景下,IEEE 802.11b协议的数据包接收率更高、有效通信距离更远,优于IEEE 802.11a协议和IEEE 802.11p协议,更适合露天煤矿临近车辆检测。     

基于WLAN快速移动接入仿真研究

IEEE 802.11（WLAN）无线局域网标准原本是为提供室内外固定接入网络服务而设计的,由于大范围无线服务应用的推动,其在快速移动环境下的接入应用需求也随之产生。通过Matlab仿真工具,对WLAN快速移动接入进行了仿真分析,研究了移动速度、数据发送速率、接收信噪比等因素对移动接入性能的影响,分析得出WLAN的移动接入应用关键是在高移动速度与高数据速率之间做出优化选择,最后提出了WLAN移动接入应用方案。     

在OFDM系统中应用时间分集对抗快衰落

针对快衰落使得OFDM系统的误码率出现严重的地板效应这一问题,在广泛用于非视距移动环境建模的Jakes谱快衰落Rayleigh信道的条件下,对Doppler扩展所造成的子载波干扰进行了理论分析,并且对在快衰落信道条件下OFDM系统应用时间分集来提高系统性能进行了分析和仿真。     

存在隐藏终端的IEEE 802.11e多跳无线网络模型分析*

首次将802.11e的接入机制放入多跳环境中进行仿真分析和定量研究。提出一种新的多跳环境下802.11e网络模型,结合M/G/1/K排队模型,定量分析在隐藏终端影响下802.11e网络的MAC层吞吐率、MAC时延和帧丢失率,研究802.11e在多跳环境下性能表现的内在原因。经过仿真实验结果与数值分析结果的对比,验证了分析模型的准确性;通过有（无）隐藏终端影响下MAC层性能的对比分析,指出了802.11e在多跳无线网络中支持QoS的局限性：受隐藏终端的影响,802.11e对不同接入等级的业务提供QoS区分的性能明显降级。因而有必要研究隐藏终端问题的解决方案,提高802.11e在多跳环境下的性能。     

存在隐藏终端的IEEE802.11e多跳无线网络模型分析

首次将802.11e的接入机制放入多跳环境中进行仿真分析和定量研究。提出一种新的多跳环境下802.11e网络模型,结合M/G/1/K排队模型,定量分析在隐藏终端影响下802.11e网络的MAC层吞吐率、MAC时延和帧丢失率,研究802.11e在多跳环境下性能表现的内在原因。经过仿真实验结果与数值分析结果的对比,验证了分析模型的准确性;通过有(无)隐藏终端影响下MAC层性能的对比分析,指出了802.11e在多跳无线网络中支持QoS的局限性:受隐藏终端的影响,802.11e对不同接入等级的业务提供QoS区分的性能明显降级。因而有必要研究隐藏终端问题的解决方案,提高802.11e在多跳环境下的性能。     

IEEE 802.11e EDCA在Ad Hoc网络中应用仿真分析

IEEE 802.11e EDCA协议通过采用MAC层业务区分的方式,提供网络的QoS支持.分析并比较了802.11e EDCA对802.11 DCF的改进,并通过NS2仿真分析EDCA在Ad Hoc网络中的应用情况,进一步讨论并仿真实现了对EDCA协议参数的分析.仿真结果表明,EDCA在多跳Ad Hoc网络中仍保持了其有效性,实现业务区分并提高了网络的吞吐量等性能,同时协议参数的不同将极大的影响系统性能,通过优化设置参数可以进一步提高系统性能.     

非视距环境下基于TOA的移动目标定位

本文研究了非视距(Non-Line-of-Sight, NLOS)环境下基于到达时间(Time-of-Arrival, TOA)的移动目标定位问题。假定移动目标速度在足够短的时间内为常数，则移动目标定位问题可转化为其初始位置和速度的估计问题。为降低非视距误差的影响，本文构造了约束最小二乘(Least Squares, LS)问题对移动目标的初始位置、移动速度和非视距误差进行联合估计，并通过合理近似减少了估计变量个数。由于所构造的约束LS问题为非凸优化问题，其全局最优解难以获得。为近似求解该问题，本文对其进行松弛，以转化为凸的半正定规划(Semidefinite Programming, SDP)问题。与现有方法相比，该方法不需要已知非视距误差的任何统计信息和路径状态。仿真结果表明，该方法缓解了非视距误差产生的负面影响，且在稀疏和密集的非视距环境下都取得了良好的性能。     

校园无线局域网性能优化途径之初探

系统吞吐率是衡量网络性能的最重要的指标之一。无线局域网不同标准的选择、无线网络标准的单一IEEE802.11g模式和IEEE802.11g/b双模式、接入点AP设备的数量以及定位原则直接影响吞吐率的提高,围绕上述三个因素概括性的阐述了校园无线网络性能优化的措施。     

不同Mac接入机制的AODV路由协议性能分析

为了提高网络的综合性能,充分考虑了数据链路层和网络层的影响因素。对比分析了不同Mac接入协议的切换方法,为无线自组网络选择了一个合适的路由和接入协议。通过在NS-2中仿真Mac层的802.11a和802.11Ext路由场景,对AODV协议的归一化路由开销、路由发现频率、端到端平均时延以及数据分组投递率进行了全面的分析比较。结果表明,802.11Ext接入协议具有明显的优势特点,更加稳定、更适合移动网络。     

A multirate MAC protocol for reliable multicast in multihop wireless networks

Many multicast applications, such as audio/video streaming, file sharing or emergency reporting, are becoming quite common in wireless mobile environment, through the widespread deployment of 802.11-based wireless networks. However, despite the growing interest in the above applications, the current IEEE 802.11 standard does not offer any medium access control (MAC) layer support to the efficient and reliable provision of multicast services. It does not provide any MAC-layer recovery mechanism for unsuccessful multicast transmissions. Consequently, lost frames cannot be detected, hence retransmitted, causing a significant quality of service degradation. In addition, 802.11 multicast traffic is sent at the basic data rate, often resulting in severe throughput reduction.In this work, we address these issues by presenting a reliable multicast MAC protocol for wireless multihop networks, which is coupled with a lightweight rate adaptation scheme. Simulation results show that our schemes provide high packet delivery ratio and when compared with other state-of-the-art solutions, they also provide reduced control overhead and data delivery delay.     

WLESS-frame: a simulation-based development environment for 802.11 stations

The IEEE 802.11 protocol is one of the most widely accepted standards for wireless local area networks (WLANs). WLESS-frame is a novel development environment for 802.11 stations. It is based on a complete model of the medium access layer (MAC) and of the direct sequence spread spectrum (DSSS) physical layer. The tool is aimed at supporting hardware/software co-design and validation of 802.11 stations, also analyzing the behaviour of a station in a network. The paper describes the implementation of the tool, its applicability and the performance analysis of a network under various workload conditions.     

一种基于信噪比的自组网功率控制算法

能量是影响无线自组网性能的一个很大的瓶颈,作为事实上的无线自组网媒体接入协议,802.11并没有动态调整传输功率的能力,网络在实际应用中的节能效果不好.为了降低网络节点的能量消耗,在媒体接入层802.11协议基础上,,提出一种功率控制算法,以物理层信噪比为基础估计控制帧和数据帧的发送功率.通过OPNET网络仿真软件对算法进行了计算机仿真,仿真结果表明.相比802.11协议,算法有效降低了节点能量消耗,提高了能量利用率,延长了网络的生存时间,同时保持了网络的吞吐量性能,对自组网的理论研究和实际应用提供了一定的参考价值.     

The status and future of 802.11-based WLANs

The main attraction of WLANs is their flexibility. They can extend access to local area networks, such as corporate intranets, as well as support broadband access to the Internet - particularly at "hot spots," public venues where people tend to gather. WLANs can provide quick, easy wireless connectivity to computers, machinery, or systems in a local environment where a fixed communications infrastructure does not exist or where such access is not permitted. These hosts can be stationary, handheld, or even mounted on a moving vehicle. Bandwidth considerations have thus far been secondary in WLAN design and implementation: the original 802.11 standard allowed a maximum channel bit rate of only 2 megabits per second, while the current 802.11 b standard supports an 11 Mbps maximum rate. However, the widespread deployment of 802.11a and 802.11g standards, which allow a bit rate of up to 54 Mbps, will pave the way for new types of mobile applications, including m-commerce transactions and location-based services.     

NDIS-based virtual polling algorithm for IEEE 802.11b for guaranteeing the real-time requirements

This paper focuses on the feasibility of IEEE 802.11b wireless LAN for real-time wireless industrial network. In general, it has been known that IEEE 802.11b is not suitable for industrial networking because its medium access control method is the contention-based CSMA/CA (Carrier Sensing Multiple Access with Collision Avoidance) that exhibits unstable performance under heavy traffic and unbounded delay distribution. As an attempt to circumvent the probabilistic nature of IEEE 802.11b, this paper presents an enhanced four-layer architecture using the Network Driver Interface Specification (NDIS) and a virtual polling algorithm. Also, the enhanced four-layer architecture's performance is compared with that of conventional IEEE 802.11b. Based on the experimental results, it is found that the enhanced four-layer architecture for IEEE 802.11b is a very promising alternative for wireless industrial networking.     

IPv6 based relay gateway protocol in hybrid networks

During the past few years, WMAN (Wireless Metropolitan Area Network) and WLAN (Wireless Local Area Network) provided fast, convenient and reliable solution for all kinds of users. With the development of the communication technology, the future wireless networks will allow mobile users access Internet anywhere, anytime. Therefore, it is a natural trend to combine different wireless networks to provide a seamless roaming for users. It is feasible for mobile terminal which have multiple modes to handoff between different networks. A relay gateway protocol for the WLAN-mode nodes (mono-mode WLAN nodes and dual-mode nodes) in the hybrid network of IEEE 802.16 WMANs and IEEE 802.11 WLANs is proposed to provide wider mobility. Dual-mode mobile nodes with good service can provide WLAN links to nearby WLAN-mode nodes with bad service or out of the WLAN coverage. Simulation results show that the relay gateway scheme can improve network performance of the hybrid networks. Moreover, the relay gateway method can bring additional service coverage and increase the capacity of WMANs and WLANs.     

Design and evaluation of a novel MAC layer handoff protocol for IEEE 802.11 wireless networks

In recent years, the IEEE 802.11 wireless network family has become one of the most important set of standards in the wireless communications industry. IEEE 802.11 compliant devices are inexpensive and easier to configure and deploy than other wireless technologies. In an IEEE 802.11 wireless network, wireless terminals can move freely. As a result, when the wireless terminal moves away from its current access point, it must switch to another access point to maintain the active connection. This is known as the MAC layer handoff process. MAC layer handoff latency should be minimized to support real-time applications and to provide mobile devices with seamless roaming in IEEE 802.11 wireless networks. This paper proposes a novel MAC layer handoff protocol over IEEE 802.11 wireless networks by introducing advertisement messages sent from other mobile nodes and from which wireless terminals are able to receive the information of access points in their neighborhood. A mobile node can try to associate with access points based on the prediction before starting the probe process. The experimental results demonstrate that our solution can reduce MAC layer handoff latency to meet the requirements of real-time applications.     

A novel Effective Capacity-based framework for providing statistical QoS guarantees in IEEE 802.11 WLANs

This article proposes a performance model of the IEEE 802.11 MAC layer that employs the notion of Effective Capacity. In particular, the paper establishes that an IEEE 802.11 mobile station can be regarded as a Semi-Markovian bursty server of the On/Off type, with known distributions for the On and Off periods, and subsequently applies known results for Semi-Markovian models to derive the Effective Capacity function of this On/Off server. The general Effective Bandwidth/Capacity theory can then be used for computing buffer overflow probabilities and for employing simple traffic control policies to enforce related QoS guarantees. The policies guarantee a soft bound on the buffer overflow probability and are suitable for real-time traffic control over WLANs. The Effective Capacity model of IEEE 802.11 stations is originally developed by assuming that the other competing stations are saturated. This is a conservative assumption that becomes very accurate in a highly loaded network. Subsequently, the model is adapted to encompass lightly loaded networks as well. In the adapted model, each mobile station directly measures a few model parameters, instead of calculating them on the basis of the saturation assumption, and uses these measurements in the computation of its Effective Capacity function. The theoretical results are checked against simulations, validating the appropriateness of the model.     

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.     

支持移动学习的无线校园网建设

随着移动技术以及数字化技术的飞速发展和广泛应用,各类便携式、多功能、可移动智能终端的普及人们对移动学习和移动办公提出了更高的要求,建设新一代支持移动学习的无线校园网应运而生.研究支持移动学习的无线校园网建设规划和设计,探讨无线校园网IPv6的支持,并提出移动学习应用需求.