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。     

一种基于空域的MIMO系统链路自适应策略研究

提出基于空域上联合SM和TD的LA策略。该策略通过检测后的符号SNR来自适应搜寻满足服务质量要求的最优空间分组方案,天线组内部使用空时分组编码(STBC),组间进行复用。仿真结果表明,同基于时域的LA策略相比,该策略能充分利用SM与TD在时域及空域的性能折中,具有更优的吞吐量性能,且复杂度较低。     

WLAN信道分配方法研究

随着时代的发展,传统有线局域网已无法满足随时随地的自由网络通信。针对有线局域网的不足,无线局域网顺势诞生,并快速进入应用阶段。全球第一个无线局域网标准是IEEE802.11标准,它是由IEEE的无线局域网委员会于1997年6月制定的。通常来说802.11协议是在2.4GHz的ISM频段上工作,信道数目相当有限。根据无线通信广播的特性,AP间的高密度状态会产生干扰、竞争的情况,降低网络整体质量,且伴随着无线局域网的继续高速增长,上述问题将愈来愈严重。全文由802.11协议出发,先从理论角度分析了产生信道分配问题的原因所在,通过对接入点间的干扰情况分析说明了信道分配算法需解决的问题,最后提出了改进型算法,特点:1)分布式、自适应;2)改进了协议扫描功能,使用嗅探技术扩大了检测干扰的范围,使扫描效率得到提升;3)使用退避机制减少信道分配冲突消耗,提高了算法收敛速度。NS-2(Network Simulator version 2,NS-2)是一种针对网络技术源代码公开的免费软件模拟平台,该文用其进行了基本实验,证明了信道分配方法的效果。     

基于链路层的WLAN加密与认证安全研究

随着网络安全的发展和无线局域网络的应用,WLAN的安全越来受到重视。文章分析了WLAN网络链路层的安全问题,结合无线局域网络安全标准802．1li,对WLAN网络中加密与认证技术中的SSID、MAC、WEP、WPA、WPA2等安全技术问题进行了介绍,并重点阐述了安全技术的应用选择方法,最终得出了在加密技术成本、安全需求成本都能兼顾的情况下应该选择WPA2加密技术来保障无线局域网络安全的结论。     

WLAN Mesh网络多信道速率自适应MAC协议

针对多种传输速率的链路共存于同一信道时引发的性能瓶颈,以及单接口多信道网络中接收端忙的问题,在DCF协议的框架内提出一种适用于WLAN Mesh网络的分布式多信道速率自适应媒体控制接入(MAC)协议。该协议允许发送节点的邻居节点通过协作应答的方式告知相应接收节点所处信道,接收节点根据当前信道质量合理选择传输速率和传输信道反馈给发送节点。通过上述机制,将不同传输速率的链路分配在不同的信道上。仿真结果证明,该协议能避免不同传输速率链路之间的相互干扰,解决接收端忙问题。与现有典型多信道速率自适应MAC协议相比,能有效提高网络的总吞吐量。     

无人机中继链路传播损耗及性能分析

针对两跳无人机中继通信场景,综合考虑机身姿态、天线特性、气候状况及信道衰落等影响,建立了两跳中继链路的传播损耗模型,分析了传播损耗均值的计算方法,推导了多径阴影复合衰落导致传播损耗随机起伏的统计特性。在此基础上,进一步推导得到了两跳无人机中继系统的中断概率与误比特率的理论表达式。最后,通过对郊区、山区和海洋场景的数值仿真,验证了本文推导的理论结果的正确性,并分析了无人机飞行高度、地面节点位置以及通信场景等因素对两跳中继系统性能的影响。     

宽带无线通信系统中的链路自适应技术研究

链路自适应算法利用统计的包错误率（PER）进行自适应的调整调制编码的方式，以使系统具有更高的吞吐量的同时，提高系统频谱的利用率，更好的满足未来宽带无线通信的传输要求。同时具有较小的时延抖动，适合TCP业务的传输。     

提高基于WLAN的CBTC无线链路可靠性方法

针对现存的基于通信的列车控制系统(Communication Based Train Control,CBTC)的无线链路在隧道中由于多径效应导致的不可靠,本文对CBTC无线通信采用的802.11g的OFDM技术进行分析,提出一种多子载波冗余传输方法。数值仿真分析表明,该方法从物理层上加强了无线链路通信的可靠性。     

基于直接链路协议的无线局域网络及其安全策略

无线局域网(WLAN)的直接链路协议(PLD)是WLAN服务质量(QoS)的重要组成部分。文章首先描述了WLAN的发展及在无线通信领域的重要作用;接着研究分析了WLAN网络中的新型协议———直接链路协议(DLP)的工作原理;在研究了无线局域网安全协议之后,针对DLP协议带来的安全问题,提出了基于DLP协议的安全算法及认证机制;该方法不仅保证了直接链路数据通信的安全性,同时可以减少接入点(AP)对移动主机过多管理带来的开销以及信息传输的时延。     

一种WLAN信道传输干扰预测方法

无线接入点(AP)在公有或私有区域的不合理规划部署容易导致基本服务集(BBS)相互重叠,增加信道干扰。在帧传输前,如何准确预测信道中潜在的干扰因素并采取应对策略,成为WLAN通信技术领域的研究热点。为此提出一种WLAN信道传输干扰预测方法,即在传输期间结合块确认帧实时采集并统计A-MPDU帧传输状态信息,计算干扰发生的概率,预测信道传输干扰因素并采取有效的应对策略。仿真结果表明,所提出的方法不仅能准确预测信道传输干扰因素,还能在多种拓扑环境下进一步利用网络带宽,提升帧传输效率。     

Dual‐port MIMO dielectric resonator antenna for WLAN applications

A dual‐port multiple‐input multiple‐output (MIMO) dielectric resonator antenna (DRA) for 5 GHz IEEE (802.11a/h/j/n/ac/ax) is discussed in this article. Two prototypes of single feed DRA and dual feed MIMO DRA are fabricated and measured results are compared with the simulated data. The proposed single feed DRA and dual feed MIMO DRA exhibits wide impedance bandwidth (IBW). Antennas have been fabricated on Rogers RT Duroid substrate with Eccostock made DRA placed over the substrate. DRAs are excited by aperture coupled feed to achieve wide bandwidth and high efficiency. The measured IBW of uniport DRA and dual‐port MIMO DRA are 26.6% (4.75‐6.21 GHz) and 27.5% (4.7‐6.2 GHz) respectively. Maximum gain of the antenna is 7.4 dBi. The results of the antennas are in good agreement with simulated data and they are suitable for WLAN applications. These antennas are also compact with area of substrate 32.8 cm².     

Design and performance investigation of a low profile MIMO/Diversity antenna for WLAN/WiMAX/HIPERLAN applications with high isolation

A low profile triband compact multiple input multiple output (MIMO) antenna operating at WLAN, WiMAX, and HIPERLAN bands is presented. The proposed MIMO antenna consists of two planar inverted‐F antenna elements located at the top two corners of printed circuit board (PCB). Dimensions of each antenna elements are reduced substantially by employing a meandered line and folded patch structure so that it occupies a small volume of 9 × 8.8 × 5.4 mm³. The proposed antenna consists of three arms namely, Main arm, Side arm 1, and Side arm 2. Each individual arm resonates corresponding to the λ/4 electrical length. Characterization of the antenna is carried out in the mobile environment as well as in user proximity. In the presence of mobile environment which includes liquid crystal display (LCD), Battery, RF components, and plastic housing, the isolation as well as reflection coefficient parameters deteriorated. To avoid the aggravation of S‐parameters, two nonradiating folded shorting strips are connected between each antenna element and ground plane of PCB. This folded shorting strip not only improves the isolation between ports but also prevent the deterioration of reflection coefficient parameter. The total efficiency, envelope correlation coefficient, and multiplexing efficiency are studied in the user proximity. The optimized structure is fabricated and measured. The measured S‐parameters cover WLAN (2.46–2.6 GHz), WiMAX (3.37–3.75 GHz), and HIPERLAN (5.2–5.87 GHz) based on ?10 dB reflection coefficient and ?24 dB isolation is achieved between antenna ports. Good agreement is obtained between the simulated and measured results. © 2015 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:510–521, 2015.     

Transmission scheduling algorithm for MIMO link ad hoc networks

Most reported works on transmission scheduling algorithm for multiple input multiple output (MIMO) link ad hoc networks do not fully exploit the spatial multiplexing potential inherent to multiple an-tennas system.In this paper,a novel transmission scheduling algorithm is proposed to further enhance the performance of MIMO link ad hoc networks.By utilizing the MIMO multi-stream reception capabilities and ordering the nodes’ allocation sequence,the proposed algorithm can dramatically shorten the scheduling frame length and more evenly distribute the data streams allocated to the network nodes.Meanwhile,allowing the nodes to transmit multiple streams in a slot,the algorithm can fully exploit the MIMO spatial multiplexing potential.Additionally,we give the theoretical analysis to quantify the throughput and delay performance with respect to this scheme.Numerical results demonstrate that the proposed algorithm can efficiently enhance the network throughput and reduce the average and maximum delay.     

基于802.11n的WLAN安全解决方案研究

随着无线局域网技术的飞速发展,IEEE 802.11n是无线局域网技术的最新标准,介绍了基于802.11n的MIMO(多入多出)、OFDM(正交频分复用)和双频带等无线通信系统的关键技术,分析了在802.11n标准下WLAN的安全问题,通过数据加密、双向认证、动态WEP密钥和虚拟专用网络等方法,提出了基于802.11n的WLAN安全解决方案。     

基于WAPI的无线局域网直连技术研究和设计

在无线局域网中同一个基本服务集下的两个终端之间经常需要进行大量数据的交换,由此导致整个网络性能下降以及网络的安全问题,通过分析无线局域网鉴别和保密基础结构和IEEE 802.11z中隧道直接链接设置的特点.提出了一种基于无线局域网鉴别和保密基础结构的,具有直连功能的设计方案.方案中的直连功能是通过对无线局域网中基本服务集和独立基本服务集两种模式之间进行快速转换而实现的.在支持无线局域网鉴别和保密基础结构的网卡程序中使用软件实现设计方案,并对软件进行测试.     

MIMO链路ad hoc网络传输调度算法

针对MIMO链路ad hoc网络中已有传输调度算法未充分利用MIMO空间复用能力的问题,提出了一种新的传输调度算法.该算法通过考虑MIMO的多流接收能力而有效地减小了调度帧长,通过合理安排节点分配顺序使得节点分配的数据流比较平均,同时,该算法通过允许节点发送多个数据流而高效利用了MIMO的空间复用能力.推导分析了传输调度算法的通过量和时延性能,结果表明,与已有算法相比,采用文中算法可以提高网络的通过量,减小网络的平均时延和最大时延.     

Dielectric resonator based two‐port dual band antenna for MIMO applications

This article investigates a dual band multiple input multiple output (MIMO) cylindrical dielectric resonator antenna (cDRA) for WLAN and WiMAX applications. It consists of two symmetrical orthogonally placed radiators. Each radiator is excited through a narrow rectangular aperture with the help of a microstrip line. For higher mode excitation, the proposed structure uses dual segment DRA which apparently looks like stacked geometry. The aperture fed dielectric resonator works as a feed for upper cDRA to generate higher order mode. The presented radiator covers the band between 3.3‐3.8 GHz and 5‐5.7 GHz. The measured isolation is better than 20 dB in the desired band. The average gain and radiation efficiency achieved for the proposed antenna is 6.0 dBi and 85%, respectively at the operating frequency band. In the proposed geometry, broadside radiation patterns are achieved by exciting HEM_11δ and HEM_12δ modes in a stacked geometry. Different MIMO performance parameters (ECC, DG, MEG, and CCL) are also estimated and analyzed. The prototype of proposed antenna is fabricated and tested. The measured outcomes are in good accord with the simulated one.     

Dual polarized triple band hybrid MIMO cylindrical dielectric resonator antenna for LTE2500/WLAN/WiMAX applications

In this communication, triple band hybrid multi‐input–multi‐output (MIMO) cylindrical dielectric resonator antenna (CDRA) with high isolation is examined. The proposed MIMO antenna includes two symmetric folded microstrip line feeding structures along with CDRA at two different ends of substrate. Two inverted L‐shaped strips on the ground plane are used to enhance the isolation (S₁₂ < ?15 dB) as well as to generates 2.7 GHz frequency band. Metallic strip on the ground plane act as an electromagnetic reflector and also enhance the isolation between two antennas (S₁₂ < ?20 dB). Archetype of proposed MIMO antenna design has been fabricated and tested to validate the simulated results. The proposed antenna operates at three different frequency bands 2.24–2.38 GHz, 2.5–3.26 GHz, and 4.88–7.0 GHz (S₁₁ < ?6 dB) with the fractional bandwidth 6.06%, 26.4%, and 35.7%, respectively. Folded microstrip lines generate path delay between the electric field lines and originate circular polarization characteristics in the frequency range 5.55–5.75 GHz with the fractional bandwidth of 3.55%. In order to satisfy the different performance requirement of MIMO antenna such as envelop correlation coefficient, mean effective gain, effective diversity gain, peak gain are also examined. The proposed antenna is found suitable for LTE2500, WLAN, and WiMAX applications. © 2016 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2016.     

Reduced mutual coupling of compact MIMO antenna designed for WLAN and WiMAX applications

This communication describes a simple compact wide band multiple input multiple output (MIMO) antenna for Wireless Local Area Network (WLAN) and Worldwide Interoperability for Microwave Access (WiMAX) applications. The proposed antenna is integrated with an electromagnetic band gap (EBG) structure which is used to reduce the mutual coupling between the ports. The structure is excited by a line feed mechanism and investigated experimentally. The antenna covers the frequency range from 2.01 to 3.92 GHz with the corresponding fractional bandwidth of 64.42%. It fulfills the bandwidth requirements of WLAN (2.35‐2.5 GHz) and WiMAX (3.2‐3.85 GHz) bands where minimum port isolation is obtained around 29 dB throughout the entire application band. The proposed MIMO antenna has very low envelope correlation co‐efficient (ECC < 0.01) and high diversity gain (DG > 9.8). It also has very low channel capacity loss (CCL) which is found to be less than 0.2 Bit/s/Hz. The simulation results are compared with the measurement outcomes and found a good agreement between them.