无线数据通信网中的信道接入协议研究

信道接入协议（MAC）是无线数据通信协议的重要组成部分。由于无线信道是一个开放、共享的信道，网络节点在通信过程中存在隐终端和暴露终端问题。基于目前无线通信常用的MAC协议存在的问题，着重分析了基于扩频码的信道接入协议，该协议利用扩频码的正变特性，使得网络中各节点可以同时占用信道而互不干扰，较好地解决了信道争用问题，提高了系统效率。同时指出，在选择MAC协议时，应根据无线网络的拓扑结构及特点灵活选择，以提高网络性能。     

适用于IEEE 802.11a/g的联合信道估计和均衡算法

基于IEEE 802.11a/g协议,提出了一种联合信道估计和均衡的算法,该算法采用自适应信道估计方式在低信噪比情况下实现精确的信道估计,并且具有较低的设计复杂度。信道均衡采用频域内MMSE(FD-MMSE)均衡方式,与自适应信道估计配合在系统性能和计算复杂度方面取得较好折中。仿真表明该算法结构性能满足IEEE 802.11a/g协议规定,与同类算法相比在低信噪比区域提高系统性能的前提下算法的计算复杂度也有所降低。     

基于DNS协议的隐蔽信道研究

隐蔽信道能够以危害系统安全策略的方式传输信息,目前,基于网络协议的隐蔽信道研究已成为热点。域名系统协议（Domain Name System,DNS）用于将主机名字和IP地址之间的转换,是双向协议,互联网正常运行离不开DNS协议,因此可以基于DNS协议建立隐蔽信道。文中首先介绍隐蔽信道、DNS隐蔽信道的概念和原理,搭建DNS隐蔽信道系统,然后演示了DNS隧道工具的使用方法,最后针对现有的DNS隐蔽信道工具提出了几点改进措施,使DNS隐蔽信道数据传输更加高效。     

基于BATMAN-adv的多跳无线网络

无线自组网具有去中心化、多信道、多跳等特点,适用于专网和临时应急网络的快速搭建。为了使网络具有更宽的覆盖、更强的绕射能力和更高的吞吐量,系统选择一种支持IEEE802.11 ac的三射频接口物理层节点,在此基础上,结合BATMAN-adv路由协议进行研究,实现了一种基于多接口多信道的自组网系统。仿真实验数据表明,该系统在5G频谱中能够全双工使用正交信道降低干扰,将网络链路拓展至5跳以上,网络损耗衰减低于30%,网络吞吐量高于300 Mbps。     

基于OFDM系统的随机信道接入方案

双忙音多址信道接入(DBTMA)协议是一种信道利用率较高的无线局域网信道随机接入控制方案。但是, 该协议的硬件实现复杂度较高。该文在对DBTMA协议进行简单修改的基础上,结合节能意识多址接入(PAMAS) 协议提出了一种基于OFDM系统的DBTMA信道接入系统,并对此系统做了简要分析。分析结果显示,该文提出的随机信道接入系统的性能良好且实现方式简单。     

无线Mesh网络多信道MAC协议

分析了WMN多信道MAC协议设计所面临的问题；介绍了几种现有典型的多信道MAC协议，并对其进行了分析；对多信道MAC协议未来的发展作了简要的总结。期望文章使读者对WMN多信道MAC协议的关键技术，特别是协议的设计有一个概括性的了解。     

CDMA20001x EV-DO 分层协议解析

本文主要研究CDMA 1x-EVDO中控制信道、前/反向业务信道、接入信道的分层协议结构，并依照协议采用由低至高逐层分析的方法对控制信道进行解析，通过实验室仪器产生信号验证该解析方法的正确性。     

基于载波监听的Ad Hoc网络双信道接入协议研究

双信道接入协议在解决隐终端和暴露终端问题上具有独特的优势.基于报文监听的Ad Hoc网络双信道接入协议无法消除数据报文的冲突.本文对基于载波监听的Ad Hoc网络双信道接入协议进行了研究,提出了DCMA载波监听信道接入协议.其中DCMA_CSBI通过载波监听和BI控制报文完全解决了隐终端和暴露终端问题.文章通过仿真对DCMA协议的性能进行了分析和比较.分析结果表明DCMA_CSBI是一种非常高效实用的Ad Hoc网络双信道接入协议,载波监听对Ad Hoc网络信道接入协议而言是至关重要的.     

新的半双工中继协议及其资源分配

传统的基于放大转发或解码转发的时分半双工中继协议避免了对于中继节点的全双工要求,却带来了频谱效率的降低.基于广播信道及多址接入信道的概念,提出了一种新的半双工中继协议,并对该协议下的资源分配问题进行了研究.结果表明,与基于放大转发或解码转发的半双工中继协议相比,新的协议能够提高频谱效率.     

Ad hoc网络技术讲座第1讲Ad hoc网络的体系结构和信道接入协议

本文首先介绍Ad hoc网络的概貌，然后分析Ad hoc网络的体系结构。在介绍了信道接入协议的地位和作用后，对Ad hoc网络特有的信道共享方式。隐终端和暴露终端问题进行了介绍和分析。文章把Ad hoc网络的信道接入协议划分成基于单信道、双信道和多信道三类，最后给出了Ad hoc网络信道接入协议的发展动向。     

IEEE802.11e EDCA和CFB下饱和吞吐量分析

IEEE802.11eMAC草案规范对IEEE802.11无线局域网标准在QOS方面加以了补充。IEEE802.11e采用2种协调机制基于控制的综合协调可控信道接入方式HCCA和基于竞争的增强型分布式信道接入方式EDCA。主要评估EDCA和竞争空闲脉冲(contention-freeburst)CFB相结合时,在系统负载过载的情况下,改变性能参数后系统饱和吞吐量的变化,并分析系统性能参数对饱和吞吐量的影响,从而达到系统参数优化的作用。     

一种基于IEEE802.11EDCF的自适应能量调整算法

简述了IEEE802.11及IEEE802.11e的两种访问控制机制DCF和EDCF,提出了一种基于IEEE802.11 EDCF的自适应能量调整算法,并在NS2上进行了仿真实现.仿真实验结果表明,该算法能够根据分组的优先权水平和节点当前能量值动态调度信道接入,延长了全网的生存期.     

Supporting multimedia streaming and best‐effort data transmission over IEEE 802.11e EDCA

As demand for broadband multimedia wireless services increases, improving quality of service (QoS) of the widely deployed IEEE 802.11 wireless LAN (WLAN) has become crucial. To support the QoS required by a wide range of applications, the IEEE 802.11 working group has defined a new standard: IEEE 802.11e. In this paper, we propose a measurement‐based dynamic media time allocation (MBDMTA) scheme combined with a concatenating window scheme to support real‐time variable bit rate (rt‐VBR) video and best‐effort (BE) data transmission using IEEE 802.11e enhanced distributed channel access (EDCA). To provide the QoS guarantee for rt‐VBR video, the proposed MBDMTA scheme dynamically assigns channel time to the rt‐VBR video based on the estimate of the required network resources. On the other hand, the concatenating window scheme controls the contention window (CW) ranges of different priority flows such that real‐time services always have higher channel access probability, thus achieving the capability of preemptive priorities. In addition, the concatenating window scheme preserves fairness among flows of the same class and attains high channel utilization under different network conditions. Simulation results demonstrate that the throughput and delay performance improve significantly for the transmission of rt‐VBR video and BE traffic as compared to those for the 802.11e EDCA specification. It is also revealed that combining the two proposed schemes provides seamless integration and reliable transmission of digital video and data service within the 802.11e EDCA framework. Copyright © 2007 John Wiley & Sons, Ltd.     

基于IEEE802.20信道模型的仿真分析

随着移动互联技术的发展,人们在日常生活和商务活动中,越来越希望能够以移动的方式灵活地接入因特网。2002年11月,IEEE802标准委员会成立了802.20——移动宽带无线接入（MBWA）工作组,致力于为移动用户开发一个新的标准。以IEEE802.20物理层关键技术为研究对象,从一般的无线信道模型入手,通过对MIMO信道进行分析,以现有MIMO信道模型和建模方法为基础,给出一个基于IEEE802.20协议的信道模型,并在此模型上进行仿真与性能分析,得到模型的频域和时域特性,从而证明给出的信道模型和仿真方法能够准确地描述IEEE802.20的信道特性。     

基于LS的STBC导频信道估计方法

结合IEEE 802.16d(e)协议的相关内容,针对多天线载波分配和导频的排列方式,提出了一种简单的性能和复杂度折衷的导频信道估计算法,该算法是通过利用LS算法求得导频位置的信道估计,然后对导频信道估计进行滤波、插值来得到数据位置的信道估计.性能仿真显示,这种复杂度很低的算法BER性能损失是可以接受的,而且802.16标准规定系统导频只能用1个symbol,因此这种方法有其实用价值.     

A robust to multi-path ranging technique over IEEE 802.11 networks

In this paper, we present a two-way TOA technique for distance estimation (i.e., ranging) over IEEE 802.11 networks. It is based on repetitively measuring the Round Trip Time (RTT) of IEEE 802.11 MAC frames and carrying out statistical processing in order to identify the multi-path condition of the channel and to improve accuracy. The study demonstrates that a multi-path condition has a negative impact on the ranging accuracy in Non-Dominant Direct Path (NDDP) and Undetectable Direct Path (UDP) situations. In this paper, an approach is presented to detect and overcome this impact. The proposed approach identifies the channel profile (i.e., the multi-path condition) from the statistical features of a set of RTT measurements and uses a ranging algorithm that is sensitive to multi-path. Results demonstrate the feasibility of channel profile identification in real time and a ranging accuracy of around one meter in all possible multi-path situations.     

Study on bandwidth control for multiple transmission rates in IEEE 802.11e EDCAF

IEEE 802.11e supports the guaranteed quality of service (QoS) by providing different transmission priorities. IEEE 802.11e improves the media access control layer of IEEE 802.11 to satisfy the different QoS requirements by introducing two channel access functions: the enhanced distributed channel access (EDCA) and the hybrid coordination function (HCF) controlled channel access (HCCA). Signal quality may affect the available bandwidth and transmission rate, because the characteristic of communication channel in wireless environment is in random time‐variation manner. Generally a station using a lower transmission rate will occupy communication channel for a longer time and degrade system performance, which causes unfairness and cannot provide the guaranteed QoS for the stations with higher transmission rates. We propose a bandwidth control scheme (BCS) by combining the IEEE 802.11e enhanced distributed channel access function (EDCAF) protocol to overcome the guaranteed bandwidth issue in multirate environments. A multirate discrete Markov chain model is analyzed for the multirate transmission system in this paper. According to the obtained results, BCS improves performance especially in throughput and makes the different QoS requirements be processed efficiently and flexibly. Copyright © 2009 John Wiley & Sons, Ltd.     

PERFORMANCE ANALYSIS OF CHANNEL ESTIMATION FOR LDPC-CODED OFDM SYSTEM IN MULTIPATH FADING CHANNEL

In this paper, the channel estimation techniques for Orthogonal Frequency Division Multiplexing （OFDM） systems based on pilot arrangement are studied and we apply Low Density Parity Check （LDPC） codes to the system of IEEE 802.16a with OFDM modulation. First investigated is the influence of channel cstimation schemes on LDPC-code based OFDM system in static and multipath fading channels. According to the different propagation environments in 802.16a system, a dynamic channel estimation scheme is proposed. A good irregular LDPC code is designed with code rate of 1/2 and code length of 1200. Simulation results show that the performance of LDPC coded OFDM system proposed in this paper is better than that of the convolution Turbo coded OFDM system proposed in IEEE standard 802.16a.     

一种基于CS的LDPC码的抗误码方法

提出了一种基于CS(压缩感知)的LDPC码(低密度奇偶校验码)的抗误码方法。在编码端对H.264视频流中的每一个NAL(网络提取层)进行基于IEEE802.16e的LDPC编码。在解码端进行两种方法的解码,一种进行传统的LDPC解码,另一种根据压缩感知线性解码和信道码线性解码之间的联系,将基于IEEE802.16e的LDPC编解码中的校验矩阵作为压缩感知重构的测量矩阵进行压缩感知重构解码。实验结果表明,在高误码率的情况下,基于CS的LDPC码解码方法相对于传统LDPC解码方法的抗误效果更明显,为视频的抗误传输提供了新的思路。     

Model-based admission control for IEEE 802.11e enhanced distributed channel access

IEEE 802.11e enhanced distributed channel access (EDCA) is a distributed medium access scheme based on carrier sense multiple access with collision avoidance (CSMA/CA) protocol. In this paper, a model-based admission control (MBAC) scheme that performs real-timely at medium access control (MAC) layer is proposed for the decision of accepting or rejecting requests for adding traffic streams to an IEEE 802.11e EDCA wireless local area network (WLAN). The admission control strategy is implemented in access point (AP), which employs collision probability and access delay measures from active flows to estimate throughput and packet delay of each traffic class by the proposed unsaturation analytical model. Simulation results prove accuracy of the proposed analytical model and effectiveness of MBAC scheme.