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.     

Improving densely deployed wireless network performance in unlicensed spectrum through hidden-node aware channel assignment

It is well known that a wireless local area network (WLAN) based on the IEEE 802.11 standard suffers from interference and scalability problems due to the limited number of non-overlapping channels. In order to mitigate the interference problem, channel assignment algorithms has been a popular research topic in recent years. It has been shown that such algorithms can greatly reduce the interference among wireless access points. However, in this paper, we show that previously proposed channel assignment algorithms may lead to an increased number of hidden nodes in dense network deployments. We also show that this can significantly decrease the performance of the network. Furthermore, we present results from experiments showing that the Request to send (RTS)/Clear to send (CTS) mechanism is unable to solve the hidden node problem in infrastructure WLANs, and therefore careful consideration needs to be taken when choosing channel assignment strategies in densely deployed wireless networks. To this end, we propose both a centralized channel assignment algorithm and a distributed channel assignment algorithm. Using a simulation study, we show that the proposed algorithms can outperform traditional channel assignment in densely deployed scenarios, in terms of QoS sensitive VoIP support without compromising the aggregate throughput, and that they are therefore a better performing alternative in such settings.     

IEEE 802.11ax密集WLAN的干扰协调策略

无线局域网(WLAN)中设备数量的指数级增长,导致激烈的信道争用及严重的同频干扰。当运行在同一频段上的两个WLAN在彼此感知范围内时,使用先监听后发送的分布式信道接入方式会引起资源浪费,降低网络吞吐率。提出一种密集WLAN场景下的干扰协调策略。根据IEEE 802.11ax中的基本服务集颜色对站点进行分组,提出协调交替目标唤醒时间策略(CAT),安排不同组内站点在不同的时间接收数据,从而避免信道争用,最小化干扰,同时通过优化下行数据包大小提高网络吞吐率。仿真结果表明,当两个WLAN距离很近时,CAT方案能够保证网络吞吐率。此外,在考虑误码率的情况下,吞吐率并非随着数据包的增大单调递增,而是存在一个阈值,超过该阈值后网络吞吐率即呈下降趋势。     

Channel allocation in multi-channel wireless mesh networks

In this article, we survey the latest progress in multi-channel wireless mesh networks, focusing on wireless interference models and channel allocation algorithms with the goal of maximizing the network performance. We present the studies of different interference models and illustrate how they could affect the design of channel assignment. We also summarize channel allocation algorithms with different strategies in both omni-directional and directional antenna networks. We conclude that both static and dynamic channel allocation strategies have advantages and disadvantages, and the design of channel allocation algorithms strongly depends on the interference model and the assumption of network traffic.     

Self-Configuration for Wireless Local Area Networks

This paper concentrates on self-configuration in wireless local area networks (WLANs). As the growth of the stringent demand of heterogeneity and re-configurability in future wireless networks, the initial configuration is getting more and more complex, especially in WLANs. In order to minimize the complexity of the configuration of the new deployed AP, this paper focuses on two key issues of self-configuration, network condition awareness and corresponding configuration decision. By designing three neighboring AP discovery mechanisms, network condition could be promptly perceived. And then, with the obtained information of neighboring APs, self-configuration decisions are made based on analytic hierarchy process (AHP) and grey relational analysis (GRA) for mode configuration and channel selection. Benefiting from self-configuration, the complexity and the cost of both configuration and maintenance for WLANs are significantly reduced. A mass of simulations prove that self-configuration works well in increasing the network capacity and reducing the inter-network interference.     

Reliable beacon transmission based MAC protocol for LR-WPANs over WLAN interferences

The use of IEEE 802.15.4 standard based application systems has been rapidly increasing, for example, in medical services, sensor networks, public safety systems, and home automation systems. However, issues arise from the fact that IEEE 802.15.4 standard based low rate wireless personal area networks （LR-WPANs） use the same frequency bands as wireless local area networks （WLANs）, and they interfere with each other. Based on past research on this issue, the interference has a more serious impact on LR-WPANs＇ performance than on WLANs＇ performance. In this paper we propose a method to improve LR-WPANs＇ performance while coexisting with WLANs, which is called the reliable beacon transmission based medium access control （MAC） protocol. Since the reliability of a beacon frame is important, in this method, only the beacon frame is transmitted in interference-free channels, and the data packets are transmitted in interfered channels instead of abandoning the channels altogether. This method increases the reliability of beacon frames as well as overall channel utilizations. The effectiveness of the proposed method was evaluated through extensive simulations, and this paper proves that this method improves the performance of IEEE 802.15.4 based wireless sensor networks （WSNs） over WLANs＇ interferences.     

A novel frequency planning algorithm for mitigating unfairness in wireless LANs

A typical objective of frequency planning in wireless networks is maximizing the overall network throughput by reducing inter-cell interference. We argue that network throughput alone, as obtained by simulations or direct measurement, is not an adequate measure for evaluating a frequency plan, so that any direct optimization of network throughput would indeed not be advisable as it is especially inadequate for fairness. Contrary to intuition, we demonstrate via simple experiments that in IEEE 802.11 wireless LANs (WLANs) a bad frequency planning, which causes higher interference, may increase the overall network throughput. Such cases occur when far users with low bit-rates get starved under bad frequency planning, while near users that use high bit-rates monopolize the transmission opportunities. Such a counter-intuitive phenomenon indicates that both network throughput and fairness must be considered for frequency planning in WLANs. In this paper we first devise a novel framework to model the load of WLAN cells considering inter-cell interference. Then, we present an efficient frequency planning algorithm which is designed on the basis of this load model. Extensive simulations show that the proposed model allows adequate evaluation of the performance of a frequency plan from both throughput and fairness perspectives. It is also shown that our algorithm provides fairer service to its users compare to existing frequency planning algorithms, while preserving high network utilization.     

路径损耗对802.11无线局域网干扰与载波监听性能的影响研究

分析了路径损耗因子对802.11无线局域网的干扰与载波监听性能的影响,揭示了增大路径损耗因子的优势。基于SNR阈值模型和固定的载波监听阈值,分析了802.11a/b/g/n的主要比特速率。首先,在绝大多数的比特速率下(除比特速率为1和2Mbps外),增大路径损耗因子有利于增加SIR和减小干扰半径,这有助于提高比特速率和空间复用。其次,基于蜂窝网络拓扑 指出了最优网络容量随路径损耗因子的增大而增大。最后,图例分析和仿真实验揭示了增大路径损耗因子有助于改善载波监听的准确率。城市中由于建筑物和人口稠密会增大路径损耗因子,因此可提升无线局域网的性能。     

基于异步时钟序列的分布式认知跳频算法

为了解决传统跳频算法对动态、复杂电磁干扰环境适应能力不足的难题,以及克服现有跳频算法在分布式通信环境下对公共控制信道和统一时钟源的依赖,提出了一种基于异步时钟序列的分布式认知跳频算法。该算法通过建立认知跳频通信系统模型,分析其通信过程中面临的关键问题,设计了一种应用于分布式跳频通信环境的异步时钟结构序列。基于该序列的分布式认知跳频算法不依赖于任何公共控制信道和统一时钟源,能够在复杂电磁干扰环境下达到高效抗干扰通信的效果。验证实验结果表明,针对典型的窄带、宽带噪声干扰、部分频段干扰以及随机干扰,所提算法的抗干扰性能均显著优于传统的跳频通信算法以及现有的公用控制信道认知跳频算法。     

How to Effectively Use Multiple Channels in Wireless Mesh Networks

Operating on a frequency band occupying several nonoverlapping channels, IEEE 802.11 is now widely used in wireless mesh networks (WMNs). Many multichannel MAC protocols are proposed to improve the spatial reuse in the network under the assumption that the transmissions on nonoverlapping channels do not interfere with each other. Some joint routing and channel assignment algorithms are also designed to increase the network throughput based on the premise that we can switch between different channels freely. Although simulations show that great improvements on network throughput can be observed in both cases, two fundamental questions remain: 1) Can we really use multiple nonoverlapping channels freely in WMNs? 2) If we can, what will be the cost when we switch channels dynamically and frequently? In this paper, by conducting extensive experiments on our testbed, we attempt to answer these questions. We find that in spite of interference between both overlapping and nonoverlapping channels, we can still use multiple channels in mesh networks under certain conditions but with care. We also show that the channel switching cost is actually very significant in WMNs. We recommend not to switch the channels too frequently when designing the channel assignment algorithms, and those channel assignment algorithms selecting one channel for each packet are not really beneficial.     

基于迭代的FBMC/OQAM系统IAM前导信道估计算法

FBMC/OQAM系统中传统的基于干扰近似(IAM)的信道估计算法存在导频开销过大,且计算复杂度高等问题。本文研究了一种新的IAM前导信道估计算法,该算法为增加导频功率采用一种新的三列导频结构,同时采用多次迭代方法获得精度更高的信道信息,来重构相邻子载波间和符号间干扰。理论分析和仿真结果表明,在ITU的车载信道环境和步行信道环境下,基于迭代的IAM前导信道估计算法在保持其传统信道估计方法性能即传输速率高且降低干扰的同时,有效地提高了数据传输速率且降低了计算复杂度。     

无线局域网同频干扰抑制关键技术研究

抑制无线局域网同频干扰的关键环节是对同频干扰的检测,针对密集无线局域网环境下同频干扰严重影响用户通信质量且不易被精确检测的情况,提出一种新的无线局域网同频干扰检测方法。该方法应用了认知无线电技术,通过计算接收信号的循环功率谱,并将此循环功率谱和判决门限比较,判断是否存在同频信号即可确定当前无线局域网环境中是否有同频干扰。仿真结果表明,提出的结合了认知无线电的同频干扰检测方法相比其他检测方法更加简单且能高效地检测出无线局域网同频干扰,在较低信噪比时有不错的干扰检测能力,在无线局域网数量较多的环境中对同频干扰的检测效果更好。     

基于自相似业务的RED算法性能研究

通过计算研究无线局域网中业务流的自相似性和仿真实验，研究无线局域网中基于自相似业务的RED算法的性能。仿真结果表明由于业务流的自相似性的负面影响和无线网络本身特点的影响，RED算法和Drop-Tail算法的性能差别不大。由于Drop-Tail算法简单易行、开销较低，建议在无线接入点采用Drop-Tail算法进行拥塞控制。     

Efficient multicasting over large-scale WLANs through controlled association

Support for efficient multicasting in WLANs can enable new services such as streaming of TV channels, radio channels, and visitor’s information. With increasing deployments of large-scale WLANs, such services can be made available to a large number of users. However, any new multicast based services must minimally impact the existing unicast services which are currently the core services offered by most WLANs. In this paper, we leverage the flexibility of associating with different access-points (APs), which occurs often due to overlapping coverage of APs, to optimize the network’s objective. Motivated by different revenue functions and network scenarios, three different optimization objectives are considered which are: maximizing the number of admitted users (MNU), balancing the load among APs (BLA), and minimizing the load of APs (MLA). We show that these problems are NP-hard and present centralized approximation algorithms and distributed approaches to solve them. These algorithms compute which AP a user should be associated with. Using simulations we evaluate their performance and compare them to a naive approach in which users associate to the AP with the best RSSI (Received Signal Strength Indicator).     

Analysis of temporal and throughput fair scheduling in multirate WLANs

The IEEE 802.11e Wireless Local Area Network (WLAN) provides controlled access features that can be used in conjunction with scheduling algorithms to provide guaranteed per-session services. However, the multirate operation of the WLAN complicates the design of scheduling and Quality of Service (QoS) provisioning algorithms. We propose a new solution based on Controlled Access Phase Scheduling (CAPS) framework, introduced for fixed rate WLANs in our earlier works, and employ a new fair central scheduler to achieve guaranteed fair services in a WLAN. We examine the fairness issues involved in designing such an algorithm and study several fair scheduling algorithms that can be used with CAPS. We present a modified start time fair queuing based scheduler as our choice and analyze its performance under dynamic and static multirate operation. The algorithm is then evaluated through several simulation experiments. We show that the enhanced CAPS is able to adapt to multirate environments and provide both temporal and throughput fair services in 802.11e WLANs.     

WLAN中非合作博弈的信道分配研究

为解决基于802.11无线局域网中固定信道结构导致的因客户需求不均衡而产生的信道频谱分配不公平和系统效用率低的问题,提出了基于认知无线电技术的可变带宽信道分配方法,建立了无线局域网中频谱分配的非合作博弈模型.在该多接入点无线网络博弈模型中,重点考虑非重叠频谱的单碰撞域方案,在证明存在公平的且是系统最优的纯纳什均衡的基础上,给出单阶段的公平的信道分配算法.仿真实验结果表明,该分配方案无论在信道使用公平性还是系统效用上都优于固定信道和非纳什均衡的分配方式.     

多径信道下数字信号调制识别的研究

在通信信号调制方式识别中,目前已有的方法大多是基于高斯白噪声干扰的通信环境.对于多径干扰强的环境下的调制识别难以实现以及识别率低的问题,首先提出基于改进的Liavas算法对信道阶数进行准确估计,在此基础上利用基于子空间的多信道盲辨识和盲均衡来克服多径信道带来的影响,然后对均衡结果进行高阶累积量计算,提取特征参数完成调制识别.与已有的算法相比,适用范围广,实用性强,所需先验知识少.仿真结果证实算法的有效性.     

iMST: A bandwidth-guaranteed topology control algorithm for TDMA-based ad hoc networks with sectorized antennas

Many topology control algorithms aim to minimize energy consumption, interference, etc. while maintaining connectivity among the nodes. Links are realized between nodes by proper adjustment of transmission power in each node. However, a link is not useful if its bandwidth is lower than what is required by the applications it supports. Therefore, topology control algorithms should also consider the minimum realizable link bandwidth when generating topologies. As the realizable bandwidth of a link depends on the amount of interference received, interference analysis must be carried out. We study the minimum realizable link bandwidth of a given network under a TDMA/TDD channel sharing mechanism. We carried out the interference analysis assuming the general multi-sectored antenna configuration, since an omni-directional antenna can be visualized as a special configuration of a sectorized antenna (i.e. with 1 sector). We then proceed to derive the transmission power to be used in each sector of all nodes in the network to maximize average channel utilization. However, a brute-force method that runs through all possible topologies takes exponential time.We thus propose an algorithm, iMST, that attempts to maximize average channel utilization by reducing interference. The iMST algorithm not only generates k-edge-connected networks, but also guarantees minimum link bandwidth. Although iMST requires global knowledge of the locations of nodes, by using a distributed MST generation method, iMST can be run in a distributed manner. The iMST scheme is evaluated and compared against a modified Fault-tolerant Cone-Based Topology Control (FCBTC) scheme, referred to as s-FCBTC, that works with sectorized antennas, on a few performance metrics: average channel utilization, network diameter, minimum link bandwidth and edge-connectedness, where edge-connectedness is a new metric defined in this paper. The iMST scheme is shown to exhibit good performance in many of these performance metrics.     

An experimental study of inter-cell interference effects on system performance in unplanned wireless LAN deployments

In this paper, we report on our experimental study of the effects of inter-cell interference on IEEE 802.11 performance. Due to growing use of wireless LANs (WLANs) in residential areas and settings supporting flash crowds, chaotic unplanned deployments are becoming the norm rather than an exception. Environments in which these WLANs are deployed, have many nearby access points and stations on the same channel, either due to lack of coordination or insufficient available channels. Thus, inter-cell interference is common but not well-understood. According to conventional wisdom, the efficiency of an IEEE 802.11 network is determined by the number of active clients. However, we find that with a typical TCP-dominant workload, cumulative system throughput is characterized by the number of actively interfering access points rather than the number of clients. We verify that due to TCP flow control, the number of backlogged stations in such a network equals twice the number of active access points. Thus, a single access point network proves very robust even with over one hundred clients, while multiple interfering access points lead to a significant increase in collisions that reduces throughput and affects media traffic. Only two congested interfering cells prevent high quality VoIP calls. Based on these findings, we suggest a practical contention window adaptation technique using information on the number of nearby access points rather than clients. We also point out the need for collision-resilient rate adaptation in such a setting. Together these techniques can largely recover the 50% loss in cumulative throughput in a setting with four strongly interfering access points.     

IEEE 802.11无线局域网中分布式协调功能的改进

结合无线局域网和TCP的特点提出了一种改进算法,当AP向无线节点发数据包时,无线节点用一个小的数据包(如:TCP层的ACK等)来代替CTS,这样可以避免无线节点单独发送小数据包,从而提高无线信道的利用率和无线网络的吞吐率。从试验结果可以看出,改进后的DCF协议不仅提高了信道的利用率和无线局域网的性能,而且更好的维护了无线节点间的公平性。