Pre-scan based fast handoff scheme for enterprise IEEE 802.11 networks

In IEEE 802.11 networks, many access points (APs) are required to cover a large area due to the limited coverage range of APs, and frequent handoffs may occur while a station (STA) is moving in an area covered by several APs. However, traditional handoff mechanisms employed at STAs introduce a few hundred milliseconds delay, which is far longer than what can be tolerated by some multimedia streams such as voice over Internet protocol (VoIP), it is a challenging issue for supporting seamless handoff service in IEEE 802.11 networks. In this paper, we propose a pre-scan based fast handoff scheme within an IEEE 802.11 enterprise wireless local area network (EWLAN) environment. The proposed scheme can help STA obtain the best alternative AP in advance after the pre-scan process, and when the handoff is actually triggered, STA can perform the authentication and reassociation process toward the alternative AP directly. Furthermore, we adopt Kalman filter to minimize the fluctuation of received signal strength (RSS), thus reducing the unnecessary pre-scan process and handoffs. We performed simulations to evaluate performance, and the simulation results show that the proposed scheme can effectively reduce the handoff delay.     

Areas of cochannel interference and multipath created by 8-VSB modulated distributed transmitters in flat terrain

Presentation is made of the methodology for defining the area in which the signal from an auxiliary on-channel transmitter appears to the receiver as multipath, and the area where the signal from an auxiliary on-channel transmitter appears as new cochannel interference. The size and location of these areas depends on how well the contour of the receiver equalizer window can be made to overlap the cochannel interference contour. The optimum overlap requires a specific adjustment to the height, power and delay of the auxiliary transmitter.     

Coverage adjustment for load balancing with an AP service availability guarantee in WLANs

In wireless local area networks, adjusting the coverage of access points (APs) may force the clients near the coverage boundaries of congested APs to associate with lightly-loaded ones, thus realizing load-balancing. Such an approach has the advantage of requiring no modification on the client software/hardware compared to other load-balancing techniques. However, its applicability is undermined by the problems of AP service cheating and AP service loophole resulted from coverage adjustment, which significantly affect the AP service availability. Nevertheless, these two problems are largely ignored by the existent research. To tackle this challenge, a variable polyhedron genetic algorithm (GA) is proposed, which not only provides an AP service availability guarantee but also yields a near-optimal beacon range for each AP when the number of evolutions is large enough. Simulation study indicates that our algorithm is superior over the default 802.11 AP association model in terms of load-balancing and network throughput enhancement. In addition, the variable polyhedron GA outperforms the traditional GA in terms of fitness value and convergence speed.     

A novel method for measurement points selection in access points localization

Precise localization of access points (APs) has become more and more important with the booming of diverse location based services. AP localization accuracy heavily relies on the locations of measurement points. However, collecting abundant information from the APs usually requires tremendous manual efforts, which greatly compromise the efficiency and accuracy of the localization of the APs. Thus, the selection of appropriate measurement points becomes an important issue to achieve a high localization accuracy with a low-cost method, which has not been fully explored in existing research. To this end, we propose a novel approach to select measurement points for AP localization under the assumption that the number of APs to be located is known in advance. Specifically, an initial point is selected randomly and the locations of APs are estimated according to the measured information. Then, subsequent measurement points are determined by locating the intersection of the coverage areas of APs in real time, so that as many APs as possible can be detected at each measurement point. Both simulation and experimental results show that our approach can reduce the number of measurement points while improving the AP localization accuracy.     

Throughput Optimization via Association Control in Wireless LANs

With the rapid development of the mobile computing, accessing the Internet everywhere is important for mobile device users. Wireless LAN is a stable and reliable technique to provide network access for mobile devices. The Wireless LAN Access Points(APs) have been densely deployed so that a user can access the Internet almost everywhere. However, this fact brings some new challenges. Since the regular AP association strategy is signal-based when a user receive the signals of multiple APs. The APs with strong signal will be too overloaded while the bandwidth resource in other APs is wasted. The throughput of the whole WLAN is not optimized. Moreover, the diverse bandwidth demands among users further exacerbate the situation. In this paper, aiming at optimizing the throughput over the whole WLAN, a joint AP association and bandwidth allocation problem is formulated. The different users’ bandwidth demands are added as new constraints. We comprehensively analyze the solution space and prove the problem NP-hard. Our trace-driven evaluations show that the throughput is improved about 23.1 % compared to the conventional schemes.     

Backbone Topology Synthesis for Multiradio Mesh Networks

Wireless local area network (WLAN) systems are widely implemented today to provide hot-spot coverage. Operated typically in an infrastructure mode, each WLAN is managed by an access point (AP). Wireless mesh networks (WMNs) are employed for the purpose of extending the wireless coverage scope by interconnecting the underlying AP nodes. The capability and performance behavior of the WMN can further be upgraded by using multiple communications channels and by having more capable nodes use multiple radio modules. In this paper, we present a fully distributed multiradio backbone synthesis algorithm, which serves to construct a mesh backbone network of APs. We assume more capable nodes, such as APs, to be equipped with two radio modules, while less capable nodes employ a single radio module. Multihop communications among distant client stations take place in accordance with a routing algorithm that uses the mesh backbone to establish inter-WLAN routes. The presented backbone construction algorithm and the associated on-demand backbone-based routing mechanism are shown to improve the system's delay-throughput performance, as well as its asynchronous and distributed behavior in a stable fashion     

WLAN中基于OpenFlow的无缝切换机制设计与实现

在传统的WLAN网络中,因为用户使用的移动终端（Station,STA）具有移动性,STA会出现离开当前网络接入点（Access Point,AP）的覆盖范围进入另一AP覆盖范围的情况,此时STA需要在AP间进行切换.传统WLAN中这样的AP间切换会产生网络延迟突然增大、吞吐量损失、以及掉线等服务质量下降的问题.本文提出并实现了一种基于AP虚拟化和OpenFlow技术的解决方案,通过搭建基于OpenFlow的交换网络对STA的流量进行细粒度的控制从而完成STA在不同AP间的无缝切换.经过实际网络环境测试,本方案具有5ms左右的切换延时、在AP切换时只会造成1秒左右的瞬时吞吐量减少和16ms左右的网络延时.相较于其他方案,本方案由于不需要再次进行认证和重新路由,因而具有更好的无缝切换性能.     

Leveraging Multi-AP Diversity for Transmission Resilience in Wireless Networks: Architecture and Performance Analysis

With the increasing development of IEEE 802.11 based wireless local area network (WLAN) devices, large-scale multi-cell WLANs with a high density of users and access points (APs) have emerged widely in various hotspots. Providing resilient data transmission has been a primary challenge for scaling the WLANs because the high density of users and APs results in too many collisions. In this paper, we analyze and point out the defect of the single association mechanism defined in IEEE 802.11 on transmission reliability from a network perspective. Then, we propose a "multi-AP" architecture with which a MAC layer device called an AP controller (AC) is employed to enable each user to associate and cooperate with multiple APs. In this way, the users can benefit from the diversity effect of multipaths with independent collisions and transmission errors. This paper concentrates on the theoretical analysis of performance comparison between the proposed ldquoMulti-APrdquo architecture and that in IEEE 802.11. Extensive simulation results show that the proposed ldquomulti-APrdquo architecture can obtain much better performance in terms of the throughput per user and the total throughput, and the performance gain is position dependent. Moreover, the unfairness issue in traditional WLANs due to capture effect can be alleviated properly in the ldquomulti-APrdquo framework.     

Power saving access points for IEEE 802-11 wireless network infrastructure

In the past decade, there has been a huge proliferation of wireless local area networks (WLANs) based on the IEEE 802.11 WLAN standard. As 802.11 connectivity becomes more ubiquitous, multihop communications will be increasingly used for access point range extension and coverage enhancement. In this paper, we present a design for an IEEE 802. 11 -based power saving access point (PSAP), intended for use in multihop battery and solar/battery powered applications. These types of APs have many practical applications and can be deployed very quickly and inexpensively to provide coverage enhancement in situations such as campuses, building complexes, and fast deployment scenarios. Unlike conventional wired access points, in this type of system, power saving on the AP itself is an important objective. A key design constraint is that the proposed PSAP be backward compatible to a wide range of IEEE 802.11 functionality and existing wired access points. In this paper, we introduce the protocols required to achieve this compatibility, show the constraints imposed by this restriction, and present performance results for the proposed system.     

Load Distribution in Non-homogeneous Wireless Local Area Networks

This paper presents an approach to distributing traffic among capability-varying access points (APs) in wireless Local Area Networks. Under consideration is a non-homogeneous environment where maximum transport rates over radio channels differ from AP to AP. We let each new station associate itself to a best-fit AP, allowing for received signal strength, potential responsiveness, and remaining channel capacity of every AP in range. Our approach is developed for alignment with standard protocols, keeping legacy stations operable while maintaining forward compatibility. Our development augments the functionality of APs such that association is decided by APs collaboratively. Simulation results show that our approach outperforms counterpart schemes by an appreciable amount in terms of throughout, transmission delay, and load distribution. Our approach is also of avail to homogeneous network settings. Featuring simplicity and efficiency, our design lends itself to other types of contention-based wireless network like ZigBee as well.     

Context Aware Inter-BSS Handoff in IEEE 802.11 Networks: Efficient Resource Utilization and Performance Improvement

IEEE 802.11 has become very popular wireless technology to offer high speed Internet access at public places called the ‘Hot-Spots’. This has enabled users to access multimedia and other real time applications using wireless local area networks (WLAN). In IEEE 802.11 WLAN technology, associations between a mobile station and an access point (AP) is controlled by the mobile station, allowing the station to select an AP with the strongest signal in terms of either ‘Received Signal Strength Identifier’ or ‘Signal to Interference and Noise Ratio’. In real time scenarios, the traffic patterns of mobile users are dynamic in nature. This leads to a situation where the traffic loads on the APs are unevenly distributed in the WLAN. Such imbalance in traffic load causes severe degradation in performance of the applications running on the mobile stations associated with the overloaded APs. In this paper, we propose a scheme which dynamically improves the performance of the overloaded APs by handing off some of its associated stations to nearby APs. This handoff decision is taken by an AP in assistance with the mobile stations. The effectiveness of the load distribution through dynamic hand-over in a WLAN is analyzed through theoretical analysis. Simulation results show the overall improvements in terms of delay, throughput and number of stations that an AP can support. The performance improvement in the proposed scheme is also justified through the results obtained from a IEEE 802.11 WLAN testbed.     

AORS: adaptive mobile data offloading based on attractor selection in heterogeneous wireless networks

The unforeseen mobile data explosion poses a major challenge to the performance of today’s cellular networks, and is in urgent need of novel solutions to handle such voluminous mobile data. Obviously, data offloading through third-party WiFi access points (APs) can effectively alleviate the data load in the cellular networks with a low operational and capital expenditure. In this paper, we propose and analyze an attractor-aware offloading ratio selection (AORS) algorithm, which can adaptive select an optimum offloading ratio based on attractor selection for the current networks environment. In the proposed algorithm, the throughput of AP and the cellular load corresponding to the coverage area of the AP, are mapped into the cell activity, which is the reflector of the current network environment. When the current attractor activity is low, the network is dominated by the noise. Then, the noise triggers the controller to select adaptive attractor for each users, the optimal offloading ratio \(\phi \), to adapt to the dynamic network environment. Hence, according to the offloading ratio \(\phi \), the part of the cellular traffic will be transmitted via WiFi networks. Through simulation, we show that the proposed AORS algorithm outperforms the existing ones with 42 % higher heterogeneous network throughput in a dense traffic environment.     

An Adaptive AP Selection Scheme Based on RSS for Enhancing Positioning Accuracy

In recent years, the indoor positioning technologies have been recognized as core technologies for realizing smart space, a ubiquitous society, context awareness, and various location-based services. There are several approaches for positioning with radio signals, but the received signal strength (RSS)-based technology is considered a promising scheme because of its simplicity and practicality in implementation. In this paper, the positioning performance of the RSS value-based scheme is analyzed with respect to the location of access points (APs) and the number of APs in an indoor environment. An adaptive AP selection scheme and a base AP changing scheme are then proposed to enhance the positioning accuracy. In order to estimate the RSS characteristics, RSS values are measured as the distance between the AP and the receiver increases. The positioning performance is evaluated with differing AP numbers, which form a triangle or a quadrilateral. The performance of the proposed schemes is evaluated via experiments using wireless local area network APs. Results show that the performance of proposed schemes is enhanced compared to that of conventional scheme.     

Balanced association algorithm for IEEE 802.11 extended service areas

Most wireless local area network performance estimations are done with the assumption of uniformly distributed (UD) users. In practice, however, stations (STAs) are distributed unevenly among access points (APs) in an extended service area, causing congested hot‐spots (HS) and under‐utilized APs. Considering a typical network is made up of multiple APs, having some nodes carrying excessive loads degrades the overall network performance. The system performance can be improved by associating STAs efficiently throughout the network, in a sense sharing the network resources fairly among APs and thus relieving congestion. The association algorithm currently employed in IEEE 802.11 systems, that is specifically designed for residential and small office environments, takes into account signal strength as the only parameter and associates STAs to the closest (in signal strength sense) AP, ignoring its load. Novel user association algorithms are required to solve the problems commonly seen in corporate network environments spanning multiple APs, namely congestion relief and resource sharing. In this work, a distributed and online association algorithm is proposed that demonstrates improved average throughput performance, a balanced load distribution as well as fairness across the network compared to the conventional algorithm. Copyright © 2008 John Wiley & Sons, Ltd.     

Cellular communications using aerial platforms

This paper is devoted to the study of cellular communications using aerial platforms (APs). A set of key equations is derived that quantify the coverage area on the ground as a function of AP elevation, the operation of the adaptive multibeam antenna on the AP, and the formulation of contiguous terrestrial cells and their shapes. Specifically, we consider the deployment of an AP to provide terrestrial mobile radio communications using the universal mobile telecommunication system operating in its wide-band code-division multiple-access mode. Calculations are made of the number of users versus E_b/N₀ for different service rates. Multitiered cellular structures having cells of different size that are steerable with the offered teletraffic are examined. The array structure to achieve this is identified. The preliminary results shows that an AP at a height of 21 km covers an area of radius 517 km. Up to 21 users per cell with a service rate of 8 kb/s can be accommodated in the 3.2-GHz band. These services can be provided within an area of radius 70 km with transmitted powers of less than 1 W. High system capacity is proved to be possible by constructing cells of radius as small as 100 m using square planar arrays with dimensions of less than 12 m ×12 m. The AP system provides high capacity and Doppler frequency shifts that only originate from roving mobiles     

Wireless LAN Access Point Placement Based on User Mobility

In a wide area campus, a university provides Wireless Local Area Network (WLAN) for users to connect to the Internet. Most users take advantage of this WLAN benefit by using their laptops. However, the number of smart phone users is growing fast. Since a smart phone is able to get an Internet connection using WLAN, users can use their smart phones without having to pay for a cellular operator. Users tend to use their smart phones more, due to their higher mobility compared to a laptop. This capability enables new services in the market, such as Fixed-Mobile Convergence (FMC), which integrates a fixed network (traditional telephony, WLAN) and a mobile network (cellular) to provide seamless voice communications anytime, anywhere. These new applications require a WLAN connection availability nearly everywhere. However, due to limited budgets, a university can only install APs in places with a high connection demand. We propose a novel WLAN AP placement technique that takes user mobility into consideration. This new approach is more complete than previous approaches, which mainly focus on coverage area and throughput data. Our technique has been implemented in our university. The results show the suitability of the WLAN access point locations in our university campus based on user mobility and activities.     

关于WLAN覆盖的研究

针对目前WLAN建设中普遍存在的覆盖范围小、覆盖信号弱及覆盖不连续等覆盖问题,通过采用衰减因子模型对2.4 GHz频段进行链路预算分析,得出室内独立型AP及室内分布型AP、室外独立型AP等3种不同覆盖方式下的WLAN理论覆盖范围。根据不同覆盖方式的覆盖范围,结合不同的应用场景,提出了AP密集覆盖和室外AP+高增益定向天线等2种加强WLAN覆盖的措施,为WLAN覆盖提供了有效的解决思路。     

基于贝叶斯博弈的WLAN节能机制研究

针对无线局域网（Wireless Local Area Networks,WLAN）中密集部署无线接入点（Access Point,AP）导致的能耗和同频干扰问题,提出了一种基于贝叶斯博弈的节能机制（BaYesian Game based Energy Saving scheme,BYG-ES）.首先,对通用AP设备的能耗进行测量与分析,构建AP发射功率-负载-能耗的关系模型;然后,基于该关系模型及软件定义网络控制器实时收集的网络状态信息,设计基于贝叶斯博弈的能耗优化模型;最后,利用社会选择函数求解能耗优化模型,获得干扰限制下最优的休眠AP集合和发射功率配置规则,完成用户流量卸载和AP发射功率的调整,同时保证AP参与博弈的诚实性.实验结果表明,BYG-ES节能机制能在减小系统能耗的同时提高网络性能.     

Mobility-driven user-centric AP clustering in mobile edge computing-based ultra-dense networks

ultra-Dense Network (UDN) has been envisioned as a promising technology to provide high-quality wireless connectivity in dense urban areas, in which the density of Access Points (APs) is increased up to the point where it is comparable with or surpasses the density of active mobile users. In order to mitigate inter-AP interference and improve spectrum efficiency, APs in UDNs are usually clustered into multiple groups to serve different mobile users, respectively. However, as the number of APs increases, the computational capability within an AP group has become the bottleneck of AP clustering. In this paper, we first propose a novel UDN architecture based on Mobile Edge Computing (MEC), in which each MEC server is associated with a user-centric AP cluster to act as a mobile agent. In addition, in the context of MEC-based UDN, we leverage mobility prediction techniques to achieve a dynamic AP clustering scheme, in which the cluster structure can automatically adapt to the dynamic distribution of user traffic in a specific area. Simulation results show that the proposed scheme can highly increase the average user throughput compared with the baseline algorithm using max-SINR user association and equal bandwidth allocation, while it guarantees at the same time low transmission delay.     

Efficient IEEE 802.11 handoff based on a novel geographical fingerprint scheme

The effective coverage distance of wireless LAN (WLAN) being small, users may leave the coverage area of the specific access point (AP) from time to time while roaming. However, the wireless network is a shared medium. The air is open for everyone. In general there is collision if a few users attempt to transmit with the same channel that is more rigorous during handoff period because of active scan mode. The active scan will perform requests for searching available AP. Unfortunately, this function consumes too much resource in wireless communication, and also affect total performance. We will propose an advanced active scan to improve it. In our proposal, we convert RF signal distribution to a simple classification problem, like as XOR classifier with artificial neural network (ANN). We combine ANN with active scan to achieve our goal. And the weight, which trained by ANN presents the connection character of geography. Moreover, the weight could be stored in AP for reusing and is called geographical fingerprint. The average enhancement of reducing the active scan area is about 62%. Copyright © 2006 John Wiley & Sons, Ltd.