QoS provisioning for large-scale multi-ap WLANs

Large-scale deployment of IEEE 802.11 wireless LANs (WLANs) with a high density of access points (APs) has become commonplace due mainly to its potential for numerous benefits, such as ubiquitous service coverage, seamless handover, and improved link quality. However, the increased AP density can induce significant channel contention among neighboring cells, thus causing severe performance degradation and throughput imbalance between cells. There have been a plethora of research efforts to improve the WLAN performance, but most of them focused only on single WLAN environments without accounting for inter-cell contention. The de facto QoS-provisioning mechanism for WLANs, i.e., the Enhanced Distributed Channel Access (EDCA), is no exception to this. The EDCA focuses only on inter-flow priority distinction and has not considered the effect of inter-cell contention which significantly restricts its efficiency. This paper presents an enhanced QoS provisioning framework that takes into account inter-cell level differentiation as well as inter-flow level priority, which may be viewed as extension of QoS provisioning from a single-WLAN domain to a multi-WLAN domain. We also propose an architecture for managing multi-AP systems in which a central controller regulates the wireless channel occupancy of APs by adaptively configuring the cell-level QoS parameters. Our extensive simulation results show that the proposed inter-AP cooperative QoS scheme overcomes the limit of legacy 802.11e and provides a high level of fairness in large-scale densely-deployed WLANs.     

Nodes organization for channel assignment with topology preservation in multi-radio wireless mesh networks

The wireless mesh network is a new emerging broadband technology providing the last-mile Internet access for mobile users by exploiting the advantage of multiple radios and multiple channels. The throughput improvement of the network relies heavily on the utilizing the orthogonal channels. However, an improper channel assignment scheme may lead to network partition or links failure. In this paper we consider the assignment strategy with topology preservation by organizing the mesh nodes with available channels, and aim at minimizing the co-channel interference in the network. The channel assignment with the topology preservation is proved to be NP-hard and to find the optimized solution in polynomial time is impossible. We have formulated a channel assignment algorithm named as DPSO-CA which is based on the discrete particle swarm optimization and can be used to find the approximate optimized solution. We have shown that our algorithm can be easily extended to the case with uneven traffic load in the network. The impact of radio utilization during the channel assignment process is discussed too. Extensive simulation results have demonstrated that our algorithm has good performance in both dense and sparse networks compared with related works.     

A dynamic and weighted spectrum decision mechanism based on SNR Tracking in CRAHNs

The spectrum decision concept in Cognitive Radio Ad-Hoc Networks (CRAHNs) introduces important challenges. These include the time-dependent SNR observations of individual CRAHN users due to the fading and shadowing effects in the licensed channels, the necessity of fusion mechanisms for accurate decisions, and the difficulties depending on multi-hop deployment. Considering these challenges, in this paper, we propose a dynamic, cooperative and distributed spectrum decision mechanism in order to decide the channel usage in CRAHNs accurately. The proposed mechanism introduces the SNR Tracking System which considers the time-varying local SNR observations and decisions of the CRAHN users. The proposed mechanism employs a distributed Weighted Fusion Scheme (WFS), to combine the individual decisions and hence, to obtain the cooperative decision. The proposed spectrum decision mechanism adapts itself dynamically to the multi-hop architecture of the network. The performance of the proposed mechanism is compared to some conventional fusion mechanisms based on the AND, OR and MAJORITY rules, and it is shown that the proposed weighted mechanism gives lower false alarm and higher detection probabilities compared to the conventional fusion mechanisms.     

Neighbor discovery in wireless networks with sectored antennas

Directional antennas offer many potential advantages for wireless networks such as increased network capacity, extended transmission range and reduced energy consumption. Exploiting these advantages requires new protocols and mechanisms at various communication layers to intelligently control the directional antenna system. With directional antennas, many trivial mechanisms, such as neighbor discovery, become challenging since communicating parties must agree on where and when to point their directional beams to communicate.In this paper, we propose a fully directional neighbor discovery protocol called Sectored-Antenna Neighbor Discovery (SAND) protocol. SAND is designed for sectored-antennas, a low-cost and simple realization of directional antennas, that utilize multiple limited beamwidth antennas. Unlike many proposed directional neighbor discovery protocols, SAND depends neither on omnidirectional antennas nor on time synchronization. SAND performs neighbor discovery in a serialized fashion allowing individual nodes to discover all potential neighbors within a predetermined time. SAND guarantees the discovery of the best sector combination at both ends of a link, resulting in more robust and higher quality links between nodes. Finally, SAND reliably gathers the neighborhood information in a centralized location, if needed, to be used by centralized networking protocols. The effectiveness of SAND has been assessed via simulation studies and real hardware implementation.     

Wireless distributed computing in cognitive radio networks

Individual cognitive radio nodes in an ad-hoc cognitive radio network (CRN) have to perform complex data processing operations for several purposes, such as situational awareness and cognitive engine (CE) decision making. In an implementation point of view, each cognitive radio (CR) may not have the computational and power resources to perform these tasks by itself. In this paper, wireless distributed computing (WDC) is presented as a technology that enables multiple resource-constrained nodes to collaborate in computing complex tasks in a distributed manner. This approach has several benefits over the traditional approach of local computing, such as reduced energy and power consumption, reduced burden on the resources of individual nodes, and improved robustness. However, the benefits are negated by the communication overhead involved in WDC. This paper demonstrates the application of WDC to CRNs with the help of an example CE processing task. In addition, the paper analyzes the impact of the wireless environment on WDC scalability in homogeneous and heterogeneous environments. The paper also proposes a workload allocation scheme that utilizes a combination of stochastic optimization and decision-tree search approaches. The results show limitations in the scalability of WDC networks, mainly due to the communication overhead involved in sharing raw data pertaining to delegated computational tasks.     

Distributed channel selection in CRAHNs: A non-selfish scheme for mitigating spectrum fragmentation

In this paper, we consider the problem of spectrum sharing in CRAHNs and propose a distributed channel selection scheme. The key functionality of our proposal, best-fit channel selection (BFC) is that it accounts both the primary channel traffic activity and CR traffic activity in channel selection. We assume CR nodes have the capability of estimating the primary channel traffic activities. In BFC, each CR selects a channel among the primary user (PU) channels for transmission that best fits to its transmission time requirement. We compare the performance of BFC to the widely known longest idle time channel selection (LITC) scheme. In LITC, a CR selects the channel that has the longest expected idle time independent of its transmission needs. In a multi-user CRN, this may degrade the network performance compared to the non-selfish BFC approach. LITC is considered selfish since each CR aims to maximize its own benefit and thus wastes resources that may be utilized by other nodes in the network. BFC providing an efficient spectrum sharing mechanism implicitly mitigates the effect of spectrum fragmentation which is a significant issue degrading the CR spectrum utilization. In CRNs, spectrum may be fragmented in various dimensions, e.g. time and frequency, such that some parts of the spectrum can not be used although being idle. Our proposal provides a solution to the spectrum fragmentation issue in time dimension at the medium access control (MAC) layer. By a set of simulations, we highlight the performance improvement by BFC over the conventional LITC under various CR/PU traffic, number of CRs, estimation accuracy and buffering capability. Simulation results show that the performance of the proposed BFC is significantly superior to that of LITC in terms of probability of successful transmission, spectrum opportunity utilization and fragmentation.     

Reproducing consistent wireless protocol performance across environments

Full scale experimentation with wireless networks in deployment environments is difficult. Therefore a common validation technique is to test a prototype network in a convenient environment prior to deployment. In this paper, we consider the problem of obtaining comparable protocol performance when the test and deployment environments differ in RF propagation environment and/or inter-node spacing. To achieve comparable protocol behavior in the two settings, we propose the concept of “link usage spectrum”. Based on the hypothesis that the link usage spectrum is a gross predictor for network performance, we show how to replicate in the test setting the link usage spectrum of the protocol that is expected in the deployment setting. We show our technique for achieving comparable protocol behavior via experiments and simulations in multiple indoor and outdoor propagation environments. The link usage spectrum is protocol specific; we illustrate for a family of protocols how the link usage spectrum is calculated analytically, from the protocol metric for choosing forwarding links in the network, and how power scaling can be used to match the link usage spectrum across networks.     

Throughput and delay analysis of multi-channel wireless infrastructure networks

Wireless infrastructure networks (WINs) provide ubiquitous connectivity to mobile nodes in metro areas. The nodes in such backbone networks are often equipped with multiple transceivers to allow for concurrent transmissions in multiple orthogonal channels. In this study, we develop an analytical model for the estimation of the delay and throughput performance of wireless infrastructure networks employing slotted ALOHA channel access and slotted Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) over multiple channels. The analytical model, which takes into account the correlation due to multi-hop transmissions, approximates the performance observed through simulations accurately.     

InRout - A QoS aware route selection algorithm for industrial wireless sensor networks

Wireless sensor networks are a key enabling technology for industrial monitoring applications where the use of wireless infrastructure allows high adaptivity and low cost in terms of installation and retrofitting. To facilitate the move from the current wired designs to wireless designs, concerns regarding reliability must be satisfied. Current standardization efforts for industrial wireless systems lack specification on efficient routing protocols that mitigate reliability concerns. Consequently, this work presents the InRout route selection algorithm, where local information is shared among neighbouring nodes to enable efficient, distributed route selection while satisfying industrial application requirements and considering sensor node resource limitations. Route selection is described as a multi-armed bandit task and uses Q-learning techniques to obtain the best available solution with low overhead. A performance comparison with existing approaches demonstrates the benefits of the InRout algorithm, which satisfies typical quality of service requirements for industrial monitoring applications while considering sensor node resources. Simulation results show that InRout can provide gains ranging from 4% to 60% in the number of successfully delivered packets when compared to current approaches with much lower control overhead.     

面向2G/3G/4G/WLAN融合接入应用的光载无线分布式天线系统

文章认为满足2G/3G/4G/WLAN四网融合业务的多样性、多业务等级、高品质要求,需要建设能够实现宽带接入的光载无线分布式天线网络。为了实现低成本、高性能、多业务融合接入的光载无线(ROF)分布式天线网络,文章基于商用的千兆以太网光器件,设计了低成本、宽带的光收发模块;利用光损耗和受激布里渊散射(SBS)效应对链路传输性能的影响,基于激光器线宽展宽技术,抑制了受激布里渊散射效应;采用副载波复用技术,实现了点到点的多业务混合传输;采用粗波分复用技术,实现了点到多点的多业务分布式传输。