Experimenting with commodity 802.11 hardware: overview and future directions

The huge adoption of 802.11 technologies has triggered a vast amount of experimentally-driven research works. These works range from performance analysis to protocol enhancements, including the proposal of novel applications and services. Due to the affordability of the technology, this experimental research is typically based on commercial off-the-shelf (COTS) devices, and, given the rate at which 802.11 releases new standards (which are adopted into new, affordable devices), the field is likely to continue to produce results. In this paper, we review and categorise the most prevalent works carried out with 802.11 COTS devices over the past 15 years, to present a timely snapshot of the areas that have attracted the most attention so far, through a taxonomy that distinguishes between performance studies, enhancements, services, and methodology. In this way, we provide a quick overview of the results achieved by the research community that enables prospective authors to identify potential areas of new research, some of which are discussed after the presentation of the survey.This work has been partly supported by the European Community through the CROWD project (FP7-ICT-318115) and by the Madrid Regional Government through the TIGRE5-CM program (S2013/ICE-2919).Publicad

Single user TCP downstream throughput probability models in IEEE802.11b WLAN system

Single User, Transmission Control Protocol Downstream Throughput (TCPDST) probability models in an IEEE802.11b WLAN have been developed, validated and evaluated for performance. Measurement of single user TCPDST were taken using Tamosoft throughput test while that of signal to noise ratio (SNR) were taken using inSSIDer 2.1. The Tamosoft throughput tests were conducted using different quality of service (QoS) traffic. These QoS traffic (which were sent through an infrastructure based network) correspond to different wireless multimedia tags. Measurements were taken in free space, small offices and open corridor environments. By assuming a normal distribution, single user TCPDST Cumulative distribution function (CDF) probability models were developed for different signal categories namely: (i) all the SNR considered, (ii) strong signals only, (iii) grey signals only and (iv) weak signals only. The models were validated and their performances evaluated using root mean square (RMS) errors. RMS errors were computed by comparing model predicted values with validation data. The RMS errors for single user CDF all signals model was 0.1466%. RMS errors for strong signals models, grey signals model and weak signals model respectively were 0.1466%, 0.6756% and 0.1233% indicating acceptable performances. All signals, strong signals, grey signals and weak signals CDF probability models predicted probabilities of obtaining TCPDST values greater than 5Mbps as 74.79%, 90.55%, 13.00% and 4.77% respectively while probabilities of obtaining TCPDST values less than 2Mbps were predicted as 4.91%, 0.00%, 18.98% and 52.41% respectively. These probability models will provide additional useful information needed to design efficient distributed data networks. Keywords: Throughput, TCP, WLAN, probability model

Feedback Mechanisms for Centralized and Distributed Mobile Systems

The wireless communication market is expected to witness considerable growth in the immediate future due to increasing smart device usage to access real-time data. Mobile devices become the predominant method of Internet access via cellular networks (4G/5G) and the onset of virtual reality (VR), ushering in the wide deployment of multiple bands, ranging from TVWhite Spaces to cellular/WiFi bands and on to mmWave. Multi-antenna techniques have been considered to be promising approaches in telecommunication to optimize the utilization of radio spectrum and minimize the cost of system construction. The performance of multiple antenna technology depends on the utilization of radio propagation properties and feedback of such information in a timely manner. However, when a signal is transmitted, it is usually dispersed over time coming over different paths of different lengths due to reflections from obstacles or affected by Doppler shift in mobile environments. This motivates the design of novel feedback mechanisms that improve the performance of multi-antenna systems. Accurate channel state information (CSI) is essential to increasing throughput in multiinput, multi-output (MIMO) systems with digital beamforming. Channel-state information for the operation of MIMO schemes (such as transmit diversity or spatial multiplexing) can be acquired by feedback of CSI reports in the downlink direction, or inferred from uplink measurements assuming perfect channel reciprocity (CR). However, most works make the assumption that channels are perfectly reciprocal. This assumption is often incorrect in practice due to poor channel estimation and imperfect channel feedback. Instead, experiments have demonstrated that channel reciprocity can be easily broken by multiple factors. Specifically, channel reciprocity error (CRE) introduced by transmitter-receiver imbalance have been widely studied by both simulations and experiments, and the impact of mobility and estimation error have been fully investigated in this thesis. In particular, unmanned aerial vehicles (UAVs) have asymmetric behavior when communicating with one another and to the ground, due to differences in altitude that frequently occur. Feedback mechanisms are also affected by channel differences caused by the user’s body. While there has been work to specifically quantify the losses in signal reception, there has been little work on how these channel differences affect feedback mechanisms. In this dissertation, we perform system-level simulations, implement design with a software defined radio platform, conduct in-field experiments for various wireless communication systems to analyze different channel feedback mechanisms. To explore the feedback mechanism, we then explore two specific real world scenarios, including UAV-based beamforming communications, and user-induced feedback systems

Robust and Interference-Resilient MAC/PHY Layer Strategies for WLANs

학위논문 (박사)-- 서울대학교 대학원 : 공과대학 전기·컴퓨터공학부, 2018. 2. 최성현.Thanks to the explosive growth of mobile devices such as smartphones and tablet PCs, IEEE 802.11 wireless local area network (WLAN), often referred to as WiFi, has become one of the most successful wireless access technologies, supporting ever increasing demand for high data rates at relatively low cost. Encouraged by this remarkable success, the state-of-the-art IEEE 802.11 WLAN provides a physical layer (PHY) data rate of Gb/s to a single user in the 5 GHz unlicensed band, by enabling multi-input and multi-output (MIMO) technology, which utilizes multiple antennas at both transmitter and receiver, and channel bonding which aggregates multiple 20 MHz channels up to 160 MHz bandwidth. Furthermore, as a key feature to enhance medium access control (MAC) efficiency, IEEE 802.11 standard defines frame aggregation called aggregate MAC protocol data unit (A-MPDU), which amortizes PHY protocol overhead over multiple frames by packing several MPDUs into a single frame. In this dissertation, we propose the following three strategies to enhance throughput performance in practice: (1) Mobility-aware PHY rate and A-MPDU length control, (2) Receiver-driven operating channel width adaptation, and (3) Receive architecture for eliminating time-domain interference not overlapping with the desired signal in frequency-domain. Firstly, a significant growth of mobile data traffic volume, primarily generated by portable devices, has led to a change of WLAN communication environmentsthe wireless channel condition in WLAN system is no longer quasi-stationary over the duration of a single frame reception. Especially, frame aggregation, i.e., A-MPDU, which lengthens frame duration significantly, causes the channel state information (CSI) obtained at the preamble can be no longer valid for successfully decoding the latter part of A-MPDUs, when the channel condition substantially changes during the A-MPDU reception. To cope with this problem, we analyze the wireless channel dynamics considering mobility through extensive measurements, and we then build a model which represents the impact of mobility with a noise vector in the I-Q plane, to investigate how the mobility affects the A-MPDU reception performance. Based on our analysis, we develop STRALE, a standard-compliant and mobility-aware PHY rate and A-MPDU length adaptation scheme with ease of implementation. Through extensive simulations with 802.11ac using ns-3 and prototype implementation with commercial 802.11n devices, we demonstrate that STRALE achieves up to 2.9 higher throughput, compared to a fixed duration setting according to IEEE 802.11 standard. STRALE simply requires to update device driver only at one end of the wireless link (i.e., transmitter), thus allowing it to be applicable to any kind of platforms. Second, IEEE 802.11ac supports bandwidth of 20, 40, and 80 MHz as a mandatory feature, and optionally supports 160 MHz bandwidth. To transmit and receive packets using such wide bandwidth, the 802.11ac devices need to increase the size of fast Fourier transform (FFT), equivalently, the baseband bandwidth, referred to as operating channel width (OCW). However, our experiment results reveal various situations where bandwidth adaptation without changing the receivers OCW, leads to poor reception performance due surprisingly to time-domain interference not overlapping with the incoming desired signal in frequency domain. To cope with this problem, we develop RECONN, a standard-compliant and receiver-driven OCW adaptation scheme with ease of implementation. Our prototype implementation in commercial 802.11ac devices shows that RECONN achieves up to 1.85x higher throughput by completely eliminating time-domain interference. To our best knowledge, this is the first work to discover the time-domain interference problem, and to develop OCW adaptation scheme in 802.11ac system. Finally, based on the observation that time-domain interference causes 1) packet detection and synchronization failure, 2) undesirable receive locking problem, and 3) automatic gain control (AGC) failure, we propose a receive architecture called REACTER to eliminate the impact of time-domain interference: REACTER digitally extracts the desired preamble signal not affected by time-domain interference, and provides interference-resilient A-MPDU reception performance by real-time AGC level adaptation during A-MPDU reception. The proposed receive architecture extensively evaluated via IT++ based link-level simulator, and the simulation results show that REACTER significantly improves the frame reception performance by completely eliminates the impact of time-domain interference. In summary, we identify the two existing problems through the extensive measurement and simulations, and we then propose compelling algorithms to improve the throughput performance. We demonstrate the feasibility of our approaches by implementing prototypes in off-the-shelf commercial 802.11n/ac devices, showing that our proposed algorithms fully comply with the 802.11 MAC and requires no PHY modification such that it can be applicable to the existing hardware platform by simply updating the device driver only at one end of the wireless link. Furthermore, we present a novel receive architecture which shows the ability to fundamentally enhance the performance of wide bandwidth operation with very low cost and complexity.1 Introduction 1 1.1 Motivation 1 1.2 Overview of Existing Approach 3 1.2.1 A-MPDU Length Adaptation 3 1.2.2 Wide Bandwidth Operation in IEEE 802.11ac WLANs 4 1.2.3 Receive Architecture for WLAN Devices 5 1.3 Main Contributions 6 1.3.1 Mobility-Aware PHY Rate and A-MPDU Length Adaptation 6 1.3.2 Receiver-Driven Operating Channel Width Adaptation 7 1.3.3 Rx Architecture for Eliminating Time-Domain Interference 7 1.4 Organization of the Dissertation 8 2 STRALE: Mobility-Aware PHY Rate and A-MPDU Length Adaptation in IEEE 802.11 WLANs 10 2.1 Introduction 10 2.2 Preliminaries . 12 2.2.1 Channel Estimation and Compensation 12 2.2.2 Frame Aggregation 14 2.2.3 Modulation and Coding Schemes 15 2.2.4 MIMO, SM, STBC and channel bonding 15 2.3 Case Study 16 2.3.1 Experimental Setting 16 2.3.2 Temporal Selectivity 17 2.3.3 Impact of Mobility 18 2.3.4 Impact of MCSs 21 2.3.5 IEEE 802.11n/ac Features 22 2.3.6 Rate Adaptation: Minstrel 23 2.4 Caudal Noise Model 25 2.4.1 Caudal Noise Modeling for n x n MIMO Channel 26 2.4.2 Impact of Caudal Noise 28 2.5 STRALE: Proposed Algorithm 30 2.5.1 Possible Solutions for Caudal Loss Problem 31 2.5.2 Operation of STRALE 32 2.6 Performance Evaluation 37 2.6.1 Methodology 37 2.6.2 Simulation Results 39 2.6.3 Prototype Implementation 44 2.7 Summary 46 3 RECONN: Receiver-Driven Operating Channel Width Adaptation in IEEE 802.11ac WLANs 48 3.1 Introduction 48 3.2 Preliminaries 51 3.2.1 Packet Detection and Initial Synchronization 51 3.2.2 Wide Bandwidth Operation 52 3.3 Cast Study 53 3.3.1 Motivation 55 3.3.2 Packet Detection and Synchronization Failure 57 3.3.3 Receive Locking to Interference Signal 59 3.3.4 AGC Failure 61 3.4 RECONN: Proposed Algorithm 64 3.4.1 Possible Solutions 64 3.4.2 RECONN 67 3.5 Performance Evaluation 70 3.5.1 One-to-One Scenario 72 3.5.2 Multi-station Scenario 74 3.6 Summary 75 4 REACTER: Receive Architecture for Eliminating Time-Domain Interference 76 4.1 Introduction 76 4.2 Preliminaries 78 4.2.1 Packet Detection and Synchronization 78 4.2.2 Automatic Gain Control in IEEE 802.11 WLAN 80 4.3 REACTER: Proposed Architecture 80 4.3.1 Simulation Methodology 80 4.3.2 Digital Low Pass Filter (DLPF) 82 4.3.3 Real-Time AGC 89 4.3.4 Structure of REACTER 96 4.4 Performance Evaluation 100 4.5 Summary 101Docto

Wi-Fi의 신뢰성 및 에너지 효율성 향상을 위한 MAC/PHY 기법

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2015. 8. 최성현.Over the last quarter century, wireless local area network (WLAN) technology has become an essential and indispensable part of our daily lives. Recently, a tremendously growing number of portable devices, such as smartphones, tablets and laptops, are being equipped with Wi-Fi, the hallmark of the IEEE 802.11 WLAN, in order to meet ever-increasing traffic demands at extremely low cost. Encouraged by this remarkable success, Wi-Fi is facing two trends. First, the state-of-the-art IEEE 802.11 specifications, e.g., IEEE 802.11n and 802.11ac, have focused on improving physical layer (PHY) rate by enabling multiple antennas, called multiple-input multiple-output (MIMO), and bandwidth widening, known as chan- nel bonding [1, 2]. Second, to achieve high throughput and transmission efficiency at medium access control (MAC) layer, IEEE 802.11n/ac standards have defined two types of frame aggregation techniques: MAC service data unit (MSDU) aggregation and MAC protocol data unit (MPDU) aggregation which amortize PHY/MAC proto- col overhead e.g., binary random backoff, physical layer convergence protocol (PLCP) preamble, and acknowledgement (ACK), over multiple frames by packing several MS- DUs and MPDUs into a single aggregate MSDU (A-MSDU) and aggregate MPDU (A-MPDU), respectively. Apparently, there is no doubt that these state-of-the-art features meet and fulfill i the requirements of Wi-Fi equipped device users by offering several hundred-fold in- creases in PHY rate and ubiquitous access. However, with respect to the demands of the battery-powered portable device users, the belief is easily broken from two per- spectives. First, since using higher PHY rate and longer A-MPDU are more vulnerable to channel errors especially for mobile users, the significant throughput performance degradation can be observed. Second, the emerging Wi-Fi chipsets, based on IEEE 802.11n/ac, consume much more energy than its legacy IEEE 802.11a/b/g counterparts due to the usage of MIMO and channel bonding. Nowadays, as the battery-powered portable device users place increasingly complex demands on the functionality of their devices which have strict power limitation, satisfaction with robust communication and battery life time is becoming increasingly important. In this dissertation, to address these challenges, we propose robust and energy efficient MAC/PHY layer strategies of Wi-Fi. First of all, to confirm those changes, we have conducted extensive experiments using state-of-the-art commercial IEEE 802.11n/ac-equipped devices and Microsofts Software Radio (Sora) platform. Our experiment results have revealed strong evidence that the use of long A-MPDU frames seriously deteriorates the Wi-Fi performance, i.e., throughput, especially for the pedestrian mobile users. Besides, we have found that the use of channel bonding remarkably consumes more energy, thus making Wi- Fi a primary energy consumer in the battery-powered portable devices. Especially, the energy cost is dominated by excessive and unnecessary listening and receiving operations. We begin an intra-frame rate control algorithm (Intra-RCA) design, called SNR- aware Intra-frame Rate Adaption (SIRA), which enhances the system performance of Wi-Fi in fast time-varying environments [3]. Widely used inter-frame rate control algorithms (Inter-RCAs), which select the PHY rate of each frame based on the time- ii averaged frame loss rate and the signal strength statistics, perform poorly for a long A-MPDU due to the channel variation in mobile environments. Unlike the previous ap- proaches, SIRA adapts the PHY rate on intra-frame basis, i.e., the PHY rate is updated in the middle of a frame according to user mobility. The performance of the proposed scheme is also evaluated by a trace-driven link level simulator employing the collected channel traces from real measurements. The simulation results show that SIRA outper- forms a standalone Inter-RCA in all tested traces. Despite its enhanced performance and considerable frame error reduction, the performance degradation caused by the impact of user mobility still remains due to the inherent limitation of IEEE 802.11 PHY design. Therefore, we conclude that this challenge should be solved with the assistance of PHY modification, and propose Channel-Aware Symbol Error Reduction (ChASER), a new practical channel estimation and tracking scheme for Wi-Fi receivers [4]. ChASER utilizes re-encoding and re-modulation of the received data symbol to keep up with the wireless channel dynamics at the granularity of orthogonal frequency division multi- plexing (OFDM) symbols. In addition, its low-complexity and feasibility of standard compliance is demonstrated by Microsofts Sora prototype implementation and experi- mentation. To our knowledge, ChASER is the first IEEE 802.11n-compatible channel tracking algorithm since other approaches addressing the time-varying channel con- ditions over a single (aggregated) frame duration require costly modifications of the IEEE 802.11 standard. Even though the above proposed approaches enhance the Wi- Fis throughput performance and robustness over conventional technique, the rest re- quirement of the portable device user, i.e., energy efficient Wi-Fi system design, should be addressed as ever. Accordingly, we propose a new power save operation as well as the corresponding protocol, called WiFi in Zizz (WiZizz), which judiciously exploits the characteristic iii of the channel bonding defined in IEEE 802.11ac and efficiently handles the channel bandwidth in an on-demand manner to minimize the traumatic energy spent by IEEE 802.11ac devices [5]. Our extensive measurement and simulation show significant per- formance improvement (as high as 73% energy saving) over a wide range of commu- nication scenarios. In addition, the feasibility of easy implementation is demonstrated by a prototype with a commercial 802.11ac device. To the best of our knowledge, WiZizz is the first IEEE 802.11ac-congenial energy efficient bandwidth management while other existing approaches require costly modifications of the IEEE 802.11ac specification. In summary, we propose a number of compelling algorithms and protocols to im- prove the robustness and energy efficiency in accordance with the paradigm shift of Wi-Fi. Moreover, our evaluation results show that the proposed schemes in this disser- tation are effective and yield considerable performance gain based on both the trace- driven link level simulation and the network level simulation which well reflects the wireless channel characteristics of the real world and the operation of IEEE 802.11 WLAN, respectively. We demonstrate the feasibility of our approaches by implement- ing prototypes in off-the-self IEEE 802.11n/ac devices and software-defined radio.Abstract Contents List of Tables List of Figures 1 Introduction 1.1 Paradigm Shift of Wi-Fi 1.2 DevelopmentTrend of IEEE802.11 1.2.1 MAC Features 1.2.2 PHY Features 1.3 Overview of Existing Approaches 1.3.1 Rate Adaptation 1.3.2 MPDU Length Optimization 1.3.3 Channel Estimation 1.3.4 Demystifying Wi-Fi Power Consumption 1.3.5 Minimizing Energy Consumption of Wi-Fi 1.4 Main Contributions 1.4.1 Impact of Mobility Analysis 1.4.2 Intra-frame Rate Adaptation 1.4.3 Enhanced Channel Estimation and Tracking 1.4.4 802.11ac Power Consumption Analysis 1.4.5 Energy Efficient Bandwidth Management 1.5 Organization of the Dissertation 2 Impact of Mobility 2.1 Introduction 2.2 Background 2.2.1 Channel Estimation and Compensation 2.2.2 Role of Pilot Subcarriers 2.2.3 Frame Aggregation 2.3 Measurement Study 2.3.1 Experimental Settings 2.3.2 Temporal Selectivity 2.3.3 Unreliability of A-MPDU in Mobile Environments 2.3.4 Relation between Symbol Dispersion and Mobility 2.4 Summary 3 SIRA: SNR-aware Intra-frame Rate Adaptation 3.1 Introduction 3.1.1 Revisit of Rate Adaptation Algorithms 3.1.2 Channel and Mobility Condition 3.2 ProposedAlgorithm 3.2.1 Pilot-based SNR Estimation 3.2.2 Mobility Detection 3.2.3 Unequal Modulation and Coding Scheme 3.2.4 Zero-overhead Feedback 3.2.5 SIRA Structure 3.3 Simulation 3.3.1 Trace-drivenLinkLevelSimulation 3.3.2 Mobility Detection Accuracy 3.3.3 Performance Comparison 3.4 Summary 4 ChASER: Channel-Aware Symbol Error Reduction 4.1 Introduction 4.2 Revisit of Channel Estimation Algorithms 4.3 ChASER Design 4.3.1 Channel Estimation using Unknown Data Symbols 4.3.2 Adaptive Filter 4.3.3 Adaptive Filter for MIMO 4.3.4 CRC-assisted Channel Correction 4.3.5 Summary of ChASER Operation 4.3.6 Impact of Step Size μ 4.4 Testbed Experiments 4.4.1 Prototype Implementation on SDR Platform 4.4.2 Testbed Settings 4.4.3 Performance Comparison 4.5 Simulation 4.5.1 Simulation Methodology 4.5.2 Estimation Accuracy 4.5.3 Impact of the A-MPDU Duration 4.5.4 Throughput Performance 4.6 Summary 5 Power Consumption of Wi-Fi: Modeling and Testbed Validation 5.1 Introduction 5.2 Revisit of IEEE 802.11ac Features 5.2.1 Wider Bandwidth Channel 5.2.2 MIMO and Higher Order Modulation 5.2.3 Relation between 802.11ac Features and Power 5.3 Modeling 802.11ac Power Consumption 5.3.1 Power Model of IEEE 802.11ac Receiver 5.3.2 Power Model of IEEE 802.11ac Transmitter 5.4 Power Consumption Measurement 5.4.1 Experimental Setting 5.4.2 Idle State Power Consumption 5.4.3 Receive State Power Consumption 5.4.4 Transmit State Power Consumption 5.4.5 PowerModelVerification 5.4.6 Summary 6 WiZizz: Energy Efficient Bandwidth Management 6.1 Introduction 6.2 WiFi Need to Zizz 6.3 WiZizz Design 6.3.1 Dynamic Mode 6.3.2 Pseudo-dynamic Mode 6.3.3 PHY-level Filtering 6.4 Testbed Experiments 6.4.1 Prototype Implementation and Testbed Setting 6.4.2 Bandwidth Switching Delay 6.4.3 Performance Evaluation 6.5 SimulationResults 6.5.1 Simulation Methodology 6.5.2 Constant Traffic Source with FixedMCS 6.5.3 Comprehensive Traffic Patterns 6.5.4 Collaboration with SMPS 6.6 Summary 7 Conclusion and Future Work 7.1 Research Contributions 7.2 Further Research Plans Abstract (In Korean) 감사의 글Docto

Rethinking Wireless: Building Next-Generation Networks

We face a growing challenge to the design, deployment and management of wireless networks that largely stems from the need to operate in an increasingly spectrum-sparse environment, the need for greater concurrency among devices and the need for greater coordination between heterogeneous wireless protocols. Unfortunately, our current wireless networks lack interoperability, are deployed with fixed functions, and omit easy programmability and extensibility from their key design requirements. In this dissertation, we study the design of next-generation wireless networks and analyze the individual components required to build such an infrastructure. Re-designing a wireless architecture must be undertaken carefully to balance new and coordinated multipoint (CoMP) techniques with the backward compatibility necessary to support the large number of existing devices. These next-generation wireless networks will be predominantly software-defined and will have three components: (a) a wireless component that consists of software-defined radio resource units (RRUs) or access points (APs); (b) a software-defined backhaul control plane that manages the transfer of RF data between the RRUs and the centralized processing resource; and (c) a centralized datacenter/cloud compute resource that processes RF signal data from all attached RRUs. The dissertation addresses the following four key problems in next-generation networks: (1) Making Existing Wireless Devices Spectrum-Agile, (2) Cooperative Compression of the Wireless Backhaul, (3) Spectrum Coordination and (4) Spectrum Coordination.PhDComputer Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/102341/1/zontar_1.pd

WLAN-paikannuksen elinkaaren tukeminen

The advent of GPS positioning at the turn of the millennium provided consumers with worldwide access to outdoor location information. For the purposes of indoor positioning, however, the GPS signal rarely penetrates buildings well enough to maintain the same level of positioning granularity as outdoors. Arriving around the same time, wireless local area networks (WLAN) have gained widespread support both in terms of infrastructure deployments and client proliferation. A promising approach to bridge the location context then has been positioning based on WLAN signals. In addition to being readily available in most environments needing support for location information, the adoption of a WLAN positioning system is financially low-cost compared to dedicated infrastructure approaches, partly due to operating on an unlicensed frequency band. Furthermore, the accuracy provided by this approach is enough for a wide range of location-based services, such as navigation and location-aware advertisements. In spite of this attractive proposition and extensive research in both academia and industry, WLAN positioning has yet to become the de facto choice for indoor positioning. This is despite over 20 000 publications and the foundation of several companies. The main reasons for this include: (i) the cost of deployment, and re-deployment, which is often significant, if not prohibitive, in terms of work hours; (ii) the complex propagation of the wireless signal, which -- through interaction with the environment -- renders it inherently stochastic; (iii) the use of an unlicensed frequency band, which means the wireless medium faces fierce competition by other technologies, and even unintentional radiators, that can impair traffic in unforeseen ways and impact positioning accuracy. This thesis addresses these issues by developing novel solutions for reducing the effort of deployment, including optimizing the indoor location topology for the use of WLAN positioning, as well as automatically detecting sources of cross-technology interference. These contributions pave the way for WLAN positioning to become as ubiquitous as the underlying technology.GPS-paikannus avattiin julkiseen käyttöön vuosituhannen vaihteessa, jonka jälkeen sitä on voinut käyttää sijainnin paikantamiseen ulkotiloissa kaikkialla maailmassa. Sisätiloissa GPS-signaali kuitenkin harvoin läpäisee rakennuksia kyllin hyvin voidakseen tarjota vastaavaa paikannustarkkuutta. Langattomat lähiverkot (WLAN), mukaan lukien tukiasemat ja käyttölaitteet, yleistyivät nopeasti samoihin aikoihin. Näiden verkkojen signaalien käyttö on siksi alusta asti tarjonnut lupaavia mahdollisuuksia sisätilapaikannukseen. Useimmissa ympäristöissä on jo valmiit WLAN-verkot, joten paikannuksen käyttöönotto on edullista verrattuna järjestelmiin, jotka vaativat erillisen laitteiston. Tämä johtuu osittain lisenssivapaasta taajuusalueesta, joka mahdollistaa kohtuuhintaiset päätelaitteet. WLAN-paikannuksen tarjoama tarkkuus on lisäksi riittävä monille sijaintipohjaisille palveluille, kuten suunnistamiselle ja paikkatietoisille mainoksille. Näistä lupaavista alkuasetelmista ja laajasta tutkimuksesta huolimatta WLAN-paikannus ei ole kuitenkaan pystynyt lunastamaan paikkaansa pääasiallisena sisätilapaikannusmenetelmänä. Vaivannäöstä ei ole puutetta; vuosien saatossa on julkaistu yli 20 000 tieteellistä artikkelia sekä perustettu useita yrityksiä. Syitä tähän kehitykseen on useita. Ensinnäkin, paikannuksen pystyttäminen ja ylläpito vaativat aikaa ja vaivaa. Toiseksi, langattoman signaalin eteneminen ja vuorovaikutus ympäristön kanssa on hyvin monimutkaista, mikä tekee mallintamisesta vaikeaa. Kolmanneksi, eri teknologiat ja laitteet kilpailevat lisenssivapaan taajuusalueen käytöstä, mikä johtaa satunnaisiin paikannustarkkuuteen vaikuttaviin tietoliikennehäiriöihin. Väitöskirja esittelee uusia menetelmiä joilla voidaan merkittävästi pienentää paikannusjärjestelmän asennuskustannuksia, jakaa ympäristö automaattisesti osiin WLAN-paikannusta varten, sekä tunnistaa mahdolliset langattomat häiriölähteet. Nämä kehitysaskeleet edesauttavat WLAN-paikannuksen yleistymistä jokapäiväiseen käyttöön

Interference Management in Dense 802.11 Networks

Wireless networks are growing at a phenomenal rate. This growth is causing an overcrowding of the unlicensed RF spectrum, leading to increased interference between co-located devices. Existing decentralized medium access control (MAC) protocols (e.g. IEEE 802.11a/b/g standards) are poorly designed to handle interference in such dense wireless environments. This is resulting in networks with poor and unpredictable performance, especially for delay-sensitive applications such as voice and video. This dissertation presents a practical conflict-graph (CG) based approach to designing self-organizing enterprise wireless networks (or WLANs) where interference is centrally managed by the network infrastructure. The key idea is to use potential interference information (available in the CG) as an input to algorithms that optimize the parameters of the WLAN.We demonstrate this idea in three ways. First, we design a self-organizing enterprise WLAN and show how the system enhances performance over non-CG based schemes, in a high fidelity network simulator. Second, we build a practical system for conflict graph measurement that can precisely measure interference (for a given network configuration) in dense wireless environments. Finally, we demonstrate the practical benefits of the conflict graph system by using it in an optimization framework that manages associations and traffic for mobile VoIP clients in the enterprise. There are a number of contributions of this dissertation. First, we show the practical application of conflict graphs for infrastructure-based interference management in dense wireless networks. A prototype design exhibits throughput gains of up to 50% over traditional approaches. Second, we develop novel schemes for designing a conflict graph measurement system for enterprise WLANs that can detect interference at microsecond-level timescales and with little network overhead. This allows us to compute the conflict graph up to 400 times faster as compared to the current best practice proposed in the literature. The system does not require any modifications to clients or any specialized hardware for its operation. Although the system is designed for enterprise WLANs, the proposed techniques and corresponding results are applicable to other wireless systems as well (e.g. wireless mesh networks). Third, our work opens up the space for designing novel fine-grained interference-aware protocols/algorithms that exploit the ability to compute the conflict graph at small timescales. We demonstrate an instance of such a system with the design and implementation of an architecture that dynamically manages client associations and traffic in an enterprise WLAN. We show how mobile clients sustain uninterrupted and consistent VoIP call quality in the presence of background interference for the duration of their VoIP sessions

Cooperation Strategies for Enhanced Connectivity at Home

WHILE AT HOME , USERS MAY EXPERIENCE A POOR I NTERNET SERVICE while being connected to their 802.11 Access Points (APs). The AP is just one component of the Internet Gateway (GW) that generally includes a backhaul connection (ADSL, fiber,etc..) and a router providing a LAN. The root cause of performance degradation may be poor/congested wireless channel between the user and the GW or congested/bandwidth limited backhaul connection. The latter is a serious issue for DSL users that are located far from the central office because the greater the distance the lesser the achievable physical datarate. Furthermore, the GW is one of the few devices in the home that is left always on, resulting in energy waste and electromagnetic pollution increase. This thesis proposes two strategies to enhance Internet connectivity at home by (i) creating a wireless resource sharing scheme through the federation and the coordination of neighboring GWs in order to achieve energy efficiency while avoiding congestion, (ii) exploiting different king of connectivities, i.e., the wired plus the cellular (3G/4G) connections, through the aggregation of the available bandwidth across multiple access technologies. In order to achieve the aforementioned strategies we study and develop: • A viable interference estimation technique for 802.11 BSSes that can be implemented on commodity hardware at the MAC layer, without requiring active measurements, changes in the 802.11 standard, cooperation from the wireless stations (WSs). We extend previous theoretical results on the saturation throughput in order to quantify the impact in term of throughput loss of any kind of interferer. We im- plement and extensively evaluate our estimation technique with a real testbed and with different kind of interferer, achieving always good accuracy. • Two available bandwidth estimation algorithms for 802.11 BSSes that rely only on passive measurements and that account for different kind of interferers on the ISM band. This algorithms can be implemented on commodity hardware, as they require only software modifications. The first algorithm applies to intra-GW while the second one applies to inter-GW available bandwidth estimation. Indeed, we use the first algorithm to compute the metric for assessing the Wi-Fi load of a GW and the second one to compute the metric to decide whether accept incoming WSs from neighboring GWs or not. Note that in the latter case it is assumed that one or more WSs with known traffic profile are requested to relocate from one GW to another one. We evaluate both algorithms with simulation as well as with a real test-bed for different traffic patterns, achieving high precision. • A fully distributed and decentralized inter-access point protocol for federated GWs that allows to dynamically manage the associations of the wireless stations (WSs) in the federated network in order to achieve energy efficiency and offloading con- gested GWs, i.e, we keep a minimum number of GWs ON while avoiding to create congestion and real-time throughput loss. We evaluate this protocol in a federated scenario, using both simulation and a real test-bed, achieving up to 65% of energy saving in the simulated setting. We compare the energy saving achieved by our protocol against a centralized optimal scheme, obtaining close to optimal results. • An application level solution that accelerates slow ADSL connections with the parallel use of cellular (3G/4G) connections. We study the feasibility and the potential performance of this scheme at scale using both extensive throughput measurement of the cellular network and trace driven analysis. We validate our solution by implementing a real test bed and evaluating it “in the wild, at several residential locations of a major European city. We test two applications: Video-on-Demand (VoD) and picture upload, obtaining remarkable throughput increase for both applications at all locations. Our implementation features a multipath scheduler which we compare to other scheduling policies as well as to transport level solution like MTCP, obtaining always better results