16 research outputs found

    Characterising the interactions between unicast and broadcast in IEEE 802.11 ad hoc networks

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    This paper investigates the relative performance of unicast and broadcast traffic traversing a one-hop ad hoc network utilising the 802.11 DCF. An extended Markov model has been developed and validated through computer simulation, which successfully predicts the respective performance of unicast and broadcast in a variety of mixed traffic scenarios. Under heavy network traffic conditions, a significant divergence is seen to develop between the performance of the two traffic classes - in particular, when network becomes saturated, unicast traffic is effectively given higher precedence over broadcast. As a result, the network becomes dominated by unicast frames, leading to poor rates of broadcast frame delivery

    Mac-Phy Cross-Layer analysis and design of Mimo-Ofdm Wlans based on fast link adaptation

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    The latestWLAN standard, known as IEEE 802.11n, has notably increased the network capacity with respect to its predecessors thanks to the incorporation of the multipleinput multiple-output (MIMO) technology. Nonetheless, the new amendment, as its previous ones, does not specify how crucial configuration mechanisms, most notably the adaptive modulation and coding (AMC) algorithm should be implemented. The AMC process has proved essential to fully exploit the system resources in light of varying channel conditions. In this dissertation, a closed-loop AMC technique, referred to as fast link adaption (FLA) algorithm, that effectively selects themodulation and coding scheme (MCS) for multicarriermultiantennaWLAN networks is proposed. The FLA algorithm determines the MCS that maximizes the throughput while satisfying a quality of service (QoS) constraint, usually defined in the form of an objective packet error rate (PER). To this end, FLA uses a packet/bit error rate prediction methodology based on the exponential effective SNRmetric (EESM). The FLA algorithm performance has been evaluated under IEEE 802.11n systems that thanks to the incorporation of a feedbackmechanismare able to implement closed- loop AMC mechanisms. Initially, this AMC technique relies only on physical layer information but it is subsequently extended to also take into account themediumaccess control (MAC) sublayer performance. At the physical layer, the FLA algorithm has demonstrated its effectivity by performing very close to optimality in terms of throughput, while satisfying a prescribed PER constraint. The FLA algorithm has also been evaluated using imperfect channel information. It has been observed that the proposed FLA technique is rather robust against imperfect channel information, and only in highly-frequency selective channels, imperfect channel knowledge causes a noticeable degradation in throughput. At the MAC sublayer, the FLA algorithm has been complemented with a timeout strategy that weighs down the influence of the available channel information as this becomes outdated. This channel information outdate is caused by the MAC sublayer whose user multiplexing policy potentially results in large delays between acquiring the instant in which the channel state information is acquired and that in which the channel is accessed. Results demonstrate the superiority of FLA when compared to open-loop algorithms under saturated and non-saturated conditions and irrespective of the packet length, number of users, protocol (CSMA/CA or CDMA/E2CA) and access scheme (Basic Access or RTS/CTS). Additionally, several analytical models have been developed to estimate the system performance at the MAC sublayer. These models account for all operational details of the IEEE 802.11n MAC sublayer, such as finite number of retries, anomalous slot or channel errors. In particular, a semi-analytical model that assesses the MAC layer throughput under saturated conditions, considering the AMC performance is first introduced. Then, an analytical model that allows the evaluation of the QoS performance under non-saturated conditions is presented. This model focuses on single MCS and it is able to accurately predict very important system performance metrics such as blocking probability, delay, probability of discard or goodput thanks to the consideration of the finite queues on each station. Finally, the previous non-saturated analytical approach is used to define a semi-analytical model in order to estimate the system performance when considering AMC algorithms (i.e. whenmultiple MCSs are available)La darrera versi贸 de l鈥檈st脿ndard deWLAN, anomenada IEEE 802.11n, ha augmentat la seva capacitat notablement en relaci贸 als sistemes anteriors gr脿cies a la incorporaci贸 de la tecnologia de m煤ltiples antenes en transmissi贸 i recepci贸 (MIMO). No obstant aix貌, la nova proposta, al igual que les anteriors, segueix sense especificar com s鈥檋an d鈥檌mplementar elsmecanismes de configuraci贸m茅s crucials, un dels quals 茅s l鈥檃lgoritme de codificaci贸 imodulaci贸 adaptativa (AMC). Aquests algoritmes ja han demostrat la seva import脿ncia a l鈥檋ora demaximitzar el rendiment del sistema tenint en compte les condicions canviants del canal. En aquesta tesis s鈥檋a proposat un algoritme AMC de lla莽 tancat, anomenat adaptaci贸 r脿pida de l鈥檈nlla莽 (FLA), que selecciona eficientment l鈥檈squema demodulaci贸 i codificaci贸 adaptativa per xarxes WLAN basades en arquitectures multiportadora multiantena. L鈥檃lgoritme FLA determina el mode de transmissi贸 capa莽 de maximitzar el throughput per les condicions de canal actuals, mentre satisf脿 un requisit de qualitat de servei en forma de taxa d鈥檈rror per paquet (PER). FLA utilitza una metodologia de predicci贸 de PER basada en l鈥檈stimaci贸 de la relaci贸 senyal renou (SNR) efectiva exponencial (EESM). El rendiment de l鈥檃lgoritme FLA ha estat avaluat en sistemes IEEE 802.11n, ja que aquests, gr脿cies a la incorporaci贸 d鈥檜nmecanisme de realimentaci贸 demodes de transmissi贸, poden adoptar solucions AMC de lla莽 tancat. En una primera part, l鈥檈studi s鈥檋a centrat a la capa f铆sica i despr茅s s鈥檋a est猫s a la subcapa MAC. A la capa f铆sica s鈥檋a demostrat l鈥檈fectivitat de l鈥檃lgoritme FLA aconseguint un rendiment molt proper al que ens proporcionaria un esquema AMC 貌ptim en termes de throughput, alhora que es satisfan els requisits de PER objectiu. L鈥檃lgoritme FLA tamb茅 ha estat avaluat utilitzant informaci贸 imperfecte del canal. S鈥檋a vist que l鈥檃lgoritme FLA proposat 茅s robust en front dels efectes d鈥檈stimaci贸 imperfecte del canal, i nom茅s en canals altament selectius en freq眉猫ncia, la informaci贸 imperfecte del canal provoca una davallada en el rendiment en termes de throughput. A la subcapa MAC, l鈥檃lgoritme FLA ha estat complementat amb una estrat猫gia de temps d鈥檈spera que disminueix la depend猫ncia amb la informaci贸 de canal disponible a mesura que aquesta va quedant desfassada respecte de l鈥檈stat actual. Aquesta informaci贸 de canal desfassada 茅s conseq眉猫ncia de la subcapa MAC que degut a la multiplexaci贸 d鈥檜suaris introdueix grans retards entre que es determina el mode de transmissi贸 m茅s adequat i la seva utilitzaci贸 per a l鈥檃cc茅s al canal. Els resultats obtinguts han demostrat la superioritat de FLA respecte d鈥檃ltres algoritmes de lla莽 obert en condicions de saturaci贸 i de no saturaci贸, i independentment de la longitud de paquet, nombre d鈥檜suaris, protocol (CSMA/CA i CSMA/E2CA) i esquema d鈥檃cc茅s (Basic Access i RTS/CTS). Am茅s, s鈥檋an desenvolupat diversosmodels anal铆tics per tal d鈥檈stimar el rendiment del sistema a la subcapa MAC. Aquests models consideren tots els detalls de funcionament de la subcapaMAC del 802.11n, comper exemple un nombre finit de retransmissions de cada paquet, l鈥檚lot an貌mal o els errors introdu茂ts pel canal. Inicialment s鈥檋a proposat unmodel semi-anal铆tic que determina el throughtput en condicions de saturaci贸, considerant el rendiment dels algoritmes AMC. Despr茅s s鈥檋a presentat un model anal铆tic que estima el rendiment del sistema per condicions de no saturaci贸, mitjan莽at elmodelat de cues finites a cada estaci贸. Aquestmodel consideramodes de transmissi贸 fixes i 茅s capa莽 de determinar de manera molt precisa m猫triques de rendimentmolt importants coms贸n la probabilitat de bloqueig de cada estaci贸, el retard mitj脿 del paquets, la probabilitat de descart o la mesura del goodput. Finalment, el model anal铆tic de no saturaci贸 s鈥檋a utilitzat per definir un model semi-anal铆tic per tal d鈥檈stimar el rendiment del sistema quan es considera l鈥櫭簊 d鈥檃lgoritmes AMC

    Performance Prediction and Tuning for Symmetric Coexistence of WiFi and ZigBee Networks

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    Due to the explosive deployment of WiFi and ZigBee wireless networks, 2.4GHz ISM bands (2.4GHz-2.5GHz) are becoming increasingly crowded, and the co-channel coexistence of these two networks is inevitable. For coexistence networks, people always want to predict their performance (e.g. throughput, energy consumption, etc.) before deployment, or even want to tune parameters to compensate unnecessary performance degradation (owing to the huge differences between these two MAC protocols) or to satisfy some performance requirements (e.g., priority, delay constraint, etc.) of them. However, predicting and tuning performance of coexisting WiFi and ZigBee networks has been a challenging task, primarily due to the lack of corresponding simulators and analytical models. In this dissertation, we addressed the aforementioned problems by presenting simulators and models for the coexistence of WiFi and ZigBee devices. Specifically, based on the energy efficiency and traffic pattern of three practical coexistence scenarios: disaster rescue site, smart hospital and home automation. We first of all classify them into three classes, which are non-sleeping devices with saturated traffic (SAT), non-sleeping devices with unsaturated traffic (UNSAT) and duty-cycling devices with unsaturated traffic (DC-UNSAT). Then a simulator and an analytical model are proposed for each class, where each simulator is verified by simple hardware based experiment. Next, we derive the expressions for performance metrics like throughput, delay etc., and predict them using both the proposed simulator and the model. Due to the higher accuracy of the simulator, the results from them are used as the ground truth to validate the accuracy of the model. Last, according to some common performance tuning requirements for each class, we formulate them into optimization problems and propose the corresponding solving methods. The results show that the proposed simulators have high accuracy in performance prediction, while the models, although are less accurate than the former, can be used in fast prediction. In particular, the models can also be easily used in optimization problems for performance tuning, and the results prove its high efficiency

    Spectrum Sharing Methods in Coexisting Wireless Networks

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    Radio spectrum, the fundamental basis for wireless communication, is a finite resource. The development of the expanding range of radio based devices and services in recent years makes the spectrum scarce and hence more costly under the paradigm of extensive regulation for licensing. However, with mature technologies and with their continuous improvements it becomes apparent that tight licensing might no longer be required for all wireless services. This is from where the concept of utilizing the unlicensed bands for wireless communication originates. As a promising step to reduce the substantial cost for radio spectrum, different wireless technology based networks are being deployed to operate in the same spectrum bands, particularly in the unlicensed bands, resulting in coexistence. However, uncoordinated coexistence often leads to cases where collocated wireless systems experience heavy mutual interference. Hence, the development of spectrum sharing rules to mitigate the interference among wireless systems is a significant challenge considering the uncoordinated, heterogeneous systems. The requirement of spectrum sharing rules is tremendously increasing on the one hand to fulfill the current and future demand for wireless communication by the users, and on the other hand, to utilize the spectrum efficiently. In this thesis, contributions are provided towards dynamic and cognitive spectrum sharing with focus on the medium access control (MAC) layer, for uncoordinated scenarios of homogeneous and heterogeneous wireless networks, in a micro scale level, highlighting the QoS support for the applications. This thesis proposes a generic and novel spectrum sharing method based on a hypothesis: The regular channel occupation by one system can support other systems to predict the spectrum opportunities reliably. These opportunities then can be utilized efficiently, resulting in a fair spectrum sharing as well as an improving aggregated performance compared to the case without having special treatment. The developed method, denoted as Regular Channel Access (RCA), is modeled for systems specified by the wireless local resp. metropolitan area network standards IEEE 802.11 resp. 802.16. In the modeling, both systems are explored according to their respective centrally controlled channel access mechanisms and the adapted models are evaluated through simulation and results analysis. The conceptual model of spectrum sharing based on the distributed channel access mechanism of the IEEE 802.11 system is provided as well. To make the RCA method adaptive, the following enabling techniques are developed and integrated in the design: a RSS-based (Received Signal Strength based) detection method for measuring the channel occupation, a pattern recognition based algorithm for system identification, statistical knowledge based estimation for traffic demand estimation and an inference engine for reconfiguration of resource allocation as a response to traffic dynamics. The advantage of the RCA method is demonstrated, in which each competing collocated system is configured to have a resource allocation based on the estimated traffic demand of the systems. The simulation and the analysis of the results show a significant improvement in aggregated throughput, mean delay and packet loss ratio, compared to the case where legacy wireless systems coexists. The results from adaptive RCA show its resilience characteristics in case of dynamic traffic. The maximum achievable throughput between collocated IEEE 802.11 systems applying RCA is provided by means of mathematical calculation. The results of this thesis provide the basis for the development of resource allocation methods for future wireless networks particularly emphasized to operate in current unlicensed bands and in future models of the Open Spectrum Alliance

    Performance Modeling and Analysis of Wireless Local Area Networks with Bursty Traffic

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    The explosive increase in the use of mobile digital devices has posed great challenges in the design and implementation of Wireless Local Area Networks (WLANs). Ever-increasing demands for high-speed and ubiquitous digital communication have made WLANs an essential feature of everyday life. With audio and video forming the highest percentage of traffic generated by multimedia applications, a huge demand is placed for high speed WLANs that provide high Quality-of-Service (QoS) and can satisfy end user鈥檚 needs at a relatively low cost. Providing video and audio contents to end users at a satisfactory level with various channel quality and current battery capacities requires thorough studies on the properties of such traffic. In this regard, Medium Access Control (MAC) protocol of the 802.11 standard plays a vital role in the management and coordination of shared channel access and data transmission. Therefore, this research focuses on developing new efficient analytical models that evaluate the performance of WLANs and the MAC protocol in the presence of bursty, correlated and heterogeneous multimedia traffic using Batch Markovian Arrival Process (BMAP). BMAP can model the correlation between different packet size distributions and traffic rates while accurately modelling aggregated traffic which often possesses negative statistical properties. The research starts with developing an accurate traffic generator using BMAP to capture the existing correlations in multimedia traffics. For validation, the developed traffic generator is used as an arrival process to a queueing model and is analyzed based on average queue length and mean waiting time. The performance of BMAP/M/1 queue is studied under various number of states and maximum batch sizes of BMAP. The results clearly indicate that any increase in the number of states of the underlying Markov Chain of BMAP or maximum batch size, lead to higher burstiness and correlation of the arrival process, prompting the speed of the queue towards saturation. The developed traffic generator is then used to model traffic sources in IEEE 802.11 WLANs, measuring important QoS metrics of throughput, end-to-end delay, frame loss probability and energy consumption. Performance comparisons are conducted on WLANs under the influence of multimedia traffics modelled as BMAP, Markov Modulated Poisson Process and Poisson Process. The results clearly indicate that bursty traffics generated by BMAP demote network performance faster than other traffic sources under moderate to high loads. The model is also used to study WLANs with unsaturated, heterogeneous and bursty traffic sources. The effects of traffic load and network size on the performance of WLANs are investigated to demonstrate the importance of burstiness and heterogeneity of traffic on accurate evaluation of MAC protocol in wireless multimedia networks. The results of the thesis highlight the importance of taking into account the true characteristics of multimedia traffics for accurate evaluation of the MAC protocol in the design and analysis of wireless multimedia networks and technologies

    Fuzzy based Channel Selection for Location Oriented Services in Multichannel VCPS Environments

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    Location-oriented services in Vehicular Cyber-Physical System (VCPS) have witnessed significant attention due to their potentiality to address traffic safety and efficiency related issues. The multichannel communication aids these services by tuning their overall performance in vehicular environments. Related literature on multichannel communication is focuses on interference as channel quality measure. However, uncertain mobility and density of vehicles significantly affect channel quality apart from interference. The static quantification of channel quality is not suitable due to the dynamic characteristics of the channel quality parameters. In this context, this paper proposes Fuzzy-based Channel Selection framework for location-oriented services in Multichannel VCPS environments (F-CSMV). A system model is presented for deriving channel access delay using Markov chain model. The channel quality is estimated using channel access delay (CAD) and signal-to-interference ratio (SIR). The fuzzy logic based channel selection framework is developed considering fuzzification and defuzzification of CAD and SIR. The comparative performance evaluation attests the benefit of the framework as compared to the state-of-the-art techniques in VCPS

    Performance and Reliability Evaluation for DSRC Vehicular Safety Communication

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    <p>Inter-Vehicle Communication (IVC) is a vital part of Intelligent Transportation System (ITS), which has been extensively researched in recent years. Dedicated Short Range Communication (DSRC) is being seriously considered by automotive industry and government agencies as a promising wireless technology for enhancing transportation safety and efficiency of road utilization. In the DSRC based vehicular ad hoc networks (VANETs), the transportation safety is one of the most crucial features that needs to be addressed. Safety applications usually demand direct vehicle-to-vehicle ad hoc communication due to a highly dynamic network topology and strict delay requirements. Such direct safety communication will involve a broadcast service because safety information can be beneficial to all vehicles around a sender. Broadcasting safety messages is one of the fundamental services in DSRC. In order to provide satisfactory quality of services (QoS) for various safety applications, safety messages need to be delivered both timely and reliably. To support the stringent delay and reliability requirements of broadcasting safety messages, researchers have been seeking to test proposed DSRC protocols and suggesting improvements. A major hurdle in the development of VANET for safety-critical services is the lack of methods that enable one to determine the effectiveness of VANET design mechanism for predictable QoS and allow one to evaluate the tradeoff between network parameters. Computer simulations are extensively used for this purpose. A few analytic models and experiments have been developed to study the performance and reliability of IEEE 802.11p for safety-related applications. In this thesis, we propose to develop detailed analytic models to capture various safety message dissemination features such as channel contention, backoff behavior, concurrent transmissions, hidden terminal problems, channel fading with path loss, multi-channel operations, multi-hop dissemination in 1-Dimentional or 2-Dimentional traffic scenarios. MAC-level and application-level performance metrics are derived to evaluate the performance and reliability of message broadcasting, which provide insights on network parameter settings. Extensive simulations in either Matlab or NS2 are conducted to validate the accuracy of our proposed models.</p>Dissertatio

    Towards Efficient and Enhanced Wireless Coexistence in the Unlicensed Spectrum

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    The 3rd Generation Partnership Project (3GPP) is developing the fifth generation (5G) of wireless broadband technology and has identified the unlicensed spectrum as a principal item on the plan of action. Listen-Before-Talk (LBT) has been recognized as the starting development point for the channel access scheme of future 5G New Radio-Unlicensed (NR-U) networks. Recent technical reports suggest that all sub-7 GHz unlicensed spectrum is targeted for 5G NR-U operation, including the 2.4 GHz Industrial, Scientific, and Medical (ISM) band. Literature is inundated with research on Wi-Fi and LBT-based long-term evolution License-Assisted Access (LTE-LAA) wireless coexistence analysis. While a treasure trove of radio spectrum has been approved for license-exempt use in the 6 GHz band, industry and standard organizations must make sure it is well utilized by enhancing their coexistence schemes. A proper assessment of the homogeneous LBT deployment is imperative under the new use cases and regulatory circumstances. The work presented herein aimed to fill the gap and underline the importance of improving channel access mechanisms in next-generation wireless systems. The research in this dissertation first analyzed the LBT channel access scheme and analytically evaluated its performance in terms of a metrics set, such as effective channel utilization, collision probability, mean access delay, and temporal fairness among coexisting nodes. Outcomes of the developed analytical model revealed inefficiencies in various cases. For example, high priority classes generally hinder overall effective channel utilization, exhibit a high collision rate, and incur long latencies compared to lower priorities; and low priority classes sustain longer delays in class-heterogeneous scenarios. The developed framework was then utilized to investigate wireless coexistence in a 5G-enabled intensive care unit, employing remote patient monitoring over 5G NR-U. A modified LBT scheme is then proposed in this work to enhance overall channel efficiency in homogeneous LBT deployments by reducing the collision probability among coexisting stations based on the analytical investigation of the LBT mechanism. It is expected that low-power, narrowband frequency hoppers will be allowed to operate in the 6 GHz spectrum based on recent European Communications Committee (ECC) mandates, which raises speculation around coexistence with incumbent radio access technologies (RATs). To address the potential operation of cellular LBT in the 2.4 GHz and frequency hopping systems in the 5- and 6-GHz bands, the coexistence of Bluetooth Low Energy (BLE) 5 and LBT was investigated empirically in an anechoic chamber. The mutual impact was explored by means of throughput, packet error rate, and interframe delays. Empirical evaluation results demonstrated how BLE throughput dropped as the intended-to-unintended signal ratio decreased and the way in which LBT classes exhibited a diminishing effect as the class priority descended. Long Range BLE physical layer (PHY) was found to sustain longer gap times (i.e., delay) than the other two PHYs; however, the LR PHY showed less susceptibility to interference. Results also demonstrated that low data rate BLE PHYs hindered LBT throughput performance since they correspond to longer airtime durations

    Energy Efficient and Cooperative Solutions for Next-Generation Wireless Networks

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    Energy efficiency is increasingly important for next-generation wireless systems due to the limited battery resources of mobile clients. While fourth generation cellular standards emphasize low client battery consumption, existing techniques do not explicitly focus on reducing power that is consumed when a client is actively communicating with the network. Based on high data rate demands of modern multimedia applications, active mode power consumption is expected to become a critical consideration for the development and deployment of future wireless technologies. Another reason for focusing more attention on energy efficient studies is given by the relatively slow progress in battery technology and the growing quality of service requirements of multimedia applications. The disproportion between demanded and available battery capacity is becoming especially significant for small-scale mobile client devices, where wireless power consumption dominates within the total device power budget. To compensate for this growing gap, aggressive improvements in all aspects of wireless system design are necessary. Recent work in this area indicates that joint link adaptation and resource allocation techniques optimizing energy efficient metrics can provide a considerable gain in client power consumption. Consequently, it is crucial to adapt state-of-the-art energy efficient approaches for practical use, as well as to illustrate the pros and cons associated with applying power-bandwidth optimization to improve client energy efficiency and develop insights for future research in this area. This constitutes the first objective of the present research. Together with energy efficiency, next-generation cellular technologies are emphasizing stronger support for heterogeneous multimedia applications. Since the integration of diverse services within a single radio platform is expected to result in higher operator profits and, at the same time, reduce network management expenses, intensive research efforts have been invested into design principles of such networks. However, as wireless resources are limited and shared by clients, service integration may become challenging. A key element in such systems is the packet scheduler, which typically helps ensure that the individual quality of service requirements of wireless clients are satisfied. In contrastingly different distributed wireless environments, random multiple access protocols are beginning to provide mechanisms for statistical quality of service assurance. However, there is currently a lack of comprehensive analytical frameworks which allow reliable control of the quality of service parameters for both cellular and local area networks. Providing such frameworks is therefore the second objective of this thesis. Additionally, the study addresses the simultaneous operation of a cellular and a local area network in spectrally intense metropolitan deployments and solves some related problems. Further improving the performance of battery-driven mobile clients, cooperative communications are sought as a promising and practical concept. In particular, they are capable of mitigating the negative effects of fading in a wireless channel and are thus expected to enhance next-generation cellular networks in terms of client spectral and energy efficiencies. At the cell edges or in areas missing any supportive relaying infrastructure, client-based cooperative techniques are becoming even more important. As such, a mobile client with poor channel quality may take advantage of neighboring clients which would relay data on its behalf. The key idea behind the concept of client relay is to provide flexible and distributed control over cooperative communications by the wireless clients themselves. By contrast to fully centralized control, this is expected to minimize overhead protocol signaling and hence ensure simpler implementation. Compared to infrastructure relay, client relay will also be cheaper to deploy. Developing the novel concept of client relay, proposing simple and feasible cooperation protocols, and analyzing the basic trade-offs behind client relay functionality become the third objective of this research. Envisioning the evolution of cellular technologies beyond their fourth generation, it appears important to study a wireless network capable of supporting machine-to-machine applications. Recent standardization documents cover a plethora of machine-to-machine use cases, as they also outline the respective technical requirements and features according to the application or network environment. As follows from this activity, a smart grid is one of the primary machine-to-machine use cases that involves meters autonomously reporting usage and alarm information to the grid infrastructure to help reduce operational cost, as well as regulate a customer's utility usage. The preliminary analysis of the reference smart grid scenario indicates weak system architecture components. For instance, the large population of machine-to-machine devices may connect nearly simultaneously to the wireless infrastructure and, consequently, suffer from excessive network entry delays. Another concern is the performance of cell-edge machine-to-machine devices with weak wireless links. Therefore, mitigating the above architecture vulnerabilities and improving the performance of future smart grid deployments is the fourth objective of this thesis. Summarizing, this thesis is generally aimed at the improvement of energy efficient properties of mobile devices in next-generation wireless networks. The related research also embraces a novel cooperation technique where clients may assist each other to increase per-client and network-wide performance. Applying the proposed solutions, the operation time of mobile clients without recharging may be increased dramatically. Our approach incorporates both analytical and simulation components to evaluate complex interactions between the studied objectives. It brings important conclusions about energy efficient and cooperative client behaviors, which is crucial for further development of wireless communications technologies

    Contention techniques for opportunistic communication in wireless mesh networks

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    Auf dem Gebiet der drahtlosen Kommunikation und insbesondere auf den tieferen Netzwerkschichten sind gewaltige Fortschritte zu verzeichnen. Innovative Konzepte und Technologien auf der physikalischen Schicht (PHY) gehen dabei zeitnah in zellul盲re Netze ein. Drahtlose Maschennetzwerke (WMNs) k枚nnen mit diesem Innovationstempo nicht mithalten. Die Mehrnutzer-Kommunikation ist ein Grundpfeiler vieler angewandter PHY Technologien, die sich in WMNs nur ungen眉gend auf die etablierte Schichtenarchitektur abbilden l盲sst. Insbesondere ist das Problem des Scheduling in WMNs inh盲rent komplex. Erstaunlicherweise ist der Mehrfachzugriff mit Tr盲gerpr眉fung (CSMA) in WMNs asymptotisch optimal obwohl das Verfahren eine geringe Durchf眉hrungskomplexit盲t aufweist. Daher stellt sich die Frage, in welcher Weise das dem CSMA zugrunde liegende Konzept des konkurrierenden Wettbewerbs (engl. Contention) f眉r die Integration innovativer PHY Technologien verwendet werden kann. Opportunistische Kommunikation ist eine Technik, die die inh盲renten Besonderheiten des drahtlosen Kanals ausnutzt. In der vorliegenden Dissertation werden CSMA-basierte Protokolle f眉r die opportunistische Kommunikation in WMNs entwickelt und evaluiert. Es werden dabei opportunistisches Routing (OR) im zustandslosen Kanal und opportunistisches Scheduling (OS) im zustandsbehafteten Kanal betrachtet. Ziel ist es, den Durchsatz von elastischen Paketfl眉ssen gerecht zu maximieren. Es werden Modelle f眉r 脺berlastkontrolle, Routing und konkurrenzbasierte opportunistische Kommunikation vorgestellt. Am Beispiel von IEEE 802.11 wird illustriert, wie der schicht眉bergreifende Entwurf in einem Netzwerksimulator prototypisch implementiert werden kann. Auf Grundlage der Evaluationsresultate kann der Schluss gezogen werden, dass die opportunistische Kommunikation konkurrenzbasiert realisierbar ist. Dar眉ber hinaus steigern die vorgestellten Protokolle den Durchsatz im Vergleich zu etablierten L枚sungen wie etwa DCF, DSR, ExOR, RBAR und ETT.In the field of wireless communication, a tremendous progress can be observed especially at the lower layers. Innovative physical layer (PHY) concepts and technologies can be rapidly assimilated in cellular networks. Wireless mesh networks (WMNs), on the other hand, cannot keep up with the speed of innovation at the PHY due to their flat and decentralized architecture. Many innovative PHY technologies rely on multi-user communication, so that the established abstraction of the network stack does not work well for WMNs. The scheduling problem in WMNs is inherent complex. Surprisingly, carrier sense multiple access (CSMA) in WMNs is asymptotically utility-optimal even though it has a low computational complexity and does not involve message exchange. Hence, the question arises whether CSMA and the underlying concept of contention allows for the assimilation of advanced PHY technologies into WMNs. In this thesis, we design and evaluate contention protocols based on CSMA for opportunistic communication in WMNs. Opportunistic communication is a technique that relies on multi-user diversity in order to exploit the inherent characteristics of the wireless channel. In particular, we consider opportunistic routing (OR) and opportunistic scheduling (OS) in memoryless and slow fading channels, respectively. We present models for congestion control, routing and contention-based opportunistic communication in WMNs in order to maximize both throughput and fairness of elastic unicast traffic flows. At the instance of IEEE 802.11, we illustrate how the cross-layer algorithms can be implemented within a network simulator prototype. Our evaluation results lead to the conclusion that contention-based opportunistic communication is feasible. Furthermore, the proposed protocols increase both throughput and fairness in comparison to state-of-the-art approaches like DCF, DSR, ExOR, RBAR and ETT
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