Wireless local area networks (WLANs) have experienced tremendous growth with the proliferation of IEEE 802.11 devices in past two decades. Wireless operators are embracing WLAN for cellular offloading in every smartphone nowadays [1]. The traffic over WLAN requires significant improvement of both WLAN throughput and efficiency. To increase throughput, multiple-input and multiple-output (MU-MIMO) has been adopted in the new generation of WLAN, such as IEEE 802.11ac. MU-MIMO systems exploit the spatial separation of users to increase the network throughput. In an MU-MIMO system, efficient channel sounding is essential for achieving optimal throughput. We propose a dynamic sounding approach for MU-MIMO systems in WLANs. We analyse and show that the optimal sounding intervals exist for single user transmit beamforming (SU-TxBF) and MU-MIMO for given channel conditions. We design a low-complexity dynamic sounding approach that adjusts the sounding interval adaptively in real-time. Through our collected over-the-air channel measurements, we demonstrate significant throughput improvements using our proposed dynamic sounding algorithm while being compliant with IEEE 802.11ac standard. Subsequently, we investigate the user grouping problem of downlink WLANs with MU-MIMO. Particularly, we focus on the problem of whether SU-TxBF or MU-MIMO should be utilized, and how many and which users should be in a multi-user (MU) group. We formulate this problem for maximizing the system throughput subject to the multi-user air time fairness (MU-ATF) criterion. We show that hypergraphs provide a suitable mathematical model and effective tool for finding the optimal or close to optimal solution. We show that the optimal grouping problem can be solved efficiently for the case where only SU-TxBF and 2-user MU groups are allowed in the system. For the general case, where any number of users can be assigned to groups of different sizes, we develop an efficient graph matching algorithm (GMA) based on graph theory principles with near-optimal performance. We evaluate the proposed algorithm in terms of system throughput using an 802.11ac emulator using collected channel measurements from an indoor environment and simulated channel samples for outdoor scenarios. We show that the approximate solution, GMA, achieves at least 93% of the optimal system throughput in all considered test cases. A complementary technique for MU-MIMO is orthogonal frequency-division multiple access (OFDMA), which will be the key enabler to maximize spectrum utilization in the next generation of WLAN, IEEE 802.11ax. An unsolved problem for 802.11ax is maximizing the number of satisfied users in the OFDMA system while accommodating the different Quality of Service (QoS) levels. We evaluate the possibility of regulating QoS through prioritizing the users in OFDMA-based WLAN. We define a User Priority Scheduling (UPS) criterion that strictly guarantees service requests of the spectrum and time resources for the users with higher priorities before guaranteeing resources to those of lower priority. We develop an optimization framework to maximize the overall number of satisfied users under this strict priority constraint. A mathematical expression for user satisfaction under prioritization constraints (scheduler) is formulated first and then linearized as a mixed integer linear program that can be efficiently solved using known optimization routine. We also propose a low-complexity scheduler having comparable performance to the optimal solution in most scenarios. Simulation results show that the proposed resource allocation strategy guarantees efficient resource allocation with the user priority constraints in a dense wireless environment. In particular, we show by system simulation that the proposed low-complexity scheduler is an efficient solution in terms of (1) total throughput and network satisfaction rate (less than 10% from the upper bound), and (2) algorithm complexity (within the same magnitude order of conventional scheduling strategy. / Ph. D.

This thesis is concerned with analytical modeling of Wireless Local Area Networks (WLANs) that are based on IEEE 802.11 Distributed Coordination Function (DCF). Such networks are popularly known as WiFi networks. We have developed accurate analytical models for the following three network scenarios: (S1) A single cell WLAN with homogeneous nodes and Poisson packet arrivals, (S2) A multi-cell WLAN (a) with saturated nodes, or (b) with TCP-controlled long-lived downloads, and (S3) A multi-cell WLAN with TCP-controlled short-lived downloads. Our analytical models are simple Markovian abstractions that capture the detailed network behavior in the considered scenarios. The insights provided by our analytical models led to two applications: (i) a faster “model-based'” simulator, and (ii) a distributed channel assignment algorithm. We also study the stability of the network through our Markov models. For scenario (S1), we develop a new approach as compared to the existing literature. We apply a “State Dependent Attempt Rate'” (SDAR) approximation to reduce a single cell WLAN with non-saturated nodes to a coupled queue system. We provide a sufficient condition under which the joint queue length Markov chain is positive recurrent. For the case when the arrival rates into the queues are equal we propose a technique to reduce the state space of the coupled queue system. In addition, when the buffer size of the queues are finite and equal we propose an iterative method to estimate the stationary distribution of the reduced state process. Our iterative method yields accurate predictions for important performance measures, namely, “throughput'”, “collision probability” and “packet delay”. We replace the detailed implementation of the MAC layer in NS-2 with the SDAR contention model, thus yielding a ``model-based'' simulator at the MAC layer. We demonstrate that the SDAR model of contention provides an accurate model for the detailed CSMA/CA protocol in scenario (S1). In addition, since the SDAR model removes much of the details at the MAC layer we obtain speed-ups of 1.55-5.4 depending on the arrival rates and the number of nodes in the single cell WLAN. For scenario (S2), we consider a restricted network setting where a so-called “Pairwise Binary Dependence” (PBD) condition holds. We develop a first-cut scalable “cell-level” model by applying the PBD condition. Unlike a node- or link-level model, the complexity of our cell-level model increases with the number of cells rather than with the number of nodes/links. We demonstrate the accuracy of our cell-level model via NS-2 simulations. We show that, as the “access intensity” of every cell goes to infinity the aggregate network throughput is maximized. This remarkable property of CSMA, namely, “maximization of aggregate network throughput in a distributed manner” has been proved recently by Durvy et al. (TIT, March, 2009) for an infinite linear chain of nodes. We prove it for multi-cell WLANs with arbitrary cell topology (under the PBD condition). Based on this insight provided by our analytical model we propose a distributed channel assignment algorithm. For scenario (S3), we consider the same restricted network setting as for scenario (S2). For Poisson flow arrivals and i.i.d. exponentially distributed flow sizes we model a multi-cell WLAN as a network of processor-sharing queues with state-dependent service rates. The state-dependent service rates are obtained by applying the model for scenario (S2) and taking the access intensities to infinity. We demonstrate the accuracy of our model via NS-2 simulations. We also demonstrate the inaccuracy of the service model proposed in the recent work by Bonald et al. (SIGMETRICS 2008) and identify the implicit assumption in their model which leads to this inaccuracy. We call our service model which accurately characterizes the service process in a multi-cell WLAN (under the PBD condition) “DCF scheduling” and study the “stability region” of DCF scheduling for small networks with single or multiple overlapping “contention domains”.

Includes bibliographical references (leaves 80-84). / Recent times has seen a tremendous increase in the deployment and use of 802.11 Wireless Local Area Networks (WLANs). These networks are easy to deploy and maintain, while providing reasonably high data rates at a low cost. In the paradigm of Next-Generation-Networks (NGNs), WLANs can be seen as an important access network technology to support IP multimedia services. However a traditional WLAN does not provide Quality of Service (QoS) support since it was originally designed for best effort operation. The IEEE 802. 11e standard was introduced to overcome the lack of QoS support for the legacy IEEE 802 .11 WLANs. It enhances the Media Access Control (MAC) layer operations to incorporate service differentiation. However, there is a need to prevent overloading of wireless channels, since the QoS experienced by traffic flows is degraded with heavily loaded channels. An admission control scheme for IEEE 802.11e WLANs would be the best solution to limit the amount of multimedia traffic so that channel overloading can be prevented. Some of the work in the literature proposes admission control solutions to protect the QoS of real-time traffic for IEEE 802.11e Enhanced Distributed Channel Access (EDCA). However, these solutions often under-utilize the resources of the wireless channels. A measurement-aided model-based admission control scheme for IEEE 802.11e EDCA WLANs is proposed to provide reasonable bandwidth guarantees to all existing flows. The admission control scheme makes use of bandwidth estimations that allows the bandwidth guarantees of all the flows that are admitted into the network to be protected. The bandwidth estimations are obtained using a developed analytical model of IEEE 802.11e EDCA channels. The admission control scheme also aims to accept the maximum amount of flows that can be accommodated by the network's resources. Through simulations, the performance of the proposed admission control scheme is evaluated using NS-2. Results show that accurate bandwidth estimations can be obtained when comparing the estimated achievable bandwidth to actual simulated bandwidth. The results also validate that the bandwidth needs of all admitted traffic are always satisfied when the admission control scheme is applied. It was also found that the admission control scheme allows the maximum amount of flows to be admitted into the network, according the network's capacity.

Malgré leur réussite remarquable, les premières versions des normes de réseaux locaux sans fil IEEE 802.11, IEEE 802. 11 a/b/g WLAN, sont caractérisées par une efficacité spectrale faible qui est devenue insuffisante pour satisfaire la croissance explosive de la demande de capacité et de couverture. Grâce aux progrès considérables dans le domaine des communications sans fil et l'utilisation de la bande de fréquence autour de 5 gigahertz le standard IEEE 802.11n et plus récemment 1'IEEE 802.11ac ont amélioré les débits offerts par la couche physique. Cela été possible grâce principalement à l'introduction des techniques multi-antennaires (MIMO, pour Multiple-Input) et des techniques avancées de modulation et de codage. Aujourd'hui, deux décennies après sa première apparition, le Wi-Fi est présenté comme une technologie WLAN permettant des débits supérieurs à 1 gigabit par seconde. Cependant, dans la plupart des scénarios de déploiement du monde réel, il n'est pas possible d'atteindre la pleine capacité offerte par la couche physique. Avec la croissance rapide de la densité des déploiements des WLANs et l'énorme popularité des équipements Wi-Fi, la réutilisation spatiale doit être optimisée. D'autre part, des nouveaux cas d’utilisation sont prévus pour décharger les réseaux cellulaires et pour couvrir des grandes surfaces (stades, gares, etc.). Ces environnements de haute densité représentent un vrai défi pour les générations actuelles de Wi-Fi qui doivent offrir une meilleure qualité à moindre coût. C'est dans ce contexte que s’inscrit l'objectif de cette thèse qui porte sur l'amélioration de l'efficacité des protocoles de la couche MAC des réseaux WLAN de haute densité. Notamment, un des buts de cette thèse est de contribuer à la préparation de la prochaine génération du standard Wi-Fi : IEEE 802.11ax High Efficiency WLAN (HEW). Plutôt que de continuer à cibler l'augmentation des débits maximums théoriques, nous nous concentrons dans le contexte de HEW sur l'amélioration du débit réel des utilisateurs. Pour cela, on prend en compte tous les autres équipements associés à des WLANs voisins, qui essayent d'accéder au même canal de transmission d’une manière simultanée. Pour améliorer la performance du Wi-Fi dans ces environnements denses, nous proposons une adaptation dynamique du mécanisme de détection de signal. Comparé au contrôle de la puissance de transmission, le mécanisme proposé est plus incitatif parce que l'utilisateur concerné bénéficie directement de son application. Les résultats de nos simulations montrent des gains importants en termes de débit atteint dans les scénarios de haute densité. Ensuite, nous étudions l’impact de la nouvelle adaptation sur les mécanismes de sélection de débit actuellement utilisés. D'après les résultats obtenus, 1'adaptation proposée peut être appliquée sans avoir besoin de modifications substantielles des algorithmes de sélection de débit. Pour améliorer l'équité entre les différents utilisateurs, nous élaborons une nouvelle approche distribuée pour adapter conjointement le mécanisme de détection de signal et le contrôle de la puissance de transmission. Cette approche est évaluée ensuite dans différents scénarios de simulation de haute densité où elle prouve sa capacité à résoudre les problèmes d'équité en particulier en présence de nœuds d'anciennes générations dans le réseau, cela tout en améliorant le débit moyen d'un facteur 4 par rapport à la performance conventionnelle du standard. Enfin, nous concevons et mettons en œuvre une solution centralisée basée sur l'apprentissage à base de réseaux de neurones. Cette approche repose sur l'adaptation conjointe de puissance de transmission et du mécanisme de détection du signal. [...] / Despite their remarkable success, the first widely spread versions of the Institute of Electrical and Electronics Engineers (IEEE) 802.11 Wireless Local Area Network (WLAN) standard, IEEE 802. 11 a/b/g, featured low spectral efficiencies that are becoming insufficient to satisfy the explosive growth in capacity and coverage demands. Thanks to the advances in the communication theory and the use of the 5 GHz frequency band, the IEEE 802.11n and recently the IEEE 802.1lac amendments improved the Physical Layer (PHY) data rates by introducing Multiple-Input Multiple Output (MIMO) techniques, higher Modulation and Coding Scheme (MCS), etc. Today, after almost two decades of its first appearance, Wi-Fi is presented as a gigabit wireless technology. However, the full potential of the latest PHY layer advances cannot be enabled in all real world deployment scenarios. With the rapidly increasing density of WLAN deployments and the huge popularity of Wi-Fi enabled devices, spatial reuse must be optimized. On another hand, the new challenging use case environments and the integration of mobile networks mainly for cellular offloading are limiting the opportunity of the current Wi-Fi generations to provide better quality at lower cost.In this thesis, we contribute to the current standardization efforts aiming to leverage the Wi-Fi efficiency in high density environments. At the time of writing this document, the IEEE 802.11ax Task Group (TG) is developing the specification for the High Efficiency WLAN (HEW) standard (next Wi-Fi evolution). Rather than continuing to target increased theoretical peak throughputs, we focus in the context of HEW on improving the throughput experienced by users in real life conditions where many other devices, belonging to neighboring overlapping networks, simultaneously contend to gain access. To enhance this performance, we propose a dynamic adaptation of the carrier sensing mechanism. Compare to controlling the transmission power, the proposed mechanism has more incentives because it benefits directly the concerned user. Extensive simulation results show impor1ant throughput gains in dense scenarios. Then, we study the impact of the new adaptation on the current rate control algorithms. We find that our adaptation mechanism operates efficiently without substantially modifying these algorithms that are widely used in today's operating WLANs. Furthermore, after analyzing the fairness performance of the proposed adaptation, we devise a new approach to jointly adapt the carrier sensing and the transmission power in order to preserve higher fairness degrees while improving the spatial reuse. This approach is evaluated in different dense deployment scenarios where it proves its capability to resolve the unfairness issues especially in the presence of legacy nodes in the network, while improving the achieved throughput by 4 times compared to the standard performance. Finally, we design and implement centralized learning-based solution that uses also an approach based on joint adaptation of transmission power and carrier sensing. This new solution takes benefit from the capability of artificial neural networks to model complex nonlinear functions to optimize the spatial reuse in dense WLANs while preserving fairness among contending nodes. The different contributions of this work have helped bring efficient solutions for future WiFi networks. We have presented these solutions to the IEEE 802.11ax TG where they were identified as important potential technical improvements for the next WLAN standard.

In this thesis, we propose the predictive multicast polling scheme for medium access control in wireless networks with multipacket reception capability. We concentrate on the case in which the packet arrival process is general and the maximum queue size is finite but larger than one. We derive both analytical results and simulation results. We use the theory of discrete-time Markov chain to analyze the evolution of the system state. In addition, we propose to use Markov reward processes to calculate the throughput. Furthermore, we obtain the average system size, the packet blocking probability, and the average packet delay. The proposed analysis approach is applicable no matter whether perfect state information is available to the controller or not. We also use simulation results to justify the usage of the proposed approach. Our study shows that the system performance can be significantly improved with a few additional buffers in the queues. The proposed medium access control scheme can be used in the single-hop wireless local area networks and the multi-hop wireless mesh networks.

It is necessary to authenticate users when they want to access services in WLANs. Extensible Authentication Protocol (EAP) is an authentication framework widely used in WLANs. Authentication mechanisms built on EAP are called EAP methods. The requirements for EAP methods in WLAN authentication have been defined in RFC 4017. Besides, low computation cost and forward secrecy, excluded in RFC 4017, are noticeable requirements in WLAN authentication. However, all EAP methods and authentication schemes designed for WLANs so far do not satisfy all of the above requirements. Therefore, we will propose an EAP method which utilizes three factors, stored secrets, passwords, and biometrics, to verify users. Our proposed method fully satisfies 1) the requirements of RFC 4017, 2) forward secrecy, and 3) lightweight computation. Moreover, the privacy of biometrics is protected against the authentication server, and the server can flexibly decide whether passwords and biometrics are verified in each round or not.

Abstract Handoff is the procedure providing the connection to the backbone network while a mobile terminal is moving across the boundaries of coverage of two wireless points of connection. The complexity of the handoff decision process has led to the examination of a number of traditional and pattern recognition handoff decision algorithms for wireless networks. Traditional algorithms use a received signal strength measurement and an optional threshold, hysteresis, or a dwell timer to determine the handoff decision. Degradation of the signal level, however, is a random process, and simple decision mechanisms result in a ping–pong effect whereby several consecutive handoffs degrade the service provided by the network. Consequently, more complex pattern recognition algorithms are needed to decide on the optimal time for handoff. In these algorithms, the handoff decision receives off line training to create a reference database of possible handoff locations in an environment with an associated handoff "fingerprint" at those locations. This dissertation introduces newly designed neural network and adaptive network based fuzzy inference system (ANFIS) pattern recognition algorithms. To select appropriate algorithms for a specific wireless network, we need to create an analytical framework for performance evaluation. The design of a framework for comparative performance evaluation of different handoff algorithms is a complex problem as different networks have different performance evaluation criteria. This dissertation divides wireless networks into three categories according to their topology and wireless service application: traditional cellular phone networks, heterogeneous wireless data networks, and rate adaptive wireless data networks. For each category of wireless networks we define a performance evaluation scenario and using Monte Carlo simulations, Monte Carlo calculations, and direct mathematical analysis we analyze the effects of different handoff decision algorithms. The Manhattan micro-cellular scenario is used for traditional cellular phone networks. Using Monte Carlo simulations on this scenario, the performance of traditional and our neural network and ANFIS handoff decision algorithms are compared. A moving-in moving-out performance evaluation scenario for heterogeneous wireless data networks is defined to characterize intertechnology roaming between two networks with substantially different data rates. We use Monte Carlo calculations to define the optimum handoff location for a mobile terminal in this scenario. Using Monte Carlo simulations and the optimal handoff location, we perform comparative performance evaluation of newly introduced asymmetric traditional and pattern recognition algorithms designed for intertechnology handoff. Finally, we introduce two performance evaluation scenarios for rate adaptive wireless networks to characterize user mobility in rate adaptive networks with random and grid deployments. For the first scenario we provide mathematical analysis for the effects of handoff using relative power to calculate the average throughput observed by the mobile terminal for different distances between the two wireless points of connection. For the second scenario designed for grid deployment we present a comparative performance analysis using Monte Carlo calculations for four handoff decision algorithms.

The unlicensed wireless spectrum offers exciting opportunities for developing innovative wireless applications. This has been true ever since the 2.4 GHz band and parts of the 5 GHz bands were first opened for unlicensed access worldwide. In recent years, the 5 GHz unlicensed bands have been one of the most coveted for launching new wireless services and applications due to their relatively superior propagation characteristics and the abundance of spectrum therein. However, the appetite for unlicensed spectrum seems to remain unsatiated; the demand for additional unlicensed bands has been never-ending. To meet this demand, regulators in the US and Europe have been considering unlicensed operations in the 5.9 GHz bands and in large parts of the 6 GHz bands. In the last two years alone, the Federal Communications Commission in the US has added more than 1.2 GHz of spectrum in the pool of unlicensed bands. Wi-Fi networks are likely to be the biggest beneficiaries of this spectrum. Such abundance of spectrum would allow massive improvements in the peak throughput and potentially allow a considerable reduction of latency, thereby enabling support for emerging wireless applications such as augmented and virtual reality, and mobile gaming using Wi-Fi over unlicensed bands. However, access to these bands comes with its challenges. Across the globe, a wide range of incumbent wireless technologies operate in the 5 GHz and 6 GHz bands. This includes weather and military radars, and vehicular communication systems in the 5 GHz bands, and fixed-service systems, satellite systems, and television pick-up stations in the 6 GHz bands. Furthermore, due to the development of several cellular-based unlicensed technologies (such as Licensed Assisted Access and New Radio Unlicensed, NR-U), the competition for channel access among unlicensed devices has also been increasing. Thus, coexistence across wireless technologies in the 5 GHz and 6 GHz bands has emerged as an extremely challenging and interesting research problem. In this dissertation, we first take a comprehensive look at the various coexistence scenarios that emerge in the 5 GHz and 6 GHz bands as a consequence of new regulatory decisions. These scenarios include coexistence between Wi-Fi and incumbent users (both in the 5 GHz and 6 GHz bands), coexistence of Wi-Fi and vehicular communication systems, coexistence across different vehicular communication technologies, and coexistence across different unlicensed systems. Since a vast majority of these technologies are fundamentally different from each other and serve diverse use-cases each coexistence problem is unique. Insights derived from an in-depth study of one coexistence problem do not help much when the coexisting technologies change. Thus, we study each scenario separately and in detail. In this process, we highlight the need for the design of novel coexistence mechanisms in several cases and outline potential research directions. Next, we shift our attention to coexistence between Wi-Fi and vehicular communication technologies designed to operate in the 5.9 GHz intelligent transportation systems (ITS) bands. Until the development of Cellular V2X (C-V2X), dedicated short range communications (DSRC) was the only major wireless technology that was designed for communication in high-speed and potentially dense vehicular settings. Since DSRC uses the IEEE 802.11p standard for its physical (PHY) and medium access control (MAC) layers, the manner in which DSRC and Wi-Fi devices try to gain access to the channel is fundamentally similar. Consequently, we show that spectrum sharing between these two technologies in the 5.9 GHz bands can be easily achieved by simple modifications to the Wi-Fi MAC layer. Since the design of C-V2X in 2017, however, the vehicular communication landscape has been fast evolving. Because DSRC systems were not widely deployed, automakers and regulators had an opportunity to look at the two technologies, consider their benefits and drawbacks and take a fresh look at the spectrum sharing scenario. Since Wi-Fi can now potentially share the spectrum with C-V2X at least in certain regions, we take an in-depth look at various Wi-Fi and C-V2X configurations and study whether C-V2X and Wi-Fi can harmoniously coexist with each other. We determine that because C-V2X is built atop cellular LTE, Wi-Fi and C-V2X systems are fundamentally incompatible with each other. If C-V2X and Wi-Fi devices are to share the spectrum, considerable modifications to the Wi-Fi MAC protocol would be required. Another equally interesting scenario arises in the 6 GHz bands, where 5G NR-U and Wi-Fi devices are likely to operate on a secondary shared basis. Since the 6 GHz bands were only recently considered for unlicensed access, these bands are free from Wi-Fi and NR-U devices. As a result, the greenfield 6 GHz bands provide a unique and rare opportunity to freshly evaluate the coexistence between Wi-Fi and cellular-based unlicensed wireless technologies. We study this coexistence problem by developing a stochastic geometry-based analytical model. We see that by disabling the listen before talk based legacy contention mechanism---which has been used by Wi-Fi devices ever since their conception---the performance of both Wi-Fi and NR-U systems can improve. This has important implications in the 6 GHz bands, where such legacy transmissions can indeed be disabled because Wi-Fi devices, for the first time since the design of IEEE 802.11a, can operate in the 6 GHz bands without any backward compatibility issues. In the course of studying the aforementioned coexistence problems, we identified several gaps in the literature on the performance analysis of C-V2X and IEEE 802.11ax---the upcoming Wi-Fi standard. We address three such gaps in this dissertation. First, we study the performance of C-V2X sidelink mode 4, which is the communication mode in C-V2X that allows direct vehicular communications (i.e., without assistance from the cellular infrastructure). Using our in-house standards-compliant network simulator-3 (ns-3) simulator, we perform simulations to evaluate the performance of C-V2X sidelink mode 4 in highway environments. In doing so, we identify that packet re-transmissions, which is a feature introduced in C-V2X to provide frequency and time diversity, thereby improving the system performance, can have the opposite effect if the vehicular density increases. In fact, packet re-transmissions are beneficial for C-V2X system performance only at low vehicular densities. Thus, if vehicles are statically configured to always use/disable re-transmissions, the maximum potential of this feature is not realized. Therefore, we propose a simple and effective, distributed re-transmission control mechanism named Channel Congestion Based Re-transmission Control (C2RC), which leverages the locally available channel sensing results to allow vehicles to autonomously decide when to switch on re-transmissions and when to switch them off. Second, we present a detailed analysis of the performance of Multi User Orthogonal Frequency Division Multiple Access (MU OFDMA)---a feature newly introduced in IEEE 802.11ax---in a wide range of deployment scenarios. We consider the performance of 802.11ax networks when the network comprises of only 802.11ax as well as a combination of 802.11ax and legacy stations. The latter is a practical scenario, especially during the initial phases of 802.11ax deployments. Simulation results, obtained from our ns-3 based simulator, give encouraging signs for 802.11ax performance in many real-world scenarios. That being said, there are some scenarios where naive usage of MU OFDMA by an 802.11ax-capable Wi-Fi AP can be detrimental to the overall system performance. Our results indicate that careful consideration of network dynamics is critical in exploiting the best performance, especially in a heterogeneous Wi-Fi network. Finally, we perform a comprehensive simulation study to characterize the performance of Multi Link Aggregation (MLA) in IEEE 802.11be. MLA is a novel feature that is likely to be introduced in next-generation Wi-Fi (i.e., Wi-Fi 7) devices and is aimed at reducing the worst-case latency experienced by Wi-Fi devices in dense traffic environments. We study the impact of different traffic densities on the 90 percentile latency of Wi-Fi packets and identify that the addition of a single link is sufficient to substantially bring down the 90 percentile latency in many practical scenarios. Furthermore, we show that while the addition of subsequent links is beneficial, the largest latency gain in most scenarios is experienced when the second link (i.e., one additional) link is added. Finally, we show that even in extremely dense traffic conditions, if a sufficient number of links are available at the MLA-capable transmitter and receiver, MLA can help Wi-Fi devices to meet the latency requirements of most real-time applications. / Doctor of Philosophy / Wireless networks have become ubiquitous in our lives today. Whether it is cellular connectivity on our mobile phones or access to Wi-Fi hotspots on laptops, tablets, and smartphones, never before has wireless communication been as integral to our lives as it is today. In many wireless communication systems, wireless devices operate by sending signals to and receiving signals from a central entity that connects to the wired Internet infrastructure. In the case of cellular networks, this entity is the cell tower deployed by the operators (such as ATandT, Verizon, etc. in the US), while the Wi-Fi router deployed in homes and offices plays this role in Wi-Fi networks. There is also another class of wireless systems, where wireless devices communicate with each other without requiring to communicate with any central entity. An example of such a distributed communication system---which is fast gaining popularity---is vehicular ommunication networks. End-user devices (e.g. cellphone, laptop, tablet, or a vehicle) can communicate with each other or the central entity only if they are both tuned to the same frequency channel. This channel can lie anywhere within the radio frequency spectrum, but some frequency channels (the collection of channels is referred to as frequency bands) are more favorable—--in terms of how far the signal sent over these channels can reach—--than others. Another dimension to these frequency bands is the licensing mechanism. Not all frequency bands are free to use. In fact, most frequency bands in the US and other parts of the world are licensed by the regional regulatory agencies. The most well-known example of this licensing framework is the cellular network. Cellular operators spend large amounts of money (to the tune of billions of dollars) to gain the privileges of exclusively operating in a given frequency band. No other operator or wireless device is then allowed to operate in this band. Without any external interfering wireless device, cellular operators can guarantee a certain quality of service that is provided to its customers. Thus, the benefits of using licensed frequency bands are obvious but these bands and their associated benefits come at a high price. An alternative to licensed frequency bands are the unlicensed ones. As the name suggests, unlicensed frequency bands are those where any two or more wireless devices can communicate with each other (subject to certain rules) without having to pay any licensing fees. Unsurprisingly, because there is no limit to who or how many devices can communicate over these bands, wireless devices in these bands frequently experience external interference, which manifests to the end-user in terms of interruption of service. The best example of a wireless technology that uses unlicensed bands is Wi-Fi. One of the greatest advantages of Wi-Fi networks is that anyone can purchase a Wi-Fi router and deploy it within their homes or offices—--flexibility not afforded by licensed bands. However, this very flexibility and ease-of-use can sometimes contribute negatively to Wi-Fi performance. Arguably, we have all faced scenarios where the performance of Wi-Fi is poor. This is most likely to happen in scenarios where there are hundreds (or even thousands) of neighboring Wi-Fi devices, such as at stadiums, railway stations, concerts, etc. Based on our discussions above, it is clear as to why Wi-Fi performance suffers in such scenarios. Thus, although unlicensed bands are lucrative in terms of low-cost, and ease of use, there is no guarantee on how good a voice/video call or a video streaming session conducted over Wi-Fi will be. The above problem is well-known and well-researched. Regulators, researchers, and service providers actively seek solutions to offer better performance over unlicensed bands. An obvious solution is to make more unlicensed bands available; if all neighboring Wi-Fi users communicate with their respective routers on different channels, everyone could communicate interference-free. The problem, however, is that frequency bands are limited. Even more limited are those bands that support wireless communications over larger distances. Another solution is to improve the wireless technology—if a Wi-Fi device can more efficiently utilize the channel, its performance is likely to improve. This fact has driven the constant evolution of all wireless technologies. However, there are fundamental limits to how much a frequency channel can be exploited. Therefore, in recent years, stakeholders have turned to spectrum sharing. Even though a wireless network may possess an exclusive license to operate on a given frequency band, its users do not use the band everywhere and at all times. Then why not allow unlicensed wireless devices to operate in this band at such places and times? This is precisely the premise of spectrum sharing. In this dissertation, we look at the problem of coexistence between wireless technologies in the 5 GHz and 6 GHz bands. These two bands are extremely lucrative in terms of their relatively favorable propagation characteristics (i.e., their communication range) and the abundance of spectrum therein. Consequently, these bands have garnered considerable attention in recent years with the objective of opening these bands up for unlicensed services. However, the 5 GHz and 6 GHz bands are home to several licensed systems, and the performance of these systems cannot be compromised if unlicensed operations are allowed. Significant activity has taken place since 2013 concerning new technologies being developed, new spectrum sharing scenarios being proposed, and new rules being adopted in these two bands. We begin the dissertation by taking a comprehensive look at these issues, describing the various coexistence scenarios, surveying the existing literature, describing the major challenges, and providing directions for potential research. Next, we look at three coexistence problems in detail: (i) coexistence of dedicated short range communications (DSRC) and Wi-Fi, (ii) coexistence of cellular V2X (C-V2X) and Wi-Fi, and (iii) coexistence of 5G New Radio Unlicensed (5G NR-U) and Wi-Fi. The former two scenarios involve the coexistence of Wi-Fi with a vehicular communication technology (DSRC or C-V2X). These scenarios arose due to considerations in the US and Europe to allow Wi-Fi operations (on an unlicensed secondary basis) in the spectrum that was originally reserved for vehicular communications. Our work shows that because DSRC and Wi-Fi are built on top of fundamentally similar protocols, they are, to an extent, compatible with each other, and coexistence between these two technologies can be achieved by relatively simple modifications to the Wi-Fi protocol. However, C-V2X, owing to its inheritance from the cellular LTE, is not compatible with Wi-Fi. Consequently, significant research is required if the two technologies are to share the spectrum. On the other hand, in the coexistence of 5G NR-U and Wi-Fi, we focus on the operations of these two technologies in the 6 GHz bands. NR-U is a technology that is built atop the 5G cellular system, but is designed to operate in the unlicensed bands (in contrast to traditional cellular systems which only operate in licensed bands). Although these two technologies can coexist in the 5 GHz and 6 GHz bands, we restrict our attention in this dissertation to the 6 GHz bands. This is because the 6 GHz bands are unique in that the entire range of the 6 GHz bands were opened up for unlicensed access all at once recently, and no Wi-Fi or NR-U devices currently operate in these bands. As a result, we can learn from the mistakes made in the 5 GHz bands, where a vast majority of today's Wi-Fi networks operate. Our work shows that, indeed, we can take decisive steps---such as disabling certain Wi-Fi functions---in the 6 GHz bands, which can facilitate better coexistence in the 6 GHz bands. Finally, in the course of identifying and tackling the various coexistence scenarios in the 5 GHz and 6 GHz bands, we identify some open issues in the performance of new wireless technologies designed to operate in these bands. Specifically, we highlight the need to better understand and characterize the performance of Multi User Orthogonal Frequency Division Multiple Access (MU OFDMA), a feature common in cellular networks but newly introduced to Wi-Fi, in the upcoming Wi-Fi 6 generation of devices. We propose and evaluate an analytical model for the same. We also characterize the performance of Multi Link Aggregation---which a novel feature likely to be introduced in future Wi-Fi 7 devices---that is aimed at reducing the worst-case delay experienced by Wi-Fi devices in dense traffic conditions. Additionally, we identify an issue in the performance of the distributed operational mode of C-V2X. We show that packet re-transmissions, which is a feature aimed at improving the performance of C-V2X, can have a counter-productive effect and degrade the C-V2X performance in certain environments. We address this issue by proposing a simple, yet effective, re-transmission control mechanism.

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Our work focuses on wireless networks in general, but deals specifically with security in wireless sensor networks and energy consumption in IEEE 802.11 infrastructure WLANs. In the first part of our work, we focus on secure communication among sensor nodes in a wireless sensor network. These networks consists of large numbers of devices having limited energy and memory. Public key cryptography is too demanding for these resource-constrained devices because it requires high computation. So, we focus on symmetric key cryptography to achieve secure communication among nodes. For this cryptographic technique to work, two nodes have to agree upon a common key. To achieve this, many key distribution schemes have been proposed in the literature. Recently, several researchers have proposed schemes in which they have used group-based deployment models and assumed predeployment knowledge of the expected locations of nodes. They have shown that these schemes achieve better performance than the earlier schemes, in terms of connectivity, resilience against node capture and storage requirements. But in many situations expected locations of nodes are not available. We propose a solution which does not use the group-based deployment model and predeployment knowledge of the locations of nodes, and yet performs better than schemes which make the aforementioned assumptions. In our scheme, groups are formed after the deployment of sensor nodes on the basis of their physical locations. Nodes in different groups sample keys from disjoint key pools, so that compromise of a node affects secure links of its group only. Because of this reason, our scheme performs better than earlier schemes as well as the schemes using predeployment knowledge, in terms of connectivity, storage requirement, and security. Moreover, the post-deployment key generation process completes sooner than in schemes like LEAP+. In the second part of our work, we develop analytical models for estimating the energy spent by stations (STAs) in infrastructure WLANs when performing TCP-controlled file downloads. We focus on the energy spent in radio communication when the STAs are in the Continuously Active Mode (CAM), or in the static Power Save Mode (PSM). Our approach is to develop accurate models for obtaining the fractions of times the STA radios spend in idling, receiving and transmitting. We discuss two traffic models for each mode of operation: (i) each STA performs one large file download, and (ii) the STAs perform short file transfers with think times (short duration of inactivity)between two transfers. We evaluate the rate of STA energy expenditure with long file downloads, and show that static PSM is worse than using just CAM. For short file downloads, we compute the number of file downloads that can be completed with a given battery capacity, and show that PSM performs better than CAM for this case. We provide a validation of our analytical models using the NS-2 simulator. Although the PSM performs better than the CAM when the STAs download short files over TCP with think times, its performance degrades as the number of STAs associated to the access point (AP) increases. To address this problem, we propose an algorithm, which we call opportunistic PSM (OPSM). We show through simulations that OPSM performs better than PSM. The performance gain achieved by OPSM increases as the file size requested by the STAs or the number of STAs associated with the AP increases. We implemented OPSM in NS-2.33, and to compare the performance of OPSM and PSM, we evaluate the number of file downloads that can be completed with a given battery capacity and the average time taken to download a file.