Enabling Techniques Design for QoS Provision in Wireless Communications

Guaranteeing Quality of Service (QoS) has become a recognized feature in the design of wireless communications. In this thesis, the problem of QoS provision is addressed from different prospectives in several modern communication systems. In the first part of the thesis, a wireless communication system with the base station (BS) associated by multiple subscribers (SS) is considered, where different subscribers require different QoS. Using the cross-layer approach, the conventional single queue finite state Markov chain system model is extended to multiple queues\u27 scenario by combining the MAC layer queue status with the physical layer channel states, modeled by finite state Markov channel (FSMC). To provide the diverse QoS to different subscribers, a priority-based rate allocation (PRA) algorithm is proposed to allocate the physical layer transmission rate to the multiple medium access control (MAC) layer queues, where different queues are assigned with different priorities, leading to their different QoS performance and thus, the diverse QoS are guaranteed. Then, the subcarrier allocation in multi-user OFDM (MU-OFDM) systems is stuied, constrained by the MAC layer diverse QoS requirements. A two-step cross-layer dynamic subcarrier allocation algorithm is proposed where the MAC layer queue status is firstly modeled by a finite state Markov chain, using which MAC layer diverse QoS constraints are transformed to the corresponding minimum physical layer data rate of each user. Then, with the purpose of maximizing the system capacity, the physical layer OFDM subcarriers are allocated to the multiple users to satisfy their minimum data rate requirements, which is derived by the MAC layer queue status model. Finally, the problem of channel assignment in IEEE 802.11 wireless local area networks (WLAN) is investigated, oriented by users\u27 QoS requirements. The number of users in the IEEE 802.11 channels is first determined through the number of different channel impulse responses (CIR) estimated at physical layer. This information is involved thereafter in the proposed channel assignment algorithm, which aims at maximum system throughput, where we explore the partially overlapped IEEE 802.11 channels to provide additional frequency resources. Moreover, the users\u27 QoS requirements are set to trigger the channel assignment process, such that the system can constantly maintain the required QoS

An Autonomous Channel Selection Algorithm for WLANs

IEEE 802.11 wireless devices need to select a channel in order to transmit their packets. However, as a result of the contention-based nature of the IEEE 802.11 CSMA/CA MAC mechanism, the capacity experienced by a station is not fixed. When a station cannot win a sufficient number of transmission opportunities to satisfy its traffic load, it will become saturated. If the saturation condition persists, more and more packets are stored in the transmit queue and congestion occurs. Congestion leads to high packet delay and may ultimately result in catastrophic packet loss when the transmit queue’s capacity is exceeded. In this thesis, we propose an autonomous channel selection algorithm with neighbour forcing (NF) to minimize the incidence of congestion on all stations using the channels. All stations reassign the channels based on the local monitoring information. This station will change the channel once it finds a channel that has sufficient available bandwidth to satisfy its traffic load requirement or it will force its neighbour stations into saturation by reducing its PHY transmission rate if there exists at least one successful channel assignment according to a predicting module which checks all the possible channel assignments. The results from a simple C++ simulator show that the NF algorithm has a higher probability than the dynamic channel assignment without neighbour forcing (NONF) to successfully reassign the channel once stations have become congested. In an experimental testbed, the Madwifi open source wireless driver has been modified to incorporate the channel selection mechanism. The results demonstrate that the NF algorithm also has a better performance than the NONF algorithm in reducing the congestion time of the network where at least one station has become congested

Optimizing multiuser MIMO for access point cooperation in dense wireless networks

As the usage of wireless devices continues to grow rapidly in popularity, wireless networks that were once designed to support a few laptops must now host a much wider range of equipments, including smart phones, tablets, and wearable devices, that often run bandwidth-hungry applications. Improvements in wireless local access network (WLAN) technology are expected to help accommodate the huge traffic demands. In particular, advanced multicell Multiple-Input Multiple-Output (MIMO) techniques, involving the cooperation of APs and multiuser MIMO processing techniques, can be used to satisfy the increasing demands from users in high-density environments. The objective of this thesis is to address the fundamental problems for multiuser MIMO with AP cooperation in dense wireless network settings. First, for a very common multiuser MIMO linear precoding technique, block diagonalization, a novel pairing-and-binary-tree based user selection algorithm is proposed. Second, without the zero-forcing constraint on the multiuser MIMO transmission, a general weighted sum rate maximization problem is formulated for coordinated APs. A scalable algorithm that performs a combined optimization procedure is proposed to determine the user selection and MIMO weights. Third, we study the fair and high-throughput scheduling problem by formally specifying an optimization problem. Two algorithms are proposed to solve the problem using either alternating optimization or a two-stage procedure. Fourth, with the coexistence of both stationary and mobile users, different scheduling strategies are suggested for different user types. The provided theoretical analysis and simulation results in this thesis lay out the foundation for the realization of the clustered WLAN networks with AP cooperation.Ph.D

Cooperative communication in wireless networks: algorithms, protocols and systems

Current wireless network solutions are based on a link abstraction where a single co-channel transmitter transmits in any time duration. This model severely limits the performance that can be obtained from the network. Being inherently an extension of a wired network model, this model is also incapable of handling the unique challenges that arise in a wireless medium. The prevailing theme of this research is to explore wireless link abstractions that incorporate the broadcast and space-time varying nature of the wireless channel. Recently, a new paradigm for wireless networks which uses the idea of 'cooperative transmissions' (CT) has garnered significant attention. Unlike current approaches where a single transmitter transmits at a time in any channel, with CT, multiple transmitters transmit concurrently after appropriately encoding their transmissions. While the physical layer mechanisms for CT have been well studied, the higher layer applicability of CT has been relatively unexplored. In this work, we show that when wireless links use CT, several network performance metrics such as aggregate throughput, security and spatial reuse can be improved significantly compared to the current state of the art. In this context, our first contribution is Aegis, a framework for securing wireless networks against eavesdropping which uses CT with intelligent scheduling and coding in Wireless Local Area networks. The second contribution is Symbiotic Coding, an approach to encode information such that successful reception is possible even upon collisions. The third contribution is Proteus, a routing protocol that improves aggregate throughput in multi-hop networks by leveraging CT to adapt the rate and range of links in a flow. Finally, we also explore the practical aspects of realizing CT using real systems.PhDCommittee Chair: Sivakumar, Raghupathy; Committee Member: Ammar, Mostafa; Committee Member: Ingram, Mary Ann; Committee Member: Jayant, Nikil; Committee Member: Riley, Georg

Relay Technologies in IEEE 802.16j Mobile Multi-hop Relay (MMR) Networks

IEEE 802.16 standard is created to compete with cable access networks. In the beginning end users are immobile and have a line of sight with base station, now it moved to mobile non line of sight (NLOS) with the new standard IEEE 802.16e and IEEE 802.16j. The new IEEE 802.16j standard which is an amendment to IEEE 802.16e is mobile multi hop relay (MMR) specification for wireless networks. This paper discusses relay modes, relay transmission schemes and relay pairing schemes of IEEE 802.16j. Relay technologies such as transparent relay modes, non transparent relay mode, relay pairing schemes such as centralized relay pairing schemes, distributed relay pairing scheme, characterises of relay based networks such as throughput enhancement, capacity increase, cost reduction , relay techniques such as time domain frequency domain relay techniques and relay placement are also discussed in this paper. The paper also discusses about integration of IEEE 802.16j with IEEE 802.11. Keywords: IEEE 802.16j, Relay pairing schemes, relay techniques, Relay modes, WIMAX, NCTUns, et

Enabling Dynamic Spectrum Allocation in Cognitive Radio Networks

The last decade has witnessed the proliferation of innovative wireless technologies, such asWi-Fi, wireless mesh networks, operating in unlicensed bands. Due to the increasing popularity and the wide deployments of these technologies, the unlicensed bands become overcrowded. The wireless devices operating in these bands interfere with each other and hurt the overall performance. To support fast growths of wireless technologies, more spectrums are required. However, as most "prime" spectrum has been allocated, there is no spectrum available to expand these innovative wireless services. Despite the general perception that there is an actual spectral shortage, the recent measurement results released by the FCC (Federal Communications Commission) show that on average only 5% of the spectrum from 30MHz to 30 GHz is used in the US. This indicates that the inefficient spectrum usage is the root cause of the spectral shortage problem. Therefore, this dissertation is focused on improving spectrum utilization and efficiency in tackling the spectral shortage problem to support ever-growing user demands for wireless applications. This dissertation proposes a novel concept of dynamic spectrum allocation, which adaptively divides available spectrum into non-overlapping frequency segments of different bandwidth considering the number of potentially interfering transmissions and the distribution of traffic load in a local environment. The goals are (1) to maximize spectrum efficiency by increasing parallel transmissions and reducing co-channel interferences, and (2) to improve fairness across a network by balancing spectrum assignments. Since existing radio systems offer very limited flexibility, cognitive radios, which can sense and adapt to radio environments, are exploited to support such a dynamic concept. We explore two directions to improve spectrum efficiency by adopting the proposed dynamic allocation concept. First, we build a cognitive wireless system called KNOWS to exploit unoccupied frequencies in the licensed TV bands. KNOWS is a hardware-software platform that includes new radio hardware, a spectrum-aware MAC (medium access control) protocol and an algorithm for implementing the dynamic spectrum allocation. We show that KNOWS accomplishes a remarkable 200% throughput gain over systems based on fixed allocations in common cases. Second, we enhance Wireless LANs (WLANs), the most popular network setting in unlicensed bands, by proposing a dynamic channelization structure and a scalable MAC design. Through analysis and extensive simulations, we show that the new channelization structure and the scalable MAC design improve not only network capacity but per-client fairness by allocating channels of variable width for access points in a WLAN. As a conclusion, we believe that our proposed concept of dynamic spectrum allocation lays down a solid foundation for building systems to efficiently use the invaluable spectrum resource

Improving the Performance of Wireless LANs

This book quantifies the key factors of WLAN performance and describes methods for improvement. It provides theoretical background and empirical results for the optimum planning and deployment of indoor WLAN systems, explaining the fundamentals while supplying guidelines for design, modeling, and performance evaluation. It discusses environmental effects on WLAN systems, protocol redesign for routing and MAC, and traffic distribution; examines emerging and future network technologies; and includes radio propagation and site measurements, simulations for various network design scenarios, numerous illustrations, practical examples, and learning aids