Rate Assignment in Wireless Networks: Stability Analysis and Controller Design

In this thesis, the problem of resource allocation in IS-856 is studied. The problem is first formulated in an interference model framework on reverse channel (uplink). A simple controller is then designed for the system and the closed-loop stability is analyzed using the Lyapunov technique. The possible oscillation in the network output caused by the limit cycles associated with the nonlinear elements in the control loop is analyzed in the framework of describing functions. A dynamic control strategy is developed subsequently for efficient rate assignment in the network. This is carried out in two steps: in the first step, the controller is designed for a simple case when the number of users in the network is fixed and known, and all users are full-buffered. The asymptotic stability property of the proposed controller is verified. Then, the designed controller is further developed for a dynamic network, where the number of active users is subject to change but is known to the users by means of the communication link between the base station and users. In this step, the activation/deactivation of users at different time instants is formulated as a switched system, and sufficient conditions on the speed of activation and deactivation of users are obtained in the control theoretic framework to achieve stability and the desired performance. In the next step, the obtained controller is adjusted properly for the case when the information about the number of active users is not communicated to the users (in order to allocate more bandwidth for data transmission). A controller is also designed to guarantee network stability and performance in the presence of time-delay in the feedback loop. Finally, the long-term fairness in rate allocation is studied. Simulation results are also provided throughout the thesis to elucidate the effectiveness of the proposed approach

SCHEDULING IN PACKET SWITCHED CELLULAR WIRELESS SYSTEMS

In cellular wireless networks where users have independent fading channels, throughput for delay tolerant applications has been greatly increased on the downlink by using opportunistic schedulers at the base station. These schedulers exploit the multiuser diversity inherent in cellular systems. An interesting question is how opportunistic schedulers will provide Quality of Service(QoS) guarantees for a mix of data traffic and traffic from delay-sensitive multimedia applications. In the first part of this dissertation, we completely characterize the scheduled rate, delay and packet service times experienced by mobile users in a packet switched cellular wireless system in terms of a configurable base station scheduler metric. The metric used has a general form, combining an estimate of a mobile user's channel quality with the scheduling delay experienced by the user. In addition to quantifying the scheduler performance, our analysis highlights the inherent trade-off between system throughput and the delay experienced by mobile users with opportunistic scheduling. We also use this analysis to study the effect of prioritized voice users on data users in a cellular wireless system with delay constrained opportunistic scheduling. Our statistical analysis of the forward link is validated by extensive simulations of a system architecture based on the CDMA 1xEV-DO system. The increase in data traffic from mobiles to the base station has led to a growing interest in a scheduled reverse link in the 1xEV-DO system. We address the reverse link scheduling problem in a multi-cell scenario with interference constraints both within and outside the cell. This approach leads to a co-operative scheduling algorithm where each base station in a cellular network maximizes the sum of mobile data transmission rates subject to linear constraints on (1) the maximum received power for individual mobiles(2) the total interference caused by scheduled mobiles to (a) traffic and control channels of other mobiles within the cell and (b) mobiles in neighboring cells. Simulations of the reverse link structure based on the 1xEV-D0 system highlight the distinct advantages of this algorithm in ensuring predictable inter-cell interference and higher aggregate cell throughputs

Fairness in dynamic networks

The main focus of this research is directed towards fairness in a dynamic network. Two specific applications are mathematically formulated: heating, ventilation, and air conditioning (HVAC), and code division multiple access (CDMA). In the first problem, namely, fair power allocation for temperature regulation of a multi-unit building, the temperature of each unit is described by a discrete-time dynamic equation. The formulation considers the effect of outside temperature and heat transfer between the adjacent rooms. Temperature regulation is then described as a constrained optimization problem, where the objective is to maintain the temperature of each unit within a prescribed thermal comfort zone with a limited amount of power. An optimal control strategy is presented to minimize the maximum mutual temperature difference between different units (long-term fairness) while maintaining the temperature of each unit in the comfort zone or close to it at all times, as much as possible (short-term fairness). Simulations demonstrate the effectiveness of the proposed control strategy in regulating the temperature of every unit in a building. Regarding the second application, an optimization-based fair reverse-link rate assignment strategy is proposed for fair resource allocation in a CDMA network. The network is modeled in a star topology, where the nodes represent either the base station (BS) or access terminals (ATs). The BS at every instant computes the fair rate for each AT by minimizing the maximum disparity in users' rates. Then, the BS sends a single bit to all ATs at every instant. It is shown that if each AT could compute a specific variable, called the coordinating variable, it can find its fair rate, which means the decision-making strategy is distributed. The proposed method is computationally efficient, and simulations confirm its efficacy in different scenarios

Quality of service optimization of multimedia traffic in mobile networks

Mobile communication systems have continued to evolve beyond the currently deployed Third Generation (3G) systems with the main goal of providing higher capacity. Systems beyond 3G are expected to cater for a wide variety of services such as speech, data, image transmission, video, as well as multimedia services consisting of a combination of these. With the air interface being the bottleneck in mobile networks, recent enhancing technologies such as the High Speed Downlink Packet Access (HSDPA), incorporate major changes to the radio access segment of 3G Universal Mobile Telecommunications System (UMTS). HSDPA introduces new features such as fast link adaptation mechanisms, fast packet scheduling, and physical layer retransmissions in the base stations, necessitating buffering of data at the air interface which presents a bottleneck to end-to-end communication. Hence, in order to provide end-to-end Quality of Service (QoS) guarantees to multimedia services in wireless networks such as HSDPA, efficient buffer management schemes are required at the air interface. The main objective of this thesis is to propose and evaluate solutions that will address the QoS optimization of multimedia traffic at the radio link interface of HSDPA systems. In the thesis, a novel queuing system known as the Time-Space Priority (TSP) scheme is proposed for multimedia traffic QoS control. TSP provides customized preferential treatment to the constituent flows in the multimedia traffic to suit their diverse QoS requirements. With TSP queuing, the real-time component of the multimedia traffic, being delay sensitive and loss tolerant, is given transmission priority; while the non-real-time component, being loss sensitive and delay tolerant, enjoys space priority. Hence, based on the TSP queuing paradigm, new buffer managementalgorithms are designed for joint QoS control of the diverse components in a multimedia session of the same HSDPA user. In the thesis, a TSP based buffer management algorithm known as the Enhanced Time Space Priority (E-TSP) is proposed for HSDPA. E-TSP incorporates flow control mechanisms to mitigate congestion in the air interface buffer of a user with multimedia session comprising real-time and non-real-time flows. Thus, E-TSP is designed to provide efficient network and radio resource utilization to improve end-to-end multimedia traffic performance. In order to allow real-time optimization of the QoS control between the real-time and non-real-time flows of the HSDPA multimedia session, another TSP based buffer management algorithm known as the Dynamic Time Space Priority (D-TSP) is proposed. D-TSP incorporates dynamic priority switching between the real-time and non-real-time flows. D-TSP is designed to allow optimum QoS trade-off between the flows whilst still guaranteeing the stringent real-time component’s QoS requirements. The thesis presents results of extensive performance studies undertaken via analytical modelling and dynamic network-level HSDPA simulations demonstrating the effectiveness of the proposed TSP queuing system and the TSP based buffer management schemes

Stability analysis and controller design for switched time-delay systems

In this thesis, the stability analysis and control synthesis for uncertain switched time-delay systems are investigated. It is known that a wide variety of real-world systems are subject to uncertainty and also time-delay in their dynamics. These characteristics, if not taken into consideration in analysis and synthesis, can lead to important problems such as performance degradation or instability in a control system. On the other hand, the switching phenomenon often appears in numerous applications, where abrupt change is inevitable in the system model. Switching behavior in this type of systems can be triggered either by time, or by the state of the system. A theoretical framework to study various features of switched systems in the presence of uncertainty and time-delay (both neutral and retarded) would be of particular interest in important applications such as network control systems, power systems and communication networks. To address the problem of robust stability for the class of uncertain switched systems with unknown time-varying delay discussed above, sufficient conditions in the form of linear matrix inequalities (LMI) are derived. An adaptive switching control algorithm is then proposed for the stabilization of uncertain discrete time-delay systems subject to disturbance. It is assumed that the discrete time-delay system is highly uncertain, such that a single fixed controller cannot stabilize it effectively. Sufficient conditions are provided subsequently for the stability of switched time-delay systems with polytopic-type uncertainties. Moreover, an adaptive control scheme is provided to stabilize the uncertain neutral time-delay systems when the upper bounds on the system uncertainties are not available a priori . Simulations are provided throughout the thesis to support the theoretical result