Browsing by Subject "Wireless systems"
Now showing 1 - 4 of 4
Results Per Page
Sort Options
Item Coordinated wireless multiple antenna networks : transmission strategies and performance analysis(2008-12) Chae, Chan-Byoung; Heath, Robert W., Ph. D.Next generation wireless systems will use multiple antenna technologies, also known as multiple-input multiple-output (MIMO), to provide high data rates and robustness against fading. MIMO communication strategies for single user communication systems and their practical application in wireless networks are by now well known. MIMO communication systems, however, can benefit from multiuser processing by coordinating the transmissions to multiple users simultaneously. For numerous reasons, work on the theory of multiuser MIMO communication has yet to see broad adoption in wireless communication standards. For example, global knowledge of channel state information is often required. Such an unrealistic assumption, however, makes it difficult in practice to implement precoding techniques. Furthermore, the achievable rates of the conventional multiuser MIMO techniques are far from the theoretical performance bounds. These and other factors motivate research on practical multiuser communication strategies for the MIMO broadcast channel (point to multi-point communication) and the analysis of those strategies. The primary contributions of this dissertation are i) the development of four novel low complexity coordinated MIMO transceiver design techniques to approach the theoretical performance bound and ii) the investigation of the optimality of the proposed coordinated wireless MIMO networks. Several coordinated beamforming algorithms are proposed, where each mobile station uses quantized combining vectors or each base station uses limited feedback from the MS. The asymptotic optimality of the proposed coordinated beamforming system for the MIMO Gaussian broadcast channel is next investigated. For multi-stream transmission, a novel block diagonalized vector perturbation is proposed and the achievable sum rate upper bound of the proposed system is derived. Finally, for multi-cell environments, linear and non-linear network CBF algorithms supporting multiple cell-boundary users are proposed. The optimality of network coordinated beamforming in terms of the number of receive antennas is also investigated.Item Exploring tradeoffs in wireless networks under flow-level traffic: energy, capacity and QoS(2009-12) Kim, Hongseok; De Veciana, GustavoWireless resources are scarce, shared and time-varying making resource allocation mechanisms, e.g., scheduling, a key and challenging element of wireless system design. In designing good schedulers, we consider three types of performance metrics: system capacity, quality of service (QoS) seen by users, and the energy expenditures (battery lifetimes) incurred by mobile terminals. In this dissertation we investigate the impact of scheduling policies on these performance metrics, their interactions, and/or tradeoffs, and we specifically focus on flow-level performance under stochastic traffic loads. In the first part of the dissertation we evaluate interactions among flow-level performance metrics when integrating QoS and best effort flows in a wireless system using opportunistic scheduling. We introduce a simple flow-level model capturing the salient features of bandwidth sharing for an opportunistic scheduler which ensures a mean throughput to each QoS stream on every time slot. We show that the integration of QoS and best effort flows results in a loss of opportunism, which in turn results in a reduction of the stability region, degradation in system capacity, and increased file transfer delay. In the second part of the dissertation we study several ways in which mobile terminals can backoff on their uplink transmit power (thus slow down their transmissions) in order to extend battery lifetimes. This is particularly effective when a wireless system is underloaded, so the degradation in the users' perceived performance can be negligible. The challenge, however, is developing a mechanism that achieves a good tradeoff among transmit power, idling/circuit power, and the performance customers will see. We consider systems with flow-level dynamics supporting either real-time or best effort (e.g., file transfers) sessions. We show that significant energy savings can be achieved by leveraging dynamic spare capacity. We then extend our study to the case where mobile terminals have multiple transmit antennas. In the third part of the dissertation we develop a framework for user association in infrastructure-based wireless networks, specifically focused on adaptively balancing flow loads given spatially inhomogeneous traffic distributions. Our work encompasses several possible user association objective functions resulting in rate-optimal, throughput-optimal, delay-optimal, and load-equalizing policy, which we collectively denote [alpha]-optimal user association. We prove that the optimal load vector that minimizes this function is the fixed point of a certain mapping. Based on this mapping we propose an iterative distributed user association policy and prove that it converges to the globally optimal decision in steady state. In addition we address admission control policies for the case where the system cannot be stabilized.Item Optimality and robustness in opportunistic scheduler design for wireless networks(2010-08) Sadiq, Bilal; De Veciana, Gustavo; Andrews, Jeffery G.; Arapostathis, Aristotle; Hasenbein, John J.; Shakkottai, SanjayWe investigate in detail two multiuser opportunistic scheduling problems in centralized wireless systems: the scheduling of "delay-sensitive" flows with packet delay requirements of a few tens to few hundreds of milliseconds over the air interface, and the scheduling of "best-effort" flows with the objective of minimizing mean file transfer delay. Schedulers for delay-sensitive flows are characterized by a fundamental tradeoff between "maximizing total service rate by being opportunistic" and "balancing unequal queues (or delays) across users". In choosing how to realize this tradeoff in schedulers, our key premise is that "robustness" should be a primary design objective alongside performance. Different performance objectives -- mean packet delay, the tail of worst user's queue distribution, or that of the overall queue distribution -- result in remarkably different scheduling policies. Different design objectives and resulting schedulers are also not equally robust, which is important due to the uncertainty and variability in both the wireless environment and the traffic. The proposed class of schedulers offers low packet delays, less sensitivity to the scheduler parameters and channel characteristics, and a more graceful degradation of service in terms of the fraction of users meeting their delay requirements under transient overloads, when compared with other well-known schedulers. Schedulers for best-effort flows are characterized by a fundamental tradeoff between "maximizing the total service rate" and "prioritizing flows with short residual sizes". We characterize two regimes based on the "degree" of opportunistic gain present in the system. In the first regime -- where the opportunistic capacity of the system increases sharply with the number of users -- the use of residual flow-size information in scheduling will 'not' result in a significant reduction in flow-level delays. Whereas, in the second regime -- where the opportunistic capacity increases slowly with the number of users -- using flow-size information alongside channel state information 'may' result in a significant reduction. We then propose a class of schedulers which offers good performance in either regime, in terms of mean file transfer delays as well as probability of blocking for systems that enforce flow admission control. This thesis provides a comprehensive theoretical study of these fundamental tradeoffs for opportunistic schedulers, as well as an exploration of some of the practical ramifications to engineering wireless systems.Item Spatial spectrum reuse in wireless networks design and performance(2011-05) Kim, Yuchul; De Veciana, Gustavo; Andrews, Jeffrey G.; Qiu, Lili; Shakkottai, Sanjay; Sanghavi, SujayThis dissertation considers the design, evaluation and optimization of algorithms/ techniques/ system parameters for distributed wireless networks specifically ad-hoc and cognitive wireless networks. In the first part of the dissertation, we consider ad-hoc networks using opportunistic carrier sense multiple access (CSMA) protocols. The key challenge in optimizing the performance of such systems is to find a good compromise among three interdependent quantities: the density and channel quality of the scheduled transmitters, and the resulting interference seen at receivers. We propose two new channel-aware slotted CSMA protocols and study the tradeoffs they achieve amongst these quantities. In particular, we show that when properly optimized these protocols offer substantial improvements relative to regular CSMA -- particularly when the density of nodes is moderate to high. Moreover, we show that a simple quantile based opportunistic CSMA protocol can achieve robust performance gains without requiring careful parameter optimization. In the second part of the dissertation, we study a cognitive wireless network where licensed (primary) users and unlicensed 'cognitive' (secondary) users coexist on shared spectrum. In this context, many system design parameters affect the joint performance, e.g., outage and capacity, seen by the two user types. We explore the performance dependencies between primary and secondary users from a spatial reuse perspective, in particular, in terms of the outage probability, node density and joint network capacity. From the design perspective the key system parameters determining the joint transmission capacity, and tradeoffs, are the detection radius (detection signal to interference and noise power ratio (SINR) threshold) and decoding SINR threshold. We show how the joint network capacity region can be optimized by varying these parameters. In the third part of the dissertation, we consider a cognitive network in a heterogeneous environment, including indoor and outdoor transmissions. We characterize the joint network capacity region under three different spectrum (white space) detection techniques which have different degrees of radio frequency (RF) - environment awareness. We show that cognitive devices relying only on the classical signal energy detection method perform poorly due to limitations on detecting primary transmitters in environments with indoor shadowing. This can be circumvented through direct use (e.g., database access) of location information on primary transmitters, or better yet, on that of primary receivers. We also show that if cognitive devices have positioning information, then the secondary network's capacity increases monotonically with increased indoor shadowing in the environment. This dissertation extends the recent efforts in using stochastic geometric models to capture large scale performance characteristics of wireless systems. It demonstrates the usefulness of these models towards understanding the impact of physical /medium access (MAC) layer parameters and how they might be optimized.