Coexistence and Energy Efficiency in Wireless Networks

Abstract: Dynamic spectrum access has been recently proposed to increase the utilization of the licensed spectrum bands, and support the constantly growing volumes of mobile traffic in the modern society. At the same time, the increasing demand for wireless connectivity, as a result of the rapid emergence of innovative wireless and mobile services, has led to the deployment of various wireless technologies in the open ISM bands. This thesis addresses the effective coexistence among the diverse wireless technologies in the above scenarios, and the energy efficiency of the deployed wireless systems, both listed among the key challenges that wireless networking is facing today.We discuss cooperative sensing, a fundamental mechanism for allowing unlicensed users perform opportunistic access in the licensed spectrum. Considering the scenario where the users perform both sensing and unlicensed spectrum access, we evaluate the efficiency of multi-channel cooperative sensing schemes with respect to the per user achievable capacity. We conclude that a careful optimization of both the number of sensed channels, and the allocation of sensing duties to the network users is necessary to achieve high capacity gains in large-scale networks of unlicensed users.We address a number of energy efficient design issues for sensor networks and wireless LANs. We study how to improve the energy efficiency of lowpower sensor networks operating under the interference from a coexisting WLAN. We propose a cognitive, cross-layer access control mechanism that minimizes the energy cost for multi-hop WSN communication, by deriving energy-optimal packet lengths and single-hop transmission distances, based on the knowledge of the stochastic channel activity patterns of the interfering WLAN. We show that the proposed mechanism leads to significant performance improvements on both energy efficiency, as well as end-to-end latency in multi-hop WSN communication, under different levels of interference. Additionally, we develop and validate the considered WLAN channel activity model and implement efficient, lightweight, real-time parameter estimation methods.We investigate how to enhance the multi-hop communication performance in ad hoc WLANs, when 802.11 stations operate under a power saving dutycycle scheme. We extend the traffic announcement scheme of the 802.11 power saving mode, allowing the stations to propagate pending frame notifications to all nodes in the end-to-end forwarding path of a network flow. We study the performance of the proposed scheme with respect to end-to-end packet delay and signaling overhead, while we investigate the impact on the achievable duty-cycle ratios of the wireless stations. For the purpose of the evaluation, and for the comparison with the standard 802.11 power saving mechanism, we implement the protocol extension in a development platform.Finally, we study how the combination of the objectives for energy efficiency and a high quality of service impacts the topology stability of selforganized ad hoc networks comprised of individual agents. Based on a noncooperative game theoretic model for topology formation, we identify key extensions in the nodes’ strategy profile space that guarantees a stable network formation under multi-objective player utility functions.