Transmission Strategies and Performance Analysis of Resource-Constrained Wireless Relay Networks

Abstract

Demand for mobile and personal communications is growing at a rapid pace, both in terms of the number of potential users and introduction of new high-speed services. Meeting this demand is challenging since wireless communications are subject to four major constraints: A complex and harsh fading channels, a scarce usable radio spectrum, and limitations on the power and size of hand-held terminals. Space-time codes provide diversity and coding gains in multiple antenna systems over fading channels. However, in ad-hoc or distributed large scale wireless networks, nodes are often constrained in hardware complexity and size, which makes multiple antenna systems impractical for certain applications. Cooperative diversity schemes have been introduced in an effort to overcome this limitation. Cooperative techniques allow a collection of radios to relay signals amongst each other, effectively creating a virtual antenna array, which combat multipath fading in wireless channels. In resource constrained networks, such as wireless sensor networks, the advantages of cooperation can be further exploited by optimally allocating the energy and bandwidth resources among users based on the available channel state information (CSI) at each node. In this thesis, we consider the design of practical distributed space-time codes and power efficient fading mitigation techniques for wireless relay networks. We show that using the proposed techniques the system performance is significantly improved under the respective resource constraints such as the energy, bit-error rate, or outage probability. Furthermore, the performance analysis of the wireless relay networks under different protocols and fading channels are investigated. We derive formulas for the symbol error rate (SER), outage probability, and diversity order of the investigated schemes in fading channels. For sufficiently large SNR, the close-form average symbol error probabilities are derived for the number of the distributed wireless systems. The simplicity of the asymptotic results provides valuable insights into the performance of cooperative networks and suggests means of optimizing them. Based on these SER expressions, power allocations are also proposed to further improve the performance of these systems. We next apply our proposed cooperative schemes to some practical wireless networks. For instance, we propose several amplify-and-forward cooperative schemes which consider the residual battery energy, as well as the statistical CSI, for the purpose of lifetime maximization in multi-branch, multihop wireless networks. We also propose new energy-efficient cooperative routing protocols in multihop wireless networks. In contrast to previous works, our proposed cooperating routings depend only on the statistics of the channels, and are implemented by both the centralized and distributed approaches.