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.
List of papers
B. Maham, A. Hjørungnes, and G. Abreu, "Distributed GABBA Space-
Time Codes in Amplify-and-Forward Relay Networks," IEEE Transactions
on Wireless Communications, volume 8, issue 2, pages 2036 -
2045, April 2009.
B. Maham and A. Hjørungnes, "Power Allocation Strategies for Distributed Space-Time Codes in Amplify-and-Forward Mode," EURASIP
Journal on Advances in Signal Processing, vol. 2009, Article ID 612719, 13 pages, 2009.
B. Maham and A. Hjørungnes, "Performance Analysis of Repetition-
Based Cooperative Networks with Partial Statistical CSI at Relays,"
IEEE Communications Letters, volume 12, issue 11, pages 828-830,
B. Maham and A. Hjørungnes, "Asymptotic Performance Analysis of
Amplify-and-Forward Cooperative Networks in a Nakagami-m Fading
Environment,"IEEE Communications Letters, volume 13, issue 5,
pages 300-302, May 2009.
B. Maham and A. Hjørungnes, "Performance Analysis of Amplify-and-
Forward Opportunistic Relaying in Rician Fading," IEEE Signal Processing Letters, volume 16, issue 8, pages 643-646, August 2009.
B. Maham, A. Hjørungnes, and M. Debbah, "Power Allocations in
Minimum-Energy SER Constrained Cooperative Networks," Annals of
Telecommunications - Annales des Télécommunications, vol. 64, no. 7,
pp. 545-555, Aug. 2009 (Published by Institut Telecom and Springer;
Indexed in ISI and Scopus databases).
B. Maham, R. Narasimhan, and A. Hjørungnes, "Energy-Efficient
Space-Time Coded Cooperative Routing in Multihop Wireless Networks,"
Proc. of IEEE Global Telecommunications Conference (GLOBECOM
2009), Honolulu, Hawaii, USA, November - December 2009.