PhD Alumni

Savazzi Stefano

Present position: Temporary researcher

Thesis title:  Analysis and Design of Cooperative Networks in Fading Channels
Advisor:  Umberto Spagnolini
Research area:  Signals
Thesis abstract:  
Multiple antenna solutions for wireless communications are widely acknowledged as an effective technology (in conjunction with channel coding) to increase the link throughput as they are known to provide spatial redundancy (or diversity order) that scales with the number of antennas deployed. Since hardware, size and cost constraints limit the number of antennas that can be deployed, virtual multiple antenna links can be created by coordinating multiple nodes with (typically) single antenna each. According to this solution, instead of a single terminal with multiple antennas, a cluster of single-antenna nodes might form a kind of coalition to cooperatively act as a transmit or receive array. This dissertation develops energy-efficient algorithms that employ certain kinds of cooperation among terminals by illustrating how these algorithms might be incorporated into different network architectures and for various applications. All the considered techniques allow a set of terminals to be reserved for relaying signals and for creating a virtual antenna array. Relay deployment costs, control overhead of relay coordination, transmit power and complexity are traded for the greater benefits that can be gained by exploiting spatial diversity. After introducing the model for the wireless MIMO channel, using information theoretic tools, we analyze the performances of various practical cooperative transmission algorithms based on different types of relay processing and coding. We show that cooperative link performances are conveniently described in terms of cooperative diversity and coding gain as parameters that have a major influence on the outage probability and are provided by the fading statistics of the channels involved in collaborative transmission. Motivated by this property, we develop an unifying view for performance analysis of distributed transmissions with arbitrary or unbalanced fading conditions. The purpose of the analysis is twofold: i) we first evaluate how the propagation settings (e.g., the fading statistic) can influence the performances of collaborative transmission; ii) next we show how this framework can help in efficient network design by defining cooperative fading regions in terms of the fading statistics and the random number of collaborating nodes where collaborative transmission is beneficial for network operation as compared to classical point-to-point network architectures. Cooperative fading regions can be used either to highlight conditions (e.g., requirements for the fading distributions) that are necessary for cooperation to enhance (or to perform as) direct (non-cooperative) multiantenna transmission, or rather more practical conditions that are sufficient to declare cooperation as not of any benefit compared to classical point-to-point transmission.
Keywords: Cooperative communication, Relay channels, Cooperative Diversity, Multihop Diversity, Distributed Space-Time coding, Randomized cooperation, Energy aware power allocation, Wireless sensors and ad-hoc networks, Optimal relay deployment, Multihop based cellular networks and mesh networks.