|dc.description.abstract||In this dissertation, two promising technologies which improve the spectrum efficiency
in wireless networks are investigated: cognitive radio (CR) and relay technology. Cognitive
radio, as a promising technology to dynamically access spectrum resources, has drawn great
research attention recently. It provides a way to further improve the spectrum efficiency by
allowing unlicensed radio devices to learn from the radio environment and adapt their transmit
and receive parameters. This dissertation addresses the design of unlicensed networks,
including their transmission scheme, resource allocation and precoding/beamforming design
when multiple antennas are deployed.
Another topic in this dissertation is that of relay networks. By introducing relay stations
into the network, multiple benefits can be obtained, such as extended network coverage,
improved throughput and higher spectrum efficiency. Also, a relay network makes it possible
to enable two-way (even multi-way) transmission among multiple users within the network.
In this dissertation, precoding design in multiuser relay networks is discussed. Also, networks
based on combined cognitive radio and relay technologies are considered to leverage higher
performance, in terms of spectrum efficiency and network throughput.
This dissertation is organized into six chapters. Chapter 1 introduces the background
of cognitive radio and cooperative relay networks. In Chapter 2, a soft-decision spectrum
sensing concept is proposed and based on which, a joint spectrum sensing, access and power
allocation problem is addressed for multi-band cognitive radio networks by means of convex
optimization. Chapters 3 and 4 deal with the precoding design in multi-user cognitive relay
networks. Chapter 3 considers the multi-way transmission among multiple users and adopts
minimum mean square error (MMSE) as the design objective while Chapter 4 considers a
two-way relay network and a joint signal and interference alignment algorithm is proposed. In
Chapter 5, a signal group based interference alignment algorithm is proposed for a generalized
MIMO Y channel where each user sends messages to all the other users. In Chapter 6,
conclusions are drawn and possible future research avenues are highlighted.
Some interesting ideas about how to improve the spectrum efficiency in wireless networks
have been proposed and analyzed in this dissertation. The proposed soft-decision
spectrum sensing method allows more flexibility in designing the radio access strategies
in cognitive radio systems and achieves significantly higher throughput compared with a
traditional hard decision spectrum sensing based algorithm. Furthermore, the proposed
precoding/beamforming algorithms in the latter part of this dissertation enable concurrent
transmission of multiple users within the same frequency band, which can significantly reduce
or completely remove the inter-user interference. These technologies make it possible to utilize
the limited radio resources more efficiently and therefore can support the ever-increasing
demand arising from various wireless devices and applications.