81 Copyright © 2013, IGI Global. Copying or distributing in print or electronic forms without written permission of IGI Global is prohibited. Chapter 4 DOI: 10.4018/978-1-4666-4189-1.ch004 Resource Allocation Strategies in Cognitive Radio Networks Under QoS Constraints ABSTRACT The rapid growth of spectral resources’ demands, as well as the increasing Quality of Service (QoS) re- quirements of wireless users have led to the necessity for new resource allocation schemes which will take into account the diferentiated QoS needs of each wireless user. Towards this direction, the researchers have introduced the concept of efective capacity, which is defned as the maximum rate that the channel can support in order to guarantee a specifed QoS requirement. This concept has been considered as a “bridge” among the physical layer characteristics and the upper-layer metrics of QoS. During the last years, it has been widely employed for resource allocation problems in various wireless networks lead- ing to efcient mechanisms. This chapter focuses on the employment of the efective capacity theory in Cognitive Radio (CR) systems, presenting an extensive survey on QoS-driven resource allocation schemes proposed in the literature. Some useful conclusions are presented and future research directions on this subject are highlighted and discussed. 1. INTRODUCTION The emerging services of modern wireless net- works raise an increasing demand for spectral bandwidth resulting in the congestion of this scarce resource. At the same time, the limited spectrum is used inefficiently most of the times leading to low utilization. To overcome these problems, the concept of CR technology has been introduced. The research community defines CR as a radio platform that can rapidly reconfigure its operating parameters through a process of cognition, based Stavroula Vassaki National Technical University of Athens, Greece Marios I. Poulakis National Technical University of Athens, Greece Athanasios D. Panagopoulos National Technical University of Athens, Greece Philip Constantinou National Technical University of Athens, Greece