I. Introduction
COGNITIVE radio (CR) has been recognized as a new technology for enhancing spectral efficiency and meeting the increasing demand for wireless communication services [1], [2]. The main idea behind employing the CR technology is to permit unauthorized users, which are known as secondary users (SUs), opportunistic access to free bands, namely, white spaces, when and where the primary users (PUs) do not use these “spectrum holes.” This technique clearly improves the spectral efficiency since more users are allowed to coexist in the same bandwidth. However, SUs are only allowed to activate their transmissions if they do not adversely affect the operation of PUs. Since SUs are considered of a lower priority than PUs, a fundamental requirement for SUs is to avoid interference with potential PUs in their vicinity. Managing such cochannel interference so as to overcome the throughput degradation of PUs has emerged as a crucial task in the physical layer design of CR systems [3], [4]. In particular, for scenarios with high PU traffic, where the number of spectrum holes becomes limited, positive cognitive data rates cannot be supported. To overcome this limitation, several approaches have been proposed in the literature [5]– [7].