Underlay cooperative af relaying in cellular networks: performance and challenges

Cognitive radio is a promising technique to resolve the growing scarcity of the indispensable spectrum resources in wireless applications. Among its three major implementation paradigms, namely, interweave, overlay and underlay, underlay CR is the most flexible. However, the transmit power of secondary users (SUs) having to be strictly limited by the prescribed tolerable interference power dictated by primary users (PUs), confines underlay CR to short-range communications. To address this limitation, cooperative amplify-and-forward (AF) relaying techniques can be exploited to extend the coverage area and reduce the interference region of secondary transmission with low implementation complexity. Nevertheless, despite the significant potential of cooperation in underlay CR systems, advances in this context have been limited so far, mainly due to the extreme difficulty of mathematical tractability involved in the performance evaluation and design of these systems. This article aims to overview recent advances in this area. In particular, important dominating factors on system performance of underlay cooperative AF relaying in cellular networks are identified, in terms of cochannel interference coming from concurrent primary transmission, relay selection, number of relaying hops, and imperfect channel state information (CSI). Moreover, we outline some future research topics in this exciting area.