Spectrum sharing networks with imperfect channel state information

Cognitive Radio (CR) systems utilizes the underutilized spectrum in two ways i.e. overlay and underlay mode. In overlay CR, a secondary user (SU) opportunistically access primary user´s spectrum when it is unused as decided based on spectrum sensing. In contrast, a SU in underlay CR system accesses the spectrum at any time as long as the interfering power received at a primary receiver is below a certain limit. For use in underlay a SU needs to control its transmit power based on channel state information (CSI) between its transmitter and primary receiver. Furthermore a cognitive relay helps to improve the performance of SU The present paper discusses the research challenges associated with underlay CR system. The imperfection associated with CSI and its impact on performance of SU is discussed. Schemes of selection of cognitive relay in underlay are discussed. Some interesting performance results based on our work are also presented. The results show the effects of imperfect CSI and primary network on the performance of SU.