The study on diversity gains in cognitive radio systems

Abstract

Cognitive radio (CR) is considered as one of promising methods to alleviate the spectrum scarcity caused by the increasing demand for wireless services and the existing exclusive spectrum allocation policy. The CR enables unlicensed users (secondary users) to access the under-utilized spectrum which is assigned to licensed users (primary users) while guaranteeing quality-of-service (QoS) requirements of primary users. However, it is hard for the secondary user to get a certain level of performance because the performance of secondary users’ communications are severely degraded by the QoS constraints imposed on the secondary users. Recently, diversity techniques in CR systems have been actively investigated that compensate the performance degradations of the secondary network by mitigating or exploiting the fluctuation of fading channels. This dissertation presents three different forms of diversity gain in cognitive radio systems: the diversity gain originated from relay, multiuser, and access mode switching.First, we consider the cooperative diversity gain in underlay CR systems where secondary users access the spectrum licensed to primary users under interference power constraint at primary receivers. Our analysis shows that the cooperative diversity gain has different characteristics from that in non-CR systems since the transmit power of a secondary transmitter is limited by the interference power constraint. If a fixed interference power constraint is imposed, the cooperative diversity gain is lost regardless of the number of relays even if peak transmit power grows to infinity. We analyze the critical value of peak transmit power beyond which increasing peak transmit power does not help in improving the outage probability. However, if a proportional interference power constraint is imposed, a full diversity gain is shown to be attained even without instantaneous interference channel information at the secondary transmitters.Second, we discuss the multius...