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Spectrum sharing in a cognitive radio network aims to improve the spectrum efficiency where a secondary user (SU) shares the spectrum band licensed to the primary user causing a limited interference to the PU. However, the performance of a SU will be affected by interference from the PU. In this paper we investigate the impact of PU interference on symbol error probability (SEP) and outage probability of SU under constraint of PU outage. The channel state information (CSI) available at SU is used to estimate the instantaneous transmit power of SU under an acceptable outage constraint of PU, while the SU transmits at the maximum power level (P m ) during the absence of PU. A power control scheme based on a scaling factor is proposed for SU so as to maintain a desired outage constraint on PU even in case of imperfect CSI. A closed form expression for outage probability of SU under PU interference is developed. A simulation test bed has been developed in MATLAB to assess the performance of SU in terms of SEP and outage probability with imperfect CSI in presence of PU interference. We also highlight the effects of imperfection in CSI on primary outage. Further the effects of constellation size (M) of the transmitted MPSK signal of SU is indicated on the outage and SEP performance of SU.Keywords-cognitive radio(CR);imperfect channel state information;outage capacity;symbol error probability.

Two novel spectrum access schemes, a hybrid spectrum access scheme (HSAS) and a modified HSAS combining underlay mode and overlay mode, are proposed for a relay based cognitive radio (CR) network. The CR employs the underlay mode with imperfect channel state information of the interfering link between CR transmitter and primary user receiver if primary user is sensed to be present else it employs overlay mode. In both the schemes, transmission of CR is assisted by relay working in amplify and forward scheme by maintaining interference below an interference threshold in underlay mode and a collision constraint in overlay mode. In HSAS, CR transmits either in underlay mode or in overlay mode only during the transmission slot of a detection cycle, whereas in modified HSAS, the CR transmits for the entire frame including the sensing time. The outage probability of CR user is considered as the performance metric. Novel expressions of outage probability for both the schemes have been developed. Impact of several sensing parameters such as sensing time, correlation coefficient, and tolerable interference threshold has been investigated. Copyright

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