New Insights to Cooperative Spectrum Sensing for Cognitive radios

In cognitive radio networks, cognitive users have to detect the presence of primary users. In this paper, we exhibit the improvement in the probability of detection of an user U1 by allowing other user U2 to act as a relay. Assuming that users experience Rayleigh fading, and the decision statistic is the instantaneous power, at the same time, there is no transmit power constraint to relay, we find that the improvement can not be always achieved when the relay U2 receives larger power from a primary user than the user U1 does, which is different from the traditional viewpoint. Conclusions are confirmed by the theoretic proof and numerical simulations. We also exhibit that the improvement can be achieved if and only if the received power at the relay due to the primary user is larger than a certain value, which is named the gain point. We then derive the expression and present an approximated function for determining the gain point.