Decode-and-Forward Relay Selection with Imperfect CSI in Cognitive Relay Networks

In this paper we analyze the performance of opportunistic decode-and-forward relay selection under cooperative spectrum sharing where the available channel state information (CSI) is imperfect due to time variations of the channel coefficients or channel estimation errors. Due to imperfect CSI the secondary user (SU) cannot satisfy its interference threshold at the primary user (PU), and in fact the probability of exceeding this threshold is higher than 0.5. Therefore we define two power margin factors for the transmission power of the secondary source and the retransmission power of the secondary relays. Power margin factors determine the level of back-off from transmitters' peak powers. We then derive the outage probability of the SU and the probability of interference to the PU. Numerical results are presented from analysis which closely match those from simulations. It is shown that with proper selection of the power margin factors, a desired value of interference probability or a desired outage probability can be achieved.