Partial relay selection with fixed-gain relays and outdated CSI in underlay cognitive networks

The impact of an imperfect channel estimation on the amplify-and-forward (AF) mode cooperative communications systems is studied, with some important factors, including the probability characteristic of the secondary user's end-to-end signal-to-noise ratio (SNR), the outage probability, the symbol error probability (SEP) and the lower bound on the capacity, etc, being analyzed. As compared to the conventional relay selection schemes, less channel state information (CSI) feedback is required in the proposed method due to an outdated channel estimation being tolerable. The proposed scheme is validated by carrying out both theoretical analysis and numerical simulation, and the theoretical closed-form expressions for some figures of merit, including the outage probability, the SEP and the lower bound on the capacity, are consistent with the numerical results. The simulation results also prove that the performance of the proposed scheme is impacted considerably by some other critical parameters, including the number of relays, the channel correlation coefficient and the interference threshold. In the presence of multiple candidate relays, an optimum solution in terms of either outage probability or SEP performance can always be found within the SNR range of [0 dB, 10 dB].