Partial relay selection with fixed-gain relays and outdated CSI in underlay cognitive networks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 channel capacity, being analyzed. Different from the conventional relay selection schemes, we assume that the primary users share their bandwidth with the secondary users to enable a secondary relay-aided communication if the interference added to the primary users is kept below a certain threshold in an underlay cognitive network. In particular, both the feedback delay and Doppler frequency shift are assumed to be within a tolerable range, and as compared with the conventional methods, less channel state information (CSI) feedback is required in the proposed method due to partial relay selection being performed in the latter. The proposed scheme is validated by carrying out both theoretical analysis and numerical simulation, and the theoretical approximations of closed-form expressions for some figures of merit, e.g., the outage probability, the SEP, and the channel capacity, are all consistent with the numerical results. The simulations also prove that the performance of the proposed scheme is considerably affected by some other critical parameters, such as 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 the outage probability or the SEP performance can always be found within the SNR range of (0, 10 dB).