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This paper evaluates the capacity of the reactive decode-and-forward (R-DF) scheme, which considers the reduced relay candidates based on the signal-to-noise ratio (SNR) of the source-relay link for relay selection in underlay cognitive relay networks. The performance of R-DF scheme is compared with that of the proactive DF (P-DF) scheme. It has been proven that R-DF scheme achieves the similar capacity of P-DF scheme as the primary receiver is located further from the secondary source. Since the transmission power of a secondary user is constrained by the location of the primary receiver, the performance of a cognitive relay network varies depending on the considered link for determining the relay candidates, between the source-relay and the relay-destination links. Hence, we also analyze the performance of the backward R-DF (BR-DF) scheme which the relay candidates are determined based on the relay-destination link. It is also verified that the performance of BR-DF scheme is better than that of R-DF scheme when the primary receiver is located close to the secondary source.
Date of Publication: October 2011