  In this paper, we quantify the benefits of employing relay station in
large-coverage cognitive radio systems which opportunistically access the
licensed spectrum of some small-coverage primary systems scattered inside.
Through analytical study, we show that even a simple decode-and-forward (SDF)
relay, which can hold only one packet, offers significant path-loss gain in
terms of the spatial transmission opportunities and link reliability. However,
such scheme fails to capture the spatial-temporal burstiness of the primary
activities, that is, when either the source-relay (SR) link or
relay-destination (RD) link is blocked by the primary activities, the cognitive
spectrum access has to stop. To overcome this obstacle, we further propose
buffered decode-and-forward (BDF) protocol. By exploiting the infinitely long
buffer at the relay, the blockage time on either SR or RD link is saved for
cognitive spectrum access. The buffer gain is shown analytically to improve the
stability region and average end-to-end delay performance of the cognitive
relay system.
