Co-existence with ARQ-based primary system through cooperate-and-access spectrum sharing

We consider spectrum sharing in a cognitive radio network where a secondary user co-exists with an ARQ-based primary user. A spectrum sharing protocol is proposed where the secondary user switches between cooperation and access modes. In the cooperation mode, the secondary system serves as a relay to assist the primary transmission, and in return accumulates credits. The credits allow the secondary user to gain spectrum access by exploiting the ARQ redundant retransmission of the primary user in access mode. We show analytically that as long as the credits accumulated in cooperation mode compensate for the degradation in primary performance during access mode, the proposed protocol achieves an equal (or higher) average throughput for the primary system than the case without spectrum sharing, while providing spectrum access opportunities for the secondary user.