Priority-based scheduling for limited energy cognitive relaying

We study the problem of cognitive relaying with limited-energy constraint in opportunistic spectrum access cognitive networks. Under this network setting, the cognitive user increases the spectrum availability for its own packets by relaying the unsuccessfully transmitted packets of the licensed user. We develop scheduling strategy at the cognitive user that maintains a strict spectrum access priority for licensed user packets and improves the average transmission delay for the cognitive and primary users. We study the effect of the relative location of the cognitive node with respect to the licensed user and its primary destination on the average transmission delay of both cognitive and primary users as well as the lifetime of the cognitive user. Our results show that there exists a tradeoff between average packet delay of the cognitive and primary users and lifetime of the cognitive user with respect to the cognitive user location. In particular, we demonstrate that there exists an optimal cognitive relay location that minimizes the average transmission delay of the primary packets.