  In this paper, we propose a cognitive protocol that involves cooperation
between the primary and secondary users. In addition to its own queue, the
secondary user (SU) has a queue to store, and then relay, the undelivered
primary packets. When the primary queue is nonempty, the SU remains idle and
attempts to decode the primary packet. When the primary queue is empty, the SU
splits the total channel bandwidth into two orthogonal subbands and assigns
each to a queue probabilistically. We show the advantage of the proposed
protocol over the prioritized cognitive relaying (PCR) protocol in which the SU
assigns a priority in transmission to the primary packets over its own packets.
We present two problem formulations, one based on throughput and the other on
delay. Both optimization problems are shown to be linear programs for a given
bandwidth assignment. Numerical results demonstrate the benefits of the
proposed protocol.
