  A single cognitive radio transmitter--receiver pair shares the spectrum with
two primary users communicating with their respective receivers. Each primary
user has a local traffic queue, whereas the cognitive user has three queues;
one storing its own traffic while the other two are relaying queues used to
store primary relayed packets admitted from the two primary users. A new
cooperative cognitive medium access control protocol for the described network
is proposed, where the cognitive user exploits the idle periods of the primary
spectrum bands. Traffic arrival to each relaying queue is controlled using a
tuneable admittance factor, while relaying queues service scheduling is
controlled via channel access probabilities assigned to each queue based on the
band of operation. The stability region of the proposed protocol is
characterized shedding light on its maximum expected throughput. Numerical
results demonstrate the performance gains of the proposed cooperative cognitive
protocol.
