Cooperate or not: The secondary user's dilemma in hierarchical spectrum sharing networks

We consider a spectrum sharing network consisting of a primary and a cognitive secondary transmitter-receiver pair, where the secondary transmitter can cooperatively relay primary traffic. If the secondary user chooses not to cooperate, it can transmit only when the channel is sensed idle. Otherwise, it relays the primary packet and transmits its own packet in the same time slot while guaranteeing the performance of the primary transmission. Choosing cooperation, the secondary user can transmit a packet immediately even if the primary queue is not empty, but it has to bear the additional cost of relaying. We consider a cognitive system, where, to solve this dilemma, the secondary user decides dynamically on when to cooperate. We derive the bounds of the stable-throughput region of the system, and formulate the problem as a Markov decision process (MDP). We prove the existence of a stationary policy that is average cost optimal. Numerical results show that the optimal dynamic secondary access can trade off between the gain and the cost of cooperation, and the average cost can be decreased significantly.