  This paper considers a cooperative OFDMA-based cognitive radio network where
the primary system leases some of its subchannels to the secondary system for a
fraction of time in exchange for the secondary users (SUs) assisting the
transmission of primary users (PUs) as relays. Our aim is to determine the
cooperation strategies among the primary and secondary systems so as to
maximize the sum-rate of SUs while maintaining quality-of-service (QoS)
requirements of PUs. We formulate a joint optimization problem of PU
transmission mode selection, SU (or relay) selection, subcarrier assignment,
power control, and time allocation. By applying dual method, this mixed integer
programming problem is decomposed into parallel per-subcarrier subproblems,
with each determining the cooperation strategy between one PU and one SU. We
show that, on each leased subcarrier, the optimal strategy is to let a SU
exclusively act as a relay or transmit for itself. This result is fundamentally
different from the conventional spectrum leasing in single-channel systems
where a SU must transmit a fraction of time for itself if it helps the PU's
transmission. We then propose a subgradient-based algorithm to find the
asymptotically optimal solution to the primal problem in polynomial time.
Simulation results demonstrate that the proposed algorithm can significantly
enhance the network performance.
