Optimal Power Allocation for Fading Cognitive Multiple Access Channels: A Two-User Case

In this paper, we consider a spectrum-sharing cognitive radio network where a two-user secondary multiple access network coexists with a primary user (PU). An interference power constraint (IPC) is imposed on the secondary network (SN) to protect the PU. Under such a constraint, we derive the optimal power allocation strategies to achieve the fundamental limits of the SN. In particular, both the ergodic capacity and the outage capacity are investigated. Closed-form solutions are obtained for both scenarios. It is also shown that concurrent transmission of the two users should be avoided if the ergodic capacity is considered, while concurrent transmission should be encouraged if the outage capacity is considered. Besides, a key parameter referred to as Interference-Transmission-Ratio (ITR) is proposed. It is shown that under a given IPC, ITR is a key factor that affects the transmission policies of the SN. In general, the user with lower ITR value has the priority to transmit.