Power Allocation and Beamforming in Overlay Cognitive Radio Systems

We consider the overlay cognitive radio channel where the cognitive user is admitted to transmit simultaneously with the primary user provided that the instantaneous rate of primary link is not degraded. Assuming causal knowledge of the primary user´s message at the cognitive transmitter (CT), we analyze the transmission rates of the cognitive user in the single-input multiple-output (SIMO) and the multiple-input single-output (MISO) configurations. In particular, the CT uses a part of its transmit power to assist the primary user in delivering the primary user´s message and the other part of its power is used to deliver its own message. The transmit power and the beamformers are designed to maximize the rate of the cognitive user while fixing the the rate of the primary user. Our simulation results show that the proposed scheme yields improved system performance compared to the interweave scheme where any interference to the primary user is prohibited.