Cognitive Radios for Preserving Primary Outage Performance over Two-Hop Relay Channels

We consider cognitive radio (CR) communications over two-hop relay channels. It is assumed that the secondary sender has only one-bit information indicating whether or not it is allowed to access, while the primary nodes still operate even without any prior knowledge on the secondary nodes. We propose three different two-hop relaying protocols according to how much the relay has prior knowledge on its outgoing channels, and show that the proposed relaying protocols make it possible for the secondary sender to transmit data without causing any loss at the primary outage probability. It is also shown that the proposed relaying protocols make the secondary outage probability as small as possible by increasing the signal-to-noise ratio (SNR) if the relay has prior knowledge on its outgoing channel to the primary receiver.