Outage analysis of causal cognitive radio channel

In this paper, we first analyze the outage performance of two user cognitive radio channel where the cognitive user causally obtains primary user´s message knowledge. Specifically, we consider cognitive rayleigh fading channel where the cognitive user uses superposition coding to relay the primary user´s message to compensate for the interference caused by its own transmission to the primary receiver. We find the mutual information expressions at primary and cognitive receivers by classifying it in different cases. Results show that non-zero capacity is obtained at cognitive receiver while maintaining outage rate of the primary unchanged. It is also shown that additional diversity is obtained at primary receiver due to relaying by the cognitive user. We then extend the single user case to multiple cognitive users. It is shown that using multiuser diversity effect and cognitive users cooperation, significant increase in the mutual information at cognitive receiver for a given outage probability is achieved.