Outage behavior of MIMO cognitive radio fading channels: A causal approach

In this paper, we study a Multiple-Input Multiple-Output (MIMO) communication scenario through Rayleigh fading cognitive radio channel in which two users, called the primary and the secondary (cognitive) users wish to communicate with two different receivers, subject to the mutual interference. The cognitive user has the causal knowledge of the message being sent by the primary user and is allowed to reuse the channel during both the idle and active periods of the primary user, as long as the primary rate remains to a particular outage rate. Here, we characterize the limits of transmission rate at which the cognitive user can reliably communicate with its receiver compensating interference to the primary user by transmitting a combined information consists of its own and primary user's information using superposition coding during the active periods of primary user. Our results show that not only cognitive user gets a non-zero outage capacity but also the outage performance of primary user is improved due to the additional diversity obtained via relaying.