Secondary Spectrum Sharing in Primary Multicasting Systems

We propose a cognitive relaying protocol with primary multicasting systems, where a primary transmitter broadcasts a common message to K licensed primary receivers. It typically adjusts its transmission rate to the user suffering from the worst channel condition in order to serve all the users simultaneously. In the limit of a large number of multicast receivers, cooperation is shown to effectively overcome this problem. In this paper, an unlicensed secondary system acts as a relay for the primary receivers. Furthermore, the secondary system achieves spectrum sharing by superimposing the secondary transmission on primary signals. We show that there exists a power allocation threshold above which the cognitive relaying protocol is able to provide better (or at least an equal) outage performance for the primary system and, at the same time, to enable the secondary system to have an opportunity to access the spectrum.