A precoding technique for joint relaying and broadcasting in cognitive radio networks

In this paper, we consider a cognitive radio network in which the primary system base station acts as relay for the cognitive or secondary system as well as transmitting the primary users´ data streams on the downlink channel. We propose a precoding technique which is used at the base station of the primary system. Using this technique, it is possible to keep the signal to interference plus noise ratio (SINR) of the primary users above a required value and in addition to help the cognitive system by relaying signals from secondary base station to desired cognitive users. Although the proposed method needs the primary base station to be modified and to perform additional processing, it does not need any modification in the primary users and moreover, the interference induced on the primary users are controlled. In this paper, we assume that the primary base station is equipped with multiple transmit antennas. The proposed precoding scheme is based on the block diagonalization technique which removes the inter-user interference and it is combined with maximal signal to jamming plus noise ratio (SJNR) precoding method which minimizes the inter-user interference.