Cooperative Beamforming in Relay Assisted MIMO-OFDMA Cognitive Radio Systems

Cognitive radio (CR) technology can effectively improve spectral efficiency by allowing secondary users (SUs) to access licensed bands without any harmful interference to primary users (PUs). Due to adverse effects of environment such as loss and fading, SUs may receive weak signals. So, in order to mitigate these effects, some SUs can act as relay nodes to improve SUs’ connectivity. Moreover, using relay nodes can increase spatial diversity and also enhance spectrum efficiency further. In this paper, a new algorithm is proposed for cooperative beamforming, power allocation and relay selection in the multiple input multiple output (MIMO) Orthogonal Frequency Division Multiple Access (OFDMA) CR systems where a pair of SU communicates with each other assisted by some single antenna relay nodes. The objective is to maximize signal to interference plus noise ratio (SINR) of SU subject to guarantee the PU's quality of service and power constraints of SU and relay nodes over all subchannels. The transmitter and receiver beamforming vectors of PU and SU are estimated by the proposed two-step iterative algorithm. The performance of the algorithm is evaluated through simulations. Simulation results show that the proposed algorithm, in addition to guarantee a required performance of the PU, increases spectrum usage efficiency by servicing the SUs. Also, by increasing the number of relay nodes, the performance of the algorithm is improved