In this paper, we study a multiple-antenna two-wayrelaying (TWR) cognitive radio (CR) system. A space alignment(SA) technique is adopted by the secondary users (SUs) to avoidinterference with the primary users (PUs). We derive the optimalpower allocation that maximizes the TWR achievable SU sum-rate while respecting the total power budget and the relay powerconstraints. We also analyze the case in which the relay is able tooptimize its gain matrix structure to enhance the SU sum-rate.In the numerical results, we quantify the sum-rate gain of usingthe SA in the TWR CR and we show that the SU sum-rate isvery limited when the relay power is low or the PU power and itsresulting interference are high. In addition, we optimize the relaygain using an iterative algorithm and compare between differentrelay matrix structures.
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