In this letter, we consider a multiple-input multiple-output two-waycognitive radio system under a spectrum sharing scenario, where primary andsecondary users operate on the same frequency band. The secondary terminalsaims to exchange different messages with each other using multiple relays whereeach relay employs an amplify-and-forward strategy. The main objective of ourwork is to maximize the secondary sum rate allowed to share the spectrum withthe primary users by respecting a primary user tolerated interferencethreshold. In this context, we derive a closed-form expression of the optimalpower allocated to each antenna of the terminals. We then discuss the impact ofsome system parameters on the performance in the numerical result section.
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