CELLULAR network operators face hurdles in supporting the escalating growth in wireless data traffic due to spectrum scarcity. Joint Optimization of Clustering and Cooperative Beamforming in Green Cognitive Wireless Networks To tackle this challenge, cognitive radio has been proposed to improve the spectrum efficiency by allowing a secondary system to opportunistically use a spectrum band licensed to a primary system provided that the former respect the interference limits imposed by the later. Joint Optimization of Clustering and Cooperative Beamforming in Green Cognitive Wireless Networks However, spectrum sharing and interference mitigation are great challenges when the BSs perform their cognitive function individually. In this work, we thus consider the cooperation between cognitive BSs. In particular, we focus on the cooperative beamforming technique, also known as coordinated multipoint transmission (CoMP), which was first proposed to improve the performance of cell-edge users . Joint Optimization of Clustering and Cooperative Beamforming in Green Cognitive Wireless Networks When cognitive BSs cooperate, not only can they reap larger capacity and diversity gains, but also they mitigate the interference to primary users more effectively. However, these benefits of CoMP come with significant costs . First, the cooperating BSs must be connected by a backhaul through which they exchange the channel knowledge and user data. Second, cooperation largely increase the energy consumption due to the extra signal processing . However, energy efficiency has become a central issue for operators as they seek to decrease their carbon footprint and operating costs . Although the theoretical benefits and practical issues of CoMP have been studied in many works, only few have looked into its energy efficiency. Joint Optimization of Clustering and Cooperative Beamforming in Green Cognitive Wireless Networks An energy consumption model for BS cooperation has just been recently developed in. With that model, the authors in analyzed the energy efficiency of an idealized CoMP system and concluded that the cooperative processing power must be kept low for CoMP to provide an energy efficiency gain. This raises questions on when and how BSs should form clusters and cooperate. Joint Optimization of Clustering and Cooperative Beamforming in Green Cognitive Wireless Networks When the service requirements is high, cooperation may help the BSs to better serve their users and protect primary users from interference. Otherwise, they should use a simpler coordination strategy to save energy.