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Extensive deployment of small cells in heterogenous cellular networks introduces both challenges and opportunities. Challenges come with reuse of limited frequency resources, which always introduce both intra- and inter-interference among small cells and macrocells. The opportunities refer to more potential inter- and intra-tier cooperation gains, in particular for ultra-dense heterogeneous and small cell networks. In addition to current spectral efficiency optimization, energy efficiency will also be a critical performance requirement for future green communications, especially when small cells are densely deployed to enhance the user's quality of experience. In this article, potential cooperation gains are explored via a cooperative bargaining game to counter challenges of mitigating interference and saving energy, thus improving both spectral and energy efficiency. We survey the current optimization and trade-offs …