  As a flexible and scalable architecture, heterogeneous cloud radio access
networks (H-CRANs) inject strong vigor into the green evolution of current
wireless networks. But the brutal truth is that energy efficiency (EE) improves
at the cost of other indexes such as spectral efficiency (SE), fairness, and
delay. It is thus important to investigate performance tradeoffs for striking
flexible balances between energy-efficient transmission and excellent
quality-of-service (QoS) guarantees under this new architecture. In this
article, we first propose some potential techniques to energy-efficiently
operate H-CRANs by exploiting their features. We then elaborate the initial
ideas of modeling three fundamental tradeoffs, namely EE-SE, EE-fairness, and
EE-delay tradeoffs, when applying these green techniques, and present open
issues and challenges for future investigations. These related results are
expected to shed light on green operation of H-CRANs from adaptive resource
allocation, intelligent network control, and scalable network planning.
