  The limited spectrum resources and dramatic growth of high data rate
communications have motivated opportunistic spectrum access using the promising
concept of cognitive radio networks. Although this concept has emerged
primarily to enhance spectrum utilization, the importance of energy consumption
poses new challenges, because energy efficiency and communication performance
can be at odds. In this paper, the existing approaches to energy efficiency
spectrum sensing and handoff are classified. The tradeoff between energy
consumption and throughput is established as function of the numerous design
parameters of cognitive radio networks, both in the case of local and of
cooperative spectrum sensing. It is argued that a number of important aspects
still needs to be researched, such as fairness, dynamic behavior, reactive and
proactive schemes for energy efficiency.
