  Limited spectrum resources and dramatic growth of high data rate applications
have motivated opportunistic spectrum access utilizing the promising concept of
cognitive networks. Although this concept has emerged primarily to enhance
spectrum utilization and to allow the coexistence of heterogeneous network
technologies, the importance of energy consumption imposes additional
challenges, because energy consumption and communication performance can be at
odds. In this paper, the approaches for energy efficient spectrum sensing and
spectrum handoff, fundamental building blocks of cognitive networks is
investigated. The tradeoff between energy consumption and throughput, under
local as well as under cooperative sensing are characterized, and what further
aspects need to be investigated to achieve energy efficient cognitive operation
under various application requirements are discussed.
