  Wireless sensor networks (WSNs) operating in the license-free spectrum suffer
from uncontrolled interference as those spectrum bands become increasingly
crowded. The emerging cognitive radio sensor networks (CRSNs) provide a
promising solution to address this challenge by enabling sensor nodes to
opportunistically access licensed channels. However, since sensor nodes have to
consume considerable energy to support CR functionalities, such as channel
sensing and switching, the opportunistic channel accessing should be carefully
devised for improving the energy efficiency in CRSN. To this end, we
investigate the dynamic channel accessing problem to improve the energy
efficiency for a clustered CRSN. Under the primary users' protection
requirement, we study the resource allocation issues to maximize the energy
efficiency of utilizing a licensed channel for intra-cluster and inter-cluster
data transmission, respectively. With the consideration of the energy
consumption in channel sensing and switching, we further determine the
condition when sensor nodes should sense and switch to a licensed channel for
improving the energy efficiency, according to the packet loss rate of the
license-free channel. In addition, two dynamic channel accessing schemes are
proposed to identify the channel sensing and switching sequences for
intra-cluster and inter-cluster data transmission, respectively. Extensive
simulation results demonstrate that the proposed channel accessing schemes can
significantly reduce the energy consumption in CRSNs.
