  We consider a cognitive radio based Internet of Things (CR-IoT) system where
the secondary IoT device (SD) accesses the licensed channel during the
transmission vacancies of the primary IoT device (PD). We focus on the impact
of the IoT devices' heterogeneous traffic pattern on the energy efficiency and
on the age of information (AoI) performance of the SD. We first derive
closed-form expressions of the energy efficiency and the average AoI, and
subsequently explore their convexity and monotonicity to the transmit power.
Following these characterizations, an optimal transmit power optimization
algorithm (TPOA) is proposed for the SD to maximize the energy efficiency while
maintaining the average AoI under a predefined threshold. Numerical results
verify the different preferences of the SD toward different PD traffic
patterns, and provides insights into the tradeoff between the energy efficiency
and the average AoI.
