  In recent years, there has been a growing interest in green cellular networks
for the sake of reducing the energy dissipated by communications and networking
devices, including the base stations (BSs) and battery-powered user terminals
(UTs). This paper investigates the joint employment of cognition and
cooperation techniques invoked for improving the energy efficiency of cellular
networks. To be specific, the cellular devices first have to identify the
unused spectral bands (known as spectrum holes) using their spectrum sensing
functionality. Then, they cooperate for exploiting the detected spectrum holes
to support energy-efficient cellular communications. Considering the fact that
contemporary terminals (e.g., smart phones) support various wireless access
interfaces, we exploit either the Bluetooth or the Wi-Fi network operating
within the spectrum holes for supporting cellular communications with the
intention of achieving energy savings. This approach is termed as
\emph{cognitive network cooperation}, since different wireless access networks
cognitively cooperate with cellular networks. In order to illustrate the energy
efficiency benefits of using both cognition and cooperation, we study the
cooperation between television stations (TVSs) and BSs in transmitting to UTs
relying on an opportunistic exploitation of the TV spectrum, where the unused
TV spectral band is utilized in an opportunistic way, depending on whether it
is detected to be idle (or not). It is shown that for a given number of
information bits to be transmitted, the total energy consumed is significantly
reduced, when both cognition and cooperation are supported in cellular
networks, as compared to the conventional direct transmission, pure cognition
and pure cooperation.
