  When a long-term energy constraint is imposed to a transmitter, the average
energy-efficiency of a transmitter is, in general, not maximized by always
transmitting. In a cognitive radio context, this means that a secondary link
can re-exploit the non-used time-slots. In the case where the secondary link is
imposed to generate no interference on the primary link, a relevant issue is
therefore to know the fraction of time-slots available to the secondary
transmitter, depending on the system parameters. On the other hand, if the
secondary transmitter is modeled as a selfish and free player choosing its
power control policy to maximize its average energy-efficiency, resulting
primary and secondary signals are not necessarily orthogonal and studying the
corresponding Stackelberg game is relevant to know the outcome of this
interactive situation in terms of power control policies.
