This paper addresses the problem of resource allocation for systems in whicha primary and a secondary link share the available spectrum by an underlay oroverlay approach. After observing that such a scenario models both cognitiveradio and D2D communications, we formulate the problem as the maximization ofthe secondary energy efficiency subject to a minimum rate requirement for theprimary user. This leads to challenging non-convex, fractional problems. In theunderlay scenario, we obtain the global solution by means of a suitablereformulation. In the overlay scenario, two algorithms are proposed. The firstone yields a resource allocation fulfilling the first-order optimalityconditions of the resource allocation problem, by solving a sequence of easierfractional problems. The second one enjoys a weaker optimality claim, but aneven lower computational complexity. Numerical results demonstrate the meritsof the proposed algorithms both in terms of energy-efficient performance andcomplexity, also showing that the two proposed algorithms for the overlayscenario perform very similarly, despite the different complexity.
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