Performance limits for cognitive small cells

Heterogeneous networks consisting of a macrocell tier and a small cell tier are foreseen as one of the solutions to meet the ever increasing mobile data demand. Since a massive deployment of small cell access points (SAPs) leads also to a considerable increase in energy consumption, the energy efficient design of those SAPs is crucial. Sleep mode techniques are a promising strategy to reduce the energy consumption, yet they require cognitive capabilities to detect the presence of a macrocell user. In this work, we define a fundamental limit on the interference density that allows robust user detection. Beyond this limit, which we call the interference wall, an energy efficient SAP design is impossible. In addition, we elucidate the relation between energy efficiency and sensing time using large deviations theory.