Area spectral efficiency and energy efficiency of multi-hop multi-antenna cognitive underlay networks

In this article, we develop a unified framework for characterizing the area spectral efficiency and the energy efficiency (EE) of multi-antenna multi-hop cognitive underlay networks (CRNs). We highlight the available degrees-of-freedom which can be exploited to maximize the EE of the network. In particular, we demonstrate that considering the primary user´s quality of service (QoS) requirements, joint tuning of the medium access probability and the transmit power results in an optimal EE performance. It is shown that an arbitrary selection of one degree of freedom while adapting another in accordance with the QoS constraint results in a sub-optimal performance. We highlight that the secondary array size and the employed modulation scheme can also be treated as the available degrees of freedom which can be exploited to further enhance the performance of an underlay CRN.