Secure spectral-energy efficiency tradeoff in random cognitive relay networks

Spectral efficiency (SE) and energy efficiency (EE) in secure communications is of primary importance due to the fact that 5G wireless networks aim to achieve high throughput, low power consumption and high level of security. Nevertheless, maximizing SE and EE are not achievable simultaneously. In this paper, we investigate the SE and EE tradeoff for secure transmission in cognitive relay networks where all nodes are randomly distributed. We first introduce the opportunistic relay selection policy, where each primary transmitter communicates with the primary receiver with the help of a secondary user as a relay. Then, we evaluate the secure SE and secure EE of the primary network based on the outage probabilities analysis. Thirdly, by applying a unified SE-EE tradeoff metric, the secure SE and EE tradeoff problem is formulated as the joint secure SE and EE maximization problem. Considering the non-concave feature of the objective function, an iterative algorithm is proposed to improve secure SE and EE tradeoff. Numerical results show that the opportunistic relay selection policy is always superior to random relay selection policy. Furthermore, the opportunistic relay selection policy outperforms conventional direct transmission policy when faced with small security threat (i.e., for smaller eavesdropper density).