Covert Communication in Two-Hop Cooperative Cognitive Radio System

Covert communication is capable of enhancing user privacy by protecting communication behavior. In this article, by employing overlay and underlay spectrum access modes, we investigate a covert cooperative cognitive radio (CCCR) system, where primary transmitter (PT) and secondary transmitter (ST) cooperate with each other to secretly transmit their own confidential information. Specifically, PT can initiate the transmission directly or transmit information with the aid of ST. In return, ST is able to transmit information by exploiting PT's spectrum. In our system, we assume that both PT and ST send artificial noise (AN) to confuse eavesdropper (Eve). That is to say, ST sends AN to Eve when PT initiates the transmission and PT sends AN to Eve when ST transmits information. Then, we explore Eve's detection scheme and obtain the closed-form expression of the minimum detection error probability at Eve in the two modes. We further analyze covert rate and covert outage probability (COP) of primary receiver (PR) and secondary receiver (SR) in different modes. Numerical results show that the interference power dominates the covert transmission, while the self-interference has little impact on the covert performance. Furthermore, it is observed that PR can obtain better covert performance in underlay mode, while SR can obtain better covert performance in overlay mode.