Secure communications with untrusted secondary users in cognitive radio networks

Spectrum sensing in a cognitive radio network is an essential technique that makes secondary users to detect the presence of primary users. Furthermore, the secondary users can make use of sensing results to help the primary users´ transmission in a way to forward them in reward for allowing the spectrum access. However, for the primary users, the spectrum sensing can be considered eavesdropping in the sense that the secondary users may try to decode primary users´ messages based on the sensing results. In this paper, by applying the notion of information-theoretic secrecy to the cognitive radio scenario, we propose a secure cooperative transmission scheme targeting at allowing the secondary users to sense and relay but making them ignorant of the primary users´ message. Wiretap channel coding is applied to an encoding process of primary users, and the secondary users help the primary users´ transmission in a way to forward the amplified sensing results combined with their own messages. We characterize an achievable secrecy rate and data rate pair that primary and secondary users can achieve and formulate three optimization problems from which the secondary transmitter carefully distributes transmit power between the relaying signal and its own message. The numerical results show that our scheme has a non-zero positive secrecy rate in the area where non-cooperative scheme achieves a zero secrecy rate.