SNR based secure communication via untrusted amplify-and-forward relay nodes using artificial noise

A two-phase beamforming solution for secure communication using untrusted relay nodes is presented. To thwart eavesdropping attempts of participant relay nodes, we deliberately introduce artificial noise in the source message. We observe that for certain scenarios, the source node prefers to allocate its full power to the message signal and, therefore, in the absence of artificial noise, such transmissions will be vulnerable to eavesdropping. Therefore, we provide a signal-to-noise ratio (SNR) based framework for secure communication. We intend to bring down the SNR at each of the untrusted relay nodes below a certain predefined threshold; whereas, using beamforming, we want to boost the SNR at the destination. With this motive, we evaluate the optimal scaling vector for the beamforming phase, which not only nullifies the artificial noise transmitted initially, but also maximizes the SNR at the destination. We discuss both the total and individual power constraint scenarios and provide an analytical optimal solution and an iterative algorithm, respectively.