Physical layer security for relay assisted MIMO D2D communication

This paper presents a secure beamforming design to prevent eavesdropping on multiple-input multiple-output (MIMO) device-to-device (D2D) communication. The devices communicate via a trusted relay which performs physical layer network coding (PNC), and multiple eavesdroppers are trying to intercept the device information. The beamforming design is based on minimizing mean square error of the D2D communication while employing signal-to-interference-plus-noise ratio (SINR) threshold constraints to prevent possible eavesdropping. The channel state information of the device-to-eavesdropper and relay-to-eavesdropper channels is imperfect at the devices and relay. The channel estimation errors are assumed with Gaussian Markov uncertainty model. Consequently, robust optimization problems are formulated considering the multiple access and broadcasting stages of the D2D communication. These problems are non-convex, and two algorithms are proposed to solve them. In the numerical analysis, we discuss the convergence of the proposed algorithms, impact of the number of eavesdroppers on the performance, and the SINR distributions at eavesdroppers.