Optimization of secrecy rate in cooperative device to device communications

In this paper, a cooperative device to device (D2D) communication system is studied while two transmitting devices communicate with one destination device in the presence of a passive eavesdropper. The second transmitting device cooperates with the first transmitting device and hence the transmission takes place is two phases. In the first phase, the first device transmits its signal to the second transmitting device. In the second phase, the second transmitting device transmits its own message along with the relaying signal from the first device simultaneously to the receiver. Our goal is to optimize this cooperation through maximizing the secrecy rate while the signal-to-noise ratio (SNR) at the destination is guaranteed to be above certain predefined threshold. We show that this optimization problem yields to polynomial fractional programming and propose an efficient numerical algorithm by using Gloptiploy and SeDuMi software packages. Simulation results show that this optimum cooperation achieves superior secrecy rate compared with fixed or maximum cooperation methods.