Symbol separation and beamforming to improve secure transmission in multi-untrusted relay networks

In multi-untrusted relay networks, the transmission should be carefully devised to guarantee both security and performance. A novel scheme with symbol separation and beamforming is proposed to improve secure transmission. First, the best two relays with the strongest channel gains are selected to deliver the separated real and imaginary parts of the confidential symbols. Then, directional beamforming is carefully designed to maximise the receptions of the real and imaginary parts at the two selected relays and to zero force the receptions at the other relays. The proposed scheme ensures a perfect physical layer security with high BER at untrusted relays, and an improved BER performance at the destination. Numerical simulations show the correctness and efficiency of the proposed scheme.