Physical Layer Security Based on Interference Alignment in K-User MIMO Y Wiretap Channels

This paper studies the secure degree of freedom (SDOF) of the multiway relay wiretap system K-user MIMO Y wiretap channel, where each legitimate user equipped with M antennas intends to convey independent signals via an intermediate relay with N antennas. There exists an eavesdropper which is equipped with N-e antennas close to the legitimate users. In order to eliminate the multiuser interference and keep the system security, interference alignment is mainly utilized in the proposed broadcast wiretap channel (BWC) and multi-access BWC (MBWC), and cooperative jamming is adopted as a supplementary approach in the MBWC model. The general feasibility conditions of the interference alignment are deduced as M >= K - 1, 2M > N and N >= ((K (K - 1))/2). In the BWC model, we have deduced the secure degrees of freedom (SDOF) as K min {M, N} min {N-e, K (K - 1)/2}, which is always a positive value. While in the MBWC model, the SDOF is given by K min {M, N}. Finally, since the relay transmits the synthesized signals of the legal signal and the jamming signal in the MBWC model, we propose a power allocation scheme to maximize the secrecy rate. Simulation results demonstrate that our proposed power allocation scheme can improve secrecy rate under various antenna configurations.