Physical layer security based on real interference alignment in K-user MIMO y wiretap channels

In this paper, we study the information-theoretical security of the K-user MIMO Y wiretap Gaaussian channels and deduce the lower bounds of the secure degrees of freedom (SDOF). Due to the different receiving links of the eavesdropper, we propose two MIMO Y wiretap models which are called Broadcast Wiretap Channel (BWC) and Multi-access Broadcast Wiretap Channel (MBWC), respectively. Each legitimate user equipped with M antennas intends to transmit independent signals via an intermediate amplify-and-forward (AF) relay with N antennas. Besides, there exists an eavesdropper with N_e antennas close to the K users (K ≥ 3). Based on the asymptotic real interference alignment and the structured cooperative jamming, the security of the MIMO Y wiretap channels can be guaranteed. From the asymptotic analysis, we can find that a lower bound of the sum SDOF in BWC is K/4. While in the MBWC model, since the relay working in the full duplex mode broadcasts cooperative jamming signals to all the other nodes especially to the eavesdropper, the lower bound of sum SDOF is K2-3K/4(K-1), which is altered along with the number of users.