Joint Secure Transmission and Interference Mitigation in Cooperative MIMO Systems: A Feasibility Study

To enhance physical layer security (PLS) and mitigate interference at legitimate receivers, artificial noise (AN) assisted interference alignment (IA) is explored in cooperative multiple-input multiple-output (MIMO) systems. The feasibility of traditional leakage minimization (LM) based IA is analyzed. We show that there exist two drawbacks which are not discussed in traditional LM based IA: 1) the necessary condition for the feasibility of IA is loose, and 2) the secret signal cancellation may occur at the desired receiver, which makes secure transmission infeasible. We analyze the reason and establish a condition under which either the secret signal cancellation is inevitable or IA is infeasible. Then a modified IA is proposed. Compared with the traditional LM based IA, a tighter necessary condition is established and the secret signal is protected against cancellation. Moreover, we prove that the proposed condition guarantees the properness of system, which is a rigorous characterization of necessary conditions for achieving IA. Under assumption that the modified IA is feasible, we provide a secrecy enhancing design to maximize the secrecy rate. subject to a secrecy outage constraint. We also establish a condition under which a positive secrecy rate is feasible. Numerical results validate the effectiveness of the proposed approach.