On Secure Degrees of Freedom of the MIMO Interference Channel With Local Output Feedback

This article studies the problem of Sum-secure Degrees of Freedom (SDoF) of the (M, M, N, N) multiple-input-multiple-output (MIMO) interference channel with local output feedback, so as to build an information-theoretic foundation and provide practical transmission schemes for 6G-enabled Vehicles-to-Vehicles (V2V). For this problem, we propose two novel transmission schemes, i.e., the interference decoding scheme and the interference alignment scheme, and thus establish a sum-SDoF lower bound. In particular, to optimize the phase duration, we analyze the security and decoding constraints and formulate a linear-fractional optimization problem. Furthermore, we show that the derived sum-SDoF lower bound is the sum-SDoF for M <= N/2, N = M, and 2N <= M antenna configurations, and reveal that for a fixed N, the optimal M to maximize the sum-SDoF is not less than 2N. Through simulations, we examine the secure sum-rate performance of proposed transmission schemes and reveal that using local output feedback can lead to a higher secure sum-rate than that by using delayed channel state information at the transmitter (CSIT).