  This paper 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.
