On the Capacity of the Discrete Time Gaussian Channel with Delayed Feedback

We give an upper bound to the finite block length capacity of discrete time nonstationary Gaussian channel with delayed feedback. With the aid of minimization of a quadratic form, it is proved that the L time delayed feedback capacity C L n,F B(P) and the nonfeedback capacity Cn(P) satisfy Cn(P) ≤ C L n,F B(P) ≤ Cn(P ∗), where P ∗ is concretely given. And we give a sufficient condition for C L n,F B(P) to be increased by L time delayed feedback. Finally we also give a necessary and sufficient condition for C 2 n,F B(P) of a class of Gaussian channel to be increased by 2 time delayed feedback