Mutual information of the white Gaussian channel with and without feedback

The following model for the white Gaussian channel with or without feedback is considered: {equation} Y(t) = ∫_o ᵗ φ (s, Y_o ˢ ,m) ds + W(t) {equation} wheremdenotes the message,Y(t)denotes the channel output at timet,Y_o ^ {t}denotes the sample pathY(θ), 0 ≤ θ ≤ t. W(t)is the Brownian motion representing noise, andφ(s, y_o ^ {s} ,m)is the channel input (modulator output). It is shown that, under some general assumptions, the amount of mutual informationI(Y_o T ,m)between the messagemand the output pathY_o ^ {T}is directly related to the mean-square causal filtering error of estimatingφ (t, Y_o ᵗ ,m)from the received dataY_o T , 0 ≤ t ≤ T. It follows, as a corollary to the result forI(Y_o ^ {T} ,m), that feedback can not increase the capacity of the nonband-limited additive white Gaussian noise channel.