Feedback Capacity of the Continuous-Time ARMA(1,1) Gaussian Channel (2302.13073v2)
Abstract: We consider the continuous-time ARMA(1,1) Gaussian channel and derive its feedback capacity in closed form. More specifically, the channel is given by $\boldsymbol{y}(t) =\boldsymbol{x}(t) +\boldsymbol{z}(t)$, where the channel input ${\boldsymbol{x}(t) }$ satisfies average power constraint $P$ and the noise ${\boldsymbol{z}(t)}$ is a first-order {\em autoregressive moving average} (ARMA(1,1)) Gaussian process satisfying $$ \boldsymbol{z}\prime(t)+\kappa \boldsymbol{z}(t)=(\kappa+\lambda)\boldsymbol{w}(t)+\boldsymbol{w}\prime(t), $$ where $\kappa>0,~\lambda\in\mathbb{R}$ and ${\boldsymbol{w}(t) }$ is a white Gaussian process with unit double-sided spectral density. We show that the feedback capacity of this channel is equal to the unique positive root of the equation $$ P(x+\kappa)2 = 2x(x+\vert \kappa+\lambda\vert)2 $$ when $-2\kappa<\lambda<0$ and is equal to $P/2$ otherwise. Among many others, this result shows that, as opposed to a discrete-time additive Gaussian channel, feedback may not increase the capacity of a continuous-time additive Gaussian channel even if the noise process is colored. The formula enables us to conduct a thorough analysis of the effect of feedback on the capacity for such a channel. We characterize when the feedback capacity equals or doubles the non-feedback capacity; moreover, we disprove continuous-time analogues of the half-bit bound and Cover's $2P$ conjecture for discrete-time additive Gaussian channels.
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