Refinements of the Half-Bit and Factor-of-Two Bounds for Capacity in Gaussian Channel with Feedback

We consider the upper bounds of the finite block length capacity Cn,FB(P ) of the discrete time Gaussian channel with feedback. We also let Cn(p) the nonfeedback capacity. We prove the relations Cn(P ) ≤ Cn,FB(P ) ≤ Cn(αP ) + 1 2 ln(1 + 1 α ) and Cn(P ) ≤ Cn,FB(P ) ≤ (1 + 1 α )Cn(αP ) for any P > 0 and any α > 0, which induce the half-bit and factor-of two bounds given by Cover and Pombra in the special case of α = 1.