On the Capacity of the Heat Channel, Waterfilling in the Time-Frequency Plane, and a C-NODE Relationship

Abstract: The heat channel is defined by a linear time-varying (LTV) filter with additive white Gaussian noise (AWGN) at the filter output. The continuous-time LTV filter is related to the heat kernel of the quantum mechanical harmonic oscillator, so the name of the channel. The channel's capacity is given in closed form by means of the Lambert W function. Also a waterfilling theorem in the time-frequency plane for the capacity is derived. It relies on a specific Szego theorem for which an essentially self-contained proof is provided. Similarly, the rate distortion function for a related nonstationary source is given in closed form and a (reverse) waterfilling theorem in the time-frequency plane is derived. Finally, a second closed-form expression for the capacity of the heat channel based on the detected perturbed filter output signals is presented. In this context, a precise differential connection between channel capacity and the normalized optimal detection error (NODE) is revealed. This C-NODE relationship is compared with the well-known I-MMSE relationship connecting mutual information with the minimum mean-square error (MMSE) of estimation theory.