Noisy feedback schemes and rate-error tradeoffs from stochastic approximation

It is known that noiseless feedback does not increase the capacity of memoryless channels. However, such feedback can considerably increase the reliability or reduce the coding complexity of schemes that approach capacity. One might hope for the same to be (at least partially) true of noisy feedback. This paper develops a new class of coding schemes for additive white noise channels with feedback corrupted by additive white noise, focusing much of the results and discussion on the Gaussian case. These schemes are variants of the well-known Schalkwijk-Kailath (SK) coding scheme and are based upon simple techniques from stochastic approximation. Specifically, instead of the classic Robbins-Munro approach to stochastic approximation originally used in the SK scheme, we employ more recent techniques from Kushner. The resulting schemes enable a tradeoff between transmission rate and error performance in the presence of noisy feedback even as the number of iterations becomes large.