Evaluation of Marton's Inner Bound for the General Broadcast Channel

Abstract

The best known inner bound on the two-receiver general broadcast channel without a common message is due to Marton [3]. This result was subsequently generalized in [p. 391, Problem 10(c) 2] and [4] to broadcast channels with a common message. However the latter region is not computable (except in certain special cases) as no bounds on the cardinality of its auxiliary random variables exist. Nor is it even clear that the inner bound is a closed set. The main obstacle in proving cardinality bounds is the fact that the traditional use of the Carathéodory theorem, the main known tool for proving cardinality bounds, does not yield a finite cardinality result. One of the main contributions of this paper is the introduction of a new tool based on an identity that relates the second derivative of the Shannon entropy of a discrete random variable (under a certain perturbation) to the corresponding Fisher information. In order to go beyond the traditional Carathéodory type arguments, we identify certain properties that the auxiliary random variables corresponding to the extreme points of the inner bound need to satisfy. These properties are then used to establish cardinality bounds on the auxiliary random variables of the inner bound, thereby proving the computability of the region, and its closedness. Lastly, we establish a conjecture of \cite{NairZizhou} that Marton's inner bound and the recent outer bound of Nair and El Gamal do not match in general.