The Mutual Information In The Vicinity of Capacity-Achieving Input Distributions
Journal
IEEE Transactions on Information Theory
Start Page
1
End Page
1
ISSN
0018-9448
1557-9654
Date Issued
2025
Author(s)
Nakiboğlu, Barış
Abstract
The mutual information is bounded from above by a decreasing affine function of the square of the distance between the input distribution and the set of all capacity-achieving input distributions ΠA, on small enough neighborhoods of ΠA, using an identity due to Topsøe and the Pinsker’s inequality, assuming that the input set of the channel is finite and the constraint set A is polyhedral, i.e., can be described by (possibly multiple but) finitely many linear constraints. Counterexamples demonstrating nonexistence of such a quadratic bound are provided for the case of infinitely many linear constraints and the case of infinite input sets. Using Taylor’s theorem with the remainder term, rather than the Pinsker’s inequality and invoking Moreau’s decomposition theorem the exact characterization of the slowest decrease of the mutual information with the distance to ΠA is determined on small neighborhoods of ΠA. Corresponding results for classical-quantum channels are established under separable output Hilbert space assumption for the quadratic bound and under finite-dimensional output Hilbert space assumption for the exact characterization. Implications of these observations for the channel coding problem and applications of the proof techniques to related problems are discussed.
Publisher
Institute of Electrical and Electronics Engineers (IEEE)
Type
journal article