Convexity of Decentralized Controller Synthesis

TL;DR

This paper establishes that quadratic invariance is both necessary and sufficient for the convexity of the set of Youla parameters in decentralized control problems, linking structural properties to convex optimization feasibility.

Contribution

It proves the converse of previous results, showing quadratic invariance characterizes when the set of Youla parameters is convex in decentralized control.

Findings

Quadratic invariance is necessary and sufficient for convexity of Youla parameter sets.

Convexity of achievable closed-loop maps aligns with quadratic invariance under certain conditions.

Results apply to bounded linear operators and causal LTI systems in discrete or continuous time.

Abstract

In decentralized control problems, a standard approach is to specify the set of allowable decentralized controllers as a closed subspace of linear operators. This then induces a corresponding set of Youla parameters. Previous work has shown that quadratic invariance of the controller set implies that the set of Youla parameters is convex. In this paper, we prove the converse. We thereby show that the only decentralized control problems for which the set of Youla parameters is convex are those which are quadratically invariant. We further show that under additional assumptions, quadratic invariance is necessary and sufficient for the set of achievable closed-loop maps to be convex. We give two versions of our results. The first applies to bounded linear operators on a Banach space and the second applies to (possibly unstable) causal LTI systems in discrete or continuous time.