# Upper-Bounding the Regularization Constant for Convex Sparse Signal Reconstruction

## TL;DR

This paper derives upper bounds on the regularization constant in convex sparse signal reconstruction, ensuring the regularization term does not dominate the data fidelity term, with practical algorithms for computing these bounds.

## Contribution

It provides necessary and sufficient conditions for the irrelevance of regularization and develops an optimization framework and ADMM algorithm to compute upper bounds.

## Key findings

- Derived bounds match empirical results in simulations.
- Established conditions for the irrelevance of regularization.
- Proposed an efficient ADMM-based method for bound computation.

## Abstract

Consider reconstructing a signal $x$ by minimizing a weighted sum of a convex differentiable negative log-likelihood (NLL) (data-fidelity) term and a convex regularization term that imposes a convex-set constraint on $x$ and enforces its sparsity using $\ell_1$-norm analysis regularization. We compute upper bounds on the regularization tuning constant beyond which the regularization term overwhelmingly dominates the NLL term so that the set of minimum points of the objective function does not change. Necessary and sufficient conditions for irrelevance of sparse signal regularization and a condition for the existence of finite upper bounds are established. We formulate an optimization problem for finding these bounds when the regularization term can be globally minimized by a feasible $x$ and also develop an alternating direction method of multipliers (ADMM) type method for their computation. Simulation examples show that the derived and empirical bounds match.