Low coherence unit norm tight frames

TL;DR

This paper introduces a method to construct low coherence unit norm tight frames in real spaces by augmenting existing equiangular tight frames with strategically chosen vectors using block designs, aiming to approximate the optimal coherence bound.

Contribution

It presents a novel construction of low coherence UNTFs from existing ETFs using combinatorial block designs, achieving near-optimal coherence levels.

Findings

Constructed low coherence UNTFs using block designs.

Achieved minimal maximum coherence for certain designs.

Provided bounds on coherence for the constructed frames.

Abstract

Equiangular tight frames (ETFs) have found significant applications in signal processing and coding theory due to their robustness to noise and transmission losses. ETFs are characterized by the fact that the coherence between any two distinct vectors is equal to the Welch bound. This guarantees that the maximum coherence between pairs of vectors is minimized. Despite their usefulness and widespread applications, ETFs of a given size $N$ are only guaranteed to exist in $\mathbb{R}^{d}$ or $\mathbb{C}^{d}$ if $N = d + 1$. This leads to the problem of finding approximations of ETFs of $N$ vectors in $\mathbb{R}^{d}$ or $\mathbb{C}^{d}$ where $N > d+1.$ To be more precise, one wishes to construct a unit norm tight frame (UNTF) such that the maximum coherence between distinct vectors of this frame is as close to the Welch bound as possible. In this paper low coherence UNTFs in $\mathbb{R}^d$ are constructed by adding a strategically chosen set of vectors called an "optimal" set to an existing ETF of $d+1$ vectors. In order to do so, combinatorial objects called block designs are used. Estimates are provided on the maximum coherence between distinct vectors of this low coherence UNTF. It is shown that for certain block designs, the constructed UNTF attains the smallest possible maximum coherence between pairs of vectors among all UNTFs containing the starting ETF of $d+1$ vectors. This is particularly desirable if there does not exist a set of the same size for which the Welch bound is attained.