On the existence of equiangular tight frames

An equiangular tight frame (ETF) is a d×N matrix that has unit-norm columns and orthogonal rows of norm . Its key property is that the absolute inner products between pairs of columns are (i) identical and (ii) as small as possible. ETFs have applications in communications, coding theory, and sparse approximation. Numerical experiments indicate that ETFs arise for very few pairs (d, N), and it is an important challenge to develop restrictions on the pairs for which they can exist. This article uses field theory to provide detailed conditions on real and complex ETFs. In particular, it describes restrictions on harmonic ETFs, a specific type of complex ETF that appears in applications. Finally, the article offers empirical evidence that these conditions are sharp or nearly sharp, especially in the real case.