Every frame is a sum of three (but not two) orthonormal bases---and other frame representations. Casazza, P. G. J. Fourier Anal. Appl., 4(6):727--732, 1998.
Paper doi bibtex @article{ MR1666009,
author = {Casazza, Peter G.},
doi = {10.1007/BF02479676},
fjournal = {The Journal of Fourier Analysis and Applications},
issn = {1069-5869},
journal = {J. Fourier Anal. Appl.},
mrclass = {47A99 (42C15 46B15 46C05)},
mrnumber = {1666009 (2000a:47046)},
mrreviewer = {Jean-Pierre Gabardo},
number = {6},
pages = {727--732},
title = {{Every frame is a sum of three (but not two) orthonormal bases---and other frame representations}},
url = {http://dx.doi.org.login.library.coastal.edu:2048/10.1007/BF02479676},
volume = {4},
year = {1998}
}