Generalised Welch Bound Equality sequences are tight frames
This paper shows what are called
Welch bound equality (WBE) sequences by the signal processing
community are precisely the isometric/equal norm/normalized/uniform tight
frames which are currently being investigated for a number of
and in the real case are the spherical $2$--designs of combinatorics.
Recent applications include wavelet expansions, Grassmannian frames,
frames robust to erasures,
and quantum measurements.
This is done by giving an elementary proof of a generalisation of
Welch's inequality to vectors which need not have equal energy,
and then showing that equality occurs in this exactly when the vectors
form a tight frame.
Welch bound equality sequences,
isometric tight frames,
normalized tight frames,
Math Review Classification:
Primary 05B30, 42C40, 94A12
; Secondary 42C15, 65D30
Length: 7 pages
Last Updated: 18 March 2003