"The method of claim 1 wherein the generating an expanded group of Fourier coefficients comprises: inserting P-1 zeros between consecutive Fourier coefficients of the initial group of Fourier coefficients, where P is a number of users; inserting p zeros before the first Fourier coefficient of the initial group of Fourier coefficients, where p identifies a current user; and inserting P-1-p zeros aft" . . . .