function [xn]=idft(Xk,N)
% calculate the idft of Xk
%[xn]=idft(Xk,N)
%------------------------
%
n=[0:1:N-1];
k=[0:1:N-1];
WN=exp(-j*2*pi/N);
nk=n'*k;
WNnk=WN.^(-nk);
xn=(Xk*WNnk)/N;