Reply by March 4, 20022002-03-04
 I meet a problem which is to convert a signal from frequency domain into time domain. I tried to use IFFT to convert it but I did not succeed. If you are interested, please see the following matlab code Thank you! function calculate_TimeVariation(N,OR,M,Dop_res,res_accu) %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% N = 10; OR = 4; M = 256; Dop_res = 0.1; res_accu = 20; %%%%%%%%%%%%%%% P = [0 -5 -10]; K = [1 0 0]; tau = [0.0 0.5 1.0]; Dop = [0.4 0.4 0.4]; ant_corr = 0.4; Fnorm = -1.5113; %%%%%%%%%%%%%%%%%%%%% P = 10.^(P/10); s2 = P./(K+1); m2 = P.*(K./(K+1)); m = sqrt(m2); %%%%%%%%%%%%%%%%%%%%% L = length(P); paths_r = sqrt(1/2)*(randn(L,N) + j*randn(L,N)).*((sqrt(s2))'*ones(1,N)) paths_c = m'*ones(1,N) %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%% for p = 1:L D = Dop(p)/max(Dop)/2 f0 = [0:M*D]/(M*D); PSW= 0.785*f0.^4 - 1.72*f0.^2 +1.0; filt = [PSW(1:end-1) zeros(1,M - 2*M*D) PSW(end:-1:2)]; %define S(f) filt = ifftshift(ifft(filt)); filt = real(filt); filt = filt/sqrt(sum(filt.^2)); path = fftfilt(filt, [paths_r(p,:) zeros(1,M)]); paths_r(p,:) = path(1+M/2:end-M/2); end; paths = paths_r + paths_c; paths = paths*10^(Fnorm/20) %Pest = mean(abs(paths).^2, 2) %fprintf('tap mean power level: %0.2f dB\n', 10*log10(Pest)); figure, psd(paths(1,:), 52, max(Dop)); %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %the above is the original code and the following code is I add t = 0:0.4/512:0.4 P = ifftshift(ifft(PSW)); plot(length(t)-1, PSW) %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%