Numerous texts are available to explain the basics of Discrete Fourier Transform and its very efficient implementation – Fast Fourier Transform (FFT). Often we are confronted with the need to generate simple, standard signals (sine, cosine, Gaussian pulse, square wave, isolated rectangular pulse, exponential decay, chirp signal) for simulation purpose. I intend to show (in a series of articles) how these basic signals can be generated in Matlab and how to represent them in frequency domain using FFT.
This article is part of the book Digital Modulations using Matlab : Build Simulation Models from Scratch, ISBN: 978-1521493885 available in ebook (PDF) format (click here) and Paperback (hardcopy) format (click here)
Wireless Communication Systems in Matlab, ISBN: 978-1720114352 available in ebook (PDF) format (click here) and Paperback (hardcopy) format (click here).
Rectangular pulse: mathematical description
An isolated rectangular pulse of amplitude A and duration T is represented mathematically as
where
The Fourier transform of isolated rectangular pulse g(t) is
where, the sinc function is given by
Thus, the Fourier Transform pairs are
The Fourier Transform describes the spectral content of the signal at various frequencies. For a given signal g(t), the Fourier Transform is given by
where, the absolute value
The amplitude spectrum peaks at f=0 with value equal to AT. The nulls of the spectrum occur at integral multiples of 1/T, i.e, (
Generating an isolated rectangular pulse in Matlab:
An isolated rectangular pulse of unit amplitude and width w (the factor T in equations above ) can be generated easily with the help of in-built function – rectpuls(t,w) command in Matlab. As an example, a unit amplitude rectangular pulse of duration
fs=500; %sampling frequency
T=0.2; %width of the rectangule pulse in seconds
t=-0.5:1/fs:0.5; %time base
x=rectpuls(t,T); %generating the square wave
plot(t,x,'k');
title(['Rectangular Pulse width=', num2str(T),'s']);
xlabel('Time(s)');
ylabel('Amplitude');
Amplitude spectrum using FFT:
Matlab’s FFT function is utilized for computing the Discrete Fourier Transform (DFT). The magnitude of FFT is plotted. From the following plot, it can be noted that the amplitude of the peak occurs at f=0 with peak value
L=length(x);
NFFT = 1024;
X = fftshift(fft(x,NFFT)); %FFT with FFTshift for both negative & positive frequencies
f = fs*(-NFFT/2:NFFT/2-1)/NFFT; %Frequency Vector
figure;
plot(f,abs(X)/(L),'r');
title('Magnitude of FFT');
xlabel('Frequency (Hz)')
ylabel('Magnitude |X(f)|');
Power spectral density (PSD) using FFT:
The distribution of power among various frequency components is plotted next. The first plot shows the double-side Power Spectral Density which includes both positive and negative frequency axis. The second plot describes the PSD only for positive frequency axis (as the response is just the mirror image of negative frequency axis).
figure;
Pxx=X.*conj(X)/(L*L); %computing power with proper scaling
plot(f,10*log10(Pxx),'r');
title('Double Sided - Power Spectral Density');
xlabel('Frequency (Hz)')
ylabel('Power Spectral Density- P_{xx} dB/Hz');
X = fft(x,NFFT);
X = X(1:NFFT/2+1);%Throw the samples after NFFT/2 for single sided plot
Pxx=X.*conj(X)/(L*L);
f = fs*(0:NFFT/2)/NFFT; %Frequency Vector
plot(f,10*log10(Pxx),'r');
title('Single Sided - Power Spectral Density');
xlabel('Frequency (Hz)')
ylabel('Power Spectral Density- P_{xx} dB/Hz');
Magnitude and phase spectrum:
The phase spectrum of the rectangular pulse manifests as series of pulse trains bounded between 0 and
clearvars;
x = [ones(1,7) zeros(1,127-13) ones(1,6)];
subplot(3,1,1); plot(x,'k');
title('Rectangular Pulse'); xlabel('Sample#'); ylabel('Amplitude');
NFFT = 127;
X = fftshift(fft(x,NFFT)); %FFT with FFTshift for both negative & positive frequencies
f = (-NFFT/2:NFFT/2-1)/NFFT; %Frequency Vector
subplot(3,1,2); plot(f,abs(X),'r');
title('Magnitude Spectrum'); xlabel('Frequency (Hz)'); ylabel('|X(f)|');
subplot(3,1,3); plot(f,atan2(imag(X),real(X)),'r');
title('Phase Spectrum'); xlabel('Frequency (Hz)'); ylabel('\angle X(f)');
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