OFDM Spectrum Analysis Using MATLAB

 

This plot shows the power distribution of an OFDM signal in frequency domain. The central flat region corresponds to the 64 subcarriers each spaced 15 kHz apart, giving a total bandwidth of 64 X 15 = 960 kHz. The PSD decreases outside this band due to the spectral shape of each subcarrier. 

MATLAB Code 

% The code is developed by SalimWireless.com clear; close all; clc; % PARAMETERS N = 64; % subcarriers Delta_f = 15e3; % subcarrier spacing Fs_base = 1e6; % base sampling rate M = 16; % 16-QAM fc = 2e6; % carrier BW = N * Delta_f; % approximate occupied BW desired_Fs = 10e6; % high sampling rate for oversampling % OVERSAMPLING L = ceil(desired_Fs / Fs_base); Fs_high = L * Fs_base; fprintf('Fs_base = %.1f kHz, Fs_high = %.1f kHz, L=%d\n', Fs_base/1e3, Fs_high/1e3, L); % GENERATE 16-QAM SYMBOLS data = randi([0 M-1], N, 1); qamSymbols = qammod(data, M, 'UnitAveragePower', true); % MAP TO IFFT BINS (centered) Nfft_time = N*L; X = zeros(Nfft_time,1); startIdx = floor(Nfft_time/2) - N/2 + 1; X(startIdx:startIdx+N-1) = qamSymbols; % TIME-DOMAIN OFDM ofdm_oversampled = ifft(ifftshift(X), Nfft_time); % SCALE TO 1 W TOTAL POWER power_signal = mean(abs(ofdm_oversampled).^2); ofdm_oversampled = ofdm_oversampled / sqrt(power_signal); % now total power ~1 W % TIME VECTOR t = (0:Nfft_time-1) / Fs_high; % BASEBAND PSD Nfft_spec = 16384; spec_base = fftshift(fft(ofdm_oversampled, Nfft_spec)); Pxx_base = (abs(spec_base).^2) / (Nfft_spec * Fs_high); % W/Hz Pxx_base_dBHz = 10*log10(Pxx_base + eps); f_axis_base = linspace(-Fs_high/2, Fs_high/2, Nfft_spec); % PASSBAND UP-CONVERSION ofdm_passband = real(ofdm_oversampled .* exp(1j*2*pi*fc*t.')); % PASSBAND PSD spec_pass = fftshift(fft(ofdm_passband, Nfft_spec)); Pxx_pass = (abs(spec_pass).^2) / (Nfft_spec * Fs_high); % W/Hz Pxx_pass_dBHz = 10*log10(Pxx_pass + eps); f_axis_pass = linspace(-Fs_high/2, Fs_high/2, Nfft_spec); % PLOTS figure('Name','Baseband PSD (dB/Hz)'); plot(f_axis_base/1e3, Pxx_base_dBHz, 'LineWidth',1.2); xlabel('Frequency (kHz)'); ylabel('PSD (dB/Hz)'); title(sprintf('Baseband PSD (Fs = %.1f kHz)', Fs_high/1e3)); grid on; figure('Name','Passband PSD (dB/Hz)'); plot(f_axis_pass/1e6, Pxx_pass_dBHz, 'LineWidth',1.2); xlabel('Frequency (MHz)'); ylabel('PSD (dB/Hz)'); title(sprintf('Passband PSD centered at %.2f MHz', fc/1e6)); grid on; figure('Name','Passband PSD zoom'); plot(f_axis_pass/1e6, Pxx_pass_dBHz,'LineWidth',1.2); xlim([(fc - 2*BW) (fc + 2*BW)]/1e6); xlabel('Frequency (MHz)'); ylabel('PSD (dB/Hz)'); title('Passband PSD (zoom around fc)'); grid on; web('https://www.salimwireless.com/search?q=ofdm%20psd', '-browser');
 

Output

 

 

 

  

 

Further Reading

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