power spectral density

Please i need help in PSD to compare between different methods of PAPR.
When apply the following code not given the same plot as in your paper .

My Matlab code

N = 256; % length of OFDM IFFT
M = 16; % number of QAM constellation
BW; % in MHz
txData= randint(N,1,M);
txDataMod = qammod(txData,M);

txDataZpadIfft = sqrt(N)*ifft(txDataMod ,N);

% power spectrum = amplitude squared / 2
a = fft(txDataZpadIfft);
amp = abs( a );
ps = amp .* amp ; % units [volts^2]
freq= linspace(-BW/2,BW/2,length(a));
plot( freq, ps )
% power spectral density = power spectrum / spectral bandwidth% for equally spaced spectral lines from FFT, spectral % bandwidth = df = fs/n = constantpsd_fft = ps / df; % units [volts^2.sec]subplot( 3,1,2 )plot( freq, psd_fft ) % units [Hz] [volts^2.sec]xlabel( 'Frequency [Hz]' )ylabel( 'FFT^2 / 2 / df' )
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