Calculate the stop band energy of a filter by its Power spectral density graph

I want to find the stop band energy of a filter by its Power spectral density graph. Stop band energy is calculated with normalized total energy.

I can not find the command for it. Below is the correct code of a rectangular filter.

%% Rectangular Filter

Ts = 0.01;                           % sampling time

time = -20:0.01:20;                  % time axis       

NFFT = 2048;                         % number of samples

freq = linspace(-0.5,0.5,NFFT);      % frequency axis

ht_Rectangular = zeros(size(time));

%% In Time Domain

for n = 1:length(time)
if time(n)>=-0.5 && time(n)<=0.5
ht_Rectangular(n) = 1;
end
end

figure(1);
plot(time,ht_Rectangular);
xlabel('t/T_s');
ylabel('h(t)');
title('Impulse Response of Rectangular Filter');
grid on,
 
%% In Frequency Domain

Hf_Rectangular = fftshift(fft(ht_Rectangular,NFFT));  % FFT with FFTshift for both negative & positive frequencies

figure(2);
plot(freq, (Hf_Rectangular(1:length(freq)))/max(Hf_Rectangular));
xlabel('f/F');
ylabel('H(F)');
title('Frequency Response of Rectangular Filter');
grid on,


%% Power Spectral Density

[PSD_Rectangular, F_Rectangular] = periodogram (ht_Rectangular, [], length (ht_Rectangular)); 

figure(3);
plot (F_Rectangular, 10 * log10 (PSD_Rectangular));
xlabel('f/F');
ylabel('PSD (dB)');
title('Power Spectral Density of Rectangular Filter');
grid on,

Answer 1

[Nitin Kapgate]

You can use the "bandpower" function to calculate the stop band energy from PSD as illustrated in this example.

[Sara Nasir]

It is for calculating the average power, not the sub-band energy