Hi Rohit,
I understand that you are trying to find the response of a system for a given input whose frequency response has been given already.
To do this, since the input is sinusoidal, you can use one of its property that the output will be just an amplified and phase differentiated version of itself when passed through the system.
Refer to the below code snippet for better understanding.
clc;
n = -25:1:25;
%system response at w=pi/4
w = pi/4;
H = 1/(1-0.9*exp(-1i*w));
mag = abs(H);
ang = angle(H);
%ouput=mag*sin(wt+ang) for sinosuidal input
output = 1/2*mag*cos((n*pi/4)+ang).*(n>=0);
stem(n,output);
You may refer to these documentation links for better understanding:
