% EXPERIMENT 3
% PULSE POSITION MODULATION AND DEMODULATION

clc;
clear all;
close all;

fm = 1000;          % Message frequency
fs = 10000;         % Pulse repetition frequency
Fs = 200000;        % Sampling rate

t = 0:1/Fs:0.01;
Ts = 1/fs;

% Message signal (1 Vpp)
m = 0.5*sin(2*pi*fm*t);

N = round(Ts*Fs);
pulse_width = round(0.1*N);

ppm = zeros(size(t));
decoded = zeros(size(t));

% PPM Generation and Decoding
for k = 0:floor(length(t)/N)-1

    index = k*N + 1;

    % Position shift according to message
    shift = round(((m(index)+0.5)/1)*0.6*(N-pulse_width));

    start_pos = index + shift;
    end_pos = min(start_pos+pulse_width-1,length(t));

    ppm(start_pos:end_pos) = 1;

    % Decode pulse position
    decoded(index:min(index+N-1,length(t))) = ...
    2*shift/(0.6*(N-pulse_width))-1;

end

% FFT based Low Pass Filter
L = length(t);

Y = fftshift(fft(decoded));

f = ((0:L-1)-floor(L/2))*(Fs/L);

cutoff = 2000;
Y(abs(f)>cutoff) = 0;

demodulated = real(ifft(ifftshift(Y)));

% Plotting
figure(1);

subplot(3,1,1);
plot(t,m,'b','LineWidth',1.5);
title('Original Message Signal');
xlabel('Time (seconds)');
ylabel('Amplitude');
grid on;

subplot(3,1,2);
plot(t,ppm,'r');
title('PPM Modulated Signal');
xlabel('Time (seconds)');
ylabel('Amplitude');
grid on;

subplot(3,1,3);
plot(t,demodulated,'g','LineWidth',1.5);
title('PPM Demodulated Signal');
xlabel('Time (seconds)');
ylabel('Amplitude');
grid on;

disp('Experiment 3 Completed Successfully');