Hi Jeffy,
Here is the annotated code with explanation. This code basically does phase modulation and demodulation.
bit_rate = 5; % how frequent the bits are transmitted, to calculate the bit duration
f = 5; % Frequency of the signal or the carrier
fs = 4000; % Sample rate
x = [0 0 0 1 1 0 1 1 1 0 0 0 1]; % Input bits
t = 0:1/fs:1/bit_rate-1/fs; % Sampling each bit (it turns out to be time duration of one symbol after sampling)
time = 0:1/fs:(1/bit_rate*length(x))-1/fs; % total duration of the signal
% Modulation
signal=[]; % Initialize the variable to store the output signal
for n=1:length(x) % Loop over each bit to perform the phase modulation
if x(n)==1 % If the value of bit is 1, transmit the carrier signal (i.e. cos(*pi*f*t)) without any phase deviation
y=1.*cos(2*pi*f*t);
else % If the value of bit is 0, transmit the carrier signal with a value shifted by pi
y=1.*cos(2*pi*f*t + pi);
end
signal=[signal y]; % Append the signal for all the bits
end
% Demodulation
result = []; % Initialize a variable to store the result
j = 0; % Loop variable to select the duration of each bit
for i=1:length(x) % Perform demodulation for each bits
a = trapz(signal(j+1:j+length(t)) .* cos(2*pi*f*t)); % This integratres the signal multiplied with cos(2*pi*f*t)
b = trapz(signal(j+1:j+length(t)) .* cos(2*pi*f*t + pi)); % This integratres the signal multiplied with cos(2*pi*f*t+pi)
if a > b % Decision threshold
% If the correlation obtained by integration of the signal multiplied
% with cos(2*pi*f*t) is greater than the correlation obtained by
% integration multiplied with cos(2*pi*f*t+pi), Then the decision is
% the transmitted signal is 1 and append this it to the result
result = [result 1];
else
% If the correlation obtained by integration of the signal multiplied
% with cos(2*pi*f*t) is less than or equal the correlation obtained by
% integration multiplied with cos(2*pi*f*t+pi), Then the decision is
% the transmitted signal is 0 and append this it to the result
result = [result 0];
end
j = j+length(t); % loop over all the bits
end
display(result); % Display the demodulated bits
Hope this helps.
Regards,
Sriram
