DQPSK Demodulation

Hello, I am implementing the DQPSK modulator and Demodulator. I was successful in implementing the DQPSK modulator. But, unfortunately, I could recover the samples from the modulated signal.

As the help indicates and I quote, "If the Output type parameter is set to Integer, then the block maps a phase difference of

? + ?m/2

to m, where ? is the Phase rotation parameter and m is 0, 1, 2, or 3."

Can you please suggest me the steps to recover the samples.

Here is what I am doing,

Code

Data =[2 0 0 1 2 3 2 3 2 0 3 1 1 1 ];

Theta=pi/4;

for i=1:length(Data)

     if(i==1)    
            DQPSK_Sig(i)=exp(1j*Theta+1j*pi*Data(i)/2);
        else
            DQPSK_Sig(i)=DQPSK_Sig(i-1)*exp(1j*Theta+1j*pi*Data(i)/2);
        end  
end

Demodulation

for i=1:length(DQPSK_Sig)

     if(i==1)    
            theta(i)=angle(DQPSK_Sig(i));
        else
            theta(i)=angle(DQPSK_Sig(i))-theta(i-1);
     end
end

*****

Please let me know, if am missing something obvious.

Regards Kiran

[Kiran Chandrashekhar]

I just found the answer for my own question.

Here is the change in the code:

for i=1:length(DQPSK_Sig)

if(i==1)

theta(i)=angle(DQPSK_Sig(i));

else

theta(i)=angle(DQPSK_Sig(i))-angle(DQPSK_Sig(i-1));%-theta(i-1);

end

% demod_dqpsk(i)=exp(1j*theta(i));

end

Answer 1

[Doug Hull]

Here is the change in the code:

for i=1:length(DQPSK_Sig) 
if(i==1) 
theta(i)=angle(DQPSK_Sig(i)); 
else 
theta(i)=angle(DQPSK_Sig(i))-angle(DQPSK_Sig(i-1));%-theta(i-1); 
end 

% demod_dqpsk(i)=exp(1j*theta(i)); 
end

Posted from comments. It is better if these have an answer.

Answer 2

[Max Div]

%Imho it more helpful.
%Compare my and matlab demodulation

hMod = comm.DQPSKModulator(pi/4, 'SymbolMapping', 'Binary');

hAWGN = comm.AWGNChannel('NoiseMethod', 'Signal to noise ratio (SNR)','SNR', 50);

hDemod = comm.DQPSKDemodulator(pi/4, 'SymbolMapping', 'Binary');

% Create an error rate calculator, account for the one symbol transient caused by the differential modulation
hError = comm.ErrorRate('ComputationDelay',1);

data = randi( [0 3], 10000, 1);

%%Modulation

modSignal = step(hMod, data);

modSignal_my = zeros(length(data), 1);
Theta=pi/4;
for i=1:length(data)
     if (i==1)
            modSignal_my(i) = exp(1i * Theta + 1i * pi * data(i) / 2);
        else
            modSignal_my(i) = modSignal_my(i-1) * exp(1i * Theta + 1i * pi * data(i) / 2);
        end  
end

%%AWGN

noisySignal = step(hAWGN, modSignal);

%%Demodulation

receivedData = step(hDemod, noisySignal);
receivedData_my = zeros( length(noisySignal), 1 );

for i=1:length(noisySignal) 
    if(i==1) 
        receivedData_my(i) = angle( noisySignal(i) ) - Theta; 
    else
        receivedData_my(i) = angle( noisySignal(i) )-angle( noisySignal(i-1) );
    end 
end

receivedData_my = receivedData_my + 8 * pi;
receivedData_my = mod(receivedData_my, 2 * pi);
receivedData_my = floor( receivedData_my / (pi / 2) );