GMSK demodulation

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raj0689
raj0689 2011-12-13
Hi all, I am trying to implement a GMSK demodulator. I tried many concepts, nothing seems to work. Finally I settled on with reversing the modulation. I almost succeeded, but stuck at the last.
num=400; %number of symbols T=4; %T is the time period fs=200; %sampling frequency num_sam=8;
% Gaussian filter OF=5; % oversampling factor NT=4; % symbol periods to the peak B=0.5; %Bandwidth 3-dB fil_op=gaussfir(B,NT,OF);
%GMSK modulation mod_sig=randsrc(1,num); %generating 1 and -1
% plot(mod_sig) % evn_bits=kron(mod_sig(1:2:end),ones(1,T)); % odd_bits=kron(mod_sig(2:2:end),ones(1,T)); evn_bits=mod_sig(1:2:end); odd_bits=mod_sig(2:2:end); % odd_bits= % evn_bits=[evn_bits zeros(1,T)]; evn_bits_vec=evn_bits(:); % odd_bits=[zeros(1,T) odd_bits]; %adding delay odd_bits_vec=odd_bits(:); % odd_bits_col=reshape(odd_bits,200,1); % fil_signal=conv2(evn_bits_vec,odd_bits,fil_op); % plot(fil_signal) fil_signal_even=conv(evn_bits,fil_op); %filtered even bits fil_signal_odd=conv(odd_bits,fil_op); %filtered odd bits
fil_signal=fil_signal_even+fil_signal_odd;
theta=zeros(1,length(fil_signal)+1);
for i=2:length(fil_signal)+1 theta(1,i)=theta(1,i-1)+(pi/2).*fil_signal(1,i-1)./num_sam; end
I=cos(theta); Q=sin(theta);
final_op=(1/sqrt(T)).*(I+1i.*Q); figure; plot(final_op)
%demodulation I1=real(sqrt(T).*final_op); Q1=imag(sqrt(T).*final_op);
theta1=acos(I1); theta2=asin(Q1);
for i=2:length(theta1+1) fil_signal_demod(1,i-1)=(2*num_sam.*(theta1(1,i)-theta1(1,i-1)))/pi; end
fil_sig_demod_odd=fil_signal_demod-fil_signal_even;%this %fil_signal_even is used from modulation so that i can check that %i m doing right fil_sig_demod_even=fil_signal_demod-fil_sig_demod_odd;
even_val=deconv(fil_sig_demod_even,fil_op); %(this is the step %where I am stuck.. this line returns NaN)
Sorry for this full page code :(
Kindly help me in solving this. Any help will be appreciated.
Thanks a lot,
Regards,
Raj.

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