problem regarding soft decision viterbi decoding

hi,
i want to decode a pi/4-dqpsk signal using a soft decision viterbi decoding. My problem is how should i weight the demapped signal to make soft bits for viterbi decoder.
Please help me.

thanks

shameem

I think you have to use soft output demodulator as shown beloww:

function y=mod_qpsk(x)
NN=size(x,1);
N=size(x,2)/2;
r=1;
for nn=1:NN
for n=1:N
if x(nn,2*n-1)==0&x(nn,2*n)==0;
y(nn,n)=r*exp(j*pi/4);
elseif x(nn,2*n-1)==0&x(nn,2*n)==1;
y(nn,n)=r*exp(j*pi*3/4);
elseif x(nn,2*n-1)==1&x(nn,2*n)==1;
y(nn,n)=r*exp(j*pi*5/4);
elseif x(nn,2*n-1)==1&x(nn,2*n)==0;
y(nn,n)=r*exp(j*pi*7/4);
else
error('qpsk input error');
end
end
end
%-------------------------------------------------------------
function y=soft_qpsk_demod(x)
%x=[-0.3162-0.3162*j];
NN=size(x,1);
N=size(x,2);
r=1;
x=x/r;

con=exp(j*pi*(1:2:7)/4);
out=[0 1 3 2];
for nn=1:NN
for n=1:N
i=1;
while (i<=4)&(x(nn,n)~=con(i))
d(i)=1/abs(x(nn,n)-con(i));
%ed(1:2,i)=d(i)*(ones(1,2)-2*deci2bin(out(i),2))';
ed(1:2,i)=d(i)*(deci2bin(out(i),2))';
i=i+1;
end
if i==5
y(nn,2*n-1:2*n)=sum(ed')/sum(d);
else
%y(nn,2*n-1:2*n)=(ones(1,2)-2*deci2bin(out(i),2));
y(nn,2*n-1:2*n)=(deci2bin(out(i),2));
end
end
end

Pl. give the program for modulator too.