% Experiment 5, CSBOOK: haykin% digital phase lock loop, S-curve% With noiseclear all% phase shifttheta=-0.1;% Generating QPSK signalsno_of_bits=400; b=round(rand(no_of_bits,1));bits=2*b-1;bitsi=bits(1:2:length(bits)-1);bitsq=bits(2:2:length(bits));s=(bitsi+i*bitsq);% Signal-to-noise-ratio SNRdB=20; snr = 10^(SNRdB/10);% Initialize S_curveS_curvef=0;for trial=1:100%Noise wn = (randn(length(s),1)+ i*randn(length(s),1) )/sqrt(snr);  s=s+wn;% Digital Phase lock loop  no_of_symbols=length(s);  x=s*exp(i*theta);  k=0;% accumulate S-curve  for theta_guess= -pi:0.1:pi;      k=k+1;        phi(k)=theta-theta_guess;        y=x*exp(-i*theta_guess);       s_predi=(sign(real(y))+i*sign(imag(y)));       error=imag(y.*conj(s_predi));       S_curve(k)=mean(error);  end% theta_guess S_curvef=S_curvef+S_curve;end % trialS_curvef=S_curvef/trial;% plot the S-curveplot(phi,S_curvef)xlabel('\phi')ylabel('S(\phi)')