error in snr estimation graph

Hello all,

I have written a following code for snr estimation of QPSK signal. My estimated snr values are matching with true snr values for snr above 4dB but for snr values less than 4dB, my estimated snr values are not matching with true snr values. The matlab code is as follows:

clc 
clear 
% Generate data. 
M = 4; % Alphabet size 
Pd = 500; % Length of data 
x = randint(Pd,1,M); % Random bit stream 
ini_phase = pi/4; %ini_phase 
psksig = pskmod(x,M,ini_phase); % PSK signal 
snr_theory = 0:2:10; % Theoretical value of SNR 
QQ = 10.^(snr_theory/10); % SNR in decimal 
c4=mean((psksig.*conj(psksig)).^2);
c6=mean((psksig.*conj(psksig)).^3); 
for a = 1:length(snr_theory) 
    for n=1:500
        rxsig = awgn(psksig,snr_theory(a));% Add Gaussian noise
        M2 = mean(rxsig.*conj(rxsig)); % Second order Moments 
        M4 = mean((rxsig.*conj(rxsig)).^2); % Fourth order Moments 
        M6 = mean((rxsig.*conj(rxsig)).^3); % Sixth order moment
        m=((M2*M4)/M6);
        v=[((m1.*c6)-c4) ((9.*m1.*c4)-c4-4) ((18.*m1)-6) ((6.*m1)-2)];
        r=roots(v);
        snr_est_M2M4M6(n) = 10*log10(abs(r(1)));
        M2M4M6_snr_est(a) = mean(snr_est_M2M4M6); % estimation value 
    end 
end 
figure(1);
%plot estimation value 
plot(snr_theory,snr_theory); 
hold on 
plot(snr_theory,M2M4M6_snr_est,'-o'); 
hold off

The plot that is generated is as follows:

Answer 1

[chiranjit]

contact at chiranjitshee at gmail dot com