modem.m

数字通信中的4QAM调制下的解调程序

%4QAM Ber simulink

Ber = zeros(1,10);
x = zeros(1,100);
x1 = zeros(1,50);
x2 = x1;
y_real = x1;
y_img = x1;
y = x;
yo_odd = x1;
yo_even = x1;
yo = x;

%simulink from 1dB to 10dB for Eb/N0
for i =1:10
    %EbN0 = idB
    EbN0 = i/1;%unit is dB
    %get SNR in ratio unit, for BPSK, without channel code & alpha index, SNR = EbN0
    SNR = 10^(EbN0/10);
    %calculate AWGN sigma^2
    sigma2 = 1/(2*SNR);
    sigma = sigma2^0.5;
    Error_bit = 0;
    iter_No = 0;%iterative No
    %start to simulink
    while (1)
        %produce random bit
        x = (sign(randn(1,100))+1)/2; % random bits
        x1 = x(1:2:99);%save x1,x3,x5,...x99
        x2 = x(2:2:100);%save x2,x4,x6...x100
        %real part
        y_real = x2.*2 -1;
        %add AWGN noise to real
        y_real = y_real + sigma*randn(1,50);
        %image part
        y_img = x1.*2 -1;
        %add AWGN noise to img
        y_img = y_img + sigma*randn(1,50);
        %4QAM 
        y = y_real + y_img.*j;
        
        %demodulation 4QAM
        yo_odd = y_img > 0;
        yo_even = y_real >0;
        yo(1:2:99) = yo_odd;
        yo(2:2:100) = yo_even;
        %record error demodulated bit No, iterative No
        iter_No = iter_No +1;
        Error_bit = Error_bit + size(find(yo-x),2);
        if (Error_bit >= 100)
            break;
        end;
    end;
    %BER calculate
    Ber(i) = Error_bit/(iter_No*100);
end;
figure, semilogy(Ber);