How can I fix an undefined function error in MATLAB?

I'm trying to write the code for quadrature phase-shift keying (QPSK) with zeroforcing when N=2, and I got an error.

Here is the code:

Modulation = 'QPSK'
Decode_Method = 'ZeroForcing'
switc开发者_JS百科h Modulation
    case {'QPSK'}
        Symbols = [ 1+j 1-j -1+j -1-j ]';
end
Symbols = Symbols.';
nSymbols = length(Symbols);

SNR_Array = [0.3 0.7 1.2 2.5 5 6.2 10 15.4 22 45 75.7 100.0];
nSNR = length(SNR_Array);

Ntest = 20;
N = 2;

for iSNR = 1 : nSNR
    SNR = SNR_Array(iSNR);
    Nerror = 0;
    for i = 1:Ntest
        H = randn(N,N) + j*randn(N,N);
        X = Symbols( ceil( nSymbols*rand(N,1) ) )';
        Noise = (randn(N,1) + j*randn(N,1))/sqrt(2)/sqrt(SNR);
        Y = H*X + Noise;
        switch Decode_Method
            case {'ZeroForcing'}
                X_Decode = Zero_Forcing(Y,H,Symbols);
        end
    end
    Nerror = Nerror + length( find( X ~= X_Decode) );
end
Symbol_Error_Rate(iSNR) = Nerror/Ntest/N;

figure(1)
loglog(SNR_Array, Symbol_Error_Rate,'b')
hold on
xlabel('SNR')
ylabel('Symbol Error Ratio')
title('Symbol Error Ratio for NxN MIMO System')

And the error is:

??? Undefined function or method 'Zero_Forcing' for input arguments of type 'double'.

Error in ==> Untitled2 at 33

X_Decode = Zero_Forcing(Y,H,Symbols);

How can I fix this error?

The error indicates that MATLAB cannot find the function Zero_Forcing. If you have a function of that name, you should make sure it's on the MATLAB path, that is, a directory MATLAB knows about. Otherwise, you should write the function. It seems rather important.

Also, you may want to not call your function 'Untitled2', but give it a more meaningful name.