How can i rectify this error "Subscripted assignment dimension mismatch."

Question

Shireen Shah 2018-3-29

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/391463-how-can-i-rectify-this-error-subscripted-assignment-dimension-mismatch

评论： Walter Roberson 2018-3-30

clc
clear all
nCP = 8; %round(Tcp/Ts);
nFFT = 64;
NT = nFFT + nCP;
F = dftmtx(nFFT)/sqrt(nFFT);
MC = 1500;
EsNodB = 0:5:40;
snr = 10.^(EsNodB/10);
beta = 17/9;
M = 16;
modObj = modem.qammod(M);
demodObj = modem.qamdemod(M);
L = 5;
N=5;
ChEstLS = zeros(1,length(EsNodB));
for ii = 1:length(EsNodB)
    disp('EsN0dB is :'); disp(EsNodB(ii)); tic;
    ChMSE_SVD=0;
      g2 = randn(L,N)+1i*randn(L,N);
      g = g2/norm(g2);
      H = fft(g,nFFT);
      X = randi([0 M-1],nFFT,5);  %BPSK symbols
      XD = modulate(modObj,X)/sqrt(10); % normalizing symbol power
      Y = randi([0 M-1],nFFT,5);  %BPSK symbols
      HhatLS = Y./XD;
      Rhh = H*H';
      [U,S,V] = svd(Rhh);
      p=8;
      for k= 1:p-1;
          lambdak=diag(S)
          delk= lambdak/(lambdak+(beta/snr(ii)));
          HhatSVD =U*delk*V*HhatLS;
          %syms k
          ChMSE_SVD= ChMSE_SVD + ((H -HhatSVD)'*(H-HhatSVD))/nFFT;
      end
      %  for j=1:ii
      %    ChMSE_SVD=ChMSE_SVD1(:,j);
      ChEstSVD(ii)=ChMSE_SVD/MC; % in this line i get an error
      %end
      toc;
  end
% Channel estimation
semilogy(EsNodB,ChMSE_SVD,'b','LineWidth',2);
legend('SVD');

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per isakson 2018-3-29

在 MATLAB Online 中打开

Preallocate ChEstSVD, e.g.

ChEstSVD = nan(1,ii);

and see

Shireen Shah 2018-3-30

@per isakson IT doesn't work either.

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Answer 1

Walter Roberson 2018-3-30

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/391463-how-can-i-rectify-this-error-subscripted-assignment-dimension-mismatch#answer_312705

在 MATLAB Online 中打开

You have

ChEstSVD(ii)=ChMSE_SVD/MC; % in this line i get an error

The right hand size, ChMSE_SVD, is a 5 x 5 array, and MC is a scalar, so ChMSE_SVD/MC will give a 5 x 5 result. You are attempting to store that 5 x 5 result into the single location ChEstSVD(ii)

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Shireen Shah 2018-3-30

by doing this way, i get an error in semilogy. figure appears on the screen but with no plot

Walter Roberson 2018-3-30

在 MATLAB Online 中打开

Well, you can use

ChEstSVD(ii,:,:)=ChMSE_SVD/MC;

That will give you a length(EsNodB) by L by N matrix (9 x 5 x 5) to be plotted against X coordinates EsNodB (1 x 9). But now what?

Especially since all of those values are complex ? You are trying to plot the results of a SVD-type computation, but SVD-type calculations do not give scalar outputs (at best you can get a diagonal vector output, if you call svd() and take the second output.)

I also have to ask whether you are aware that when you compute

delk= lambdak/(lambdak+(beta/snr(ii)))

that because lambdak is a vector, you are doing algebraic matrix operations, with the 64 x 1 vector lambdak resulting in a 64 x 64 operation being returned as a least-squared fit because of the / fitting operation. The / operation is not an element-by-element division operator! And if you were to change the / to ./ to get element-by-element division then the U*delk*V operation that follows would fail. So maybe you do want the / operator -- but if so then I would recommend adding a comment describing the meaning of the line.

You might use abs() to convert the 9 x 5 x 5 array ChEstSVD to real values, but that would still leave you with the question of what it means to plot that.

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