Hi Igor,
As per my understanding, the matrix with pilot subcarriers is gnerating an error while performing the OFDM modulation.
The shape of the matrix, as per the definition mentioned in the documentation, is 
-by-
-by-
, where 
is the length of the pilotIdx array. However, in our case we are defining the pilot array in terms of the order of the modulation, MCS7.
-by--by-, where is the length of the pilotIdx array. However, in our case we are defining the pilot array in terms of the order of the modulation, MCS7.A workaround for this issue would be to define the pilot indices such that the number of elements is equal to the order of modulation for QAM.
The script below might be helpful:
MCS7 = 16;
nfft = 64;
cplen = 16;
nSym = 100;
npilot = 4;
nantena = 1;
pilotIdx = [12:23 40:42 54]';
nullIdx = [1:6 33 64-4:64]';
numDataCarrs = nfft-length(nullIdx)-length(pilotIdx);
inSig = randi([0 MCS7-1],numDataCarrs,nSym);
pilots = repmat(pskmod((0:MCS7-1).',MCS7),1,nSym);
qamSym = qammod(inSig,MCS7,'UnitAveragePower',true);
outSig64 = ofdmmod(qamSym,nfft,cplen,nullIdx,pilotIdx,pilots);
Hope this helps!
