Your code has
%Plot commands j=1;
Up to that point, j has not been given a value in the code. That line is a comment so nothing happens with it. You then have
for i=M,
plot(EbN0dB,log10(simulatedSER(i,:)),sprintf('%s-',plotColor(j))); hold on;
j=j+1; end
The "for i" line does not assign a value to j. So by the time the plot() call is made, j has not been given a value in the code. You might have expected that to result in an error message about undefined function or variable j, but notice that I said that your code has not given a value to j, not that j is undefined. In MATLAB, i and j are both functions with no inputs, which return complex(0,1), which is sqrt(-1), the imaginary unit. When you assign a value to i or j, you are "overshadowing" that meaning as functions, making them become variables in that scope until that meaning gets released (such as if a "clear" is encountered.) And since you did not give j a value in your code, j is still that function that returns sqrt(-1) . Which, as a complex number, is not a valid subscript.
What you probably wanted was:
%Plot commands
j=1;
on two different lines. Then j would have been assigned a value.
To avoid this situation, experienced MATLAB programmers avoid using "i" and "j" as variable names. Depending on their background, they might avoid using "I" as well (it can refer to sqrt(-1) in symbolic work.)
