The line
symbols = randi([0 3], 1, 256)
generates random values from the values [0 1 2 3]. Those zero values are not a valid index int the constellation vector, in other words
constellation(0)
would throw an error, because there is no "zeroth element". So, I presume this person adds the 1 to get valid indexing.
They could also have done
symbols = randi([1 4], 1, 256); % Note that I changed the random numbers generated
constellation = [1+1i, -1+1i, -1-1i, 1-1i];
signal = constellation(symbols); % Note that I took away the +1
I'm not sure why they did not code it this way, but maybe the zeros are need later.
