I do not know why is this happening
Allresult is 1x16, Noise is 1x48(assuming it didn't exist before you create it). Of course, at the error message tells you, you can't add vectors of different size.
It's really unclear why you create a noise vector with 48 elements.
Noise = rand(size(allresult)) * SNR;
would ensure that Noise always has as many elements as allresult.