matlab - Find Preamble (pattern) in signal, using Corelation

I have a signal which always has the same preamble (4x 0xAA - 10101010...).

I also have a signal with additive noise, and I want to find my preamble in that a noisy signal using corelation.

U used xcorr, but I was a little confused by that. Can you help me? It looks like that signal was too noisy so that no preamble was found?

I have just used

corel=xcorr(preamble(:,2),u_noisy(:,2));