cross and auto correlation timing estimation....

HÝ,

I have searched for information for synchronization project and its
about IEEE 802.11a preamble.I have one question that is very important
for me. why do we use both auto correlation and cross correlation
together .I mean exactly that we use cross correlation (using known
symbol such as long symbols) for symbol timing but for packet
detection we use auto correlation.why dont we use auto correlation for
symbol timing or why dont we use cross correlation for packet
detection. In the receiver we can know the short symbols as a reference
symbol and I think we can use cross correlation again for packet
detection if you answer me or show me any reference ,paper or book
anthing else I will really be pleased because I have searching it for
one week I have looked up almost every paper about it but I have not
got any satisfied answer.
Thank you very much...
Mehmet Karaca
Sabanci University