search XML schema for UML metamodel(UML XMI schema)

Hi Folks,
My name is Fei Huang, a graduate student of University of Saskatchewan. My
email address is feh273@mail.usask.ca.
    I am facing a problem regarding XML schema. I need the whole XML schema for
UML metamodel to accomplish the task of XML schema mapping. It seems that there
is UML XMI DTD available. Would you please give some suggestion or information
about UML XMI schema or the UML tool which can generate it?    Your reply will
be appreciated. Many thanks.

Best regards,
Fei Huang