Setting up SSO using reference system

Hi,

I am trying to setup SSO in EP 7 with backend R/3 and BW systems. The portal user id's and the backend system userid's are different. However backend systems have the same user id. I was referring to the SAP documentation and found that we have to define a Reference System. I have also exported the certificate from Portal into R/3 and BW and configured the STRUSTSSO2 setup. Can any one let me know how do I create the reference system (R/3 or BW) and how do I map the users now from the reference system.

Your help would be most appreciated.