Hello Shashank
EAP-FAST can indeed be used with RADIUS and you can see how this can be achieved in the following Cisco documentation:
In it, it refers specifically to RADIUS as the authentication server. This is a typical setup, although other documentation that talks about an overview of EAP-FAST as a technology, does not refer specifically to RADIUS, but it states “authentication server” as you said, much like what you will find in the following document:
In section 2.1 of RFC 4851 which describes EAP-FAST, it speaks about the architectural model of the technology, and it references a RADIUS server as performing some of the authentication mechanisms in the event you use an external authenticator. Alternatively, you can use the AP itself as an authenticator.
So you see RADIUS is used, although it is not the only option to be used.
I hope this has been helpful!
Laz