The present invention relates to a Femtocell providing services to a UE, and it proposes a method for authenticating a UE registered in a first operating domain of a communication network (e.g. a mobile core network), when the UE requests the service provided by a second operating domain (e.g. a fixed access network, a backhaul network). An authentication server in the first operating domain allocates the needed information to access the service provided by the second operating domain for the UE, and stores. After receiving the needed information, the UE sends an authentication request message to an authentication server in the second operating domain, wherein the authentication server in the second operating domain forwards the authentication request message to the authentication server in the first operating domain.