LTE per definition is the radio access network part consisting of base stations also know as eNodeBs. Base stations are connected to the packet core. Core network is termed as Evolved Packet Core (EPC ) in 4G , specified by 3GPP in a separate project named System Architecture Evolution (SAE).Let’s just say EPC is not a completely new core network for 4G. It is possible to
upgrade existing Packet Switch Core nodes (SGSN and GGSN) of 3G to be EPC compliant. There is no Circuit Switch core network for EPC, so no need of upgrading Circuit switch core network for 4G.