How to build 4G architecture on top of previous architectures?
UTRAN in 3G becomes e-UTRAN in 4G.
UE needs to support LTE. LTE is NOT backward compatible to previous technologies BUT has interworking capabilities with them. Also, you need an upgraded SIM which supports LTE.
NodeB in 3G becoms evolved-NodeB in 4G.
Why? Because RNC functionalities in 3G move to e-NodeB (hence the name ‘evolved’-NodeB). This leads to lower latency (lesser number of hops) and a flatter architecture.
Core in 4G is an ‘evolved’ form of its previous version (hence the name ‘Evolved’ Packet Core). But you can point out similarities.
HSS in 4G–similar to HLR+AUC in GSM
S-GW in 4G–similar to SGSN in GPRS
P-GW in 4G–similar to GGSN in GPRS
2 new elements were added to EPC:
MME (Mobility Management Entity)–brains of EPC
PCRF (Policy and Charging Rules Function)–manages QOS requirements