LTE Radio Interface, X2 Interface ,S1-MME Interface, S1-U Interface

LTE-Uu
•Air interface of EUTRAN
•Based on OFDMA in downlink and SC-FDMA in uplink
•FDD and TDD duplex methods
•Scalable bandwidth: from 1.4 up to 20 MHz
•Data rates up to 100 Mbps(DL), 50Mbps (UL)
•MIMO (Multiple Input Multiple Output) is a major component although optional.

X2
•Inter eNB interface
•Handover coordination without involving the EPC
•X2AP: special signaling protocol
•During HO, Source eNB can use the X2 interface to forward downlink packets still buffered or arriving from the serving gateway to the target eNB.
•This will avoid loss of a huge amount of packets during inter-eNB handover.

S1-MME
•Control interface between eNB and MME
•MME and UE will exchange non-access stratum signaling via eNB through this interface.
•E.g.: if a UE performs a tracking area update the TRACKING AREA UPDATE REQUEST message will be sent from UE to eNB and the eNB will forward the message via S1-MME to the MME.
•S1AP:S1 Application Protocol
•S1flex -> 1 eNB to connect to several MME

S1-U
•User plane interface between eNB and serving gateway.
•It is a pure user data interface (U=User plane).
•S1flex-U also supported: a single eNB can connect to several Serving GWs.
•Which Serving GW a user’s SAE bearer will have to use is signaled from the MME of this user.