X2 - Handover without MME/SGW relocation

X2 Handover without MME/SGW Relocation

X2 is the Interface present between the Macro eNB's & HeNB's & between Macro eNB & a HeNB. Basically, X2 is the interface between the eNB's. If you are aware of the WiMax architecture, then you will come to know where basically the X2 interface is adopted from, in LTE. The Concept of Call Anchor is also adopted from WiMax architecure only.

Lets discuss the X2 Handover (X2HO) here with a detailed callflow.

1. When the source cell Signal is weaker than the Target cell, then the UE sends the Measurement Report to Source EnodeB. Prior to this, Soure EnodeB would have been configured for the Measurement Events (A1, A6, as i have explained in the previous blogs).

2. UE reports Measurement report A1, A2, A3, A4 & A5 once by one. Once the A4 & A5 events informed to Source EnodeB, Source EnodeB will decide to move the call to the Target Cell, by performing a Handover.

3. The Handover decision is Taken by the Source EnodeB. It follows the following call flow to do the handover.

i. SeNB sends Handover Request to TeNB.
ii. TeNB sends Handover request Ack to SeNB
iii. SeNB sends RRC Connection Reconfiguration to UE.
iv. SeNB sends Sn Status Transfer to TeNB.
v. SeNB sends the Buffered Data to TeNB
vi. UE sends RRC Reconfiguration Complete to TeNB
vii. TeNB sends Path Switch Request to MME.
viii. MME sends Modify Bearer request to SGW
ix. SGW sends Modeify Bearer Response to MME.
x. MME sends Patch Switch request Ack to TeNB.
xi. TeNB sends UE Context Release to SeNB

Now you might have understood the Call flow for an X2 HO in LTE.

You have to follow many specs to get the Details of each and every message & also get into the details of the Message Contents.

I have attached the callflow of an X2HO for the same explained above.

Spec References: 
RRC - 36.331
X2 Inteface - 36.423
X2HO - 36.300
S1AP - 36.413