Handover Decision

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Handover Decision:

As soon as the event A3(for instance) is reported the eNB decides what kind of handover to perform to which target cell and then initiates a handover procedure. Handovers can be classified in many ways one of the classification is as below –

(1) Depending on weather EPC Entities that UE is connected to are changed after a handover or not a handover can be categorized as one of the following kinds:

(Interview Question: What are the types Handover?)  

Intra LTE handover
  • Intra MME/SGW handover – Neither UE’s serving MME nor SGW is changed after handover.

Inter LTE handover
UE’s serving MME and/or SGW is changed after handover
  • Inter MME HO: UE’s serving MME is changed, SGW remains unchanged after handover.
  • Inter SGW handover: UE’s MME is unchanged, SGW is changed after handover.
  • Inter MME/SGW Handover: Both MME and SGW are changed after handover.

Inter RAT handover – 
It is a HO between networks that use different radio access technologies UTRAN to EUTRAN, EUTRAN to UTRAN, etc.

(2) LTE hand over can also be classified as either X2 handover or S1 handover depending on whether and EPC entity is involved in preparing and executing of a hand over between source eNB and a target eNB or not.

(Interview Question: What is X2 Handover?)  

X2 Handover: this handover happens or occurs when there is a X2 interface between two eNBs. (refer below figure) If there is a X2 connection between source eNB and the target eNB and the X2 connection is available for handover, X2 handover is initiated.  

Here the two eNBs will negotiate directly when the X2 handover is performed. After the connection is established between UE and the target eNodeB, the MME is notified to convert the path.

(Interview Question: What is S1 Handover?)  

S1 handover: S1 interface connects eNB (E-UTRAN) and EPC. S1 handover is initiated if :

(I) there is no X2 connection between a source cell (eNodeB) and a target cell (eNodeB). (refer below figure)
(II) There is a X2 connection but the connection is not allowed for handover or
(III) Handover preparation between a serving cell (eNodeB) and a target cell (eNodeB) fails.

Once the handover is completed the source eNB starts to communicate with the target eNB through MME to control the handover.

Using which event measurement event triggers? Or when does UE starts to report “measurement report”?
Ans: Using A2 event (most operator case)

Using which event UE stops to report “measurement report”?
Ans: Using A1 event (most operator case)

Which event is used for intra-Freq Handover?
Ans: A3 event is used for intra-Freq Handover.

Which event is used for inter-Freq Handover?
Ans: A5 event is used for inter-Freq Handover.

Which events are used for inter-RAT Handover?
Ans: B1 and B2 events

Where you can find Handover timer information?
Ans: T304 from RRC Reconfiguration Msg (and other timer SIB2).

Which event is used Configuration for Scell release?
Ans: A2 event (in the most operator cases)

Which event is used to configuration for Scell addition?
Ans: A4 event (in the most operator cases)

Which event is used for Configuration of Scell Change (replacement)?
Ans: A6 event (in the most operator cases)

Note: For inter frequency we can use A3 event with measurement gap also, for inter RAT we can use either B1 or B2.

In Upcoming Part we will learn more about X2 Handover and S1 Handover.

Stay tune for more Update…………………

Pinal Dobariya…

1)3GPP TS 36.300
2)3GPP TS 36.331
3)Netmanias Technical Document, “Eleven EMM Cases in an EMM Scenario


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