LTE : How does Forward handover works ?

Question

I have a use case for forward handover.

What will happen, when UE moves to target eNodeB through forward handover and target eNodeB is connected to a new MME.
I mean to ask what will happen with old session which is already maintained at MME, SGW and PGW. Definitely there is a loss of data packet since UE has already moved to some other eNodeB (target eNodeB) due to RLF.

When UE shal attach to new MME, MME shall send create session request to SGW, and SGW forward it to PGW.
Now question is that how PGW handle this situation, when it receives a new create session request from a new MME for an already existing session(via old mme).

Answer 1

I have not much detailed information about forward handover, but I would like to mention whatever I know.

Handover term is used when an UE moves from current cell to another target cell. If both the cells are covered under same eNodeB, it called as intra-eNodeB handover. When both the cells (source cell as well as target cell) covered under different eNodeB then handover is known as inter-eNodeB.

Normal handover, In which handover command message is sent by source eNodeB to UE. Normal handover is known as backward handover.

Now, I come to your actual query.
To make forward handover happen following two conditions need to met.
1. Source eNodeB should not able to decode Ue measurement report.
2. RLF detection should be aggressively.

when these above conditions met and UE detects RLF, UE send RRC Re-establishment request message to target eNodeB. Since target eNodeB is not prepared , it responds back to UE with RRC Re-establishment Reject message. Once UE receives Re-establishment Reject message from eNodeB. It leaves RRC - Connected state and moves to RRC -IDLE state. It starts from initial cell selection procedure and sends fresh RRC Connection Request message to eNodeB. In this way , UE goes to target ; this is known as forward handover.

Comments

I agree on what Vimal Kumar said, but I thing even though the target eNB is not prepared for the HO, after receiving the RRC connection Re-establishment request the target eNB will fetach the UE context (If it is not present)from the source eNB and sends Re-establishment to the UE. The forward HO is used to stop the UE to go to idle state and start from the fresh.
There is no such is message defined as of now to request the UE context.
refer to the below link:
http://www.qualcomm.com/media/documents/files/lte-mobility-enhancements.pdf

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Is it mandatory to be present X2 link for forward handover ? what will happen if X2 link not in between source eNodB and target eNodeB ? Is there any message over X2 or S1-AP interface specification as of now, by which target eNodeB can fetch the Ue context .
Please share the link or spec section.

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The forward HO concept should not depend on X2 link, it should work basically on S1AP also. But I don't have any idea on the forward HO is achieved on S1Ap.
As of now I'm aware the Forward HO on the X2AP link,  The specs have not defined any message in X2AP.

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There are two situations:

-- if eNB B is already a "prepared" cell, i.e eNB B had received HO Request from eNB A which used the UE's old-CRNTI, then eNB B has UE context and will continue with successful RRC Re-establishment.

-- Else, eNB B is not a prepared cell, i.e had not previously received HO Request from eNB A. From Rel-12 onwards, eNB-B will send a RLF indication message to eNB A. eNB A will verify the UE' CRNTI and then will proceed with Handover Request message to eNB B. (See 36.300 Section 20.2.2.12, If the previous serving eNB matches the correct context, it may also trigger the Handover Preparation procedure towards the eNB that initiated the Radio Link Failure Indication procedure.).
This way eNB-B gets UE's context from eNB-A. But this (context fetch) is only supported from Rel-12 onwards.