While analyzing the RCA
for handover we need to understand - which part of the handover
procedure the failure has occurred.Let us discuss the various handover counters triggered during the different stages of the handover procedure
The X2 handover:
The procedure is
performed between the 2 enode B's
a>Before
handover-Reporting
The UE sends the source
ENB an event A3 that contains monitored neighboring cells
based on the reported measurement results/event it chooses the best
reported target cell by the UE
Counter Triggered--Handover Preparation Attempt number
b>Handover
Preparation
The Source eNodeB sends
a handover request to the Target eNodeB, which performs admission control after which a handover request acknowledgement is sent by the target ENB to the source ENB .On receiving the handover request acknowledgement from the target ENB
Counter Triggered--Handover Preparation Success Number
c>Handover
Execution
After successful
preparation, the Source eNodeB sends a handover command to the UE that encloses the RRC reconfiguration message to the UE
Counter Triggered--Handover Execution Attempt Number
SN Status Transfer
The source eNB sends the Sequence Number (SN) status
transfer message that contains the Packet Data Convergence Protocol (PDCP) sequence numbers to the target
eNB through X2 interface.
For UL the first missing
data unit is included and for DL the next sequence
number to be allocated.
RRC Reconfiguration Complete
UE is synchronized with the
target based on the given parameters received from the RRC Connection Reconfiguration message and send the HO Confirm message that
encloses the RRC Connection Reconfiguration Complete to acknowledge the
successful handover to the target eNB
If a dedicated random access
preamble has been received in the RRC Connection Reconfiguration message, the
UE does not need to perform the random access
procedure, i.e., contention free Random Access Channel (RACH) process.
If this is not the case, the UE performs the normal
random access procedure described (contentionbased RACH)
The target eNB receives the
RRC Connection Reconfiguration Complete message and
Path Switch procedure
The path switch procedure is initiated between the target eNB and the MME/S-GW
The target eNB starts to forward all the packets received from the X2 interface to the UE before any new ones coming from the Serving Gateway (S-GW)
The path switch procedure is basically used to receive the end-marker
from the old path switch and starts transmitting packets
from the new path switch
Context Release
The source eNB
UE context is released via receiving UE release context
message from the target eNB
Counter Triggered--Handover Execution Success Number
Handover Formula :
Handover Preparation*Handover Execution*100
Handover Preparation=Handover Preparation success
----------------------------------------
Handover Preparation Attempts
Handover Execution = Handover Execution
success
-------------------------------------------
Handover Execution Attempts
REFERENCE DOCS::http://www.eurecom.fr/en/publication/4854/download/comsys-publi-4854.pdf
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