The LTE standards come with well defined interface between eNB(X2), which makes handover between eNB possible.
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LTE X2-based Handover |
The steps of the handover as in the figure above are described below:
(1) The old eNB upon receiving the appropriate measurement reports from the UE will initiate the Handover procedure by sending a HANDOVER_REQ to the new eNB.This requires intelligent eNB which are aware of the other eNB in the vicinity.
(2) The new eNB sends a HANDOVER_ACK. This will contain a RRC_CONNECTION_RECONFIG message containing the parameters of the new cell for the UE.It will also have the GTP-U TEID for the data connection for DL data to be sent from the old eNB.
(3) The old eNB relays the RRC_CONNECTION_RECONFIG message to the UE.
(4) At this point, the old eNB starts forwarding the DL data destined for the UE to the new eNB using the GTP-U TEID from step 2.
(5) The UE does a Random Access Procedure in the new cell under new eNB to get the Timing Advance and UL grant.
(6) UE sends the RRC_CONNECTION_RECONFIG_COMPLETE message as a handover confirmation to the new eNB.
(7) The new eNB will now ask the MME to switch the GTP-U tunnel for DL data connection from the old eNB to the new eNB by sending the PATH_SWITCH_REQ message which has the new GTP-U TEID.
(8) The MME will in turn send the UPDATE_UP_REQ having the GTP-U TEID from the last step to S-GW.
(9) The UL and DL data starts in the new eNB. The new eNB however has to buffer the UL data till the data connection to the S-GW is switched completely.
(10) (11)The old eNB forwards the last packet to new eNB before the swicth.
(12) The S-GW will confirm the switch by sending UPDATE_UP_RESP to the MME.
(13) MME in turn confirms the switch to the new eNB with PATH_SWITCH_ACK.
(14) Now that the new eNB has completely taken over the role of the eNB serving the UE, it sends a class 2 one-sided message (UE_RESOURCE_REL) to the old eNB.
(15) The data on new data link continues.