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1、Handover1 Intra-E-UTRAN handover1.1 X2-based handover1.1.1 GeneralThese procedures are used to hand over a UE from a source eNodeB to a target eNodeB using the X2 reference point. In these procedures the MME is unchanged. Two procedures are defined depending on whether the Serving GW is unchanged or
2、 is relocated. In addition to the X2 reference point between the source and target eNodeB, the procedures rely on the presence of S1-MME reference point between the MME and the source eNodeB as well as between the MME and the target eNodeB.The handover preparation and execution phases are performed
3、as specified in TS36.3005. If emergency bearer services are ongoing for the UE handover to the target eNodeB is performed independent of the Handover Restriction List. The MME checks, as part of the Tracking Area Update in the execution phase, if the handover is to a restricted area and if so MME re
4、leases the non-emergency bearers as specified in clause5.10.3.If the serving PLMN changes during X2-based handover, the source eNodeB shall indicate to the target eNodeB (in the Handover Restriction List) the PLMN selected to be the new Serving PLMN.When the UE receives the handover command it will
5、remove any EPS bearers for which it did not receive the corresponding EPS radio bearers in the target cell. As part of handover execution, downlink and optionally also uplink packets are forwarded from the source eNodeB to the target eNodeB. When the UE has arrived to the target eNodeB, downlink dat
6、a forwarded from the source eNodeB can be sent to it. Uplink data from the UE can be delivered via the (source) Serving GW to the PDN GW or optionally forwarded from the source eNodeB to the target eNodeB. Only the handover completion phase is affected by a potential change of the Serving GW, the ha
7、ndover preparation and execution phases are identical.If the MME receives a rejection to a NAS procedure (e.g. dedicated bearer establishment/modification/release; location reporting control; NAS message transfer; etc.) from the eNodeB with an indication that an X2 handover is in progress (see TS36.
8、3005), the MME shall reattempt the same NAS procedure either when the handover is complete or the handover is deemed to have failed, except in case of Serving GW relocation. The failure is known by expiry of the timer guarding the NAS procedure.If during the handover procedure the MME detects that t
9、he Serving GW needs be relocated, the MME shall reject any PDN GW initiated EPS bearer(s) request received since handover procedure started and shall include an indication that the request has been temporarily rejected due to handover procedure in progress. The rejection is forwarded by the Serving
10、GW to the PDN GW, with the same indication.Upon reception of a rejection for an EPS bearer(s) PDN GW initiated procedure with an indication that the request has been temporarily rejected due to handover procedure in progress, the PDN GW start a locally configured guard timer. The PDN GW shall re-att
11、empt, up to a pre-configured number of times, when either it detects that the handover is completed or has failed using message reception or at expiry of the guard timer.If the MME receives a rejection to a UE Context Modification Request message with a CS Fallback indicator from the eNodeB with an
12、indication that an X2 handover is in progress, the MME shall resend a UE Context Modification Request message with CS Fallback indicator to the target eNodeB when the handover is complete or to the source eNB when the handover is deemed to have failed.1.1.2X2-based handover without Serving GW reloca
13、tionThis procedure is used to hand over a UE from a source eNodeB to a target eNodeB using X2 when the MME is unchanged and decides that the Serving GW is also unchanged. The presence of IP connectivity between the Serving GW and the source eNodeB, as well as between the Serving GW and the target eN
14、odeB is assumed.Figure 1.1.2-1: X2-based handover without Serving GW relocationNOTE1:For a PMIP-based S5/S8, procedure steps (A) are defined in TS23.4022.1.The target eNodeB sends a Path Switch Request message to MME to inform that the UE has changed cell, including the TAI+ECGI of the target cell a
15、nd the list of EPS bearers to be switched. If the target cell is a CSG cell , the target eNodeB includes the CSG ID of the target cell in Path Switch Request message. If the target cell is in hybrid mode, it includes the CSG ID of the target cell and CSG Access Mode set to hybrid in the Path Switch
16、Request message. The MME determines the CSG membership based on the CSG ID and CSG Access Mode received from the target eNodeB.The MME updates the User CSG information based on the CSG ID and CSG Access Mode received from the target eNodeB and CSG membership if one of the parameters has changed.NOTE2:X2 handover between HeNBs is possible when the handover is between closed/hybrid access HeNBs having the same CSG ID or when the target
