Section: 5.5.1.1 X2-based handover 5.5.1.1.1 General
Content: These 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 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 as specified in TS 36.300[ Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 ]  [5]. 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 releases the non-emergency bearers as specified in clause 5.10.3. 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 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 data 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 handover 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 TS 36.300[ Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 ]  [5]), the MME shall reattempt the same NAS procedure either when the handover is complete or the handover is deemed to have failed, except in the case of Serving GW relocation. The failure is known by expiry of the timer guarding the NAS procedure. If the X2 handover includes the Serving GW relocation, and if the MME receives a rejection to a NAS message transfer for a Downlink NAS Transport or Downlink Generic NAS Transport message from the eNodeB with an indication that an X2 handover is in progress, the MME should resend the corresponding message to the target eNodeB when either the handover is complete or to the source eNodeB when the handover is deemed to have failed if the MME is still the serving MME. If the MME receives a rejection to a NAS message transfer for a CS Service Notification or to a UE Context Modification Request message with a CS Fallback indicator from the eNodeB with an indication that an X2 handover is in progress, the MME shall resend the corresponding message to the target eNodeB when the handover is complete or to the source eNodeB when the handover is deemed to have failed. If during the handover procedure the MME detects that the 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 GW to the PDN GW, with the indication that the request has been temporarily rejected. 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-attempt, 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. For handover the following applies related to handling of radio capabilities: - If the source eNodeB and target eNodeB support RACS as defined in clause 5.11.3a, the source eNodeB provides the UE's UE Radio Capability ID to the target eNodeB. - If the source eNodeB has knowledge that the target eNodeB might not have a local copy of the Radio Capability corresponding to the UE Radio Capability ID (i.e. because the source eNodeB had itself to retrieve the UE's Radio Capability from the MME) then the source eNodeB may also send some (or all) of the UE's Radio Capability to the target eNodeB (the size limit based on local configuration) in X2 signalling as defined in TS 36.423[ Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 Application Protocol (X2AP) ]  [76]. In the case of inter-PLMN handover, when the source and target eNodeB support RACS as defined in clause 5.11.3a and the source eNodeB determines based on local configuration that the target PLMN does not support the UE Radio Capability ID assigned by the source PLMN, then the source eNodeB shall provide the UE radio capabilities to the target eNodeB and shall not send the UE Radio Capability ID. - If the target eNodeB does not have mapping between the specific UE Radio Capability ID and the UE radio capabilities and no UE radio capabilities are provided by source eNodeB, it shall use the procedure described in TS 36.413[ Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1AP) ]  [36] to retrieve the mapping from the Core Network. - If, as permitted in TS 36.423[ Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 Application Protocol (X2AP) ]  [76], the target eNodeB during the handover preparation received the UE radio capabilities but did not receive the UE Radio Capability ID the target eNodeB shall proceed with handover using the received UE radio capabilities. - If the target eNodeB received both the UE radio capabilities and the UE Radio Capability ID, then the target eNodeB shall use any locally stored UE radio capability information corresponding to the UE Radio Capability ID. - If none are stored locally, the target eNodeB may request the full UE radio capability information from the core network. - If the full UE radio capability information is not promptly received from the core network, or the target eNodeB chooses not to request it, then the target eNodeB shall proceed with the UE radio capabilities sent by the source RAN node. The target eNodeB shall not use the UE radio capability information received from the source eNodeB for any other UE with the same UE Radio Capability ID. - If the target eNodeB knows (e.g. by configuration) that the UE's E-UTRA radio capabilities applicable to the target eNB may be different to the E-UTRA radio capabilities stored in the source eNodeB (e.g. for handover to/from an E-UTRA eNodeB that supports the NTN enhancements as defined in TS 36.300[ Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 ]  [5]), then the target eNodeB shall trigger retrieval of the radio capability information again from the UE. NOTE: In order to support handover from an E-UTRA eNodeB that supports the NTN enhancements as defined in TS 36.300[ Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 ]  [5] the target eNodeB needs to be upgraded to support the above behaviour.