INTRA-LTE MOBILITY

In LTE, network controlled UE assisted handovers with context transfer between eNB’s are used to support UE mobility. An intra-LTE handover can be executed either via the X2 or via the S1 interface. As long as the UE moves between eNB’s that belong to the same pooling area where the UE is currently registered, the handovers are always executed via the X2 interface.

In cases when the UE moves between eNB’s that belong to different pooling areas the handover procedure necessarily has to be executed via the S1 interface. In such cases at least the MME function, holding the UE context has to be relocated from one MME node in the first pool to another MME node in the second pool. There is possibility to relocate also the S-GW node during the S1 handover procedure, if it is needed (e.g., if no IP connectivity exists between the target eNB and the current S-GW). However, it is assumed that in the typical case the S-GW will not have to be relocated during an S1 handover, since IP connectivity will be configured in the network such that all S-GW’s will have connectivity to all eNB’s.

Figure 1 shows the procedure flow for intra frequency handover. It consists of Measurement configuration, handover evaluation, preparation, execution and completion. These procedures are also indicated in Figure 2.

The most important aspects of the UP and CP handling in case of intra-LTE mobility have been already agreed in 3GPP. According to the agreed principles, the EPC is not involved in the handover preparation signaling (unless it is an inter-pool mobility with S1 handover); instead the CP signaling is done between the eNBs directly on the X2 interface. The UP is handled by packet forwarding from source eNB to target eNB.

1. The source eNB configures the UE measurement procedures according to the area restriction information.

2. The UE is triggered to send a MEASUREMENT REPORT according to the rules set by the measurement configuration.

3. Based on the measurement report the source eNB decides to perform a handover and selects the target cell.

4. The source eNB issues a HANDOVER REQUEST message to the target eNB passing necessary information to prepare the HO at the target side (UE X2 signalling context reference at source eNB, UE S1 EPC signalling context reference, target cell ID, RRC context including the CRNTI of the UE in the source eNB, AS-configuration (excluding physical layer configuration), EPC bearer context and physical layer ID of the source cell + MAC for possible RLF recovery). UE X2 / UE S1 signalling references enable the target eNB to address the source eNB and the EPC. The EPC bearer context includes necessary RNL and TNL addressing information, and QoS profiles of the EPC bearers.

5. Admission Control may be performed by the target eNB dependent on the received EPC bearer QoS information to increase the likelihood of a successful HO, if the resources can be granted by target eNB. The target eNB configures the required resources according to the received EPC bearer QoS information and reserves a C-RNTI and optionally a RACH dedicated preamble. The AS-configuration to be used in the target cell can either be specified independently (i.e., an “establishment”) or as a delta compared to the ASconfiguration used in the source cell (i.e., a “reconfiguration”).

6. Target eNB prepares the handover with L1/L2 and sends the HANDOVER REQUEST ACKNOWLEDGE to the source eNB. The HANDOVER REQUEST ACKNOWLEDGEmessage includes a transparent container to be sent to the UE as part of the Handover Command. The container includes the new C-RNTI, optionally a dedicated RACH preamble, indication of the expiry time of the dedicated RACH preamble and possibly some other parameters i.e.access parameters, SIBs, etc. The HANDOVER REQUEST ACKNOWLEDGE message may also include RNL/TNL information for the forwarding tunnels, if necessary.

NOTE: As soon as the source eNB receives the HANDOVER REQUEST ACKNOWLEDGE, or as soon as the transmission of the handover command is initiated in the downlink, data forwarding may be initiated.

7. The source eNB generates the HANDOVER COMMAND (RRC message) towards the UE. The HANDOVER COMMAND includes the transparent container, which has been received from the target eNB. The source eNB performs the necessary integrity protection and ciphering of the message.

8. The source eNB sends the SN STATUS TRANSFER message to the target eNB to convey the uplink PDCP SN receiver status and the downlink PDCP SN transmitter status of EPC bearers for which PDCP status preservation applies. The uplink PDCP SN receiver status includes atleast the PDCP SN of the next expected in-sequence UL SDU (upper window edge) and may include a list of the PDCP SN of the out of sequence missing UL SDUs that the UE needs to retransmit in the target cell, if there are any such SDUs. The downlink PDCP SN transmitter status indicates the next PDCP SN that the target eNB shall assign to new SDUs, not having a PDCP SN yet. The source eNB may omit sending this message if none of the EPC bearers of the UE shall be treated with PDCP status preservation.

9. After receiving the HANDOVER COMMAND, UE performs synchronisation to the target eNB and accesses the target cell via RACH following a contention-free procedureif a dedicated RACH preamble was allocated in HANDOVER COMMAND or following a contention based procedure if no dedicated preamble was allocated.

10. Network responds with UL allocation and timing advance.

11. When the UE has successfully accessed the target cell, the UE sends the HANDOVER CONFIRM message (C-RNTI) to the target eNB to indicate that the handover procedure is completed for the UE. The target eNB verifies the C-RNTI sent in the HANDOVER CONFIRM message.

12. The target eNB sends a PATH SWITCH message to the MME to inform it that the UE has changed cell.

13. The MME sends a USER PLANE UPDATE REQUEST message to the Serving Gateway to trigger the switch of the DL tunnels (optionally the UL tunnel endpoints may also be changed).

14. The Serving Gateway switches the downlink data path to the target side and can release any U-plane/TNL resources towards the source eNB.

15. User Plane update response sent to the MME

16. The MME confirms the PATH SWITCH message with the PATH SWITCH ACK message (optionally including new UL tunnel endpoint in case they have been changed).

17. By sending RELEASE RESOURCE the target eNB informs the source eNB about the successful completion of the handover and triggers the release of resources.

18. Upon reception of the RELEASE RESOURCE message, the source eNB can release radio and C-plane related resources associated to the UE context. However, the data forwarding may be still be ongoing even after that point.