White PaperInteroperable UE Handovers in LTEBy: V. Srinivasa Rao, Senior Architect & Rambabu Gajula, Lead EngineerOverview CONTENTSWith the fast-changing mobile landscape and convergence in all aspects of Objectives of Handover Procedures pg. 2telecommunications, “seamless handover” is important for any technology Intra-LTE (Intra-MME/SGW) Handoverto succeed. Operators and consumers both benefit from seamless handover Using the X2 Interface pg. 2in terms of cost effectiveness, enhanced features, location independenceand ease of use, not only within a Long Term Evolution (LTE) network but Intra-LTE (Intra-MME/SGW) Handoveralso between networks including UMTS, GSM and CDMA. Using the S1 Interface pg. 4 Intra-LTE Handover (Intra-MME/SGW) pg. 4In this paper we briefly touch upon the procedures executed by the userequipment (UE) and the various network elements to provide the handover Inter-MME Handover Using theservices requested by the UE. We cover Intra-LTE and LTE to/from UMTS S1 Interface (Without Changing S-GW) pg. 5handovers. Inter-MME/SGW Handover Using the S1 Interface pg. 6 Inter-RAT Handover: E-UTRAN to UTRAN Iu Mode pg. 7 Inter-RAT Handover: UTRAN to E-UTRAN pg. 9 Conclusions pg. 11 References pg. 11
Interoperable UE Handovers in LTE | Radisys White Paper 2Objectives of In LTE there are three types of handovers:Handover Procedures • Intra-LTE: Handover happens within the current LTE nodes (intra-MME and Intra-SGW)1. t is important that QoS is maintained, not I just before and after a handover, but during • Inter-LTE: Handover happens toward the other the handover as well. LTE nodes (inter-MME and Inter-SGW)2. Handover shall not drain the UE battery power. • Inter-RAT: Handover between different radio technology networks, for example GSM/UMTS3. ervice continuity shall be maintained S and UMTS (i.e., minimal handover latency).4. Seamless handoff to 3G / 2G / CDMA technology. We’ll look at Intra-LTE handovers with X2AP signaling and S1AP signaling first, then Inter-RAT handoversThere are two ways a handoff can be decided: in LTE (i.e., handover between LTE and UMTS).• Network Evaluated: the network makes the handover decision Intra-LTE (Intra-MME/SGW)• Mobile Evaluated: the UE makes the handoff decision Handover Using the X2 and informs the network about it. In this instance, Interface the final decision will be made by the network based This procedure is used to handover a UE from a source upon on the Radio Resource Management. eNodeB (S-eNB) to a target eNodeB (T-eNB) usingIn 3G and LTE networks, a hybrid approach is used to the X2 interface when the Mobility Management Entitydecide on the handover. In this case, the UE will assist (MME) and Serving Gateway (SGW) are unchanged.in the handoff decision by measuring the neighboring It is possible only if direct connectivity exists betweencells and reporting the measurements to the network, the source and target eNodeB’s with the X2 interface.which in turn decides upon the handoff timing and the The X2 handover procedure is performed withouttarget cell/node. The parameters to measure and the Evolved Packet Core (EPC) involvement, i.e. preparationthresholds for reporting are decided by the network. messages are directly exchanged between the S-eNBFigure 1. Intra-LTE (Intra-MME/SGW) Handover Using the X2 Interface
Interoperable UE Handovers in LTE | Radisys White Paper 3and T-eNB. The release of the resources at the source 7. he S-eNB generates the RRC message to perform the Tside during the handover completion phase is triggered handover, i.e, RRCCONNECTION RECONFIGURATIONby the T-eNB. The message flow is shown in Figure 1 message including the mobilityControlInformation.followed by the description. The S-eNB performs the necessary integrity protection1. data call is established between the UE, S-eNB A and ciphering of the message and sends it to the UE. and the network elements. Data packets are 8. The S-eNB sends the eNB STATUS TRANSFER transferred to/from the UE to/from the network message to the T-eNB to convey the PDCP and in both directions (DL as well as UL). HFN status of the E-RABs.2. he network sends the MEASUREMENT CONTROL T 9. The S-eNB starts forwarding the downlink data REQ message to the UE to set the parameters to packets to the T-eNB for all the data bearers measure and set thresholds for those parameters. Its (which are being established in the T-eNB during purpose is to instruct the UE to send a measurement the HANDOVER REQ message processing). report to the network as soon as it detects the 10. In the meantime, the UE tries to access the T-eNB thresholds. cell using the non-contention-based Random3. he UE sends the MEASUREMENT REPORT to the T Access Procedure. If it succeeds in accessing S-eNB after it meets the measurement report criteria the target cell, it sends the RRC CONNECTION communicated previously. The S-eNB makes the RECONFIGURATION COMPLETE to the T-eNB. decision to hand off the UE to a T-eNB using the 11. The T-eNB sends a PATH SWITCH REQUEST handover algorithm; each network operator could message to the MME to inform it that the UE have its own handover algorithm. has changed cells, including the TAI+ECGI of4. he S-eNB issues the RESOURCE STATUS REQUEST T the target. The MME determines that the SGW message to determine the load on T-eNB (this is can continue to serve the UE. optional). Based on the received RESOURCE STATUS 12. The MME sends a MODIFY BEARER REQUEST RESPONSE, the S-eNB can make the decision to (eNodeB address and TEIDs for downlink user proceed further in continuing the handover procedure plane for the accepted EPS bearers) message using the X2 interface. to the SGW. If the PDN GW requested the UE’s5. he S-eNB issues a HANDOVER REQUEST message T location info, the MME also includes the User to the T-eNB passing necessary information to Location Information IE in this message. prepare the handover at the target side (e.g., UE 13. he SGW sends the downlink packets to the target T Context which includes the Security Context and eNB using the newly received addresses and TEIDs RB Context (including E-RAB to RB Mapping) and (path switched in the downlink data path to T-eNB) the Target cell info). and the MODIFY BEARER RESPONSE to the MME.6. he T-eNB checks for resource availability and, if T 14. The SGW sends one or more “end marker” packets available, reserves the resources and sends back on the old path to the S-eNB and then can release the HANDOVER REQUEST ACKNOWLEDGE message any user plane / TNL resources toward the S-eNB. including a transparent container to be sent to the UE as an RRC message to perform the handover. The 15. The MME responds to the T-eNB with a PATH container includes a new C-RNTI, T-eNB security SWITCH REQ ACK message to notify the completion algorithm identifiers for the selected security of the handover. algorithms, and may include a dedicated RACH 16. The T-eNB now requests the S-eNB to release preamble and possibly some other parameters the resources using the X2 UE CONTEXT RELEASE (i.e., access parameters, SIBs, etc.). message. With this, the handover procedure is complete.
Interoperable UE Handovers in LTE | Radisys White Paper 4Figure 2. Intra-LTE (Intra-MME/SGW) Handover Using the S1 InterfaceIntra-LTE (Intra-MME/SGW) Intra-LTE HandoverHandover Using the (Intra-MME/SGW)S1 Interface • As mentioned in the previous section, based on theThe S1-based handover procedure is used when MEASUREMENT REPORT from the UE, the S-eNBthe X2-based handover cannot be used—e.g., no decides to Handover the UE to another eNodeBX2 connectivity to the target eNodeB; by an error (T-eNB). The handover procedure in this section isindication from the T-eNB after an unsuccessful very similar to that in the previous section (Intra-X2-based handover; or by dynamic information LTE Handover Using the X2 Interface), except thelearnt by the S-eNB using the STATUS TRANSFER involvement of the MME in relaying the handoverprocedure. The S-eNB initiates the handover by signaling between the S-eNB and T-eNB.sending a Handover required message over the • There are two differences here:S1-MME reference point. The EPC does not changethe decisions taken by the S-eNB. ˸˸ No need for the PATH SWITCH Procedure between the T-eNB and MME, as MME is aware of theThe availability of a direct forwarding path is Handover.determined in the S-eNB (based on the X2 connectivity ˸˸ The SGW is involved in the DL data forwardingwith the T-eNB) and indicated to the source MME. if there is no direct forwarding path availableIf a direct forwarding path is not available, indirect between the S-eNB and T-eNB.forwarding will be used. The source MME uses theindication from the S-eNB to determine whether to • Once the Handover is complete, the MME clearsapply indirect forwarding or not. The message flow the logical S1 connection with the S-eNB byis depicted in Figure 2 followed by the description of initiating the UE CONTEXT RELEASE procedure.the procedures.
Interoperable UE Handovers in LTE | Radisys White Paper 5Figure 3. Inter-MME Handover (Intra-SGW)Inter-MME Handover 2. The S-MME uses GTP signaling to communicate the handover signaling to the T-MME and vice versa.Using the S1 Interface The FORWARD RELOCATION procedure in GTP-C is(Without Changing S-GW) being used here.In an inter-MME handover, two MMEs are involved in 3. After receiving the S1 HANDOVER REQUIRED, the the handover, the source MME (S-MME) and target S-MME detects that the target cell requested forMME (T-MME). The S-MME controls the S-eNB and the handover belongs to another MME and initiatesT-MME controls the T-eNB; both MMEs are connected the GTP FORWARD RELOCATION REQ messageto the same SGW. This handover is triggered when the to the T-MME.UE moves from one MME area to another MME area. 4. he T-MME creates the S1 logical connection toward T the T-eNB and sends the S1 HANDOVER REQ on it.1. s mentioned in the previous section (Intra-MME/ A SGW handover), based on the MEASUREMENT 5. he T-eNB prepares the requested resources and T REPORT from the UE, the S-eNB decides to handover responds with a HANDOVER REQ ACK to the T-MME. the UE to another eNodeB (T-eNB). The handover 6. The T-MME sends a GTP FORWARD RELOCATION procedure in this section is very similar to that in the RESP to the S-MME, to notify the resource previous section except for the involvement of two reservation at the T-eNB. From this point onwards, MMEs coordinating the handover signaling between the interaction between the S-MME and S-eNB the source and target eNodeBs. is very similar to the S1-based Intra-MME/SGW handover described in the previous section.
Interoperable UE Handovers in LTE | Radisys White Paper 6Figure 4. Inter-MME/SGW Handover7. L data packets are forwarded from the S-eNB D to T-eNB via the SGW during the handover as Inter-MME/SGW Handover the SGW is not changed here. Using the S1 Interface8. nce the T-eNB detects the UE in its area, it notifies O This scenario is similar to the previous section with the the T-MME with a S1 HANDOVER NOTIFY message. difference being the Source and Target eNodeBs are served by different MME/SGW nodes. Figure 4 depicts9. he T-MME notifies the completion of the handover T the procedures and is followed by the explanation. to the S-MME with a GTP FORWARD RELOCATION COMPLETE NOTIFY message. 1. s described in the previous section (Inter-MME, A Intra-SGW handover), based on the MEASUREMENT10. he S-MME acknowledges the GTP FORWARD T REPORT from the UE, the S-eNB decides to handover RELOCAION COMPLETE NOTIFY to the T-MME the UE to another eNodeB (T-eNB). The handover and proceeds with clearing the S1 logical procedure in this section is very similar to that in connection and the associated bearer resources. the previous section, except for the involvement of two SGWs (S-SGW and T-SGW) to transfer the data packets during the handover.
Interoperable UE Handovers in LTE | Radisys White Paper 7Figure 5. Inter-RAT LTE-to-UMTS Handover: Preparation Phase2. fter receiving the GTP: FORWARD RELOCATION A REQ from the S-MME, the T-MME detects the SGW Inter-RAT Handover: change and initiates the bearer creation toward the E-UTRAN to UTRAN Iu Mode target SGW (T-SGW) using a GTP: CREATE SESSION Preparation Phase REQ message. In the LTE-to-UMTS Inter RAT handover, the source3. fter the creation of the requested bearers, the A eNodeB connects to the S-MME and S-SGW while T-SGW responds back to the MME with a GTP: the target RNC connects to the T-SGSN and T-SGW; CREATE SESSION RESPONSE message. both the source and target SGWs connect to the same4. rom this point onward, the message flow is very F PGW. This procedure is divided into two parts for similar to that in the previous section (Inter-MME, clarity, Preparation and Execution. In the Preparation Intra-SGW handover) except for the following phase, resources are reserved in the target network. differences: In the Execution phase, the UE is handed over to the target network from the source network. The • While processing the S1 HANDOVER NOTIFY Preparation phase message flow is given in Figure 5, message from the T-eNB, the T-MME updates followed by the description. the T-eNB endpoint information to the T-SGW using GTP: MODIFY BEARER REQ. 1. nce the inter-RAT handover is decided at the O S-eNB based on the measurement report procedure, • After updating the T-eNB information in the it prepares and sends a HANDOVER REQUIRED bearers (successfully set up during the handover), message to the S-MME. the T-SGW responds with a GTP: MODIFY BEARER RESPONSE message to the T-MME. 2. he S-MME detects that it is an Inter-RAT handover T from the message contents, retrieves the target SGSN • After successful completion of the handover, the details from the database based on the information S-MME takes care of releasing bearer resources in the message. It now prepares and sends a GTP-C: with the S-SGW for this UE by initiating the GTP: FORAWRD RELOCATION REQUEST to the T-SGSN. DELETE SESSION procedure. 3. The T-SGSN detects the change of SGW and creates the bearer resources in the T-SGW by initiating the GTP: CREATE SESSION procedure.
Interoperable UE Handovers in LTE | Radisys White Paper 8Figure 6. Inter-RAT LTE-to-UMTS Handover: Execution Phase4. nce the resources are reserved at the T-SGW, O Execution Phase it responds to the T-SGSN with a GTP: CREATE 1. The S-MME sends the HANDOVER COMMAND SESSION RESPONSE message. message to the S-eNB with the target to source5. he T-SGSN now reserves the resources at the T transparent container (i.e., it has the reserved T-RNC by sending a RANAP: RELOCATION REQUEST resource information at the target). message to it. 2. The S-eNB prepares and sends the MOBILITY FROM 6. he T-RNC reserves the radio resources and T EUTRA COMMAND message to prepare the UE for responds to the T-SGSN with a RANAP: RELOCATION the handover toward the target network. REQUEST ACK message. 3. After accessing the target UMTS cell, the UE sends 7. he T-SGSN creates the indirect data forwarding T a HO TO UTRAN COMPLETE message to the T-RNC tunnels in the T-SGW for the DL packets transfer signaling the successful handover. from the S-SGW to T-SGW during the handover. 4. he S-eNB forwards the DL data packets toward T8. fter the Indirect Data forwarding tunnel creation, A the T-SGW via the S-SGW during the handover. This the T-SGSN responds with a GTP: FORWARD step can happen any time after it receives the S1AP RELOCATION RESPONSE message to the S-MME. HANDOVER COMMAND message from the S-MME.9. he S-MME has to create the indirect data T This step is executed in case a direct forwarding path forwarding tunnels as the resources are reserved is not available with the T-RNC, otherwise it will successfully in the target network to forward forward the DL data packets to the T-RNC directly. the DL packets to the target network. With this, Both the options are shown in Figure 6. the preparation phase is complete.
Interoperable UE Handovers in LTE | Radisys White Paper 9Figure 7. Inter-RAT UMTS-to-LTE Handover: Preparation Phase5. nce the T-RNC detects the UE in its area, it O phase, resources are reserved in the target network, notifies the T-SGSN about the completion of the while in the execution phase the UE is handed over handover by sending a RANAP: RELOCATION to the target network from the source network. The COMPLETE message. Preparation phase message flow is given in Figure 7,6. he T-SGSN notifies the completion of handover T followed by the description. to the S-MME by sending a GTP: FORWARD 1. Once the inter-RAT handover is decided at RELOCATION COMPLETE NOTIFICATION ACK the S-RNC based on the measurement report message. The S-MME acknowledges this message procedure, it prepares and sends a RANAP and proceeds with release of the resources RELOCATION REQUIRED message to the S-SGSN. associated with this UE at the S-SGW and S-eNB. 2. he S-SGSN detects that it is an Inter-RAT handover T7. he T-SGSN modifies the E-RAB resources at T from the message contents and retrieves the T-MME the T-SGW by initiating the GTP MODIFY BEARER details from the database based on the information procedure. in the message. It now prepares and sends a GTP-C:8. he T-SGW notifies the bearer parameters with T FORAWRD RELOCATION REQUEST to the T-MME. the PGW by initiating the GTP MODIFY BEARER 3. The T-MME detects the change of SGW and creates procedure. the bearer resources in the T-SGW by initiating the GTP: CREATE SESSION procedure.Inter-RAT Handover: 4. Once the resources are reserved at the T-SGW, UTRAN to E-UTRAN it responds to the T-MME with a GTP: CREATE SESSION RESPONSE message.Preparation Phase 5. The T-MME now reserves the resources at the In the UMTS-to-LTE Inter-RAT handover, the S-RNC T-eNB by sending aS1AP: HANDOVER REQUESTconnects to the S-SGSN and S-SGW, the T-eNB message to it.connects to the T-MME and T-SGW, and both thesource and target SGWs connect to the same PGW. 6. The T-eNB reserves the radio resources and As above, this procedure is divided into two parts for responds to the T-MME with a S1AP: HANDOVERclarity, Preparation and Execution. In the Preparation REQUEST ACK message.
Interoperable UE Handovers in LTE | Radisys White Paper 10Figure 8. Inter-RAT UMTS-to-LTE Handover: Execution Phase7. he T-MME creates the indirect data forwarding T 3. After accessing the T-eNB, the UE sends an RRC tunnels in the T-SGW for the DL packets transfer CONNECTION RECONFIGURATION COMPLETE from the S-SGW to the T-SGW during the handover message to the T-eNB signaling the successful if there is no direct forwarding path available from handover. source to target. 4. The S-RNC forwards the DL data packets toward 8. fter the Indirect Data forwarding tunnel creation, A the T-SGW via the S-SGW during the handover. the T-MME responds with a GTP: FORWARD This step can happen any time after it receives the RELOCATION RESPONSE message to the S-SGSN. RANAP RELOCATION COMMAND message from9. he S-SGSN has to create the indirect data T the S-SGSN. This step is executed in case a direct forwarding tunnels, as the resources are reserved forwarding path is not available with the T-eNB, successfully in the target network to forward otherwise it will forward the DL data packets to the the DL packets to the target network. With this, T-eNB directly. Both the options are shown above in the preparation phase is complete. Figure 8. 5. Once the T-eNB detects the UE in its area, it notifies Execution Phase the T-MME about the completion of the handover by1. he S-SGSN sends the RANAP RELOCATION T sending an S1AP: HANDOVER NOTIFY message. COMMAND message to the S-RNC with the target 6. The T-MME notifies the completion of handover to source transparent container (it has the reserved to the S-SGSN by sending a GTP: FORWARD resource information at the target). RELOCATION COMPLETE NOTIFICATION ACK2. he S-RNC prepares and sends the HO FROM T message. The S-SGSN acknowledges this message UTRAN COMMAND message to prepare the UE and proceeds with the release of the resources for the handover toward the target network. associated with this UE at the S-SGW and S-RNC.