LTE Mobility

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The terminal performs continuous measurements on its own and neighboring cells and send measurement reports when required. Once the measurement report is sent the serving eNB can take a handover decision and start the handover preparation phase. When the terminal successfully receives the handover command no more communication is required in the source cell T1 is the time from when the handover should occur until it do occur.

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Nội dung Text: LTE Mobility

LTE Mobility Prepared for LTE seminar (2007-12-06) by EAB/TBU Gunnar Mildh
Content of presentation Background, Requirements and Solutions for  Intra-LTE mobility  Inter-RAT mobility with GERAN/UTRAN  Inter-RAT mobility with CDMA2000 Ericsson Internal 2 LTE Mobility 2007-12-17
Intra-LTE mobility
Service requirements on intra-LTE mobility  Not so strong compared to other requirements such as user plane latency (< 10 ms), capacity, … – 3GPP TR 25.913 only states that the service interruption should be less or equal than GSM CS  Looking at possible services today it is difficult to find some services with strong requirements – Voice works with < 200 ms – TCP should probably cope with 100 ms interruption as long as there are not packet losses – Play out buffers for streaming would probably be much larger than 200 ms  Requirements for future services are unknown Ericsson Internal 4 LTE Mobility 2007-12-17
Radio requirements  The characteristics of the LTE radio system will lead to quite tough requirements on the intra-LTE mobility solution – No soft handover – Reuse 1 (potential for strong interference at street corners etc.) – Problems seen today when using hard handover in HSPA  This leads to a risk that UE is looses connection to serving cell before handover is completed (illustrated on next slide) – UE experiences a radio link failure, leading to longer service interruption Ericsson Internal 5 LTE Mobility 2007-12-17
Risk for radio link failure power The serving cell (1) is no longer Cell 1 best cell Cell 2 Threshold (e.g. 3 dB) After this point the serving cell Cell 2 is X dB better than Cell 1 will no longer be able to (handover is initiated) communicate with terminal Cell 3 Mobility procedure need to time finish in shorter time Internal this Ericsson than 6 LTE Mobility 2007-12-17
Handover Preparation Time in LTE Source Target UE eNode B eNode B • The terminal performs continuous measurements on its own and neighboring cells UE filtering (e.g. 100-200 ms) and send measurement reports 1. Measurement Report when required. • Once the measurement report 2. Handover Decision is sent the serving eNB can take a handover decision and 3. Handover Request start the handover preparation T1 phase. When the terminal 4. Admission successfully receives the Control handover command no more communication is required in 5. Handover Request Ack. the source cell 6. Processing • T1 is the time from when the 7. Handover Command handover should occur until it do occur Ericsson Internal 7 LTE Mobility 2007-12-17
Conclusion / Way forward  Although current service requirements are not so strong it is still probably important to have a mobility solution in the standard that allows for very short service interruption – It is good to have a future proof solution since it might be difficult to add improvements at a later stage – PS VoIP performance in LTE need to as good as existing CS networks  Fast handover execution is important – Flat architecture and faster signaling channels etc. could help a bit – We will also continue to study handover performance to understand the issues – In case this turns out to be an issue we have introduced support for handover failure recovery (see later slides) which can be further improved in the products if needed Ericsson Internal 8 LTE Mobility 2007-12-17
Intra-LTE mobility solution Basic principles  eNB controls the handover (network controlled handover) – Allows tuning, more predictable mobile behavior – Works well with network prepared resources (measurement report triggers preparation) – Some companies have pushed UE controlled handover (resources are setup when UE arrive) – Operators like network controlled handover probably for interoperability and tuning reasons  Lossless handover is supported – Good for TCP and the UDP Streaming performance Ericsson Internal 9 LTE Mobility 2007-12-17
UE Source eNB Target eNB Core Network UL/DL Data DL allocation Legend 7. Handover Command (incl . pre-amble) L3 signalling Deliver buffered and in transit L1/L2 signalling packets to target eNB User Data ~20 ms service DL Data Forwarding Handover Execution interruption Buffer packets from Source eNB 8. Synchronisation (on Random Access Channel using dedicated pre-amble) 9. UL allocation + TA for UE 10. Handover Confirm packet data 11. Handover Complete Handover Completion Path Switching 12. Handover Complete Ack 13. Release Resource Flush DL buffer, continue delivering in - transit packets Ericsson Internal 10 LTE Mobility 2007-12-17
Radio link failure Previous hot topic in 3GPP  In case UE looses contact with source cell it will select a new cell and send a RRC recovery request message (incl. UE identity and “shared secret”)  In case the eNB who receives knows the UE it would be possible to recover the connection with low service interruption – In case the serving eNB looses contact with the UE it can prepare neighbor eNBs about the potential arrival of the UE LTE NodeB LTE NodeB Ericsson Internal 11 LTE Mobility 2007-12-17
Inter-RAT mobility with GERAN / UTRA
Requirements  3GPP TR 25.913 requires service interruption below 300 ms  Most important is probably to support fall back from LTE (initial spotty coverage) to GERAN/UTRAN (high coverage), mobility in opposite direction also supported  Strong requirements have also lately come for supporting fall back from LTE PS VoIP to GSM/WCDMA CS as well as other forms of CS inter-working – So far multiple solutions are being studied. PS to CS fallback requires both radio and CN domain handover. Solutions for CS paging over LTE is studied.  Operators also put requirement on more flexible terminal steering mechanism (e.g. subscription based) both for Camping and Active users Ericsson Internal 13 LTE Mobility 2007-12-17
Inter-RAT (GERAN/UTRAN) mobility (PS) Packet Data Networks  Basic principles Control Interface Gi User Data Interface similar to intra- P/S -GW AGW --UP AGW UP LTE S4(Gn)  Loss-less network S3 (Gn) S4 -CP(Gn) S11 2G SGSN 2G SGSN 3G SGSN controlled 3G SGSN Iu-CP MME AGW -- AGW handover Gb Iu-UP / S4 -UP S3(Gn)  The UE context is BSC BSC RNC RNC S1 -CP S1 -UP “converted” in the Iub network during BTS BTS Node B Node B eNode B eNode B the handover GERAN UTRAN(incl. Ev. HSPA) LTE RAN preparation phase Ericsson Internal 14 LTE Mobility 2007-12-17
Terminal Steering Packet Data Networks Control Interface Gi User Data Interface P/S -GW AGW--UP AGW UP  The basic concept is that the S4(Gn) terminal is assigned a list with the S3 (Gn) S4 -CP(Gn) S11 priorities of the different RATs and 2G SGSN 2G SGSN 3G SGSN 3G SGSN MME AGW -- AGW Iu-CP frequency bands (based on Gb Iu-UP / S4 -UP S3(Gn) BSC BSC RNC RNC “subscription” information) Iub S1-CP S1 -UP  Different UEs can get different lists BTS BTS Node B Node B eNode B eNode B  A terminal in idle only searches for GERAN UTRAN(incl. Ev. HSPA) LTE RAN higher priority accesses  In active mode the eNB controls the Access Selection access selection (can also be based 1. LTE1 on “subscription” information) 2. LTE2 3. WCDMA 4. GSM Ericsson Internal 15 LTE Mobility 2007-12-17
Inter-RAT mobility with CDMA2000
Requirements  Work item was created at RAN plenary in December and Stage 2 CR was agreed (the solution is stable)  Requirements are the same as for inter-RAT handover with GERAN/UTRAN – Driven mainly by Verizon Wireless, but also KDDI, US Cellular, Alltel – Goal is to support smooth migration from 3GPP2 systems to LTE  Focus is on handover for “single radio” terminals from LTE to CDMA2000 EV-DO (HRPD) and also CS fallback for LTE VoIP (PS) to CDMA2000 1xRTT (CS) – CS fallback is already supported between EV-DO (PS) and 1xRTT (CS) Ericsson Internal 17 LTE Mobility 2007-12-17
Principles for CDMA2000 mobility  Although the radio principles for CDMA2000 EV-DO is in many ways similar to HSPA the higher layer protocols stacks are very different from 3GPP accesses – Different security, UE context, addressing, identities, mobility concepts etc.  It was concluded it would not be possible to “convert” the UE context in the network without involving the UE  Instead a solution was adopted where the UE communicates with the target RAT in a tunnel over the source RAT – This minimizes the dependencies with between the different access (LTE doesn’t need to know so much about CDMA2000) Ericsson Internal 18 LTE Mobility 2007-12-17
LTE to EV-DO handover CDMA2K RNC/PCF EV-DO PDSN Access Operator's IP services S101 S2a UE Rx+ PCRF command Handover S1-MME MME S11 S7 S10 S5 E-UTRAN Serving PDN SGi GW GW S1-U Similar solution is used for CS fallback Ericsson Internal 19 LTE Mobility 2007-12-17
LTE to EV-DO handover Comments  The service interruption of the proposes solution should be similar to the service interruption experienced for handover to GERAN/UTRAN  The preparation time will however likely be longer since UE need to be involved in the preparation signaling  Solution is to separate the preparation into two phases – “pre-registration” which takes longer time and is performed in advance – “handover execution” which is faster and is performed at the handover instance Ericsson Internal 20 LTE Mobility 2007-12-17