LTE Mobility LTE Mobility

Content of presentation LTE Mobility Content of presentation Background, Requirements and Solutions for Intra-LTE mobility Inter-RAT mobility with GERAN/UTRAN Inter-RAT mobility with CDMA2000

LTE Mobility 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

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

Risk for radio link failure power The serving cell (1) is no longer best cell Cell 1 Cell 2 Threshold (e.g. 3 dB) Cell 2 is X dB better than Cell 1 (handover is initiated) After this point the serving cell will no longer be able to communicate with terminal Cell 3 Mobility procedure need to finish in shorter time than this time

Handover Preparation Time in LTE Source Target UE eNode B eNode B 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 T1 1 . Measurement Report UE filtering ( e g 100 - 200 ms ) 2 . Handover Decision 3 Handover Request 4 Admission Control 5 Handover Request Ack 7 Handover Command 6 Processing

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

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

~20 ms service interruption

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

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

Inter-RAT (GERAN/UTRAN) mobility (PS) Basic principles similar to intra-LTE Loss-less network controlled handover The UE context is “converted” in the network during the handover preparation phase

Terminal Steering The basic concept is that the terminal is assigned a list with the priorities of the different RATs and frequency bands (based on “subscription” information) Different UEs can get different lists A terminal in idle only searches for higher priority accesses In active mode the eNB controls the access selection (can also be based on “subscription” information) Access Selection 1. LTE1 2. LTE2 3. WCDMA 4. GSM

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)

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)

LTE to EV-DO handover Similar solution is used for CS fallback Access CDMA2K RNC/PCF EV - DO PDSN Access Operator's IP services S101 S2a UE Rx+ Handover command PCRF S1 - MME MME S11 S7 S10 S5 Serving PDN E - UTRAN SGi GW GW S1 - U Similar solution is used for CS fallback

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

Conclusions on LTE Mobility The LTE standard have good support for intra and inter-RAT mobility Service interruption will be around 20 ms for intra-LTE handover and probably below 200 ms for inter-RAT handover It is likely that the handover triggering will be faster in LTE compared to existing 3G networks Fast handover execution is important in order to minimize the risk for radio link failure CS fallback is important for operators, in order to be able to deploy VoIP on LTE