1. LTE Mobility
LTE Mobility

2. 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

3. LTE Mobility
Intra-LTE mobility

4. Service requirements on intra-LTE mobility
Not so strong compared to other requirements such as user plane latency (< 10 ms), capacity, …
3GPP TR 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. ~20 ms service
interruption

11. 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

12. Inter-RAT mobility with GERAN / UTRA

13. 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

14. 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

15. 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

16. Inter-RAT mobility with CDMA2000

17. 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)

18. 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)

19. 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

20. 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

21. 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