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LTE Mobility
Top right corner for field-mark, customer or partner logotypes. See Best practice for example.
Slide title 40 pt
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Bullets level 2-520 pt
Ericsson Internal LTE Mobility2
Content of presentation
Background, Requirements and Solutions for Intra-LTE mobility Inter-RAT mobility with GERAN/UTRAN Inter-RAT mobility with CDMA2000
Intra-LTE mobility
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Slide title 40 pt
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Text 24 pt
Bullets level 2-520 pt
Ericsson Internal LTE Mobility4
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
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Ericsson Internal LTE Mobility5
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
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Ericsson Internal LTE Mobility6
Cell 1
Cell 2
The serving cell (1) is no longer
best cell
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
time
power
Mobility procedure need to finish in shorter time than this
Cell 3
Risk for radio link failure
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Ericsson Internal LTE Mobility7
Handover Preparation Time in LTE
• 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
UE Source eNode B
Target eNode B
2. Handover Decision
3. Handover Request
4. Admission Control
5. Handover Request Ack.
7. Handover Command
6. Processing
1. Measurement Report
UE filtering (e.g. 100-200 ms)
T1
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Ericsson Internal LTE Mobility8
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
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Ericsson Internal LTE Mobility9
Intra-LTE mobility solutionBasic 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
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Ericsson Internal LTE Mobility10
packet data
7. Handover Command (incl. pre-amble)
DL allocation
DL Data Forwarding
10. Handover Confirm
13. Release Resource
11. Handover Complete
UE Source eNB Target eNB Core Network
Deliver buffered and in transit packets to target eNB
Buffer packets from Source eNB
8. Synchronisation (on Random Access Channel using dedicated pre-amble)
9. UL allocation + TA for UE
Flush DL buffer, continue delivering in-
transit packets
Path Switching
12. Handover Complete Ack
Han
dove
r C
ompl
etio
nH
ando
ver
Exe
cutio
n
Legend
L3 signalling
L1/L2 signalling
User Data
UL/DL Data
~20 ms serviceinterruption
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Ericsson Internal LTE Mobility11
Radio link failurePrevious 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 NodeBLTE NodeB
Inter-RAT mobility with GERAN / UTRA
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Ericsson Internal LTE Mobility13
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
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Ericsson Internal LTE Mobility14
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
S1 - UP
AGW - UP AGW - UP
Gi
Packet Data Networks
S4 - CP(Gn)
Node B Node B
RNC RNC
BTS BTS
BSC BSC
2G SGSN 2G SGSN
S4(Gn)
S3 (Gn)
Iu - CP
Iub
Gb Iu - UP / S4 - UP
S3(Gn)
LTE RAN UTRAN(incl. Ev. HSPA) GERAN
Control Interface User Data Interface
AGW - AGW -
eNode B eNode B
S1 - CP
S11
3G SGSN 3G SGSN
S1 - UP
AGW - UP P/S -GW
Gi
Packet Data Networks
S4 - CP(Gn)
Node B Node B
RNC RNC
BTS BTS
BSC BSC
2G SGSN 2G SGSN
S4(Gn)
S3 (Gn)
Iu - CP
Iub
Gb Iu - UP / S4 - UP
S3(Gn)
LTE RAN UTRAN(incl. Ev. HSPA) GERAN
Control Interface User Data Interface Control Interface User Data Interface
AGW MME
eNode B eNode B
S1 - CP
3G SGSN 3G SGSN
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Ericsson Internal LTE Mobility15
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)
1. LTE12. LTE23. WCDMA4. GSM
S1 - UP
AGW - UP AGW - UP
Gi
Packet Data Networks
S4 - CP(Gn)
Node B Node B
RNC RNC
BTS BTS
BSC BSC
2G SGSN 2G SGSN
S4(Gn)
S3 (Gn)
Iu - CP
Iub
Gb Iu - UP / S4 - UP
S3(Gn)
LTE RAN UTRAN(incl. Ev. HSPA) GERAN
Control Interface User Data Interface
AGW - AGW -
eNode B eNode B
S1 - CP
S11
3G SGSN 3G SGSN
S1 - UP
AGW - UP P/S -GW
Gi
Packet Data Networks
S4 - CP(Gn)
Node B Node B
RNC RNC
BTS BTS
BSC BSC
2G SGSN 2G SGSN
S4(Gn)
S3 (Gn)
Iu - CP
Iub
Gb Iu - UP / S4 - UP
S3(Gn)
LTE RAN UTRAN(incl. Ev. HSPA) GERAN
Control Interface User Data Interface Control Interface User Data Interface
AGW MME
eNode B eNode B
S1 - CP
3G SGSN 3G SGSN
Access Selection
Inter-RAT mobility with CDMA2000
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Ericsson Internal LTE Mobility17
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)
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Ericsson Internal LTE Mobility18
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)
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Ericsson Internal LTE Mobility19
LTE to EV-DO handover
PDN GW
S1-MME
S10
MME
S11
UE
E-UTRAN
S7
SGi S1-U
PCRF
Operator's IP services
Rx+
S2a
S101
RNC/PCF PDSN
CDMA2K EV-DO
Serving GW
S5
Han
dove
rco
mm
and
Acc
ess
Similar solution is used for CS fallback
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Ericsson Internal LTE Mobility20
LTE to EV-DO handoverComments
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
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Ericsson Internal LTE Mobility21
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
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Ericsson Internal LTE Mobility22
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