LTE Mobility by tfGBNfa


									LTE Mobility
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
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
Radio requirements

 The characteristics of the LTE radio system will lead to
  quite tough requirements on the intra-LTE mobility
     – No soft handover
     – Reuse 1 (potential for strong interference at street corners
     – 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
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 will                       Cell 2 is X dB better than Cell 1
        no longer be able to                                      (handover is initiated)
    communicate with terminal

            Cell 3

  Mobility procedure need to finish                                           time
     in shorter time than this 6
                    Ericsson Internal                LTE Mobility
Handover Preparation Time in LTE
                                     Source                             Target
                                    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.

                                 2. Handover
                                                                                 •        Once the measurement report
                                   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
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
   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
Intra-LTE mobility solution
Basic principles

 eNB controls the handover (network controlled
     – 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
 UE                                        Source eNB                                    Target eNB                                      Core Network
                                                                    UL/DL Data
                  DL allocation
      7. Handover Command (incl. pre-amble)
                                                                                                                  L3 signalling

                                                                                                                 L1/L2 signalling
                                  Deliver buffered and in transit
                                      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
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
     – 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
Inter-RAT mobility with

   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

                   Ericsson Internal   13        LTE Mobility
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 --
  handover                       Gb
                                                                 Iu-UP / S4 -UP

 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
Terminal Steering
                                                                                                                    Packet Data Networks

                                                               Control Interface                                           Gi
                                                               User Data Interface
                                                                                                                         P/S -GW
 The basic concept is that the                                          AGW-UP

  terminal is assigned a list with the                                 S3 (Gn)
                                                                                      S4 -CP(Gn)                          S11

                                        2G SGSN
  priorities of the different RATs and   2G SGSN       3G SGSN
                                                       3G SGSN       MME
                                                                     AGW --
                                                                     AGW      Iu-CP

  frequency bands (based on                               Gb
                                                                                       Iu-UP / S4 -UP

                                          BSC     RNC
  “subscription” information)             BSC     RNC
                                                                                                                 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
Inter-RAT mobility with

   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
       –   Driven mainly by Verizon Wireless, but also KDDI, US Cellular,
       –   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

                  Ericsson Internal   17        LTE Mobility
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
LTE to EV-DO handover

                                RNC/PCF               EV-DO       PDSN

                                                                                               Operator's IP
   UE                                                                                         Rx+


                                                            S11                         S7
            E-UTRAN                                                 Serving             PDN
                                                                     GW                 GW

                               Similar solution is used for CS fallback
                      Ericsson Internal         19                       LTE Mobility
LTE to EV-DO handover

 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
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
     – 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

                Ericsson Internal   21       LTE Mobility
Ericsson Internal   22   LTE Mobility

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