Dr. Stefan Brück Qualcomm Corporate R&D Center Germany 3G/4G Mobile Communications Systems
Dr. Stefan BrückQualcomm Corporate R&D Center Germany
3G/4G Mobile Communications Systems
Chapter IX: Mobility Control
2
Chapter IX: Mobility Control
Slide 2
Mobility Control
� Handover Types
� Mobility Measurements in UMTS
� Mobility Procedures for HSDPA and HSUPA (E-DCH)
� Mobility Measurements in LTE
� X2/S1 based Mobility Procedures in LTE
3 Slide 3
Cell (Re-)Selection and Handover
� Cell (Re-)Selection� Procedure that allows the UE to
change the cell it is camped on
� (E)-UTRAN provides parameters to control (re-)selection
� Handover (Hand-off)� Procedure that allows the UE to
change from one cell to another,
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change from one cell to another, while the UE has radio resources allocated to it
Slide 4
Types of Cell (Re-)Selection and Handover
� Intra-Frequency� Within the same carrier frequency
� Inter-Frequency� Between different carrier frequencies
� Inter-RAT� Between different Radio Access technologies (RAT), e.g. from UMTS
FDD to GSM
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FDD to GSM
� Soft handover� Multiple radio links exist to cells of different Node Bs
� Softer handover� Multiple radio links exist to cells of the same Node B
� Hard handover� Existing radio links are dropped before a new link is established
Slide 5
� General: Mechanism of changing a cell or base station during a call or session
� UE may have active radio links to more than one Node B
� Mobile-assisted & network-based handover in UMTS:� UE reports measurements to UTRAN if reporting criteria (which are set by the
UTRAN) are met
� UTRAN then decides to dynamically add or delete radio links depending on the measurement results
Handover Control: Basics
6
measurement results
� Types of Handover:� Soft/Softer Handover (dedicated channels)
� Hard Handover (shared channels)
� Inter Frequency (Hard) Handover
� Inter System Handover (e.g. UMTS-GSM)
� Cell selection/re-selection (inactive or idle)
� All handover types require heavy support from the UMTS network infrastructure!
Slide 6
� In soft/softer handover the UE maintains active radio links to more than one Node B
� Combination of the signals from multiple active radio links is necessary
� Soft Handover� The mobile is connected to (at least) two cells belonging to different Node Bs� In uplink, the signals are combined in the RNC,
e.g. by means of selection combining using CRC
� Softer Handover
Soft/Softer Handover
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� Softer Handover� The mobile is connected to two sectors within one Node B� More efficient combining in the uplink is possible like
maximum ratio combining (MRC) in the Node B instead of RNC
� Note: In uplink no additional signal is transmitted, while in downlink each new link causes interference to other users, therefore:� Uplink: HO general increase performance� Downlink: Trade-off
Slide 7
Soft Handover – Example: UMTS
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Combining and Selecting• UE combines symbols received from each Node B• RNC selects the best radio frame from each Node B
Multiple Node Bs are involved
Slide 8
Softer Handover – Example: UMTS
Only one Node B, but multipleCells are involved
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Combining and Selecting� UE combines symbols
received from each cell� Node B combines symbols
received from each cell
Slide 9
Hard Handover – Example: UMTS
10
Reasons for Hard Handover� Inter-frequency handover� Inter-RAT handover� Shared transport channels
Slide 10
� Measurement quantity, e.g. EC/I0 on CPICH
� Relative thresholds δ & MeasurementMeasurement
NodeB 1NodeB 1 NodeB 2NodeB 2UEUE
soft handoversoft handoverareaarea
Example: Soft Handover Control – UMTS
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� Relative thresholds δadd & δdrop for adding & dropping
� Preservation time Tlink to avoid “ping-pong” effects
� Event triggered measurement reporting to decrease signalling load
MeasurementMeasurementQuantityQuantity
Link to 1Link to 1 Link to 1 & 2Link to 1 & 2 Link to 2Link to 2 timetime
CPICH 1CPICH 1
CPICH 2CPICH 2
δδdropdrop
δδaddadd TTlinklink
Slide 11
UMTS Soft/Softer Handover in Practice
12 Slide 12
10%
15%
20%
25%O
uta
ge
Pro
bab
ilit
y(B
lock
ing
an
d D
rop
pin
g)
1 link
max 2 SHO links
max 4 SHO links
max 6 SHO links
Soft Handover – Simulation Results
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Soft handover significantly improves the performance, but …
0%
5%
10%
5 15 25 35 45 55
Offered Traffic [Erlang per site]
Ou
tag
e P
rob
abil
ity
(Blo
ckin
g a
nd
Dro
pp
ing
)
max 6 SHO links
Slide 13
1
1,5
2M
ean
Nu
mb
er o
f A
ctiv
e L
inks
Soft Handover – Simulation Results II
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… the overhead due to simultaneous connections becomes higher!
0
0,5
1 2 4 6
Max. Active Set Size
Mea
n N
um
ber
of
Act
ive
Lin
ks
Slide 14
Hierarchical cell structure (HCS)
Macro Micro Macro
f1
f2
f1
Hot-spot
f1
f2
f1 f1
Hot spot
Inter-Frequency Handover in UMTS
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Handover f1 ⇔ f2 always needed between layers
f1 f1
Handover f1 ⇔ f2 needed sometimes at hot spot
f1 f1 f1
� Hard handover
� Inter-frequency measurements of target cell needed in both scenarios
Slide 15
Measurement Control and Reporting in UMTS
� Categories of Cells� Active Set
� Cells for which a radio links is established between UE and UTRAN
� UE is in soft/softer handover with all cells in the active set
� Monitored (Neighbor) Set� UTRAN instructs UE to perform measurements on a list of cell in the geographic
neighborhood
� All such cells that are not in the active set are in the monitored set
� Most likely candidates for soft/softer handover
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� Most likely candidates for soft/softer handover
� Detected Set� All other cells which UE has detected and measured
� The UE may report such cells to the UTRAN to be added to the monitored set
� Reporting is either event-triggered or periodic� Periodic reports generate a high load on the uplink
� Event triggered reporting is therefore usually preferred
Slide 16
Parameters for Event Triggered Reporting (UMTS)
� Events as a function of the Measurement Types� Intra-frequency: Events 1a to 1f
� Inter-frequency: Events 2a to 2f
� Inter-RAT: Events 3a to 3d
� Each event is associated with a set of parameters � What cells can trigger the events?
� Absolute and relative threshold
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� Time-to-Trigger
� Time-to-Trigger� Interval between event detection and report sent
� Time-to-Trigger interval ranges between 0 and 5 seconds
Slide 17
Intra-Frequency Reporting Events (UMTS)
� Event 1a: A P-CPICH enters the reporting range� Used to indicate to UTRAN when a new cell should be added to the active set
� Event 1b: A P-CPICH leaves the reporting range� Used to indicate to UTRAN when a new cell should be removed from the active set
� Event 1c: A non-active P-CPICH becomes better than an active P-CPICH� Used to indicate to UTRAN to replace a cell in the active set with a different cell
(active set is full)
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(active set is full)
� Event 1d: Change of best cell� Used for changing cells in HSDPA
� Event 1e: A P-CPICH becomes better than an absolute threshold� Used to indicate to UTRAN when a new cell should be added to the active set
� Event 1f: A P-CPICH becomes worse than an absolute threshold� Used to indicate to UTRAN when a new cell should be removed from the active set
Slide 18
Event 1a: A P-CPICH enters the Reporting Range
19 Slide 19
Event 1c: Active Set is full
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� If Event 1c is received the UTRAN should replace the weakest active set with the new rising cell
� Event 1c may be configured such that the UE begins using periodic reporting if the UTRAN does not send active set update message
Slide 20
HSDPA Mobility Procedures I
� HS-DSCH for a given UE belongs to only one of the radio links assigned to the UE (serving HS-DSCH cell)
� The UE uses soft handover for the uplink, the downlink DCCH and any simultaneous CS voice or data
� Using existing triggers and procedures for the active set update (events 1A, 1B, 1C)
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(events 1A, 1B, 1C)
� Hard handover for the HS-DSCH, i.e. Change of Serving HS-DSCH Cell within active set
� Using RRC procedures, which are triggered by event 1D
Slide 21
HSDPA Mobility Procedures II
MAC-hs
MAC-hs
Source HS-DSCH Node B
Target HS-DSCH Node B
CRNC CRNC
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� Inter-Node B serving HS-DSCH cell change
� Note: MAC-hs needs to be transferred to new Node B !
NodeB
NodeB
NodeB
NodeB
Serving HS-DSCH radio link
Serving HS-DSCH radio link
s t
Slide 22
HS-DSCH Serving Cell Change
Measurement quantity
CPICH 2
CPICH 1 Hysteresis
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� Event 1D: change of best cell within the active set
� Hysteresis and time to trigger to avoid ping-pong(HS-DSCH: 1…2 dB, 0.5 sec)
Reporting event 1D
Time
CPICH3 Time to trigger
Slide 23
HSDPA Handover Procedure SRNC
= DRNC
Target HS-DSCH cell
UE
RL Reconfiguration Prepare
RL Reconfiguration Ready
Radio Bearer Reconfiguration
Radio Bearer Reconfiguration Complete
Source HS-DSCH cell
ALCAP Iub HS-DSCH Data Transport Bearer Setup If new NodeB
Synchronous Reconfiguration with Tactivation RL Reconfiguration Commit
Reset MAC-
Serving HS-DSCH cell change decision i.e. event 1D
RL Reconfiguration Prepare
RL Reconfiguration Ready
RL Reconfiguration Commit
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Radio Bearer Reconfiguration Complete
ALCAP Iub HS-DSCH Data Transport Bearer Release
DATA
Reset MAC-hs entity
Slide 24
� The RNC determines the activation time for the serving HS-DSCH cell change� Time is populated by the RNC
� In the RRC message TRANSPORT CHANNEL RECONFIGURATION to the UE
� In the NBAP message RADIO LINK RECONFIGURATION COMMIT to the involved Node Bs
� At this time the Node B commits and the UE activates a new transport channel configuration for HS-DSCH serving cell change
� After the transport channel configuration is completed the UE sends the RRC message RADIO BEARER RECONFIGURATION COMPLETE
E-DCH Operation in Soft Handover
UEUE
scheduling grantHARQ ACK/ NACK
scheduling grantHARQ ACK/ NACK
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� Macro-diversity operation on multiple Node Bs� Softer handover combining in the same Node B� Soft handover combining in RNC (part of MAC-es)
� Independent MAC-e processing in both Node Bs� HARQ handling rule: if at least one Node B tells ACK, then ACK� Scheduling rule: relative grants “DOWN” from any Node B have precedence
NodeB 1NodeB 1 NodeB 2NodeB 2
Slide 25
EDCH Mobility Handling
� The UE uses soft handover for associated DCH as well as for E-DCH� Using existing triggers and procedures for the active set update
(events 1A, 1B, 1C)
� E-DCH active set is equal or smaller than DCH active set� New event 1J: non-active E-DCH link becomes better than active one
� The UE receives AG on E-AGCH from only one cell out of the E-DCH active set (serving E-DCH cell)� E-DCH and HSDPA serving cell must be the same
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� E-DCH and HSDPA serving cell must be the same
� Hard Handover, i.e. change of serving E-DCH cell
� Using RRC procedures, which maybe triggered by event 1D� Could be also combined with Active Set Update
Slide 26
EDCH Mobility Procedures
MAC-e
MAC-e
SRNC SRNC
MAC-es MAC-es
MAC-e MAC-e
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� Inter-Node B serving E-DCH cell change within E-DCH active set
� Note: MAC-e still established in both Node Bs !
NodeB
NodeB
NodeB
NodeB
Serving E-DCH radio link
Serving E-DCH radio link
s t
Slide 27
Serving E-DCH Cell Change SRNC
= DRNC
Target serving E-DCH cell
UE
RL Reconfiguration Prepare
RL Reconfiguration Ready
Radio Bearer Reconfiguration
Source serving E-DCH cell
If new NodeB
Synchronous Reconfiguration with Tactivation RL Reconfiguration Commit
Serving E-DCH cell change decision i.e. event 1D
RL Reconfiguration Prepare
RL Reconfiguration Ready
RL Reconfiguration Commit
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Radio Bearer Reconfiguration
Radio Bearer Reconfiguration Complete
RL Reconfiguration Commit
UE receives now AG & dedicated RG from target cell
� Handover of E-DCH scheduler control� No changes in UL transport bearer� No MAC-es RESET
� Handover of HS-DSCH serving cell� DL transport bearer setup� MAC-hs RESET
Slide 28
Mobility Measurement Reporting in LTE
� LTE mobility measurements are similar as in UMTS� Event triggered
� Event triggered with periodic reporting
� Periodic reporting
Event Purpose
A1 Serving cell becomes better than an absolute threshold
29 Slide 29
A2 Serving cell becomes worse than an absolute threshold
A3 Neighbor E-UTRA cell becomes an offset better than the serving cell
A4 Neighbor E-UTRA cell becomes better than an absolute threshold
A5 Serving cell becomes worse than an absolute threshold AND neighborE-UTRA cell becomes better than another absolute threshold
B1 Inter-RAT neighbor cell becomes better than an absolute threshold
B2 Serving cell becomes worse than an absolute threshold AND inter-RAT neighbor cell becomes better than an absolute threshold
Intra-LTE Handover Types
� LTE supports two types of handover signaling� X2 based handover
� S1 based handover
� From the air interface perspective� Handovers are hard
� Procedure is identical for intra- and inter-frequency� Random access is required for synchronization
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� From the core network perspective, handover type depends on the network topology� Intra- or Inter-MME/S-GW handover
Slide 30
LTE Handover
� LTE uses UE-assisted network controlled handover� UE reports measurements; network decides when handover and to which cell
� Relies on UE to detect neighbor cells → no need to maintain and broadcast neighbor lists� Allows "plug-and-play" capability; saves BCH resources
� For search and measurement of inter-frequency neighboring cells only carrier frequency need to be indicated
� X2 interface used for handover preparation and forwarding of user data
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� Target eNB prepares handover by sending required information to UE transparently through source eNB as part of the Handover Request Acknowledge message� New configuration information needed from system broadcast
� Accelerates handover as UE does not need to read BCH on target cell
� Buffered and new data is transferred from source to target eNB until path switch →prevents data loss
� UE uses contention-free random access to accelerate handover
Slide 31
LTE Handover: Preparation Phase
UEUESource
eNB
Source Source
eNB
Measurement Control
Target
eNB
Target
eNBMMEMME sGWsGW
Packet Data Packet Data
UL allocation
Measurement Reports
HO decision
HO Request
L1/L2
signaling
L3 signaling
User data
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Admission Control
HO Request
HO Request Ack
DL allocation
RRC Connection Reconfig.
� HO decision is made by source eNB based on UE measurement report
� Target eNB prepares HO by sending relevant info to UE through source eNB as part of HO request ACK command, so that UE does not need to read target cell BCH
SN Status Transfer
Slide 32
LTE Handover: Execution Phase
UEUESource
eNB
Source Source
eNB
Target
eNB
Target
eNBMMEMME sGWsGW
Detach from old cell,
sync with new cellDeliver buffered packets and forward
new packets to target eNB
DL data forwarding via X2
Synchronisation
L1/L2
signaling
L3 signaling
User dataBuffer packets from
source eNB
Packet Data
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Synchronisation
UL allocation and Timing Advance
RRC Connection Reconfig. Complete
Packet Data
� RACH is used here only so target eNB can estimate UE timing and provide timing advance for synchronization
� RACH timing agreements ensure UE does not need to read target cell P-BCH to obtain SFN (radio frame timing from SCH is sufficient to know PRACH locations)
UL Packet Data
Slide 33
LTE Handover: Completion Phase
UEUESource
eNB
Source Source
eNB
Target
eNB
Target
eNBMMEMME sGWsGW
DL Packet Data
Path switch req
User plane update req
Switch DL path
DL data forwarding
Packet Data
End Marker
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User plane update responsePath switch req ACK
Release resources
Packet Data Packet Data
L1/L2
signaling
L3 signaling
User data
Flush DL buffer,
continue delivering
in-transit packets
End Marker
Release resources
Slide 34
LTE Handover: Illustration of Interruption Period
UL
U- plane active
UEUESource
eNB
Source
eNB
Target
eNB
Target
eNB
UL
U- plane active
UEs stops
Rx/Tx on the old cell
DL synchronisation+
Timing advance
DL sync
+ RACH (no contention)
+ Timing Adv
HO Request
HO Confirm
HandoverLatency
(approx 55 ms)approx
Measurement
Report
HO Command
Handover Preparation
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U- plane activeU- plane active
Timing advance+
UL resource request/grant
+ Timing Adv
+ UL Resource Req and Grant
ACK
Latency(approx 55 ms)
approx 20 ms
HO Complete
HandoverInterruption(approx 35 ms)
Slide 35