3g/4g mobile communications systems - universität … & 4g mobile... · mobility control...

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Dr. Stefan Brück Qualcomm Corporate R&D Center Germany 3G/4G Mobile Communications Systems

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Page 1: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

Dr. Stefan BrückQualcomm Corporate R&D Center Germany

3G/4G Mobile Communications Systems

Page 2: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

Chapter IX: Mobility Control

2

Chapter IX: Mobility Control

Slide 2

Page 3: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

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

Page 4: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

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,

4

change from one cell to another, while the UE has radio resources allocated to it

Slide 4

Page 5: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

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

5

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

Page 6: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

� 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

Page 7: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

� 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

7

� 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

Page 8: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

Soft Handover – Example: UMTS

8

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

Page 9: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

Softer Handover – Example: UMTS

Only one Node B, but multipleCells are involved

9

Combining and Selecting� UE combines symbols

received from each cell� Node B combines symbols

received from each cell

Slide 9

Page 10: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

Hard Handover – Example: UMTS

10

Reasons for Hard Handover� Inter-frequency handover� Inter-RAT handover� Shared transport channels

Slide 10

Page 11: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

� 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

11

� 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

Page 12: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

UMTS Soft/Softer Handover in Practice

12 Slide 12

Page 13: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

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

13

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

Page 14: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

1

1,5

2M

ean

Nu

mb

er o

f A

ctiv

e L

inks

Soft Handover – Simulation Results II

14

… 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

Page 15: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

Hierarchical cell structure (HCS)

Macro Micro Macro

f1

f2

f1

Hot-spot

f1

f2

f1 f1

Hot spot

Inter-Frequency Handover in UMTS

15

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

Page 16: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

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

16

� 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

Page 17: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

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

17

� 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

Page 18: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

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)

18

(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

Page 19: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

Event 1a: A P-CPICH enters the Reporting Range

19 Slide 19

Page 20: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

Event 1c: Active Set is full

20

� 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

Page 21: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

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)

21

(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

Page 22: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

HSDPA Mobility Procedures II

MAC-hs

MAC-hs

Source HS-DSCH Node B

Target HS-DSCH Node B

CRNC CRNC

22

� 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

Page 23: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

HS-DSCH Serving Cell Change

Measurement quantity

CPICH 2

CPICH 1 Hysteresis

23

� 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

Page 24: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

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

24

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

Page 25: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

E-DCH Operation in Soft Handover

UEUE

scheduling grantHARQ ACK/ NACK

scheduling grantHARQ ACK/ NACK

25

� 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

Page 26: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

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

26

� 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

Page 27: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

EDCH Mobility Procedures

MAC-e

MAC-e

SRNC SRNC

MAC-es MAC-es

MAC-e MAC-e

27

� 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

Page 28: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

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

28

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

Page 29: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

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

Page 30: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

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

30

� From the core network perspective, handover type depends on the network topology� Intra- or Inter-MME/S-GW handover

Slide 30

Page 31: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

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

31

� 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

Page 32: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

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

32

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

Page 33: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

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

33

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

Page 34: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

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

34

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

Page 35: 3G/4G Mobile Communications Systems - Universität … & 4G Mobile... · Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH)

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

35

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