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Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
WCDMA Handover Principle
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Foreword
Why mobile systems need handover? It is decided by the characters of mobile system:
The mobility of UE The mobile system is composed by cells which the cov
erage ability is limited. Providing the continuous service in mobile system is t
he basic element in QoS.
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Objectives
Upon completion of this course, you will be able to:
The basic definitions of Handover
The algorithms of handover decision
The handover flow
The parameters of handover
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Contents
1. Introduction of Handover
2. Measurement of Handover
3. The Basic Handovers
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The Purpose of Handover
Providing the continuous service in mobile system is the basic element in QoS.
The load balance: sharing the resource The hierarchy divided by speed and service: high efficiency of u
sing resource
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The Basic Concepts of Handover Active Set Monitored Set Detected set Event reporting
Event reporting to Periodic reporting Periodic reporting Radio Link (RL) Radio Link Set (RLS) Combination way:
maximum ratio combination selection combination
The soft handover gain CPICH Soft handover, softer handover, hard handover
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The Categories of Handover According to the signaling characters:
Soft handover (softer handover) Hard handover
According to the properties of source cell and target cell Intra-frequency handover Inter-frequency handover Inter-system handover (UMTS <-> GSM)
According to the purpose of handover Based on Coverage Based on Load (Optional) Based on mobility of UE (Optional) Based on Service (Optional)
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The Characters of Different Handovers
Item Soft Handover Hard Handover
The numbers of RL in
active set after
handover
Several One
Interruption during
handover
No Yes
The frequencies of
cells
Only happened in
Intra-frequency
cells
Can be happened in Intra-
frequency cells or Inter-
frequency cells
Comparison between soft handover and hard
handover:
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The Characters of Different Handovers Comparison between soft handover and softer hando
ver During softer handover, the uplink signaling are combined in
NodeB by maximum ratio combination, but during soft handover they are combined in RNC by selection combination
Compare to later one, the maximum ration combination can get more gain. So the performance of maximum ration combination is better
Since softer handover is completed in NodeB, it do not consume transport resource of Iub
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RNC
NodeB 2NodeB 1
Soft Handover
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RNC (WFMR)
NodeB
Softer Handover
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Hard Handover
RNC
NodeB 2NodeB 1
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Three Steps of Handover
Decision
Execute
Measurement
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Questions
The differences among Soft handover, softer
handover and hard handover
Typical application scenarios
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Summary
This chapter focus on the purpose of handovers and
the categories of handover in WCDMA.
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Contents
1. Introduction of Handover
2. Measurement of Handover
3. The Basic Handovers
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Contents
2. Measurement of Handover
2.1 Measurement control and measurement report
2.2 The basic definitions of measurement
2.3 Measurement event
2.4 Compressed mode
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Measurement Control, normal case
UE UTRAN
MEASUREMENT CONTROL
Measurement Control
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Measurement Control
Measurement Control, normal case
UE UTRAN
MEASUREMENT CONTROL
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Measurement Control, normal case
UE UTRAN
MEASUREMENT REPORT
Measurement Report
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Contents
2. Measurement of Handover
2.1 Measurement control and measurement report
2.2 The basic definitions of measurement
2.3 Measurement event
2.4 Compressed mode
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The Basic Concepts of Measurement The measurement values of Handover
Soft handover and Intra-frequency hard handover : CPICH RSCP, CPICH Ec/N0, Path loss
Inter-frequency : CPICH RSCP, CPICH Ec/N0 Inter-system : GSM Carrier RSSI, BSIC Identification, BSIC Reconfirmati
on The reporting methods of measurement
Periodic reporting Event reporting
The events of reporting Intra-frequency events : 1A,1B,1C,1D,1F Inter-frequency events : 2D,2F,2B,2C Inter-system events : 3A,3C
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Measurement Model
Layer 1filtering
Layer 3filtering
Evaluationof reporting
criteria
A DB C
C'
parameters parameters
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Reporting Criterion
),2/(10)1(1010 11
aBest
N
iiNew HRLogMWMLogWLogM
A
),2/(10)1(1010 11
aBest
N
iiNew HRLogMWMLogWLogM
A
Reporting Criterion Decision formula: for example, 1A event : Path Loss
Other measurement :
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Key Parameters
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Contents
2. Measurement of Handover
2.1 Measurement control and measurement report
2.2 The basic definitions of measurement
2.3 Measurement event
2.4 Compressed mode
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Intra-frequency Measurement Events Intra-frequency measurement report events are identified
with 1X 1A : A primary pilot channel enters the reporting range. When
the active set of UE is full, UE stops reporting 1A event; 1B : A primary pilot channel leaves the reporting range; 1C : The primary pilot channel in a non active set is better tha
n the primary pilot channel in an active set; 1D : The best cell changes; 1E : The measurement value of a primary pilot channel excee
ds the absolute threshold
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Inter-frequency Measurement Events Inter-frequency measurement events are identified with
2X 2B : The estimated quality value of the used frequency is lo
wer than a certain threshold, and that of the non-used frequency is higher than a certain threshold;
2C : The estimated quality value of the non-used frequency is higher than a certain threshold;
2D : The estimated quality value of the used frequency is lower than a certain threshold
2F : The estimated quality value of the used frequency is higher than a certain threshold
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Inter-system Measurement Events Inter-system measurement events are identified with 3X
3A : The estimated quality value of the used UTRAN frequency is lower than a certain threshold, and that of the other system is higher than a certain threshold
3C : The estimated quality value of the other system is higher than a certain threshold
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Contents
2. Measurement of Handover
2.1 Measurement control and measurement report
2.2 The basic definitions of measurement
2.3 Measurement event
2.4 Compressed mode
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The Purpose of Compressed mode Purpose
Measure the inter-frequency cell or inter-system cell under FDD mode
Categories Downlink compressed Uplink compressed
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Compressed Mode Sketch Map
One frame(10 ms) Transmission gap available for
inter-frequency measurements
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The realization Methods of Compressed mode Realization Methods
SF/2 Rate matching/puncturing Higher layer scheduling
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Questions
How many events are there in intra-frequency
measurement? What are they?
How many events are there in UE internal
measurement?
How many events are there in inter-frequency
measurement? What are they?
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Summary
This chapter covers intra-frequency measurement,
inter-frequency measurement, inter-system
measurement and UE internal measurement from
their definitions and application scenarios
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Contents
1. Introduction of Handover
2. Measurement of Handover
3. The Basic Handovers
Page37Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Contents
3. The Basic Handovers
3.1 Soft Handover
3.2 Intra-frequency Hard Handover
3.3 Inter-frequency Hard Handover
3.4 Inter-system Hard Handover
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Brief Introduction of Soft Handover Characters of Soft Handover
During handover, UE has several RLs with different cells - active set
The handover among different cells which are in same RLS can be softer handover
Soft handover:– Selection combination in uplink
– Maximum combination in downlink
Softer handover– Maximum combination in uplink and downlink
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Brief Introduction of Soft Handover Advantages
Soft handover gain Load balance Decrease the possibility of call drop caused by ping-pong handove
r
Disadvantages More resource needed in downlink, especially for the code resourc
e of BE service Usually, the gain of downlink power is negative When the downlink power from different cells is not balanced, it wi
ll bring side-effect
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Measurement of Soft Handover
The measurement of soft handover/softer handover Measurement value : CPICH RSCP 、 CPICH Ec/No 、 Pathlo
ss Process of Measurement : Layer 1 filter, Layer 3 filter Reporting way
Periodic reporting Event reporting
– Event type : 1A 、 1B 、 1C 、 1D 、 1F
– UE measures the time difference between CFN and SFN
– Reporting rules: trigger condition, Relative threshold, Absolute threshold, Hysteresis, Time to trigger
Event reporting to periodic reporting
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The Decision Algorithm of Soft Handover Soft/softer handover decision
1A : Radio Link Addition 1B : Radio Link Removal 1C : Combined Radio Link Addition and Removal 1D : change best cell
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The Execution of Soft Handover Soft handover execution
The update principal of measurement control Configure the neighbor cells and the parameters according to b
est cell RLC mode
AM mode is used for measurement control UM mode is used for measurement report
The compensation and restriction of soft handover From event reporting to periodic reporting
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Soft Handover Flow (add RL)
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Soft Handover Flow (del RL)
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Key Parameters
Relative threshold Time to trigger Layer 3 filter coefficient Absolute threshold of soft handover
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Contents
3. The Basic Handovers
3.1 Soft Handover
3.2 Intra-frequency Hard Handover
3.3 Inter-frequency Hard Handover
3.4 Inter-system Hard Handover
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Brief Introduction of Intra-frequency Hard Handover Characters
UE only has one RL with one cell in the process of handover Disconnect UE with the original cell, then hand it over to target cell The working frequency of source cell and target cell are same
Advantages Enhance the using efficiency of the OVSF code and hardware resour
ce Disadvantages
High call drop possibility because of the intra-frequency interference
Application scenarios Without Iur interface between two RNCs or the Iur interface jam (onl
y happened in inter-RNC handover) The different strategies can be used for different conditions, such a
s code resource condition, the QoS condition and so no
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The Measurement, Decision and Execution of Intra-frequency Hard Handover Measurement
Similar to soft handover Decision
1D event Execution
UE reports CFN-SFN information Synchronization hard handover Use the original DOFF value Continuous CFN
UE does not report CFN-SFN information The hard handover based on timing Re-configure DOFF Calculate CFN according to DOFF
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The Measurement, Decision and Execution of Intra-frequency Hard Handover Measurement
Similar to soft handover Decision
1D event Execution
UE reports CFN-SFN information Synchronization hard handover Use the original DOFF value Continuous CFN
UE does not report CFN-SFN information The hard handover based on timing Re-configure DOFF Calculate CFN according to DOFF
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Intra-frequency Hard Handover Flow
UE
1.RADIO LINK SETUP REQUEST
Target NODEB
Source NODEB SRNC
2. RADIO LINK SETUP RESPONSE
3.ALCAP establish
4. PHYSICAL CHANNEL RECONFIGURATION
5. RADIO LINK FAILURE INDICATION
6. PHYSICAL CHANNEL RECONFIGURATION COMPLETE
7. RADIO LINK DELETION REQUEST
8. RADIO LINK DELETION RESPONSE
9.ALCAP release
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Key Parameters
Handover decision threshold based on BE speed UE should do soft handover when the speed of BE service is
less than the threshold. UE should do intra-frequency hard handover when the spee
d of BE service is greater than the threshold.
The parameters about 1D event Time to trigger, hysteresis The parameters should be set accord with the Qos
Page52Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Contents
3. The Basic Handovers
3.1 Soft Handover
3.2 Intra-frequency Hard Handover
3.3 Inter-frequency Hard Handover
3.4 Inter-system Hard Handover
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The Brief Introduction of Inter-frequency Hard Handover
Characters The working frequency is different after handover The compressed mode needed if the UE only has one
receiver Usually, the timing re-initiation hard handover is used
for hard handover
Application scenarios Disconnected coverage Handover based on load Hierarchy cells
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Measurement Values and Events The Inter-frequency hard handover measurement values
Measurement values CPICH RSCP 、 CPICH Ec/N0
Different handover purpose for different measurement type In the edge of carrier coverage: CPICH RSCP In the center of carrier coverage: CPICH Ec/No
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Compressed Mode Initiation Conditions for measurement initiated
2D event : The estimated quality value of the used frequency is lower than a certain threshold
Conditions for Measurement stopping 2F event : The estimated quality value of the used frequency
is higher than a certain threshold
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Cells Hierarchy Algorithm Based on UE Moving Speed
When UE is in one of the hierarchy cells, the moving speed estimation algorithm is initiated
Handover events in a while decides whether the speed of UE satisfies the current cell hierarchy condition
UE’s moving speed decided the result
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Decision Algorithm
The inter-frequency hard handover decision The coverage triggers the handover
2B event :– the quality in the used frequency is lower than absolute threshold, bu
t the quality in another non-used frequency is higher than another absolute threshold.
The load triggers handover 2C event :
– the quality in another frequency is higher than an absolute threshold
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Inter-frequency Hard Handover Signaling Flow
UE
1.RADIO LINK SETUP REQUEST
Target NODEB
Source NODEB SRNC
2. RADIO LINK SETUP RESPONSE
3.ALCAP established
4. PHYSICAL CHANNEL RECONFIGURATION
5. RADIO LINK FAILURE INDICATION
6. PHYSICAL CHANNEL RECONFIGURATION COMPLETE
7. RADIO LINK DELETION REQUEST
8. RADIO LINK DELETION RESPONSE
9.ALCAP released
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Key Parameters (I) Inter-frequency coverage handover parameters
Inter-frequency measurement reporting: periodic reporting or event reporting
Inter-frequency measurement values: CPICH Ec/No or CPICH RSCP Inter-frequency measurement layer 3 filter coefficient, delay
trigger time and hysteresis Inter-frequency initiated and stopped threshold: Considering the
different demands of CPICH Ec/No and CPICH RSCP for PS domain and CS domain, the different 2D and 2F parameters are configured.
Inter-frequency handover based on coverage: the quality threshold of target cell
Using frequency quality threshold in inter-frequency hard handover
The minimum access threshold for inter-frequency handover
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Key Parameters (II)
Inter-frequency handover parameters caused by non-coverage ability
Inter-frequency measurement layer 3 filter coefficient, delay trigger time and hysteresis
Inter-frequency handover based on non-coverage ability
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Contents
3. The Basic Handovers
3.1 Soft Handover
3.2 Intra-frequency Hard Handover
3.3 Inter-frequency Hard Handover
3.4 Inter-system Hard Handover
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The Brief Introduction of Inter-system Hard Handover
Application scenarios WCDMA FDD < - >GSM
Characters Different radio access technology is used after handover Usually, compressed mode should be used to help the measure
ment Advantages
For coverage, it can solve the problems from one system to another system
For capacity, it can enhance the utilizing efficiency of old equipments(2G->3G)
Disadvantages The flow is complicated, and it demands higher compatibility for
equipments Demands more complicated UE
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Compressed Mode Initiated The inter-system measurement (GSM measurement)
Measurement type:
GSM Carrier RSSI
BSIC Identification
BSIC Reconfirmation
The process of measurement: layer 1 filter, layer 3 filter
Measurement reporting
Event reporting
– 2D Event: initiate GSM measurement
– 2F Event: stop GSM measurement
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Decision Algorithm Inter-system handover caused by coverage
Event reporting: 3A event : The estimated quality value of the used UTRAN freq
uency is lower than a certain threshold, and that of the other system is higher than a certain threshold
Periodic reporting: Evaluation : According to the periodic reported GSM RSSI mea
surement value and the BSIC confirming state of target cell of GSM system, and meanwhile the UE evaluates the GSM RSSI of target cell is greater than the absolute threshold, then consider the cell confirmed by BSIC
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Decision Algorithm Inter-system handover caused by non-coverage
Event reporting
3C event: The estimated quality value of the other
system is higher than a absolute threshold
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1. Relocation Required 2. Prepare
Handover 3. Handover
Request
4. Handover Request Ack
12. Iu Release Command
13. Iu Release Complete 14. Send End
Signal Response
5. Prepare Handover Response
6. Relocation Command
7. DCCH : Handover from UTRAN Command
8. Handover
Detect
9. Handover Complete
10. Handover Complete
11. Send End
Signal Request
UE Node B SRNC CN MSC BSC
Inter-system Handover Flow
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Key Parameters (I) Inter-system handover caused by coverage parameters
Inter-system measurement initiated and stopped threshold: Considering the different demands of CPICH Ec/No and CPICH RSCP for PS domain and CS domain, the different 2D and 2F parameters are configured
Inter-system measurement values (2D, 2F) CPICH Ec/No CPICH RSCP
BSIC confirmed (Optional) Inter-system handover caused by coverage parameters
configure the GSM RSSI threshold of CS domain and PS domain separately
Using inter-system frequency quality handover threshold Configure the delay trigger time, hysteresis for each event
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Key Parameters (II)
Inter-system handover caused by non-coverage
parameters
The period of inter-system measurement report
The decision threshold for inter-system handover:
configure the CS, PS, signaling separately
Time delay trigger : configure BSIC with
acknowledged and unacknowledged mode
Inter-frequency measurement layer 3 filter
coefficient and hysteresis
Penalty time
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Questions
What are the differences between soft handover and
softer handover?
What is compressed mode?
Draw out the hard handover signaling flow.
Page70Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Summary This chapter focus on the basic handovers in WCDMA:
soft handover, softer handover, intra-frequency hard
handover, inter-frequency hard handover and inter-
system hard handover. It also introduced the
application scenarios for these different handovers
Meanwhile, compressed mode is introduced in this
chapter
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