Download - Principles of Handover in WCDMA
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Huawei Confidential
WCDMA Handover Principle
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Why mobile systems need handover?
Mobile systems are composed by cells
which the coverage is limited.
The mobility of the UE.
Providing the continuous service - the
basic element in QoS.
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TS25.331 Flow of Handover and
Related Parameters
TS25.133 Measurement Process
TS25.215 Definitions of Measurement
Indexes
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After studying this course, you will:
Understand basic concepts about
handover
Understand the flow of soft handover
Master basic parameters about soft
handover
Master soft handover decision algorithms
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Chapter 1 Overview of Handover
Chapter 2 Handover Measurement
Chapter 3 Soft Handover
Chapter 4 Hard Handover
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Chapter 1 Overview of Handover
1.1 Purposes of Handover
1.2 Modes and Status of the UE
1.3 Methods and Types of
Handover
1.4 Comparison between Soft
Handover and Hard Handover
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Purposes of Handover
Providing the continuous service in mobile system is the basic
element in QoS
Providing continuous communication service is the main purpose
of handover.
The load balance: sharing the resource
The hierarchy divided by speed and service: high efficiency of
using resource
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Chapter 1 Overview of Handover
1.1 Purposes of Handover
1.2 Modes and Status of the UE
1.3 Methods and Types of
Handover
1.4 Comparison between Soft
Handover and Hard Handover
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CELL_DCH CELL_FACH
CELL_PCHURA_PCH
IDLEDEAD RRC connection
UE Modes and StatusUE Modes and Status
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CELL_DCH CELL_FACH
CELL_PCHURA_PCH
IDLE
DEAD
- Searching a PLMN- Find a cell to camps on
- Monitering the paging channel- Cell reselecting
- Dedicated channel - Common channel, low speed service- Triggered by theCN
- Reducing activities, saving power and resource
RRC connection
These statuses are only applied in the UTRAN and UE. They are transparent to the CN.
UE Modes and Status (Status migration)UE Modes and Status (Status migration)
SMSVP/Voice/PS
Low speed PS
Cell Update No trafficAfter several Cell Updates
URA Update
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Chapter 1 Overview of Handover
1.1 Purposes of Handover
1.2 Modes and Status of the UE
1.3 Methods and Types of Handover
1.4 Comparison between Soft
Handover and Hard Handover
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Handover Methods
Changing cells in the status of CELL_DCH
Handover
Directly retry
Changing residing cells in the statuses of IDLE, CELL_FACH,
CELL_PCH, and URA_PCH
Cell reselection (forward handover)
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Handover Types in CELL_DCH
Classify by System Change
Classify by Signaling Flow
Classify by Frequency
ChangeDescription
Intra- system
Soft handover
Intra-frequency
Low-rate services (e.g. AMR, VP)
Hard handover
High-rate services (e.g. PS384K)
No Iur interface between to RNCs
Inter-frequency
For the purpose of coverage or load balancing (e.g. handover between carriers with different load, handover to the carrier that supports the HSDPA service.
WCDMA-to-GSM handover)
Inter-system (Inter-RAT)
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Chapter 1 Overview of Handover
1.1 Purposes of Handover
1.2 Modes and Status of the UE
1.3 Methods and Types of Handover
1.4 Comparison between Soft
Handover and Hard Handover
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Features Comparison between Soft Handover and Hard Handover
Item Soft Handover (Softer Handover) Hard Handover
Number of radio links in the activate set
Several One
Interruption during handover
No Yes
Frequencies in the cells before and after
handoverIntra-frequency
Intra-frequency
Inter-frequency or
Inter-RAT
Handover gainMaximum-ratio combining or selective combining which can reduce the impact of fading and transmission power of the UE.
None
DisadvantageOccupies more resources.
Problems occur when the power of soft handover cells is unbalanced.
Call drop ratio is relatively higher
In a word, some resources are wasted to obtain optimal system performance in soft handover.
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Soft Handover – (Connection before Disconnection)
RNCNodeB1 NodeB2
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Softer Handover – (A Special Case of Soft Handover)
RNCNode B
Cell A Cell B
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Hard Handover- (Disconnection before Connection)
RNCNodeB1 NodeB2
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Questions
What are the differences between
soft handover, softer handover, and
hard handover?
Why intra-frequency handover is not
always soft handover?
How does handover gain arise? How
much are various handover gains?
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Summary
This chapter describes the purposes of handover, working modes of the UE and common handover types in the WCDMA system, compares soft handover and hard handover and describes respective advantages and disadvantages.
In addition, this chapter describes a brief flow of each type of handover.
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Chapter 1 Overview of
Handover
Chapter 2 Handover
Measurement
Chapter 3 Soft Handover
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Chapter 2 Handover Measurement
2.1 Basic Concepts about
Handover
2.2 Three Steps of Handover
2.3 Measurement Control
2.4 Measurement Report
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Basic Concepts about Handover
Active set
Monitor set
Detection set
Event report
Event to periodic report
Periodic report
Radio link (RL)
Radio link set (RLS)
Maximum-ratio combining
Selective combining
Soft handover gain
Pilot channel (CPICH)
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Chapter 2 Handover Measurement
2.1 Basic Concepts about Handover
2.2 Three Steps of Handover
2.3 Measurement Control
2.4 Measurement Report
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Three Steps of Handover
Decision
Execution
Measurement
The following figure shows handover procedure (measurement quality set to PCPICH Ec/Io)
UE measures the CPICH Ec/Io of its serving cell(s) and neighbor cells. And sends the measurement report to RNC.
UE measures the CPICH Ec/Io of its serving cell(s) and neighbor cells. And sends the measurement report to RNC.
Are handover criteria meet?Are handover criteria meet?
Perform a handover and update relative parameters
Perform a handover and update relative parameters
Measure
Decision
Execution
Y
N
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Chapter 2 Handover Measurement
2.1 Basic Concepts about Handover
2.2 Three Steps of Handover
2.3 Measurement Control
2.4 Measurement Report
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Measurement Control
Measurement control is used to notify the UE of the object to be measured, neighbor cell list, report method, and event parameters.
When measurement conditions are changed, the RNC notifies the UE of new conditions.
Measurement Control, normal case
UE UTRAN
Measurement Control
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Measurement Control
Measurement Control
The RNC sends a
measurement control message
according to the best cell in the
active set.
− Measurement identity
− Measurement command
− Measurement type
− Measurement object
− Criteria of measurement report
− Measurement report mode
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Measurement Objects
Measurement objects
Intra-frequency measurement
− CPICH RSCP, CPICH Ec/Io, or Pathloss (normally Ec/Io)
Inter-frequency measurement
− CPICH RSCP, CPICH Ec/Io
Inter-RAT measurement
− GSM Carrier RSSI, BSIC Identification, and BSIC Reconfirmation
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Measurement Model
Measurement time of layer 1 is 200 ms. The vendor can decide the filtering method.
Filtering parameters of layer 3 can be configured.
Measurement results are reported when the reporting criteria are met.
Layer 1filtering
Layer 3filtering Evaluation
of reportingcriteria
A DB C
C'
parameters parameters
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Measurement Model - Layer 3 Filtering
Filtering of measurement results by the UE is calculated according to the following formula:
Fn = (1-α)Fn-1+αMn
Fn: new measurement result after L3 filtering
Fn-1: the last measurement result after filtering
Mn: the latest measurement result from the physical layer of the UE
α = 0.5k/2, K is the parameter in filter coefficient in the measurement control message (default=3 ). If it is set to 0 , layer 3 filtering is unavailable.
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Chapter 2 Handover Measurement
2.1 Basic Concepts about
Handover
2.2 Three Steps in Handover
2.3 Measurement Control
2.4 Measurement Report
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Measurement Report
When the measurement reporting criteria are met, the UE
reports the results to the RNC as an event.
Measurement Report
UE UTRAN
Measurement Report
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Measurement Report Methods
Event report
The measurement report is sent when the reporting criteria are met.
Periodic report (event to periodic report)
If the RNC does not respond after several events are reported, the
UE switches to periodic report.
The interval between report and total report times are controlled
through parameters.
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Intra-frequency Measurement Events
Intra-frequency measurement events are identified through 1X:
1A: A CPICH signal from monitor set enters the reporting range. Means a
monitor cell is good enough that the UE can add it into active set.
1B : A CPICH signal in active set leaves the reporting range. Means a cell
in the active set is not good enough according to the best cell.
1C: Replace event. A CPICH signal from a monitor set is better than one in
the active set.
1D: Event of changing the best serving cell.
1F: Event that measurement results of the cells in the active set are lower
than an absolute threshold.
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Inter-frequency Measurement Events
Inter-frequency measurement events are identified by 2X:
2B event: The quality of the serving cell is lower than an absolute
threshold value and the quality of another inter-frequency neighbor
cell is higher than another threshold value. - Handover based on
Coverage
2C event: The quality of the inter-frequency neighbor cell is higher
than an absolute threshold value. - Handover based on Non-Coverage
(e.g. Load)
2D event: The quality of the serving cell is lower than an absolute
threshold value. It is used to start the compress mode.
2F event: The quality of the serving cell is higher than an absolute
threshold value. It is used to stop the compress mode.
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Inter-RAT Measurement Events
Inter-RAT Measurement events are identified through 3X:
3A event: The quality of the currently used frequency is lower than an
absolute threshold value, but the quality in a GSM cell is higher than
another absolute threshold value. - Handover based on Coverage
3C event: The quality in a GSM cell is higher than an absolute threshold
value. - Handover based on Non-Coverage (e.g. Load)
2D event: The quality of the currently used frequency is lower than an
absolute threshold value. It is used to start the compress mode.
2F event: The quality of the currently used frequency is higher than
another absolute threshold value. It is used to stop the compress mode.
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Some Event Report Control Parameters Relative threshold (reporting range)
Unidirectional difference between two measurement results, for example,
relative threshold value between 1A event and 1B event
Absolute threshold (threshold)
Measurement results reach an absolute value, for example, absolute
threshold values of 1E event and 1F event
Hysteresis
Bi-directional difference between two measurement results
Time to trigger
Time that lasts until the previous threshold value is met
Weight factor (W)
Control calculation of comparative benchmark values
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Event Report Decision
Deciding according to the judgment inequation
Example: decision inequation of 1A event
If W = 0, for RSCP and Ec/Io,
10LogM(New)≥10LogM(Best) - (R1a-H1a/2)
M: measurement results
R: relative threshold values
H: hysteresis
W: weight factor. It is 0 generally.
),2/(10)1(1010 11
aBest
N
iiNew HRLogMWMLogWLogM
A
2. Other measurement variables (e.g. RSCP, Ec/Io)
),2/(10)1(1010 11
aBest
N
iiNew HRLogMWMLogWLogM
A
1. Path loss
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Event Report Decision - Example of 1A Event Measurement Report
Measurement results in the primary cell
1A event occurs
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Event Report Decision (Continued)
Trigger criteria of 1B event:
)2/(10)1(1010 111
bbBest
N
iinew HRLogMWMLogWLogM
A
,2/11 ffNew HTM Trigger criteria of 1F event:
,2/1dBestNotBest HMM Trigger criteria of 1D event:
,2/1cInASNew HMM Trigger criteria of 1C event:
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Event Report Decision - Example of 1B Event Measurement Report
1B event is reported
Delete the cell which primary scrambling code is 23
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Event Report
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Event Report
Replace cell 1 by the cell 2
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Event Report
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Questions
In which cases does the RNC trigger
measurement control?
Is the start time of intra-frequency,
inter-frequency, and Inter-RAT
measurement the same?
When is an event reported?
How to report an event?
What events does intra-frequency
measurement contain?
What events does inter-frequency
measurement contain?
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Summary
This chapter describes concepts related to handover, explains measurement control, which covers measurement objects, event report.
Page 48HUAWEI TECHNOLOGIES CO., LTD. Huawei Confidential
Chapter 1 Overview of Handover
Chapter 2 Handover
Measurement
Chapter 3 Soft Handover
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Chapter 3 Soft Handover
3.1 Flow of Soft Handover
3.2 Common Parameters and Intra
Frequency Neighbor Cell
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Flow of Soft Handover - Adding Radio Link
(Reporting 1A event)
(Add new radio link)
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Flow of Soft Handover - Adding Radio Link
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Flow of Soft Handover - Deleting Radio Link
(Reporting 1B event)
(Del old radio link)
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Chapter 3 Flow of Soft Handover
3.1 Flow of Soft Handover
3.2 Common Parameters and Intra
Frequency Neighbor Cell
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Setting Common Parameters of Soft Handover Relative threshold
1A and 1B event have their individual values.
The value of 1B should be larger than that of 1A, to avoid ping-pong
handover. (1A < 1B)
− Default setting: 1A: 3 dB; 1B: 6 dB
Hysteresis
Configure it for each event separately.
Usually the threshold values of 1A event and 1B event are 0 dB and those
of 1C, 1D, 1F are 4 dB.
This value affects the soft handover area of the UE.
MML command for adding cell-class intra-frequency handover parameters: ADD CELLINTRAFREQHOMML Command for setting RNC-class intra-frequency handover parameters: SET INTRAFREQHO
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Common Parameters of Soft Handover (Continued)
Time to trigger
Configured it for each event separately.
The value of 1B should be larger than that of 1A, to avoid ping-pong
handover. (1A < 1B)
− Default setting: 1A: 320 ms; 1B: 640 ms; 1C, 1D: 640 ms
Lay 3 filtering coefficient
It is shared by all types of intra-frequency measurement.
This parameter is sensitive to the delay of event triggering and ping-
pong handover.
It is usually set to 3.
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Intra Frequency Neighbor Cell
The Handover can only happened among the neighbor cells
MML: Add IntraFreqNCell:
Configure neighbor cells with the same frequency
The maxim number of IntraFreqNCell for each cell is 31
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Questions
Draw the signaling flow chart of soft
handover.
What is the impact of parameters in
soft handover on system performance?
What parameters can affect
performance of soft handover besides
the parameters described in this
chapter?
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Summary
This chapter describes the signaling
flow and common parameters of soft
handover in WCDMA.
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Other Handovers
The WCDMA system supports many handover technologies.
Besides soft (softer) handover described previously, there are
intra-frequency hard handover, inter-frequency hard handover,
and Inter-RAT handover.
If the system supports HSDPA, the WCDMA system also
supports handover between carrier frequency of HSDPA and
ordinary carrier frequency to balance load and to utilize network
resources efficiently.
Refer to supplementary contents for description of intra-
frequency hard handover, inter-frequency hard handover, and
Inter-RAT handover. This part is provided for self-study.
Page 60HUAWEI TECHNOLOGIES CO., LTD. Huawei Confidential
Course Summary
Course Summary
This course describers the principles, algorithm, related parameters and general flow of soft handover in the WCDMA system.
Thank you
www.huawei.com
Page 62HUAWEI TECHNOLOGIES CO., LTD. Huawei Confidential
Chapter 4 Hard Handover
4.1 Intra-frequency Hard Handover
4.2 Inter-frequency Hard Handover
4.3 Inter-RAT Hard Handover
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Intra-frequency Hard Handover - Measurement and Event
Measurement
It is similar to that in soft handover
Related events
1D event
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Intra-frequency Hard Handover - Handover Flow
TARGET NODEB SOARCE NODEB
3. ALCAP ESTABLISHMENT
9. ALCAP RELEASE
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Intra-frequency Hard Handover - Common Parameters BeBitrateThd: BE service handover rate decision threshold.
If the maximum rate of transmission channel of the BE service is not more than this threshold, the system performs soft handover for the users using this service.
If the maximum rate of transmission channel of the BE service is greater than this threshold, the system performs intra-frequency hard handover for the users using this service.
MML: SET HOCOMM
Parameters related to the 1D event
Time to trigger (640ms), hysteresis (4dB)
The parameters should be appropriate so the system can not only track changes of the signal to perform handover, but also avoid ping-pong handover.
Page 66HUAWEI TECHNOLOGIES CO., LTD. Huawei Confidential
Chapter 4 Hard Handover
4.1 Intra-frequency Hard
Handover
4.2 Inter-frequency Hard
Handover
4.3 Inter-RAT Hard
Handover
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Inter-frequency Hard Handover Overview
Features
The frequency is different after handover.
For the UE with only one transceiver, the compress mode should be
started to assist measurement.
Advantages
The handover success rate is higher than that of intra-frequency hard
handover.
The load on carrier frequencies are balanced.
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Inter-frequency Hard Handover Overview (Continued)
Disadvantages
The compress mode causes extra radio resource
Application scenarios
Uncontinuous coverage of carrier
Load caused handover
Hierarchical cell
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Inter-frequency Hard Handover - Measurement and Event
Measurement in inter-frequency hard handover
Measurement variables:
− CPICH RSCP, CPICH Ec/N0
Different measurement types are used for different handover purposes:
− Edge of carrier coverage area: CPICH RSCP
− Center of carrier coverage area: CPICH Ec/Io
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Inter-frequency Hard Handover - Measurement and Event (Continued)
Measurement Report Methods
Event report
− 2D event: The quality of current frequency is lower than a certain absolute
threshold value. It is used to start the compress mode.
− 2F event: The quality of current frequency is higher than a certain absolute
threshold value. It is used to stop the compress mode.
− 2B event: The quality of current frequency is lower than the absolute
threshold value, and the quality of the neighbor frequency is higher than
another absolute threshold value. It is used in coverage handover trigger.
− 2C event: The quality of the neighbor frequency is higher than a certain
absolute threshold value. It is used in load handover trigger.
Periodic report
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Inter-frequency Hard Handover - Starting and Stopping of the Compression Mode Starting conditions of measurement
2D event: If the quality of the currently frequency is lower than a certain absolute threshold value, the compress mode is started.
Use the velocity estimation algorithm to decide starting of inter-frequency handover measurement.
Stopping conditions of measurement 2F event
After the best cell is changed, the new best cell does not have any inter-frequency neighbor cell.
The timer of inter-frequency measurement expires.
The speed estimate status of the UE changes, and related inter-frequency handover measurement is disabled according to the hierarchical cell algorithm
In hard handover, if the coverage reason is triggered, handover is stopped simultaneously.
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Inter-frequency Hard Handover Flow
Signaling flow
9. ALCAP RELEASE
3. ALCAP ESTABLISHMENT
SOARCE NODEBTARGET NODEB
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Inter-frequency Hard Handover - Common Parameters Inter-frequency coverage handover parameters:
Methods for report inter-frequency measurement: periodic or event
Inter-frequency measurement variables: CPICH Ec/Io or CPICH RSCP.
Inter-frequency measurement layer 3 filtering coefficient, time to trigger, and hysteresis
Starting and stopping threshold values of inter-frequency measurement: Configure it respectively for CPICH Ec/Io, CPICH RSCP, and configure 2D, 2F event for CS and PS.
Inter-frequency coverage handover threshold value: quality threshold value of the target cell
Quality threshold value of used frequency in inter-frequency hard handover
Lowest access threshold value in inter-frequency handover
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Inter-frequency Hard Handover - Questions Why the compress mode threshold for the CS and PS are
configured different?
The Eb/No required by these two services are different, so the RSCPs are also different.
If the UE uses the CS service and PS service at the same time, the 2D and 2F parameters of which service are used to decide starting the compress mode?
If the parameters of any service meet the threshold, the compress mode will started.
Is the compress mode started immediately after the UE sends the 2D parameter?
The compress mode is started after the RNC reconfigure the RB.
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Chapter 4 Hard Handover
4.1 Intra-frequency Hard Handover
4.2 Inter-frequency Hard Handover
4.3 Inter-RAT Hard Handover
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Inter-RAT Hard Handover
Overview
Measurement and Event
Handover Decision Algorithm
Handover Flow
Common Parameters
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Inter-RAT Hard Handover - Overview Category
WCDMA FDD < - >GSM WCDMA FDD < - >WCDMA TDD WCDMA FDD < - >CDMA2000
Features The radio access technology (RAT) adopted in the system after handover
is different. Need compress mode to assist measurement.
Advantages Coverage: Solve the problem of transition between different systems. Capacity: Improve the utilization of old equipments to the greatest extent
(2G->3G)
Disadvantages The flow is complicated, and the requirement for compatibility of
equipment is high. The UE is complicated.
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Inter-RAT Hard Handover - Measurement and Event
Measurement in Inter-RAT hard handover (GSM measurement)
Measurement types:
− GSM Carrier RSSI
− BSIC Identification
− BSIC Reconfirmation
Measurement processing: layer-1 filtering, layer-3 filtering
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Inter-RAT Hard Handover - Measurement and Event (Continued)
Measurement Report
− Event report
▪ 2D event: starting GSM measurement
▪ 2F event: stopping GSM measurement
▪ 3A event: The quality of WCDMA is lower than an absolute
threshold but quality of a GSM neighbor cell is higher than
another absolute threshold. - Handover based on Coverage
▪ 3C event: The quality of a GSM neighbor cell is higher than an
absolute threshold. - Handover based on Non-Coverage (e.g. Load)
Page 80HUAWEI TECHNOLOGIES CO., LTD. Huawei Confidential
Inter-RAT Hard Handover - Decision Algorithm
Decision of Inter-RAT handover
Inter-RAT handover based on coverage
− Event
▪ 3A event: The quality of the currently used frequency is lower
than an absolute threshold value, but quality of a GSM cell is
higher than another absolute threshold value.
− Periodic
▪ Evaluation: Confirm the status according to the GSM RSSI
measurement value and the BSIC of the target cell in GSM and
evaluate the cell of which the GSM RSSI exceeds the absolute
threshold value. If both conditions are met, consider the cell of
which the BSIC is confirmed.
Page 81HUAWEI TECHNOLOGIES CO., LTD. Huawei Confidential
Inter-RAT Hard Handover - Decision Algorithm (Continued)
Inter-RAT handover based on non-coverage
− Event
▪ The 3C event means that the quality of the GSM cell is
higher than an absolute threshold value.
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Inter-RAT Handover Flow
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Inter-RAT Handover - Common Parameters
Inter-RAT coverage handover parameters
Starting and stopping thresholds of Inter-RAT measurement: They
determine the thresholds of the 2D event and 2F event. Configure CPICH
Ec/Io, CPICH RSCP for the CS service and PS service respectively.
Measurement variables for the 2D event and 2F event in Inter-RAT
handover: Select CPICH Ec/Io or PICH RSCP.
BSIC selection confirmation switch
Inter-RAT coverage handover threshold: GSM RSSI threshold in Inter-RAT
coverage handover. Configure it for the CS service and PS service
respectively.
Quality threshold of used frequency in Inter-RAT handover
Configure time to trigger and hysteresis for each event respectively
Penalty duration
Page 84HUAWEI TECHNOLOGIES CO., LTD. Huawei Confidential
Summary - 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-RAT handover (UMTS ↔ GSM/GPRS)
According to the purpose of handover:
− Based on Coverage
− Based on Noncoverage