Handover in WCDMA

Handover in WCDMA

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Course Contents

Chapter 1 Overview

Chapter 2 Handover parameters

Chapter 3 Handover procedure

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Introduction

Handover is the most important part of mobility management .

The purpose of handover is that a UE in CELL_DCH state is

served continuously as it moves.

There are

• Intra-frequency handovers,

• Inter-frequency handovers,

• Inter-system , a.k.a. Inter-Radio Access Technology (RAT),

handovers.

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Intra-Frequency Handovers

Softer handover

Handover between two sectors of the same NodeB (handled by NodeB itself)

Maximum Ratio Combining (MRC) occurs in both the UL and DL

Soft handover (SHO)

Handover between two sectors of the different NodeBs (handled by RNC)

MS is simultaneously connected to multiple cells from different Node Bs

DL: MRC by UE & UL: Frame Selection Combining by RNC.

Hard handover

Occurs when

• inter-RNC SHO is not possible (Iur not supported or Iur congestion)

• Some high speed data service is required/requested, e.g. HSPA

Decision procedure is the same as SHO (i.e. RNC controlled)

Causes temporary disconnection of the user

Handover types

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Handover types

Inter-Frequency Handover

Can be intra-NodeB, intra-RNC, inter-RNC,

Decision algorithm is located in and performed by RNC.

Inter-RAT Handover

Handovers between GSM and WCDMA

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Comparison between Soft and Softer

Handovers

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Comparison between Soft and Hard

Handovers

The situation in hard handover. Cells are using the same frequency:

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Handovers (cont.)

The situation in soft handover. Cells are using the same frequency:

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Handovers (cont.)

Item Soft Handover Hard Handover

The number of cells in

the active set

Several (normally

up to 3, but max.

could be 6)

Only one

Interruption during

handover

No Yes

The frequencies of

cells

Only happens in

Intra-frequency

cells

Can happen in Intra-

frequency cells or Inter-

frequency cells

Utilization of downlink

and transmission

resources

Occupies more

resources

Occupies less resources

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Handovers (cont.)

Soft handover scenario Hard handover scenario

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Handovers (cont.)

Controlling RNC (C-RNC): A role an RNC can take with respect to a specific

set of Node B's.

There is only one Controlling RNC for any Node B.

The Controlling RNC has the overall control of the logical resources of its

Node B's.

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Handovers (cont.)

Serving RNC (S-RNC): A role an RNC can take with respect to a specific

connection between an UE and UTRAN.

There is one Serving RNC for each UE that has a connection to UTRAN.

The Serving RNC is in charge of the radio connection between a UE and the

UTRAN.

The Serving RNC terminates the Iu for this UE.

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Handovers (cont.)

Drift RNC (D-RNC): A role an RNC can take with respect to a serving RNC.

It provides macro-diversity combining and splitting.

There could be more than one Drift RNC for a UE.

It performs transparent routing of data on Iub and Iur interfaces.

Maximum Ratio Combination

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• Each signal reception branch is multiplied by a weight factor that is

proportional to the signal amplitude.

• Branches with a stronger signal are further amplified, while weak

signals are attenuated.

Frame Selection Combination

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• Decodes the signal received by each cell individually and selects only

one of the correctly decoded blocks for further processing by higher

layers.

• Compared to Frame selection combination, Maximum ratio

combination provides both diversity and power gain.

• Maximum ratio combination has a gain of 2-3 dB relative to Frame

selection combination.

Frame Selection Combination (cont.)

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Page 17

Three Steps of Handover

Decision

Execute

Measurement

Measurement PhaseMeasurement control vs. Measurement report messagesMeasurement execution and the result processingMainly accomplished by UE

Decision PhaseBased on Measurement reports from UEConsiders distribution ofresourcesMainly accomplished by RRM in RNC

Execution PhaseBased on signalingShould support rollback in case of failureMeasurement control refresh

Measurement Control vs. Message Report

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Measurement Control: From network to UE

Measurement Report: From UE to network


(cont.)

Page 19

Intra-frequency soft(er) handover


(cont.)

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Hard handover (intra-frequency or inter-frequency)

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Course Contents

Chapter 1 Overview



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2.1 Intra-frequency handover

2.2 inter-frequency handover

2.3 inter-system handover

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Intra-frequency Measurement Events

Event 1A，A primary pilot channel enters the reporting range. When

the active set of UE is full, UE stops reporting 1A event;

Event 1B，A primary pilot channel leaves the reporting range;

Event 1C，A primary pilot channel out of active set is better

than a primary pilot channel in the active set;

Event 1D，The best cell in the active set changes;

Event 1F , A primary pilot in the active set becomes worse than an

absolute threshold

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Measurement parameters

Report range or relative threshold: As higher it is, it is easier to

trigger a particular event or vice-versa.

Hysteresis: Related to slow-fading characteristics. The greater it

is, the less ping-pong effect and misjudgement can be caused.

However, this would bring a delay for triggering the event.

Time to Trigger: Also related to slow-fading characteristics. Has

similar characteristics as hysteresis.

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Measurement parameters (cont.)

Cell-individual Offsets: used as an offset to control promoting or

avoiding handover to a particular cell.

Layer3 filter coefficient: controls the impact of each cell in the

active set on the triggering decision.

The greater it is, the higher the soft handover relative threshold.

When equal to 0, only the best cell in the active set could impact

on the decision.

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Condition for measurement reporting:

• Periodic Reporting,

• Event Reporting.

Metric to be measured:

• CPICH Ec/No,

• CPICH RSCP.

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Measurement parameters and conditions are transferred within

Measurement Control messages:

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Triggering of Event 1A

Event 1A，A primary pilot channel enters the reporting range. When

the active set of UE is full, UE stops reporting 1A event.

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Triggering of Event 1A (cont.)

From UE point of view, WCDMA cells are divided into

• Active Set cells: Cells with which there is a radio link established

through soft(er) handover,

• Monitored Set cells: Neighbours of the cells in the active set. They

could be intra-frequency, inter-frequency and/or inter-system,

• Detected Set cells: Other cells than the above that are detected.

Unless implemented otherwise, total number of cells is limited to 32

for each set.

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Event 1A Chart

Reporting

event 1A

Measurement

quantity

Time

Time-to-trigger

P CPICH 1Reporting

range

P CPICH 2

P CPICH 3

Best cellBest cell-Relative threshold

Event 1A from TEMS

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Triggering of Event 1B

Event 1B，A primary pilot channel leaves the reporting range.

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Event 1B Chart

Best cellBest cell-Relative threshold

Event 1B from TEMS

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Impact of Parameters for Events 1A and 1B

Report Range or Relative threshold• Set values for Events 1A and 1B separately,

• 1A < 1B，which makes deleting RL is more difficult, and it can avoid

ping-pong handover,

• Usually 1A: 3dB; 1B: 6dB,

• Relative thresholds are set separately for CS and PS services!

Time to trigger• Value for each event can be set separately,

• Usually, 1B>1A，which makes deleting RL is more difficult, and it can

avoid ping-pong handover,

• Usually, 1A: 320ms, 1B: 640ms.

Layer 3 filter coefficient• Only one value for all intra-frequency measurements,

• Sensitive to the delay of event trigger and ping-pong handover,

• Usually：3

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Triggering of Event 1C

Event 1C，A primary pilot channel out of active set is better

than a primary pilot channel in the active set;

It can only be triggered if the active set is full. Otherwise, Event 1A

is triggered!

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Event 1C Chart

Reporting

event 1C

Reporting

event 1C

Measurement

quantity

Time

P CPICH 2

P CPICH 1

P CPICH 3

P CPICH 4

Event 1C from TEMS

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Page 41

Triggering of Event 1D

Event 1D，The best cell in the active set changes;

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Event 1D Chart

Reporting

event 1D

Measurement

quantity

Time

P CPICH 2

P CPICH 1

P CPICH3

Event 1D from TEMS

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Page 44

Triggering of Event 1F

Event 1F，A primary pilot in the active set becomes worse than an

absolute threshold;

If there is no more primary pilot left in the active set, RNC may

decide for a blind handover of UE to an inter-frequency cell, if

available.

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Inter-frequency handover overviewInter-frequency handover can be triggered based on

• Coverage: for continuity of calls while avoiding drops.

• Load and/or service: for balancing the traffic between the

cells and moving certain users/applications to the other

carrier in case required, e.g. UMTS carrier vs. HSPA carrier.

• Speed: In what so called Hierarchical Cell Structure (HCS),

cells are divided into macro and micro cells. Those users

with higher speeds stay at macro cells whereas those with

lower speed are served by micro cells.

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Inter-frequency Measurement Events

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Event 2D，When the current signal quality is lower than the preset

threshold, the system enables compressed mode and starts inter-

frequency measurement

Event 2D from TEMS

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After Event 2D

After receiving event 2D, the RNC would perform either

• Periodical measurement reporting, or,

• Event 2B triggered measurement reporting,

depending on if inter-frequency measurement reporting mode is

chosen as periodical or event based.

Inter-frequency handover mode will continue till event 2F is received

or inter-frequency measurement timer expires!

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After Event 2D (cont.)

Periodical measurement reporting is more recommended than to

event triggered measurement reporting.

It leads to a higher handover success ratio since even if handover

fails, RNC tries handover to the same cell upon reception of a

periodical report from UE.

RNC makes the handover decision based on the periodic

measurement reports provided by UE.

Compressed Mode

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UE cannot operate on two different frequencies at the same time.

Compressed mode is required to be able to match with the

frequency of a cell that uses another frequency than the one

currently used by the serving cell.

There are two possible ways of moving to compressed mode:

• Spreading factor reduction, i.e. SF/2,

• Higher layer scheduling.

Periodical Measurement

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Inter-frequency handover based on periodical measurement

requires holding following conditions during hard handover trigger

delay time:

1.CPICH RSCP and Ec/No of the inter-frequency cell should be

higher than the corresponding minimum access thresholds,

2.

Periodical Measurement (cont.)

Page 54

Inter-frequency handover based on periodical measurement

is stopped if:

1.Either CPICH RSCP or Ec/No of the inter-frequency cell falls

below the corresponding minimum access threshold,

2.The following condition is no longer satisfied:

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Triggering of Event 2F

Event 2F，When the current signal quality is higher than the preset

threshold, the system disables compressed mode and stops inter-

frequency measurement

Event 2F from TEMS

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Triggering of Event 2B

Event 2B，When the current signal quality is lower than the preset

threshold and the signal quality of an inter-frequency neighbouring

cell is also higher than the preset threshold, the system triggers an

inter-frequency handover based on coverage.

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Event 2C，The estimated quality of an inter-frequency cell is higher

than the preset threshold.

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Inter-system handover overviewApplication scenario: WCDMA <－> GSM

• Different radio access technology is used after handover,

• From WCDMA to GSM, UE switches to compressed mode to be

able to perform measurement process in GSM.

Advantages

• For coverage, it can solve the problems from one system to another

system,

• For capacity, it can enhance the utilizing efficiency of equipments in

terms of load (GSM->WCDMA).

Disadvantages

• The flow is complicated, and it demands higher compatibility for

equipments,

• Demands more complicated UE.

Page 61

Inter-system handover overview (cont.)

When UE is in CELL-DCH state, moving between these two

technologies is called as handover for CS services whereas as

cell reselection for PS services.

WCDMA to GSM Handover could be based on:

• Coverage: When CPICH RSCP or CPICH Ec/No of the serving

cell is lower than a threshold while RSSI of the GSM neighbour

cell is higher than the corresponding threshold,

• Load: The load of serving cell is higher than a threshold,

• Service: When requested service could be supplied by GSM

network.

Page 62

Inter-system handover overview (cont.)

If handover is based on coverage, then it is either periodical or

based on Event 3A reporting.

However, if handover is based on load or service, then it is based

on Event 3C reporting.

Confirmation of BSIC of the GSM cell is optional. However, it would

make handover more reliable.

Periodical Measurement

Page 63

Inter-system handover based on periodical measurement

requires holding of following condition during handover trigger

delay time:

Page 64

Event reporting

Event 2D: initiate GSM measurement.

Event 2F: stop GSM measurement.

Event 3A : The estimated quality of the used UTRAN frequency is lower

than a certain threshold, and that of the other system is higher than a

certain threshold.

Event 3C: The estimated quality value of the other system is higher

than an absolute threshold.

Inter-system handover decision

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Event 2D，When the current signal quality is lower than the preset

threshold, the system enables compressed mode and starts

measuring inter-system cells:

Event 2D from TEMS

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After Event 2D

After receiving event 2D, the RNC would perform either

• Periodical measurement reporting or

• Event 3A triggered measurement reporting

depending on if inter-system measurement reporting mode is

chosen as periodical or event based.

Inter-frequency handover mode will continue till event 2F is received

or inter-frequency measurement timer expires!

Page 68

Triggering of Event 3A

Event 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.

Event 3A from TEMS

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Event 3C : The estimated quality value of the other system is higher

than a absolute threshold.

Page 71

Course Contents

Chapter 1 Overview



Page 72





Page 73

Signaling Flow for Soft Handover (add RL)

Page 74

Signaling Flow for Soft Handover (del RL)

Page 75

Signaling Flow for Intra-frequency

Hard Handover

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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Page 77

Signaling Flow for Inter-frequency

Hard Handover (same RNC)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

Page 78

Signaling Flow for Inter-frequency

Hard Handover (different RNC)

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Page 80

Signaling Flow for WCDMA to GSM

Handover in CS Domain

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

Page 81

Signaling Flow for WCDMA to GSM

Handover in PS Domain

GSM -> WCDMA Handover and Cell

Reselection

Page 82

In Motorola BSS, GSM -> WCDMA Inter-RAT Handover and Cell-Reselection

functionalities are controlled by the features:

• GSM BSS Inter-RAT Handover

• FR 22879 Enhanced 2G-3G Handovers and Cell Reselection


Reselection (cont.)

Page 83

GSM BSS Inter-RAT Handover feature enables a multi-RAT MS:

– to perform cell reselection from GSM to WCDMA while in idle mode,

– to perform handover from a WCDMA cell to a GSM cell while in

dedicated mode.


Reselection (cont.)

Page 84

FR 22879 Enhanced 2G-3G Handovers and Cell Reselection feature provides

a multi-RAT MS:

– to perform handovers while in dedicated mode from a GSM cell to a

WCDMA cell,

– idle mode cell reselection functionality by providing blind search

support.

For this purpose, WCDMA frequencies are broadcast in the System

Information messages.


Reselection (cont.)

Page 85

On the other hand, MSC may trigger service-based handovers to WCDMA in

case requested service is not available in GSM.

To enable Inter-RAT Handover and Cell-Reselection between

a GSM and WCDMA cell, they should be defined as neighbours.

Depending on if a WCDMA cell is defined as BCCH and/or SACCH

neighbor during neighbor addition with add_neighbour command,

umts_ba_type parameter gets a value.


Reselection (cont.)

Page 86

• inter_rat_enabled: It can have values from 0 to 7, where setting of 7

allows idle and bi-directional dedicated handovers and cell reselections. In

fact, setting 0 to 3 for this parameter require the first feature enabled,

whereas 4 to 7 require the second feature enabled.

• blind_search_preference: decides for if blind search is to be performed

in the idle mode or not. If it is set, it reduces SI messages in DL, but

increases the time to synchronize with WCDMA cells.

• qsearch_i: it controls the RXLEV of the serving cell in the idle mode, i.e.

BCCH level, below or above which measurements on WCDMA neighbour

cells are triggered.

BSS-CELL Parameters


Reselection (cont.)

Page 87

• qsearch_c: it determines the RXLEV of the serving cell in the dedicated

mode, i.e. SACCH level, below or above which measurements on WCDMA

neighbour cells are triggered.

• qsearch_c_initial: till a valid qsearch_c is available, MS will use

qsearch_c_initial value. When it is equal to 0, it means to use qsearch_c

equal to qsearch_i.

• fdd_rep_quant: it decides for if the signal level or quality of a WCDMA

neighbor would be considered for handover.

BSS-CELL Parameters (cont.)


Reselection (cont.)

Page 88

• fdd_qoffset: minimum threshold in idle mode for RSCP measurements.

• fdd_qmin: minimum threshold in idle mode for Ec/No measurements.

• umts_cpich_ec_no_min: minimum threshold in dedicated mode for

Ec/No measurements.

• umts_cpich_rscp_min: minimum threshold in dedicated mode for RSCP

measurements.



Reselection (cont.)

Page 89

• umts_ncell_avg_period: it sets the period of SACCH multiframes for the

averaging of WCDMA neighbour cell measurements.

• umts_meas_margin: is defined per neighbour to promote or avoid

handovers and cell reselections to a particular cell.

• umts_band_preferred: decides for the selection of a neighbor based on

its radio access technology if both GSM and WCDMA qualify.



Reselection (cont.)

Page 90

• Service Handover: Normally this parameter belongs to MSC. It controls

handovers mandated by the MSC based on service. For example, it is

possible to handover all CS calls from WCDMA to GSM upon call setup. It

can be overwritten by umts_band_preferred parameter.

• inter_cell_handover: Setting this parameter as 3 avoids sending

handover required message from a BSC. It is very useful for test purposes.

• 3G_search_prio: It enables/disables BSIC decoding. Moreover, it works

with Service Handover. For restrictions on access to WCDMA, it is

disabled, set to 0.



Reselection (cont.)

Page 91

• umts_ba_type: It takes a value following the parameter combination for

add_neighbour command in terms of UTRAN_BCCH and UTRAN_SACCH

neighbourhood. It is not possible to set this parameter manually. It is rather

the consequence of add_neighbour command.

• service_band_reporting: specifies the number of cells to be reported

from the GSM frequency band.

• fdd_multirat_reporting: determines the number of cells to be reported

from the WCDMA frequency band in the measurement report.



Reselection (cont.)

Page 92

When a multi-RAT MS is in idle mode, these parameters are broadcast from

BSS via System Information Type 2quater.

System Information Type 2quater message includes

• 3G neighbor cells,

• qsearch_i,

• qsearch_c_initial,

• fdd_qoffset,

• fdd_rep_quant,

• fdd_multirat_reporting,

• fdd_qmin.


Reselection (cont.)

Page 93


Reselection (cont.)

Page 94

The concept of blind search in the idle mode enables BSS to broadcast

only WCDMA ARFCN information within the System Information Type

2quater message.

Although it reduces the amount of information within the System

Information Type 2quater message, it would increase the delay for MS to

perform cell reselection in the idle mode.


Reselection (cont.)

Page 95

In order for a reselection to a WCDMA cell,

• CPICH RSCP of this cell should be greater than the sum of receive level of

serving GSM cell and all of the six strongest GSM neighbor cells by a

margin of fdd_qoffset for 5 seconds.

• Ec/No value of WCDMA cell should be greater than or equal to the value

of fdd_qmin.

This phenomenon also applies for cell-reselections in packet-dedicated

mode.


Reselection (cont.)

Page 96

While moving to circuit-dedicated mode, MS communicates its Inter-

RAT capabilities to BSS within Classmark Change and UMTS Classmark

Change messages.


Reselection (cont.)

Page 97

Upon reception of Classmark Change and UMTS Classmark

Change messages, BSS starts sending Measurement Information

messages to the MS.

These messages include

• 3G neighbor cell description,

• qsearch_c,

• 3g_search_prio,

• fdd_rep_quant ,

• fdd_multirat_reporting,

values.


Reselection (cont.)

Page 98


Reselection (cont.)

Page 99

Upon reception of Measurement Information messages, measurements from

UTRAN neighbours are included in the list of MS measurements from 6 best

cells within Measurement Report and sent on the SACCH.

Based on the value of fdd_rep_quant parameter, MS reports

• RSCP, or

• Ec/No.


Reselection (cont.)

Page 100


Reselection (cont.)

Page 101

MS reported levels received within Measurement Report are averaged for

umts_ncell_avg_period.

If averaged value is higher than the minimum threshold

• utran_cpich_rscp_min, when fdd_rep_quant=0

• utran_cpich_ec_no_min, when fdd_rep_quant_min=1

plus the margin (utran_meas_margin), then reported 3G neighbor is qualified

for the handover and Handover Required message is sent from BSS to

MSC.


Reselection (cont.)

Page 102

Statistics and counters

Documents

Handover in WCDMA