WCDMA 3G I-Rat (Inter-Rat) Handover Principle and Its Parameters

WCDMA 3G (Inter-Rat)" I-Rat " Handover Principle and Its Parameters

WCDMA 3G Inter-RAT Handover

(1.)     Inter-RAT Handover Overview

(2.)     Inter-RAT Handover Procedure

• IMP. LINK:- 4G Lte Modulation technique,Physical layer & Cell planning Tutorial

Inter-RAT Handover Application Scenario

a)        Coverage-based Inter-RAT handover

b)          QoS-based Inter-RAT handover

c)           Load-based Inter-RAT handover

d)          Service-based Inter-RAT handover

e)           UMTS-to-GSM Multimedia Fallback

f)         PS UMTS-to-GSM Handover with NACC

g)          UMTS-to-GSM Handover retry

(3.)  Signaling Procedures for Inter-RAT Handover

• LINK:- 3G wcdma Intro and Principles
• LINK:- 3G wcdma Channels & Techniques
• LINK:- 3G wcdma DETAILED Tutorial

(1.)     Inter-RAT Handover Overview:-

Inter-RAT Handover Application Scenario

•  LINK:-Inter frequency 3G wcdma handover
• LINK:- Intra frequency 3G wcdma handover

Inter-RAT handover provides coverage expansion, load sharing, and layered services. It saves cost by utilizing the existing GSM network resources.
·       Inter-RAT handover refers to the handover between UMTS and GSM. The reason for the handover can be coverage limitation, link stability control or load limitation of the 3G system.
·       Inter-RAT handover can be UMTS-to-GSM or GSM-to-UMTS handover.
·       Strategy of 2G and 3G cooperation is shown in the picture:
==>>    Based on coverage, QoS , Service, load and speed, RNC can trigger UE handover from 3G to 2G; When UE return back to Idle Mode, trigger UE Cell reselect to 3G.
·       In this handover, however, GSM and UMTS dual-mode UEs (MSs) are required, and both the GSM MSC and the GSM BSS must be upgraded.

WCDMA 3G Classification of Inter-RAT Handover:

ü  Based on the triggering causes of handover, inter-frequency handover can be categorized into four types .

·       Coverage-based

·       QoS-based

·       Load-based

·       Service-based

·       Speed-based 

Coverage-based inter-frequency handover:-

The coverage of the UMTS is incontinuous at the initial stage of the 3G network. On the border of the coverage, the poor signal quality of UMTS triggers the UMTS-to-GSM measurement. If the signal quality of GSM is good enough and all the services of the UE are supported by the GSM, the coverage-based UMTS-to-GSM handover is triggered.

QoS-based inter-frequency handover:-

According to the Link Stability Control Algorithm, the RNC needs to trigger the QoS-based UMTS-to-GSM handover to avoid call drops.

Load-based inter-frequency blind handover:-

If the load of the UMTS is heavy and all the RAB of a UE are supported by the GSM, the load-based UMTS-to-GSM handover is triggered.

Service-based UMTS-to-GSM handover:-

Based on layered services, the traffic of different classes is handed over to different systems. For example, when an Adaptive Multi Rate (AMR) speech service is requested, this service can be handed over to the GSM.

Speed-based inter-frequency handover:-

When the Hierarchical Cell Structure (HCS) is used, the cells are divided into different layers on the basis of coverage. Typically, a marco cell has large coverage and low priority, whereas a micro cell has small coverage and high priority.UMTS-to-GSM handover can be triggered by the UE speed estimation algorithm of the HCS. A UE moving at high speed is handed over to a cell with larger coverage to reduce the times of handover, whereas a UE moving at low speed is handed over to a cell with smaller coverage.   

Preconditions for UMTS-to-GSM Handover :

>>  Service Handover Indicator

ü  Capabilities of Deciding UMTS-to-GSM Handover

·       GSM neighboring cell capability

·       service capability

·       UE capability

Ø Before handover, the RNC checks whether the preconditions meet the triggering requirements of the UMTS-to-GSM handover. The preconditions include the service handover indicator, GSM cell capability, service capability, and UE capability.

Ø The parameter Service handover indicator indicates the CN policy for the service handover to the GSM. This parameter is indicated in the Radio Access Bearer (RAB) assignment signaling assigned by the CN, or can be configured on the RNC side by ADD/MOD TYPRABBASIC .

Ø Before deciding UMTS-to-GSM handover, the RNC considers GSM cell capability, service capability and UE capability.

Ø GSM cell capability could be “GSM”,”GPRS”,”EDGE”, it should be the real capability of the GSM neighbor, and configured in RNC data base  by LMT correctly

Ø Service capability could be “GSM”,”GPRS”,”EDGE” also, it should be properly configured in RNC data base  by LMT

Ø UE capability is reported by the UE itself in “RRC Setup Complete” message.

Before handover, the RNC checks service handover indicator, This parameter is indicated in the Radio Access Bearer (RAB) assignment signaling assigned by the CNThe service handover indicators are as follows:
·       HO_TO_GSM_SHOULD_BE_PERFORM: means that the handover to the 2G network is performed when 2G signals are available.
·       HO_TO_GSM_SHOULD_NOT_BE_PERFORM: means that the handover to the 2G network is performed when 3G signals are weak but 2G signals are strong.
·       HO_TO_GSM_SHALL_NOT_BE_PERFORM: means that the handover to the 2G network is not performed even when 3G signals are weak but 2G signals are strong.
·       For the UE accessing combined services (with CS services), the RNC sets the service handover indicator of the UE to that of the CS service, because the CS service has the highest QoS priority.
·       For the UE accessing combined services (with only PS services), the RNC sets the service handover indicator of the UE to that of the PS service, which has the highest QoS priority.
·       If service handover indicators are not configured by the CN, each indictor can be set to Service parameter index of a service on the RNC.
·       Based on different service handover indicators .RNC may initiate different action, for example, handover based on service are not not performed for the services whose handover indicator is “HO_TO_GSM_SHOULD_NOT_BE_PERFORM” or “HO_TO_GSM_SHALL_NOT_BE_PERFORM”

Capabilities of Deciding UMTS-to-GSM Handover

ü  GSM neighboring cell capability
·       NO_CAPABILITY, GSM, GPRS, EDGE
ü  Service required capability
·       GSM, GPRS, EDGE
ü  UE capability
·       GSM, GPRS, or EDGE
The rules for enabling UMTS-to-GSM handover are based on the parameter Service Handover Indicator and the three types of capability parameters. The rules vary with different types of Inter-RAT handover , that is  , the 4 factors will decide if the Inter-RAT handover is allowed.
The rules are:
ü  Coverage-based and QoS-based UMTS-to-GSM handover
·       when Service Handover Indicator is set as follows:
·       HO_TO_GSM_SHOULD_BE_PERFORM
·       HO_TO_GSM_SHOULD_NOT_BE_PERFORM
ü  In addition, the RNC initiates Inter-RAT handover based on the following capabilities:
·       GSM cell capability (can be set on RNC)
·       Service required capability (can be set on RNC)
·       UE capability (reported from UE “RRC setup complete” message )

ü  Parameters for Inter-RAT Handover precondition

·       Inter-RAT cell type  
·       NO_CAPABILITY, GSM, GPRS, EDGE

ü  Inter-RAT Handover RNC Algorithm Switch

·       INTER_RAT_PS_OUT_SWITCH
·       Default value is ON
·       INTER_RAT_CS_OUT_SWITCH
·       Default value is ON

3G WCDMA Inter-RAT cell type

·       Parameter ID : RatCellType
·       Content: Identifying the type of the inter-RAT cell NO_CAPABILITY indicates that the capacity of the GSM cell remains unknown. It is not recommended to set the cell type to NO_CAPABILITY. If the cell type is NO_CAPABILITY, the inter-RAT handover cannot be triggered.
·       Set this parameter through ADD GSMCELL
Handover algorithm switch is set in the command SET CORRMALGOSWITCH.
  INTER_RAT_PS_OUT_SWITCH
·       The switch decides whether the RNC will initiate Inter-RAT measurement to trigger Inter-RAT handover of the PS domain from the UTRAN.
INTER_RAT_CS_OUT_SWITCH
The switch decides whether the RNC will initiate Inter-RAT measurement to trigger Inter-RAT handover of the CS domain from the UTRAN.
Set this parameter through SET CORRMALGOSWITCH

3G WCDMA Inter-RAT Handover

1)    Inter-RAT Handover Overview
2)    Inter-RAT Handover Procedure
    a)     Coverage-based Inter-RAT handover
    b)    QoS-based Inter-RAT handover
    c)     Load-based Inter-RAT handover
    d)    Service-based Inter-RAT handover
    e)     UMTS-to-GSM Multimedia Fallback
    f)      PS UMTS-to-GSM Handover with NACC
    g)    UMTS-to-GSM Handover retry
3)    Signaling Procedures for Inter-RAT Handover

a)     Coverage-based Inter-RAT handover:-

Procedure of Coverage-based Inter-RAT handover :

    The handover procedure is divided into four phases: handover triggering, handover measurement, handover decision, and handover execution.

In the triggering phase

·       The RNC sends a MEASUREEMNT CONTROL message to the UE, notifying the UE to measure the current carrier quality. This message defines the reporting rules and thresholds of events 2D and 2F. If the quality of the pilot signal in the current cell deteriorates, the CPICH Ec/No or CPICH RSCP of the UMTS cell that the UE accesses is lower than the corresponding threshold and the UE reports event 2D.

In the measurement phase

·       If the RNC receives a report of event 2D, the RNC may request the NodeB and UE to start the compressed mode to measure the qualities of GSM cells. Then, the RNC may send an Inter-RAT measurement control message that defines the neighboring cell information, reporting period, and reporting rule.

   In the measurement phase, either periodical measurement report mode or event-triggered measurement report mode can be used.

["In the decision phase"]

   After the UE reports event 3A, the RNC makes a handover decision. Or, after the UE periodically sends the measurement reports, the RNC evaluates the reports first and then makes a handover decision.

In the execution phase

    The RNC initiates a handover procedure.

MEASUREMENT EVENTS

When the estimated quality of the currently used frequency is below a certain threshold,2D Event will be triggered, Then RNC will initiate the compress Mode to start inter-frequency or Inter-RAT handover measurement.

During compress mode, if the the estimated quality of the currently used frequency is above a certain threshold, 2F Event will be triggered, Then RNC will stop the compress Mode.

2D EVENT

·       Event 2D is triggered on the basis of the following formula

ü  Q_Used <= T_Used2d - H_2d/2

ü  Q_Used is the measured quality of the used frequency.

·       T_Used2d is the absolute quality threshold of the cell that uses the current frequency. Based on the service type (CS , PS domain R99 service, or PS domain HSPA service) and measurement quantity (CPICH Ec/No or RSCP),

·       H_2d is the event 2D hysteresis value 2D hysteresis.

Inter-RAT CS measure start Ec/No THD

·       The default value of this parameter is -14dB

Inter-RAT R99 PS measure start Ec/No THD

·       The default value of this parameter is -15dB

Inter-RAT H measure start Ec/No THD

·       The default value of this parameter is -15dB

Inter-RAT CS measure start RSCP THD

·       The default value of this parameter is -100dBm

Inter-RAT R99 PS measure start RSCP THD

·       The default value of this parameter is -110dBm

Inter-RAT H measure start RSCP THD

·       The default value of this parameter is -110dBm

2D hysteresis

·       The default value of this parameter is 4 (2dB)

2D event trigger delay time

·       The default value of this parameter is D320 (320 ms)

2F EVENT

·       Event 2F is triggered on the basis of the following formula

·       Q_Used >=  T_Used2f + H_2ff2

·       Q_Used is the measured quality of the used frequency.

T_Used2f is the absolute quality threshold of the cell that uses the current frequency. Based on the service type (CS , PS domain R99 service or PS domain HSPA service) and measurement quantity (CPICH Ec/No or RSCP),

·       H_2f is the event 2F hysteresis value 2F hysteresis.

Inter-freq CS measure stop Ec/No THD

·       The default value of this parameter is -12dB

Inter-freq R99 PS measure stop Ec/No THD

·       The default value of this parameter is -13dB

Inter-RAT H measure stop Ec/No THD

·       The default value of this parameter is -13dB

Inter-freq CS measure stop RSCP THD

·       The default value of this parameter is -97 dBm

Inter-freq R99 PS measure stop RSCP THD

·       The default value of this parameter is -107dBm

Inter-RAT H measure stop RSCP THD

·       The default value of this parameter is -107dBm

2F hysteresis

·       The default value of this parameter is 4 (2dB)

2F event trigger delay time

·       The default value of this parameter is D1280  (1280 ms)

Handover Measurement

When the UE enters the compress mode, RNC will trigger the Inter-RAT handover measurement by one additional measurement control signaling , so as to request UE test Inter-RAT neighbor cell.

In this Measurement control message, RNC should inform the UE Inter-RAT measurement parameter (Neighbor list, reporting mode…)

Handover Measurement

•        The measurement report mode of Inter-RAT handover is configured through the parameter Inter-frequency measure report mode. By default ,periodically reporting is recommended.

•        Based on the measurement control message received from the RNC, the UE periodically reports the measurement quality of the target cell. Then, based on the measurement report, the RNC makes the handover decision and performs handover.

•        If the reporting mode is periodically reporting : UE only test the Inter-RAT neighbor RSSI only.

•        If the reporting mode is event trigger reporting : UE test the Inter-RAT neighbor RSSI and current cell Ec/No or RSCP ( depend on the 3A Measure Quantity ) .

3G WCDMA Inter-RAT Handover

1)    Inter-RAT Handover Overview
2)    Inter-RAT Handover Procedure
    a)     Coverage-based Inter-RAT handover
    b)    QoS-based Inter-RAT handover
    c)     Load-based Inter-RAT handover
    d)    Service-based Inter-RAT handover
    e)     UMTS-to-GSM Multimedia Fallback
    f)      PS UMTS-to-GSM Handover with NACC
    g)    UMTS-to-GSM Handover retry
3)    Signaling Procedures for Inter-RAT Handover

    b)    QoS-based Inter-RAT handover

Procedure of QoS-based Inter-RAT handover

The handover procedure is divided into four phases: handover triggering, handover measurement, handover decision, and handover execution.

    Besides the triggering step, the rest 3 steps are the same with Coverage-based Inter-RAT handover

l  In the triggering phase

   If the service quality of the current cell deteriorates, the Link Stability Control Algorithm makes a handover measurement decision.

l  In the measurement phase

   The RNC requests the NodeB and the UE to start the compressed mode to measure the qualities of inter-frequency and Inter-RAT neighboring cells. Then, the RNC sends measurement control messages for inter-frequency measurement  and Inter-RAT measurement

  In the measurement phase, the method of periodical measurement report or event-triggered measurement report can be used.

l  In the decision phase

After the UE reports event 3A, the RNC performs the handover. Otherwise, the UE periodically generates measurement reports, and the RNC makes a decision after evaluation.In the execution phase

  The RNC executes the handover procedure.

Note :

  About “Link Stability Control Algorithm” :

  When the uplink transmit power of the UE or downlink transmitted code power of the NodeB exceeds the  associated threshold  :

l  For AMR, a fixed  sequence of rate downsizing, inter-frequency handover, and then Inter-RAT handover are performed,

l  for VP ,inter-frequency handover are performed,

l  For BE service, rate downsizing, inter-frequency handover, and then Inter-RAT handover are performed according to the configured sequence

Parameters of 3G WCDMA Inter-RAT handover

l  InterRAT Handover Switch based on Uplink Traffic AMR

p Parameter ID: UlQoSAmrInterRATHoSwitch  

p The default value of this parameter is NO

l  InterRAT Handover Switch based on Downlink Traffic AMR

p Parameter ID: DlQoSAmrInterRATHoSwitch   

p The default value of this parameter is NO

l  First / Second / Third Uplink QOS Enhancement Action for Traffic BE

p Parameter ID: BeUlAct1/ BeUlAct2/ BeUlAct3

p The default value of this parameter is RateDegrade/ InterFreqHO/ InterRatHO

l  First / Second / Third Downlink QOS Enhancement Action for Traffic BE

p Parameter ID: BeDlAct1/ BeDlAct2/ BeDlAct3

p The default value of this parameter is RateDegrade/ InterFreqHO/ InterRatHO

l  Down Link QoS Measure timer length

p Parameter ID: DLQoSMcTimerLen  

p The default value of this parameter is 20 (20s)

l  Up Link QoS Measure timer length

p Parameter ID: UpQoSMcTimerLen  

p The default value of this parameter is 20 (20s)

l  3A Used-Freq Measure Quantity for QoS

p Parameter ID: UsedFreqMeasQuantityForQoS3A   

p The default value of this parameter is CPICH_RSCP

 3G WCDMA Inter-RAT Handover

1)    Inter-RAT Handover Overview
2)    Inter-RAT Handover Procedure
    a)     Coverage-based Inter-RAT handover
    b)    QoS-based Inter-RAT handover
    c)     Load-based Inter-RAT handover
    d)    Service-based Inter-RAT handover
    e)     UMTS-to-GSM Multimedia Fallback
    f)      PS UMTS-to-GSM Handover with NACC
    g)    UMTS-to-GSM Handover retry
3)    Signaling Procedures for Inter-RAT Handover

    c)     Load-based Inter-RAT handover

Procedure of Load-based  Inter-RAT handover

   The handover procedure is divided into three phases: handover triggering, handover decision, and handover execution

l  In the triggering phase

   When the load of the UMTS cell that the UE accesses is higher than the related threshold, the Load Reshuffling (LDR) algorithm makes a handover decision.

l  In the measurement phase

   The RNC enables the compressed mode and starts the Inter-RAT handover measurement.

l  In the decision phase

    After the UE reports event 3C, the RNC makes a handover decision.

l  In the execution phase

   The RNC initiates a handover procedure.

Based on the service ARP, Traffic class, Channel type(R99, HSDPA), RNC will choose the users with lower priority to execute handover .

l  Inter-RAT measure timer length

p Parameter ID: InterRATMeasTime    

p The default value of this parameter is 60 s

l  Inter-RAT measure timer length

p Parameter ID : InterRATMeasTime

p Value range 0 to 512

p The default value of this parameter is 60 s

p Content: If no Inter-RAT handover occurs upon expiry of the Inter-RAT measurement timer, the system stops the Inter-RAT measurement and disables the compressed mode. If this parameter is 0, the system does not start the Inter-RAT measurement timer.

3C EVENT

l  Event 3C is triggered on the basis of the following formula

l  M_OtherRAT + CIO_OtherRAT >= T_OtherRAT + H_3c/2

p M_OtherRAT is the measurement value of the cell (in another RAT) in the reporting range.

CIO_OtherRAT is the cell individual offset value of the cell (in another RAT) in the reporting range T_OtherRAT is the absolute Inter-RAT handover threshold. Based on the service type (CS , PS domain R99 service, or PS domain HSPA service)

l  Inter-RAT CS handover decision THD

p Parameter ID: InterRATNCovHOCSTh     

p The default value of this parameter is 21 (-90dBm)

l  Inter-RAT PS handover decision THD

p Parameter ID: InterRATNCovHOPSThd     

p The default value of this parameter is 21 (-90dBm)

l  Inter-RAT CS handover decision THD

p Parameter ID : InterRATNCovHOCSTh

p Value range 0 to 63 ,step:1dB

p The default value of this parameter is 21 (-90dBm)

p Content: This parameter is used to set measurement control on the event 3C. The event 3C is triggered when the quality of the target frequency is higher than this threshold. Note that the value 0 means the physical value is smaller than -110 dBm .

p Inter-RAT PS handover decision THD

p Parameter ID : InterRATNCovHOPSTh

p Value range 0 to 63 ,step:1dB

p The default value of this parameter is 21 (-90dBm)

p Content: This parameter is used to set measurement control on the event 3C. The event 3C is triggered when the quality of the target frequency is higher than this threshold. Note that the value 0 means the physical value is smaller than -110 dBm .

l  3C hysteresis

p Parameter ID: Hystfor3C     

p The default value of this parameter is 0 dB

l  3C event trigger delay time

p Parameter ID: TrigTime3C

p The default value of this parameter is D640 (640 ms)

l  3C hysteresis

p Parameter ID : Hystfor3C

p Value range 0 to 15 ,step:0.5 dB

p The default value of this parameter is 0 dB

p Content: This parameter specifies the event 3C trigger hysteresis, which is related to slow fading . The larger the value of this parameter, the smaller the probability of ping-pong effect and decision mistakes. In this case, however, event 3C cannot be triggered in time .

p 3C event trigger delay time

p Parameter ID : TrigTime3C  

p Value range D0, D10, D20, D40, D60, D80, D100, D120, D160, D200, D240, D320, D640, D1280, D2560, D5000

p The default value of this parameter is D640 (640 ms)

p Content: This parameter specifies the time of event 3C trigger delay, which is related to slow fading. The larger the value of this parameter, the smaller the probability of decision mistakes. In this case, however, event 3C responds to the changes of measured signals more slowly.

l  Decision and Execution Procedure

p Decision

l  3C Event

l  load information interchanging between the 3G and 2G cell

p Execution

After receiving the event 3C measurement report of GSM cells, the RNC performs the following handover decision and execution procedure:

p Put all the GSM cells that trigger event 3C into a cell set and arrange the cells according to the measurement quality in descending order.

p Select the cells in sequence from the cell set.

The load status between the source cell and the target cell can be acquired by interchanging load information between a UMTS cell and a GSM cell during the load-based and service-based UMTS-to-GSM handover. Thus, whether to further conduct the handover can be determined to avoid the 2G cell overload and possible handover to the congested cell.

l  Send Load Info to GSM Ind

p Parameter ID: SndLdInfo2GsmInd     

p The default value of this parameter is ON

l  NCOV Reloc Ind based on GSM cell load

p Parameter ID: NcovHoOn2GldInd 

p The default value of this parameter is ON

l  Send Load Info to GSM Ind

p Parameter ID : SndLdInfo2GsmInd

p Value range ON, OFF

p The default value of this parameter is ON

p Content:

l   When the parameter is set to ON, the RNC sends UMTS cell load information to the GSM CN during the non-coverage based system relocation in or out process.

l  When the parameter is set to OFF, the RNC does not send UMTS cell load information to the GSM during the system relocation in or out process.

p Set this parameter through SET INTERRATHONCOV

p NCOV Reloc Ind based on GSM cell load

p Parameter ID : NcovHoOn2GldInd

p Value range ON, OFF

p The default value of this parameter is ON

p Content:

When the parameter is set to ON, the RNC stops the non-coverage based system relocation out process if the GSM cell load exceeds the CS dormain Reloc GSM load THD or PS dormain Reloc GSM load THD.

When the parameter is set to OFF, the RNC continues the system relocation out process without considering the thresholds. This parameter specifies the time of event 3C trigger delay, which is related to slow fading. The larger the value of this parameter, the smaller the probability of decision mistakes. In this case, however, event 3C responds to the changes of measured signals more slowly.

l  CS domain Reloc GSM load THD

p Parameter ID: CSHOOut2GLoadThd      

p The default value of this parameter is 80 (80%)

l  PS domain Reloc GSM load THD

p Parameter ID: PSHOOut2GLoadThd 

p The default value of this parameter is 60 (60%)

l  CS domain Reloc GSM load THD

p Parameter ID : CSHOOut2GLoadThd

p Value range 0 to 100 ,step:1%

p The default value of this parameter is 80 (80%) 

p Content:

l   When the parameter is set to ON, the RNC sends UMTS cell load information to the GSM CN during the non-coverage based system relocation in or out process.

l  When the parameter is set to OFF, the RNC does not send UMTS cell load information to the GSM during the system relocation in or out process.

p Set this parameter through SET INTERRATHONCOV

p PS domain Reloc GSM load THD

p Parameter ID : PSHOOut2GLoadThd

p Value range 0 to 100 ,step:1%

p The default value of this parameter is OFF

p Content:

l   When the parameter is set to ON, the RNC sends UMTS cell load information to the GSM CN during the non-coverage based system relocation in or out process.

l  When the parameter is set to OFF, the RNC does not send UMTS cell load information to the GSM during the system relocation in or out process.

l  Inter-RAT handover max attempt times

p Parameter ID: InterRATHOAttempts       

p The default value of this parameter is 16

l  Inter-RAT handover max attempt times

p Parameter ID : InterRATHOAttempts

p Value range 1 to 16

p The default value of this parameter is 16 

p Content: This parameter specifies the maximum number of attempts of load-based and service-based Inter-RAT handover.

3G WCDMA Inter-RAT Handover

1)    Inter-RAT Handover Overview
2)    Inter-RAT Handover Procedure
    a)     Coverage-based Inter-RAT handover
    b)    QoS-based Inter-RAT handover
    c)     Load-based Inter-RAT handover
    d)    Service-based Inter-RAT handover
    e)     UMTS-to-GSM Multimedia Fallback
    f)      PS UMTS-to-GSM Handover with NACC
    g)    UMTS-to-GSM Handover retry
3)    Signaling Procedures for Inter-RAT Handover

    d)    Service-based Inter-RAT handover

Procedure of Service-based Inter-RAT handover

The handover procedure is divided into four phases: handover triggering, handover measurement handover decision, and handover execution

l  In the triggering phase

   When a service is established (RAB Assignment ), If the Service Handover Indicator is set to HO_TO_GSM_SHOULD_BE_PERFORM, the RNC requests the handover to the GSM

l  In the measurement phase

   The RNC enables the compressed mode and starts the Inter-RAT handover measurement.

l  In the decision phase

   After the UE reports event 3C, the RNC makes a handover decision.

l  In the execution phase

   The RNC initiates a handover procedure.

l  service type is defined by parameters on cell level:

   Inter-RAT CS handover switch  and Inter-RAT PS handover switch

p When a single CS service is initially set up by the UE, the RNC allows the UMTS-to-GSM service-based handover if Inter-RAT CS handover switch is set to ON.

p When a single PS service is initially set up by the UE, the RNC allows the UMTS-to-GSM service-based handover if Inter-RAT PS handover switch is set to ON.

p For the CS and PS combined services, no service-based handover is trigged

l  service handover indicator assigned by the Core Network. Only the services with the indicator “HO_TO_GSM_SHOULD_BE_PERFORM” can trigger Service-based Inter-RAT handover

l  Inter-RAT CS handover switch

l  Inter-RAT PS handover switch

l  Parameter ID:

p CSServiceHOSwitch

p PSServiceHOSwitch      

l  The default value of this parameter is OFF

l  Inter-RAT CS handover switch

l  Inter-RAT PS handover switch

p Parameter ID :

l   CSServiceHOSwitch

l  PSServiceHOSwitch

p Value range ON, OFF

p The default value of this parameter is OFF 

p Content:

l  This parameter indicates whether the cell allows the service-based Inter-RAT handover for the CS or PS services

3G WCDMA Inter-RAT Handover

1)    Inter-RAT Handover Overview
2)    Inter-RAT Handover Procedure
    a)     Coverage-based Inter-RAT handover
    b)    QoS-based Inter-RAT handover
    c)     Load-based Inter-RAT handover
    d)    Service-based Inter-RAT handover
    e)     UMTS-to-GSM Multimedia Fallback
    f)      PS UMTS-to-GSM Handover with NACC
    g)    UMTS-to-GSM Handover retry
3)    Signaling Procedures for Inter-RAT Handover

e)     UMTS-to-GSM Multimedia Fallback

Compared with the traditional speech service of the GSM, the VP service of the UMTS can transmit not only speech services but also the images and videos captured by both parties

For the UMTS-to-GSM handover, network-initiated multimedia fallback on the following occasions:

VP service:

•speech

•videos 

AMR service:

•speech

l  The RNC decides to send an Inter-RAT handover request after receiving periodical measurement reports or event 1F, 3A, or 3C.

l  The service is combined with a VP, and the "Alternative RAB Para" in the RAB ASSIGNMENT message is a valid AMR speech format.

The procedure for the fallback service is described as follows:

Procedure of Multimedia Fallback

l  In the service set up stage, the CN sends the SRNC a RANAP RAB ASSIGNMENT REQUIREMENT message to set up the VP service. The message includes the "Alternative RAB Para" that has QoS parameters required for setting up the speech service.

l  During UMTS-to-GSM handover, the SRNC sends a RANAP MODIFY REQUEST message to change the VP service to the AMR speech service. In the 3GPP R6 protocol, the Alternative RAB Configuration is also added to the RAB MODIFY REQUEST message, which enables the RNC to request the CN to change the VP service to the AMR speech service.

l  The MSC initiates the Bearer Capability (BC) negotiation with the UE.

l  After the negotiation is modified, the RNC is informed of performing service change. The multimedia fallback ends when the service change is completed.

l  When the multimedia fallback ends, the RNC decides whether to perform the UMTS-to-GSM handover according to the current measurements reported by the UE.

   At the beginning of the service setup, the RNC saves the RAB Para and "Alternative RAB Para" in the RAB ASSIGNMENT or REQUEST RELOCATION REQUEST message. This makes preparations for notifying the CN of changing the VP service to the AMR speech service.

The CN initiates the RAB reconfiguration to inform the two calling parties of performing the multimedia fallback. The multimedia fallback of the calling party is consistent with that of the called party. The single VP service falls back to the single AMR speech service. The multi-RAB service combined with VP falls back to the multi-RAB service combined with AMR. If the multimedia fallback succeeds, that is, the video phone in the service falls back to speech successfully, the Inter-RAT handover is initiated. Otherwise, the Inter-RAT handover fails.

3G WCDMA Inter-RAT Handover

1)    Inter-RAT Handover Overview

2)    Inter-RAT Handover Procedure
    a)     Coverage-based Inter-RAT handover
    b)    QoS-based Inter-RAT handover
    c)     Load-based Inter-RAT handover
    d)    Service-based Inter-RAT handover
    e)     UMTS-to-GSM Multimedia Fallback
    f)      PS UMTS-to-GSM Handover with NACC
    g)    UMTS-to-GSM Handover retry
3)    Signaling Procedures for Inter-RAT Handover

f) PS UMTS-to-GSM Handover with NACC:-

ü  What is NACC?

 Network Assisted Cell Change

ü  What is the use of NACC

To reduce the delay of PS UMTS-to-GSM handover

Procedure of NACC

·       Normal PS is realized by cell reselection, the time delay can not be guaranteed. But Some PS services have requirements for the delay. If the handover takes too long, TCP may start slowly or data transmission of the stream service may be interrupted due to the overflow of the UE buffer.

·       The introduction of NACC enables the system information exchange between BSS and RAN , Thus the inter-system delay in PS domains, can be reduced.

·       With NACC, the RNC sends the cell change order to the UE, which contains the GSM EDGE Radio Access Network (GERAN) system information, when the UMTS-to-GSM handover in the PS domain is triggered.

·       After the SRNC receives a measurement report from the UE, the UE is reselected to the GERAN cell according to the decision.

·       The SRNC sends a RAN INFORMATION REQUEST message to the SGSN.

·       The SGSN forwards the message to the corresponding BSS.

·       The BSS sends a GERAN SI/PSI message to the SRNC via the SGSN. RAN INFORMATION message can either be On-demand (single report) or On-modification (multiple reports).

·       The SGSN forwards the report message to the SRNC through Iu interface.

·       If there are several report messages, the SRNC terminates reporting by the TERMINATION/END message.

ü  To enable the NACC function, do as follows:

>> Run the SET CORRMALGOSWITCH command to set PS_3G2G_CELLCHG_NACC_SWITCH to ON.

>> Run the ADD GSMCELL/MOD GSMCELL command to set Inter-RAT cell support RIM indicator to TRUE.

ü  PS 3G to 2G Cell change order NACC Switch

·       Parameter ID: PS_3G2G_CELLCHG_NACC_SWITCH     

·       The default value of this parameter is OFF

ü  Inter-RAT cell support RIM indicator

·       Parameter ID: SuppRIMFlag      

·       The default value of this parameter is FALSE

ü  PS 3G to 2G Cell change order NACC Switch

·       Parameter ID : PS_3G2G_CELLCHG_NACC_SWITCH

·       Value range OFF, ON

·       The default value of this parameter is OFF

·       Content: When it is checked, and Inter-RAT handover of the PS domain from UTRAN use cell change order method, Inter-RAT handover support NACC(Network Assisted Cell Change) function.

·       Set this parameter through SET CORRMALGOSWITCH

ü  Inter-RAT cell support RIM indicator

·       Parameter ID: SuppRIMFlag

·       Value range FALSE, TRUE

·       The default value of this parameter FALSE

·       Content: The parameter indicates whether the Inter-RAT cell supports RIM.   

·       Set this parameter through ADD GSMCELL/MOD GSMCELL

3G WCDMA Inter-RAT Handover

1)    Inter-RAT Handover Overview

2)    Inter-RAT Handover Procedure
    a)     Coverage-based Inter-RAT handover
    b)    QoS-based Inter-RAT handover
    c)     Load-based Inter-RAT handover
    d)    Service-based Inter-RAT handover
    e)     UMTS-to-GSM Multimedia Fallback
    f)      PS UMTS-to-GSM Handover with NACC
    g)    UMTS-to-GSM Handover retry
3)    Signaling Procedures for Inter-RAT Handover

g) UMTS-to-GSM Handover retry

In case of event triggered Inter-RAT handover failure, if the cause of the failure is not a configuration failure and the retry timer expires, the handover attempts to the cell again until the retry number exceeds the maximum retry number

Ø Parameters

ü  3A event retry period

Parameter ID: PeriodFor3A      

The default value of this parameter is 1 (500ms)

ü  3A event retry max times

Parameter ID: AmntOfRpt3A      

The default value of this parameter is 63 (infinity)

ü  3C event retry period

Parameter ID: PeriodFor3C       

The default value of this parameter is 4 (2000ms)

ü  3C event retry max times

Parameter ID: AmntOfRpt3C     

The default value of this parameter is 5

3G WCDMA Inter-RAT Handover

1)    Inter-RAT Handover Overview

2)    Inter-RAT Handover Procedure
    a)     Coverage-based Inter-RAT handover
    b)    QoS-based Inter-RAT handover
    c)     Load-based Inter-RAT handover
    d)    Service-based Inter-RAT handover
    e)     UMTS-to-GSM Multimedia Fallback
    f)      PS UMTS-to-GSM Handover with NACC
    g)    UMTS-to-GSM Handover retry
3)    Signaling Procedures for Inter-RAT Handover

3) Signaling Procedures for Inter-RAT Handover:-

The signaling procedures for PS and CS Inter-RAT handover are different:

1.) UMTS-to-GSM Handover in CS Domain

2.) UMTS to GSM Handover in PS Domain

1.) UMTS-to-GSM Handover in CS Domain:-

•        The SRNC sends the 3G MSC a RANAP message RELOCATION REQUIRED if the condition of Inter-RAT outgoing handover is met.

•        As indicated in the received message, the 3G MSC forwards this request to the 2G MSC on the MAP/E interface through a MAP message PREPARE HANDOVER.

•        The 2G MSC forwards the request to the BSC. The message shown in the figure is for reference only and is subject to the actual condition of the GSM.

•        The BSC responds to this request. The message shown in the figure is for reference only and is subject to the actual condition of the GSM.

•        Once the initial procedures are completed in the 2G MSC/BSS, the 2G MSC returns a MAP/E message PREPARE HANDOVER RESPONSE.

•        The 3G MSC sends the SRNC a RANAP message RELOCATION COMMAND.

•        The SRNC sends the UE an RRC message HANDOVER FROM UTRAN through the existing RRC connection. This message may include information from one or several other systems.

•        The BSC performs handover detection. The figure does not show such procedures as GSM BSS synchronization. The message shown in the figure is for reference only and is subject to the actual condition of the GSM.

•        The UE sends the BSC a HANDOVER COMPLETE message.

•        The BSC sends the MSC a HANDOVER COMPLETE message. The message shown in the figure is for reference only and is subject to the actual condition of the GSM.

•        After detecting the UE in the coverage area of the GSM, the MSC sends the CN a MAP/E message SEND END SIGNAL REQUEST.

•        The CN sends the former SRNC an IU RELEASE COMMAND message, requesting the former SRNC to release the allocated resource.

•        After the bearer resource is released in the UMTS, the former SRNC sends the CN an IU RELEASE COMPLETE message.

•        After the call ends, the CN sends the MSC a MAP/E message SEND END SIGNAL RESPONSE.

2.) UMTS to GSM Handover in PS Domain

•        The signal quality of the WCDMA cell where the UE camps on is dissatisfactory or the load of

•        the serving cell is heavy.

•        When the UE is in CELL_DCH state, the UTRAN sends a CELL CHANGE ORDER message to the UE to perform a handover to GSM by cell reselection.

•        The NodeB sends a RADIO LINK FAILURE INDICATION message, because the UE shuts down transmission towards the WCDMA cell after cell reselection to a GSM cell.

•        After the UE accesses a GSM cell, the SGSN directly sends an IU RELEASE COMMAND message to the SRNC, if the Packet Data Protocol (PDP) context does not need to be transferred.

•        The UE in CELL_DCH state or the UTRAN (when the UE is in CELL_FACH state) decides to initiate an Inter-RAT handover in the PS domain to hand over the UE to a new GSM cell and stop the data transmission between the UE and the network.

•        The UE sends a ROUTING AREA UPDATE REQUEST message to the 2G SGSN. The Update Type in the message indicates RA update, combined RA/LA update, or combined RA/LA update with IMSI attach. The BSS adds the CGI including the RAC and LAC of the cell to the received message before forwarding the message to a new 2G SGSN.

•        The new 2G SGSN sends an SGSN CONTEXT REQUEST message to the old 3G SGSN to obtain the MM and PDP contexts. The old 3G SGSN validates the old P-TMSI Signature. If the old P-TMSI Signature is valid, the old 3G SGSN starts a timer. Otherwise, the old 3G SGSN responds with an error cause.

•        If the UE stays in connected mode before handover, the old 3G SGSN sends an SRNS CONTEXT REQUEST message. After receiving this message, the SRNS buffers the DPUs, stops sending the PDUs to the UE, and sends an SRNS CONTEXT RESPONSE message to the old 3G SGSN.

•        The old 3G SGSN sends an SGSN CONTEXT RESPONSE message to the 2G SGSN, including the MM and PDP contexts.

•        The security functions can be executed.

•        The new 2G SGSN sends an SGSN CONTEXT ACKNOWLEDGE message to the old 3G SGSN. This informs the old 3G SGSN that the new 2G SGSN is ready to receive the PDUs belonging to the activated PDP contexts.

•        The old 3G SGSN sends a DATA FORWARD COMMAND message to the SRNS. The SRNS starts a data-forwarding timer and sends the buffered PDUs to the old 3G SGSN.

•        The old 3G SGSN tunnels the GTP PDUs to the new 2G SGSN. In the PDUs, the sequence numbers in the GTP header remain unchanged.

•        The new 2G SGSN sends an UPDATE PDP CONTEXT REQUEST message to each related GGSN. Each GGSN sends an UPDATE PDP CONTEXT RESPONSE message after updating its PDP context fields.

•        The new 2G SGSN sends an UPDATE GPRS LOCATION message, requesting the HLR to modify the SGSN number.

•        The HLR sends a CANCEL LOCATION message to the old 3G SGSN. The old 3G SGSN responds with a CANCEL LOCATION ACK message. After the timer expires, the old 3G SGSN removes the MM and PDP contexts.

•        The old 3G SGSN sends an IU RELEASE COMMAND message to the SRNS. After the data-forwarding timer expires, the SRNS responds with an IU RELEASE COMPLETE message.

•        The HLR sends an INSERT SUBSCRIBER DATA message to the new 2G SGSN. The 2G SGSN constructs an MM context and PDP contexts for the UE and returns an INSERT SUBSCRIBER DATA ACK message to the HLR.

•        The HLR sends an UPDATE GPRS LOCATION ACK message to the new 2G SGSN.

•        If the association has to be established, the new 2G SGSN sends a LOCATION UPDATE REQUEST message to the VLR. The VLR stores the SGSN number for creating or updating the association.

•        If the subscriber data in the VLR is marked as not confirmed by the HLR, the new VLR informs the HLR. The HLR cancels the old VLR and inserts subscriber data in the new VLR.

•        The new VLR sends an UPDATE LOCATION message to the HLR.

•        The HLR cancels the data in the old VLR by sending a CANCEL LOCATION message to the old VLR.

•        The old VLR acknowledges the message by responding with a CANCEL LOCATION ACK message.

•        The HLR sends an INSERT SUBSCRIBER DATA message to the new VLR.

•        The new VLR acknowledges the message by responding with an INSERT SUBSCRIBER DATA ACK message.

•        The HLR responds with a UPDATE LOCATION ACK message to the new VLR.

•        The new VLR allocates a new TMSI and responds with a LOCATION UPDATE ACCEPT message to the 2G SGSN.

•        The new 2G SGSN checks the presence of the MS in the new RA. If all checks are successful, the new 2G SGSN constructs the MM and PDP contexts for the MS. A logical link is established between the new 2G SGSN and the UE. The 2G SGSN responds to the UE with a ROUTING AREA UPDATE ACCEPT message.

•        The UE acknowledges the new P-TMSI by returning a ROUTING AREA UPDATE COMPLETE message, including all PDUs successfully sent to the UE before the routing area update procedure.

•        The new 2G SGSN sends a TMSI REALLOCATION COMPLETE message to the new VLR if the UE confirms the VLR TMSI.

•        The 2G SGSN and the BSS perform the BSS PACKET FLOW CONTEXT procedure.