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IRAT Handover Basics

Commercial in confidence 2008-11-272

Introduction� IRAT (Inter-Radio Access Technology) handover and cell change

allows 3G voice and data services WCDMA RAN to GSM transition to maintain the connection with the UE between coverage areas to prevent dropped calls.

� The Inter-RAT Handover Evaluation algorithm performs the Inter-RAT (WCDMA RAN to GSM) handover evaluation based on the UE GSM measurement reports.


UMTS/GSM Coverage

GSM coverageUMTS and GSM coverage


Connection Quality Monitoring

� The connection quality is monitored in DL based on both CPICH Ec/No and CPICH RSCP and in UL based on UE TX power.

� The UE informs the RNC of connection quality with 2d, 2f, 6d and6b events, and RNC continually keeps track of whether the DL Ec/No, the DL Rscp or the UL coverage is good or bad.

� When at least one of these gets bad, an GSM HO (or IF) attempt will be started, and if the coverage then gets good for all then the IRAT HO (or IF HO) attempt will be aborted.

� In P6 WCDMA RAN it allows the operator to set different offsets for triggering event 2d for each UeRc Type at RNC level


Reporting Events 2d, 2f and 3aStart measuring on GSM cells (event 2d):- The quality (CPICH Ec/No) on the UTRAN side below threshold.- The coverage (RSCP) on the UTRAN side below threshold.Stop measuring on GSM cells (event 2f):- The quality on the UTRAN side above threshold.-The coverage on the UTRAN side above threshold.Handover trigger condition (event 3a):- The quality of the UTRAN cell drops below threshold.- Signal Strength (RSSI) of GSM cell is above threshold.

usedFreqRelThresh2f

utranRelThresh3aEcno


P6 IRAT Parameters

1 = IFHO_PREFERRED2 = GSM_PREFERRED3 = None

Default: GSM_PREFERRED

Indicates whether GSM HO, IFHO or nohandover shall be attempted when ahandover from the current WCDMAfrequency is triggered by the connectionquality monitoring, indicating that theWCDMA quality is low.

hoType

FALSE/TRUE

Default: FALSE

Indicates if the RNC supports inter-RAT HO(Inter Radio Access Technology Handover).

fddGsmHoSupp

0 = not allowed1 = allowed

Default = not allowed

Enables/disables the possibility ofHO from UMTS to GSM per RABtype.

gsmHoAllowed

RangeDescriptionParameter Name


Default P6 IRAT Parameters

-12 dBValid range for CPICH Ec/No.-24 – 0 dB

usedFreqThresh2dEcno

-1 dBRange is valid for CPICH Ec/No.-10 – 10 dB

utranRelThresh3aEcno

100 ms0; 10; 20; 40; 60; 80; 100; 120; 160;200; 240; 320; 640; 1280; 2560; 5000ms

timeToTrigger3a

-102 dBmRange is valid for GSM carrier RSSI.-115 – 0 dBm

gsmThresh3a

1280 ms0; 10; 20; 40; 60; 80; 100; 120; 160; 200; 240; 320;640; 1280; 2560; 5000 ms

timeToTrigger2fEcno

1 dB0 – 20 dBusedFreqRelThresh2fEcno

320 ms0; 10; 20; 40; 60; 80; 100; 120; 160; 200; 240;320; 640; 1280; 2560; 5000 ms

timeToTrigger2dEcno

DefaultRangeParameter Name


Default P6 IRAT Parameters (cont)

-97 dBmValid range for CPICH Rscp. -115— -25 dBm

usedFreqThresh2dRscp

-3 dBRange is valid for CPICH Rscp. -20— 20 dB

utranRelThresh3aRscp

3 dB0 – 20 dBusedFreqRelThresh2fRscp

DefaultRangeParameter Name


IRAT Handover Sequence


IRAT Cell Change

� IRAT Cell Change handles the cell change and cell reselection when a UE is connected to a Packet-Switched service.

� One difference between Inter-RAT Handover and Inter-RAT Cell Change is that in the IRATCC case there are no resources reserved in the target cell beforehand.


IRAT Cell Change Sequence


GSM to WCDMA - Measurement

� Measurement on WCDMA cells while the UE is on the GSM network is defined by the parameters QSI and QSC

� The parameters QSI and QSC define thresholds and also indicate whether these measurements shall be performed when signal strength (SS) of the serving cell is below or above the threshold


IRAT GSM to WCDMA handover

� IRAT can be performed from GSM to WCDMA even if the radio conditions are good.

� IRAT 2G to 3G speech handover is triggered due to capacity, radio quality or priority reasons (UTRAN Preferred)

� Two criteria must be fulfilled to trigger a GSM to WCDMA handover

– The percentage of idle TCH in the serving cell � ISHOLEV– CPICH Ec/No > MRSL

MRSL – GSM to WCDMA handover triggerISHOLEV – Defines GSM traffic load for enabling GSM to WCDMA handover. Handover evaluations start when the percentage of idle full rate Traffic Channels (TCHs) left in the cell is less than or equal to ISHOLEV.Note – In practise the reliance on TCH can be removed by setting the ISHOLEV low


GSM to WCDMA Cell Change

� IRAT Cell Change and Cell Reselection is triggered due to radio quality and priority reasons (UTRAN preferred)


Conditions for Cell Reselection� CPICH E c /N o > FDDQMIN

and � CPICH RSCP > RLA (serving +neighboring GSM cells) + FDDQOFF

� CPICH RSCP is a measure of the signal strength for a WCDMA cell. � RLA (Received Level Average) is the average of the received signal levels

measured in dBm for all monitored GSM frequencies in the BCCH Allocation list. � FDDQMIN is the minimum quality of a WCDMA cell for cell reselection. This

parameter should not be used to control the cell reselection, but provide a sufficient quality of the candidate WCDMA cell

� FDDQOFF is the key parameter to control the behavior of the IRATCC. It defines an offset between signal quality of WCDMA and GSM cells. Lower values of the parameter FDDQOFF could be used to off-load the GSM system. Higher values of the parameter FDDQOFF could be used to keep Multi-RAT MSs in the GSM system, for example, if an WCDMA to GSM off-load priority setting is applied in the WCDMA system

� Cell reselection back to WCDMA shall not occur within 5 seconds after a previous cell reselection from WCDMA. If a cell reselection occurred within the previous 15 seconds, an additional offset to FDDQOFF is increased by 5 dB by the multi RAT MS.


Directed Retry to GSM

� Directed Retry is a feature in which excess traffic may be off-loaded to GSM, with the following benefits:

– It allows the WCDMA RAN to focus on providing data services.

– It makes it possible for the combined system to fully utilize the existing spectrum in GSM


Directed Retry

� Directed Retry is a one-way diversion of voice traffic from WCDMA to GSM.

� Directed retry occurs at RAB establishment, where when the load of the WCDMA network exceeds a certain threshold, the WCDMA RAN will request a “blind” IRAT handover to a pre-configured GSM cell via the core network.

� Network load is based on downlink transmitted carrier power since this is most often the limiting factor.

� The power for HSDPA services is not considered in the load measurements

� It is assumed that the WCDMA RAN is co-sited with the GSM RAN and a 1-to-1 correspondence between WCDMA and GSM cells exists


Directed Retry Message Flow

� The load is shared between WCDMA and GSM by sending RAB Assignment Reject Messages with ‘Directed retry’ as the cause for Speech only calls. The CN will redirect the call to the GSM cell. The CN will reinitiate the RAB procedure if the GSM connection fails

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