IRAT handover allows the transition of 3G voice and data services between WCDMA and GSM networks to maintain connections when users move between coverage areas. The process involves monitoring connection quality and signal strength on both networks, and triggering handovers when certain thresholds are met. Directed retry is also used to offload excess traffic from WCDMA to GSM networks by rejecting calls on WCDMA and redirecting them to GSM when WCDMA network load exceeds a threshold.

1. IRAT Handover
Basics

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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.

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UMTS/GSM Coverage
GSM coverage
UMTS and
GSM coverage

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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 and
6b 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

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Reporting Events 2d, 2f and 3a
Start 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

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P6 IRAT Parameters
1 = IFHO_PREFERRED
2 = GSM_PREFERRED
3 = None
Default: GSM_PREFERRED
Indicates whether GSM HO, IFHO or no
handover shall be attempted when a
handover from the current WCDMA
frequency is triggered by the connection
quality monitoring, indicating that the
WCDMA quality is low.
hoType
FALSE/TRUE
Default: FALSE
Indicates if the RNC supports inter-RAT HO
(Inter Radio Access Technology Handover).
fddGsmHoSupp
0 = not allowed
1 = allowed
Default = not allowed
Enables/disables the possibility of
HO from UMTS to GSM per RAB
type.
gsmHoAllowed
RangeDescriptionParameter Name

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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; 5000
ms
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

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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

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IRAT Handover Sequence

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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.

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IRAT Cell Change Sequence

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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

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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 trigger
ISHOLEV – 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

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

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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.

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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

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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

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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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