Irat handover basics

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.

Commercial in confidence 2 2008-11-27

UMTS/GSM Coverage

UMTS and
GSM coverage 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 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


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


P6 IRAT Parameters

Parameter Name Description Range

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


Default P6 IRAT Parameters
Parameter Name Range Default

usedFreqThresh2dEcno Valid range for CPICH Ec/No. -12 dB
-24 – 0 dB

timeToTrigger2dEcno 0; 10; 20; 40; 60; 80; 100; 120; 160; 200; 240; 320 ms
320; 640; 1280; 2560; 5000 ms

usedFreqRelThresh2fEcno 0 – 20 dB 1 dB

timeToTrigger2fEcno 0; 10; 20; 40; 60; 80; 100; 120; 160; 200; 240; 320; 1280 ms
640; 1280; 2560; 5000 ms

utranRelThresh3aEcno Range is valid for CPICH Ec/No. -1 dB
-10 – 10 dB

timeToTrigger3a 0; 10; 20; 40; 60; 80; 100; 120; 160; 100 ms
200; 240; 320; 640; 1280; 2560; 5000
ms

gsmThresh3a Range is valid for GSM carrier RSSI. -102 dBm
-115 – 0 dBm


Default P6 IRAT Parameters (cont)
Parameter Name Range Default

usedFreqThresh2dRscp Valid range for CPICH Rscp. -115 -97 dBm
— -25 dBm

usedFreqRelThresh2fRscp 0 – 20 dB 3 dB

utranRelThresh3aRscp Range is valid for CPICH Rscp. -20 -3 dB
— 20 dB


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 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


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


Irat handover basics

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