IRAT HO
Inter Radio Access Technology
Handover
Node B
(UMTS)
RBS
(GSM)
Sonny Rafiq
Types of Handovers in UMTS
1. Soft Handover
2. Softer Handover
3. Hard Handover
Inter-Frequency Handover
Inter-Radio Access Technology (Inter-RAT) Handover
IRAT HO
IRAT HO is a type of hard handover where the
UE is ordered by the network to tune to
another RAT, establish contact with it and then
continue the traffic using resources from the
new contacted system.
This means that there will be small
interruptions in the data flow to and from the
UE.
IRAT HO Case IRAT HO is handover that occurs between cells on different
Radio Access Technologies.
For example, handover from UMTS to GSM in the
edge of UMTS coverage is considered an IRAT
handover.
Another Cases:
UMTS Coverage Hole
Missing 3G Site
Non UMTS In Building
Purpose
IRAT HO prevents dropped calls and thus allow service continuation on
dedicated channels for circuit-switched services when the UE is moving
out from WCDMA RAN coverage to an area where only GSM/GPRS
network coverage exists.
IRAT Cell Change prevents dropped calls and thus allow service
continuation on dedicated channels for Packet-Switched-services when the
UE is moving out from WCDMA RAN coverage to a GSM/GPRS network
coverage area.
The major difference between Inter-RAT Handover and Inter-RAT Cell
Change is that in the Inter-RAT Cell Change case there are no resources
reserved in the target cell before the Inter-RAT Cell Change is executed.
Feature activation
C_GsmHoAllowed: IRAT handover is only attempted if this parameter is set to Allowed for the current UeRc state. This is a constant hard coded value and is currently set to Allowed by Ericsson.
FddGsmHoSupp: This parameter at the RNC level should be set to TRUE to enable IRAT functionality.
hoType: This cell parameter controls if IRAT Handover or Inter-Frequency HO or None shall be evaluated in case both Inter RAT and Inter-Frequency neighboring cells have been configured.
defaultHoType: This parameter set per DRNC indicates the hoType for cells in the neighboring RNC.
Neighbor definition
Max number of 32 GSM neighbors.
Minimize/Optimize GSM neighbor definition. Need more time to measure higher number of GSM neighbor
Consume more power on the UE while in CM
Slightly higher interference may produce
Measurement accuracy may also decrease
Most important GSM neighbor that need to be defined: Edge of UMTS coverage
Non UMTS In-building site
Missing UMTS site due to deployment issue
Coverage Hole due to unplanned site
IRAT HO Evaluation
IRAT HO evaluation is based on connection quality
monitored in downlink and uplink.
Downlink:
• CPICH Ec/No
• CPICH RSCP
Uplink:
• UE Transmit Power
IRAT HO Evaluation
Ec/No : usedFreqThresh2dEcno
RSCP : usedFreqThresh2dRscp
UE TxPwr : ueTxpowerThreashold6b
No hyst for UE Tx Pwr
3G to 2G Handover procedure
1. Connection quality monitoring based on
event triggers.
2. Event based GSM measurements reporting.
3. Identification of target GSM cell for
Handover/Cell Change.
4. 3G to 2G IRAT Handover/Cell Change
execution.
Connection quality monitoring based on event triggers
Start measuring on GSM cells (start CM) - event 2d : the quality (CPICH
EC/No) on the UTRAN side below threshold for the specified Time to trigger.
Stop measuring on GSM cells (stop CM)- event 2f : the quality (CPICH
EC/No) on the UTRAN side above threshold for the specified Time to trigger.
Hyst and T2T
To avoid event triggering
for unsignificant
measurement fluctuations
Compressed Mode
Physical layer function that allows UE
temporarily tune to another
frequency/technology while
maintaining an existing dedicated
channel.
Compressed Mode
How does Compressed Mode work?
• Transmission and reception are halted for a short time to create a
gap.
• The gap is created by compressing the information in a radio
frame in time.
• Methods for reducing the transmission time are spreading factor
reduction (SF/2), and higher layer scheduling (HLS).
• UTRAN commands the UE to configure and activate compressed
mode.
SF/2
When compressed mode is activated, the spreading factor of the compressed radio frames is
divided by two, allowing the same number of bits to be sent in a smaller amount of time.
This allows a transmission gap of up to seven slots (of the fifteen slots per frame) to be
available for measurements on other frequency. However this 50% reduction in the spreading
factor has many side effects. To maintain the appropriate Bit Error Rate (BER) the power of
the compressed frame will have to be increased by approximately 3 dB. . In addition to
downlink power utilization, compressed mode will also affect capacity by consuming more
codes in the downlink, increasing channel element usage, and increasing uplink noise rise due
to increased UE transmit power.
High Layer Scheduling
In the case of HLS, the transmission gap is generated by simply reducing the user throughput in the higher layers. This is achieved in Layer 2 where the MAC utilizes a subset of the available transport format combinations (TFC) within the transport format combination set (TFCS). For example, the transport block set size for a PS 64 kbps transport channel may contain 0, 1, 2, 3 or 4 transport blocks of 336 bits within every 20 msec transport time interval (TTI). By only utilizing 0, 1, or 2 transport blocks every TTI, a transmission gap of up to 7 slots can be generated, while maintaining a maximum throughput of 50% of the original transport channel. Unlike the SF/2 method, HLS does not require additional power, code usage, channel element usage, or UE transmit power to achieve compressed mode operation.
Event based GSM measurements reporting.
Handover trigger condition - event 3a : the quality of the 3G cell
drops below threshold and the signal strength (RSSI) of GSM
cell is above threshold.
Event 3a
Measurement Quantity used for the UMTS event 3a
threshold trigger is dependant on the original event 2d
or event 6d trigger. If event 2d was triggered based
on Ec/No, then the event 3a threshold trigger will be
based on Ec/No. If event 2d was triggered based on
RSCP, or an event 6d was the trigger, then the event
3a threshold trigger will be based on RSCP.
In the case of UE Tx power, the event 3a is always
evaluated only on CPICH RSCP and not on CPICH
Ec/No.
Parameter Example
WCDMA Parameters Abbr. Value Description
utranThresh3aEcno 3a -13 U2G HO trigger
utranThresh3aRSCP
usedFreqThresh2dEcno 2d -11
Trigger start of GSM measurements and
Compressed Mode CPM
usedFreqThresh2dRscp
usedFreqThresh2fEcno 2f -9
Trigger stop of GSM measurements and exit of
compressed mode
usedFreqThresh2fRscp
gsmThresh3a -95 U2G HO condition
Identification of target GSM cell for
Handover/Cell Change
The RNC uses the measurement report from event 3a and orders the GSM cells according to their GSM quality measure.
While doing this, RNC discards all GSM cells that are not BSIC verified and cells whose GSM carrier RSSI is less than gsmThresh3a.
RNC then chooses the target GSM cell as the cell which has the best carrier RSSI and starts an IRAT handover attempt towards this cell.
IRAT HO Execution
If handover the UE connection to a
GSM/GPRS cell succeed, the UE is finally
connected to the GSM/GPRS cell and
WCDMA RAN resources related to the UE
connection are released.
The UE connection remains on the WCDMA
RAN if a handover to GSM/GPRS failed.
IRAT HO Execution
IRAT Cell Change Execution
IRAT Measurements and Reporting
UTRAN sends Measurement Control message that contains information about inter-
RAT measurements. Much of the information is the same as for intra-frequency
measurements.
The differences are:
• Cell Info List – Similar information as for intra-frequency cells, but also includes GSM
specific
information, such as frequency, band indicator, and BSIC.
• Measurement Quantity – Defines quantity that the UE should measure for the GSM cells
and for the UTRAN cells. Indicates whether BSIC identification is required on the GSM cells.
• Reporting Quantity – Defines reporting quantities in addition to those quantities that are
mandatory for the event, including observed time difference and GSM Carrier RSSI.
• Inter-RAT Reporting Criteria – Defines a reporting event 3a to 3d and the
corresponding parameters for inter-RAT measurements, such as thresholds, hysteresis, and
time to trigger.
Measurement Control DL
Measurement Report UL
Event 2d
Event 2d: When estimated quality
of currently used freq is below a
certain threshold, UE sends a
event 2d to SRNC, which on
receiving this message asks the
UE to set up event 3a
RNC send a Physical
channel reconfiguration
message to UE which tells
the UE to enter
compressed mode to
measure GSM neighbor
cells.
The method to
induce
compressed
mode is SF/2
IRAT Events Contd..
Contd…
Measurement Control and Report
HO Complete
HO Complete
3a
Summary
Purpose of IRAT HO: Avoid Drop Calls due to lack of UMTS coverage (example: edge coverage, coverage hole, non UMTS in building sites)
Optimize neighbor definition to minimize CM delay time
Measurement quantities for IRAT HO evaluation CPICH Ec/No and CPICH RSCP on Downlink
UE TX Power on Uplink
Compressed Mode Sf/2
HLS
Event 2d, 2f, 3a
THANK YOU
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