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 T2TTo 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 -11Trigger start of GSM measurements and Compressed Mode CPM

usedFreqThresh2dRscp

usedFreqThresh2fEcno 2f -9Trigger 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 specificinformation, 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 aremandatory for the event, including observed time difference and GSM Carrier RSSI.• Inter-RAT Reporting Criteria – Defines a reporting event 3a to 3d and thecorresponding parameters for inter-RAT measurements, such as thresholds, hysteresis, andtime 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

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