ericsson irat optimization and troubleshooting

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IRAT Optimization and Troubleshooting

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Ericsson Confidential Ivan Krga Aug 20072 IRAT Optimization and Troubleshooting

Preparation Phase


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Pre-optimization Activities

Missing IRAT neighbors is one of major causes of 3G-2G handover failures so thorough check of IRAT neighbor definitions is recommended

Consistency of IRAT related network parameters (fddGsmHOSupp, hoType, usedFreqThresh2dEcno, usedFreqThresh2dRscp, utranRelThresh3aEcno, utranRelThresh3aRscp, gsmThresh3a..) should be checked

UMTS network need to be well tuned otherwise poor Ec/No would lead to frequent 3G-2G handovers, long time spent in Compressed mode, and/or 3G-2G handovers prevented by sudden drops on UMTS


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Common Problems


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1. Missing IRAT Neighbors

Missing IRAT neighbors problem particularily affects areas lacking UMTS signal dominance

UMTS overshooters should be contained if possible GSM cells located at the end of UMTS network

coverage might need 2nd tier UMTS neighbors defined Balanced approach needed since long IRAT neighbor

list decreases Compressed mode measurement accuracy so non-optimal cell may be targeted

Prioritization of IRAT neighbors based on location and/or STS data is highly recommended to avoid neighbor truncation


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1. Missing IRAT Neighbors cont.

In example shown non-optimal GSM cell IL02503 instead IL02453 was targeted due to missing IRAT neighbors – handover attempt failed due to Physical Channel Failure.


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2. IRAT Neighbor List Truncation

IRAT neighbors get truncated if combined number of unique neighbors for cells included in Active set exceeds 32

Same as for missing neighbor problem, IRAT neighbor list truncation mainly affects areas lacking UMTS signal dominance

Handover attempt will fail even if IRAT neighbor gets truncated after e3a is sent – UTRAN doesn’t have the mechanism to keep targeted GSM cell from being truncated

Prioritization of IRAT neighbors based on location and/or STS data is essential for overcoming the problem


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2. IRAT Neighbor List Truncation cont.

In example shown handover attempt failed due to truncation – e3a reported handover target cell IL15428 (129/20) got truncated from Measurement Control list and replaced with 129/22 after active set was updated with two additional cells.


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3. Wrong IRAT Parameter Settings

Compressed mode thresholds (usedFreqThresh2dEcno and usedFreqThresh2dRscp) highly affect 3G-2G handover performance – set to high could lead to frequent 3G-2G handovers, decreased 3G traffic level and weaken HSDPA performance, while set to low could increase 3G DCR and make IRAT neighbor list hard to maintain

Typically two sets of e2d thresholds are introduced – default set and the one for UMTS border cells where Compressed mode is triggered earlier to avoid excessive dropped calls

3G-2G handover thresholds have to be set consistently with e2d thresholds to allow 3G-2G transition once UE enters Compressed mode


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3. Wrong IRAT Parameter Settings cont. hoType parameter need to

be set carefully in IF network border areas – set to IFHO_PREFERRED even for one of the cells included in Active set it prevents IRAT handovers which can be a problem if no suitable IF handover target exists within the area


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4. BSIC Decoding Issue

Non-optimal GSM cell may be targeted if UE failed to decode BSIC of all measured cells

BSIC decoding issues can be caused by too long IRAT neighbor list, poor GSM radio conditions or insufficient time UE spent in Compressed mode (as in example shown)

Keeping IRAT neighbor lists at optimal length and prioritized by location and/or STS data should minimize BSIC decoding issue


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5. SI Decoding Issue

PS mobile is obliged to decode set of GSM System Information messages before sending LA Updating Request

Time available for completing handover procedure is limited by T309 (8s by default)

SI decoding issues can be caused by poor GSM radio conditions or UE issues


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6. Poor UMTS Radio Conditions

Poor UMTS radio conditions may prevent UE to enter Compressed mode (due to missing Physical Channel Reconfiguration message) or may cause connection to drop before 3G-2G handover is attempted

The problem should be addressed temporarily by increasing e2d thresholds while on a long term basis by improving UMTS coverage


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7. Physical Channel Failure (As the Only Reason)

Handover attempts failed due to Physical Channel Failure and not caused by any of previously described problems require GSM traces to determine reason behind it.

In example shown handover attempt failed due Physical Channel Failure – subsequent one succeeded within 3s at same optimal GSM target cell.


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IRAT Troubleshooting


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IRAT Troubleshooting Flowchart

Check if UE sent e2d measurement report

Check UMTS radio conditions at e2d sent

Check if UE decoded Physical Channel Reconfiguration message and entered Compressed mode

Check if UE sent e3a measurement report

Check GSM radio conditions at e3a sent; Identify UMTS cells included in Active set and check IRAT neighbor definitions

Check e3a for measured GSM cells; Identify cells included in cellToReportList and cells UE failed to decode BSIC for

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IRAT Troubleshooting Flowchart

Search for IRAT Measurement Control messages backwardsto resolve BCCH and BSIC of cells included in cellToReportList

Search for IRAT Measurement Control messages forward to determine anychange of ‘interRAT CellId’ due to truncation or UMTS neighbor removal

Confirm BCCH/BSIC of targeted GSM cell from Handover command,check its location and identify any missing neighbors

In case of PS handover attempt check if all System Information messages are decoded

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IRAT Troubleshooting Flowchart Details

Check if UE sent e2d measurement report1

In case of missing e2d measurement report check UMTS radio conditions and determine if Compressed mode thresholds have been set correctly


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Check UMTS radio conditions at e2d sent2

Poor UMTS radio conditions may prevent UE to enter Compressed mode or cause connection to drop before 3G-2G handover is attempted.


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Check if UE decoded Physical Channel Reconfiguration message and entered Compressed mode


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Poor UMTS radio conditions may prevent UE to decode Physical Channel Reconfiguration message needed to enter Compressed mode and start measuring 2G cells.


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Check if UE sent e3a measurement report4

In case of missing e3a measurement report check UMTS radio conditions and determine if both UMTS and GSM related e3a thresholds (utranRelThresh3aEcno, utranRelThresh3aRscp, hysteresis3a, timeToTrigger3a, gsmThresh3a) have been set consistently with e2d thresholds.


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Check GSM radio conditions at e3a sent; Identify UMTS cells included in Active set and check IRAT neighbor definitions


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In case GSM scanner data is not available, GSM radio conditions can be roughly estimated from e3a measurement report.

Poor GSM radio conditions may prevent UE from reading BSIC, SI or other L3 messages. MCOM tool can be used to check IRAT neighbor

definitions for UMTS cells included in the Active set and identify any missing neighbors.

In example shown UMTS cells ILU02629 (126) and ILU00149 (21) were included in the Active set.


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Check e3a for measured GSM cells; Identify cells included in cellToReportList and cells UE failed to decode BSIC for


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In example shown test UE managed to decode BSIC for all measured GSM cells.Cell positioned as 17th in IRAT Measurement Control list was included in cellToReportList.

Non-optimal GSM cell may be targeted if UE failed to decode BSIC of all measured cells.


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Search for IRAT Measurement Control messages backwardsto resolve BCCH and BSIC of cells included in cellToReportList


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In example shown cellToReportList cell IL15428 (129/20) was defined at position 17 in IRAT Measurement Control list.

Actix search function used


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Search for IRAT Measurement Control messages forward to determine anychange of ‘interRAT CellId’ due to truncation or removed UMTS neighbor


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Actix search function used

In example shown cellToReportList cell IL15428 (129/20) got truncated from Measurement Control list and replaced with 129/22 after active set was updated with two additional cells. Handover attempted on wrong target cell failed due to Physical Channel Failure.


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Search for IRAT Measurement Control messages forward to determine anychange of ‘interRAT CellId’ due to truncation or UMTS neighbor removal


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Another possible reason for targeted ‘interRAT CellId’ change is removal of UMTS neighbor from the Active set.

In example shown targeted ‘interRAT CellId’ 1 (169/16) was replaced with (568/05) after UMTS neighbor counterpart ILU01767 (SC 30) was removed from the Active set. Handover attempted on wrong target cell failed due to Physical Channel Failure.


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Confirm BCCH/BSIC of targeted GSM cell from Handover command;Check targeted GSM cell location and identify any missing neighbors


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BCCH/BSIC of targeted GSM cell should match cell reported in e3a, otherwise targeted ‘interRAT CellId’ has been changed either due to truncation or removed UMTS neighbor.

In example shown 3G-2G handover was attempted at optimal 2G handover target cell IL01232 (146/67).


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In case of PS handover attempt check if all System Information messages are decoded


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PS mobile is obliged to decode set of GSM System Information messages (Type 1, Type 2, Type 2ter, Type 3, Type 4 and Type 13) before initiate LA Update. SI decoding issues can be caused by poor GSM radio conditions or UE issues.

In example shown UE failed to decode SI Type 13 before T309 expiry.


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ericsson irat optimization and troubleshooting

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