This page shows how a TD-SCDMA network asks the UE to measure a GSM cell during a packet call, and how the UE reports it. The flow needs only two RRC messages, one MEASUREMENT CONTROL and one MEASUREMENT REPORT. The measurement is event triggered with event 3c, so the UE stays silent until the GSM cell is strong enough. The same GSM cell, ARFCN 20 with NCC 1 and BCC 5, is the target on the Redirection to GSM page.
Let's go through the call flow first, and then decode the two messages field by field.
What does the call flow look like?
Steps 1 to 15 set up an ordinary PS data call on TD-SCDMA. The network sends the MEASUREMENT CONTROL after the radio bearer is set up, while the UE is in CELL_DCH. Steps 16 and 17 are the two messages that the rest of this page decodes.
(1) UE <--> NW : Registration (Combined or Non-Combined)
(2) UE <--> NW : < IDLE >
(3) UE : < Make a MO Packet Call >
(4) UE --> NW : RRC CONNECTION REQUEST
(5) UE <-- NW : RRC CONNECTION SETUP
(6) UE --> NW : RRC CONNECTION SETUP COMPLETE
(7) UE --> NW : GMM SERVICE REQUEST
(8) UE <-- NW : AUTHENTICATION AND CIPHERING REQUEST
(9) UE --> NW : AUTHENTICATION AND CIPHERING RESPONSE
(10) UE <-- NW : SECURITY MODE COMMAND
(11) UE --> NW : SECURITY MODE COMPLETE
(12) UE --> NW : ACTIVATE PDP CONTEXT REQUEST
(13) UE <-- NW : RADIO BEARER SETUP
(14) UE --> NW : RADIO BEARER SETUP COMPLETE
(15) UE <-- NW : ACTIVATE PDP CONTEXT ACCEPT
The two steps do not have the same timing. The network can send the MEASUREMENT CONTROL at any time during the call. The MEASUREMENT REPORT has no fixed time, because the UE sends it only when the GSM cell meets the event 3c condition. If the GSM cell stays weak, step 17 never happens. So a missing report in a log does not always mean a failure. First check the reported level against the threshold.
Two messages carry the whole measurement : MEASUREMENT CONTROL configures it, and MEASUREMENT REPORT returns the result.An event-triggered report can be absent : the UE reports only when the GSM cell rises above the threshold.
Measurement Control
A MEASUREMENT CONTROL for GSM has to answer four questions. Which cell does the UE measure, and what quantity? When does it report, and how does it deliver the report? The capture below answers all four in one setup command.
Decoded RRC message from a tester log,
+-message ::= CHOICE [measurementControl]
+-measurementControl ::= CHOICE [later-than-r3]
+-later-than-r3 ::= SEQUENCE
+-rrc-TransactionIdentifier ::= INTEGER (0..3) [0]
+-criticalExtensions ::= CHOICE [later-than-r4]
+-later-than-r4 ::= SEQUENCE
+-rrc-TransactionIdentifier-MSP ::= INTEGER (0..3) [0]
+-criticalExtensions ::= CHOICE [criticalExtensions]
+-criticalExtensions ::= CHOICE [criticalExtensions]
+-criticalExtensions ::= CHOICE [criticalExtensions]
+-criticalExtensions ::= CHOICE [r9]
+-r9 ::= SEQUENCE [00]
+-measurementControl-r9 ::= SEQUENCE [1000]
| +-measurementIdentity ::= INTEGER (1..16) [1]
| +-measurementCommand ::= CHOICE [setup]
| | +-setup ::= CHOICE [interRATMeasurement]
| | +-interRATMeasurement ::= SEQUENCE [1110]
| | +-interRATMeasurementObjects ::= CHOICE [interRATCellInfoList] OPTIONAL:Exist
| | | +-interRATCellInfoList ::= SEQUENCE [100]
| | | +-removedInterRATCellList ::= CHOICE [removeAllInterRATCells]
| | | | +-removeAllInterRATCells ::= NULL
| | | +-newInterRATCellList ::= SEQUENCE OF SIZE(1..maxCellMeas[32]) [1]
| | | | +-NewInterRATCell ::= SEQUENCE [1]
| | | | +-interRATCellID ::= INTEGER (0..maxCellMeas-1[31]) [1] OPTIONAL:Exist
| | | | +-technologySpecificInfo ::= CHOICE [gsm]
| | | | +-gsm ::= SEQUENCE [00]
| | | | +-cellSelectionReselectionInfo ::= SEQUENCE OPTIONAL:Omit
| | | | +-interRATCellIndividualOffset ::= INTEGER (-50..50) [0]
| | | | +-bsic ::= SEQUENCE
| | | | | +-ncc ::= INTEGER (0..7) [1]
| | | | | +-bcc ::= INTEGER (0..7) [5]
| | | | +-frequency-band ::= ENUMERATED [dcs1800BandUsed]
| | | | +-bcch-ARFCN ::= INTEGER (0..1023) [20]
| | | | +-dummy ::= NULL OPTIONAL:Omit
| | | +-cellsForInterRATMeasList ::= SEQUENCE OF OPTIONAL:Omit
| | | +-interRATCellInfoIndication-r6 ::= INTEGER OPTIONAL:Omit
| | +-interRATMeasQuantity ::= SEQUENCE [1] OPTIONAL:Exist
| | | +-measQuantityUTRAN-QualityEstimate ::= SEQUENCE [1] OPTIONAL:Exist
| | | | +-filterCoefficient ::= ENUMERATED [fc0] OPTIONAL:Exist
| | | | +-modeSpecificInfo ::= CHOICE [tdd]
| | | | +-tdd ::= SEQUENCE
| | | | +-intraFreqMeasQuantity-TDDList ::= SEQUENCE OF SIZE(1..4) [1]
| | | | +-IntraFreqMeasQuantity-TDD ::= ENUMERATED [primaryCCPCH-RSCP]
| | | +-ratSpecificInfo ::= CHOICE [gsm]
| | | +-gsm ::= SEQUENCE [1]
| | | +-measurementQuantity ::= ENUMERATED [gsm-CarrierRSSI]
| | | +-filterCoefficient ::= ENUMERATED [fc0] OPTIONAL:Exist
| | | +-bsic-VerificationRequired ::= ENUMERATED [required]
| | +-interRATReportingQuantity ::= SEQUENCE OPTIONAL:Exist
| | | +-utran-EstimatedQuality ::= BOOLEAN [FALSE]
| | | +-ratSpecificInfo ::= CHOICE [gsm]
| | | +-gsm ::= SEQUENCE
| | | +-gsm-Carrier-RSSI ::= BOOLEAN [TRUE]
| | +-reportCriteria ::= CHOICE [interRATReportingCriteria]
| | | +-interRATReportingCriteria ::= SEQUENCE [1]
| | | +-interRATEventList ::= SEQUENCE OF SIZE(1..maxMeasEvent[8]) [1] OPTIONAL:Exist
| | | +-InterRATEvent ::= CHOICE [event3c]
| | | +-event3c ::= SEQUENCE [0]
| | | +-thresholdOtherSystem ::= INTEGER (-115..0) [-65]
| | | +-hysteresis ::= INTEGER (0..15) [0]
| | | +-timeToTrigger ::= ENUMERATED [ttt0]
| | | +-reportingCellStatus ::= CHOICE OPTIONAL:Omit
| | +-idleIntervalInfo ::= SEQUENCE OPTIONAL:Omit
| +-measurementReportingMode ::= SEQUENCE OPTIONAL:Exist
| | +-measurementReportTransferMode ::= ENUMERATED [acknowledgedModeRLC]
| | +-periodicalOrEventTrigger ::= ENUMERATED [eventTrigger]
| +-additionalMeasurementList ::= SEQUENCE OF OPTIONAL:Omit
| +-cellDCHMeasOccasionInfo-TDD128 ::= SEQUENCE OPTIONAL:Omit
| +-dpch-CompressedModeStatusInfo ::= SEQUENCE OPTIONAL:Omit
+-measurementControl-r9-add-ext ::= BIT STRING OPTIONAL:Omit
+-v970NonCriticalExtensions ::= SEQUENCE OPTIONAL:Omit
The message uses the Rel-9 critical extension, measurementControl-r9. The measurementIdentity is 1, and measurementCommand is a setup of an interRATMeasurement. The UE keeps the measurement under this identity, and the MEASUREMENT REPORT carries the same value.
The measurement object is an inter-RAT cell list. The network first clears the list with removeAllInterRATCells. It then adds one GSM cell as interRATCellID 1, with an individual offset of 0, BSIC NCC 1 and BCC 5, and BCCH ARFCN 20. The band indicator dcs1800BandUsed has no effect on ARFCN 20, because 45.005 Table 2-2 places this channel in P-GSM 900.
The measurement quantity for GSM is gsm-CarrierRSSI, with filterCoefficient fc0. 25.331 clause 8.6.7.2 says that k = 0 means no layer 3 filtering. So the UE evaluates each RSSI result as it arrives. The bsic-VerificationRequired field is set to required. In this case clause 14.3.1.3 lets event 3c trigger only for GSM cells that match both the BCCH ARFCN and the BSIC in the list. The message also carries a UTRAN quality estimate based on P-CCPCH RSCP. Event 3c does not use it, and utran-EstimatedQuality in the reporting quantity is FALSE.
The report criterion is event 3c, which 25.331 clause 10.3.7.30 defines as the estimated quality of the other system rising above a threshold. Here thresholdOtherSystem is -65, which means a GSM carrier RSSI of -65 dBm. The hysteresis IE is 0, and 25.331 scales it by 0.5 dB, so the hysteresis is 0 dB. The timeToTrigger is ttt0, so the UE reports as soon as the condition holds. The optional reportingCellStatus is omitted.
There is also a timing condition that the message does not show. 25.331 clause 8.6.7.5 has the UE evaluate the event only after it has verified the BSIC of the GSM cell. 25.123 clause 8.1A.2.5.2.1 allows Tidentify abort, 5000 ms, for the initial BSIC identification of one GSM cell. So the first report can come several seconds after the MEASUREMENT CONTROL, even when the GSM cell is already strong.
The measurementReportingMode asks for the report on acknowledged mode RLC, with periodicalOrEventTrigger set to eventTrigger.
Finally, look at what the network leaves out. The capture has no idleIntervalInfo, no cellDCHMeasOccasionInfo-TDD128 and no dpch-CompressedModeStatusInfo. Compressed mode is an FDD mechanism, and 25.331 clause 10.3.6.33 marks DPCH compressed mode info as FDD only. Clause 10.3.7.12a describes the idle interval IE as a tool for E-UTRAN measurement in CELL_DCH. The Rel-9 CELL_DCH measurement occasion info, clause 10.3.7.126, can reserve occasions for GSM carrier RSSI, initial BSIC identification and BSIC re-confirmation. This network uses none of them. 25.123 clause 8.1A.2.5 covers both kinds of UE, one that needs idle intervals for GSM measurements and one that does not.
The cell id links the two messages : interRATCellID 1 is the value that the report returns for this GSM cell.BSIC verification delays the event : the UE evaluates event 3c only for a GSM cell whose BSIC it has verified.fc0 turns layer 3 filtering off : the event reacts to each RSSI result, which is quick but also noisy.No compressed mode is configured : TD-SCDMA has no compressed mode, and this message configures no measurement occasions either.
Measurement Report
The report answers the MEASUREMENT CONTROL above, and it is very short. It reuses the identities from that message, so read the two side by side. Let's check what the UE put in the report, and what it left out.
Decoded RRC message from a tester log,
+-message ::= CHOICE [measurementReport]
+-measurementReport ::= SEQUENCE [00010]
+-measurementIdentity ::= INTEGER (1..16) [1]
+-measuredResults ::= CHOICE OPTIONAL:Omit
+-measuredResultsOnRACH ::= SEQUENCE OPTIONAL:Omit
+-additionalMeasuredResults ::= SEQUENCE OF OPTIONAL:Omit
+-eventResults ::= CHOICE [interRATEventResults] OPTIONAL:Exist
| +-interRATEventResults ::= SEQUENCE
| +-eventID ::= ENUMERATED [e3c]
| +-cellToReportList ::= SEQUENCE OF SIZE(1..maxCellMeas[32]) [1]
| +-CellToReport ::= SEQUENCE
| +-bsicReported ::= CHOICE [verifiedBSIC]
| +-verifiedBSIC ::= INTEGER (0..maxCellMeas[32]) [1]
+-v390nonCriticalExtensions ::= SEQUENCE OPTIONAL:Omit
The measurementIdentity is 1, which ties the report to the MEASUREMENT CONTROL above. The eventResults carry interRATEventResults with eventID e3c. The cellToReportList holds one cell, reported as verifiedBSIC 1. In 25.331 clause 10.3.7.28 the verified BSIC choice carries the inter-RAT cell id. So the value 1 points back to interRATCellID 1 in the MEASUREMENT CONTROL, which is the cell at ARFCN 20. The report does not repeat the ARFCN or the BSIC, because the cell id already identifies them.
The capture shows verifiedBSIC as INTEGER (0..maxCellMeas), and the current ASN.1 has the same range. A comment in the ASN.1 adds that the value maxCellMeas is not allowed, and clause 10.3.7.28 gives the range as 0 to maxCellMeas-1.
The report also leaves measuredResults out, and here the capture and the current text differ. 25.331 clause 14.3.1.3 tells the UE to set the measured results in an event 3c report as clauses 8.6.7.5 and 8.4.2 describe. Clause 8.6.7.6 then says that, when gsm-Carrier-RSSI is TRUE, the UE includes the GSM carrier RSSI as the measured RXLEV of the cell. Its exceptions concern a UE that needs compressed mode, which does not apply to TD-SCDMA. The MEASUREMENT CONTROL above asks for gsm-Carrier-RSSI, so a report that follows the current text would carry the RSSI. Treat this capture as a record of what one UE sent, not as the expected content.
verifiedBSIC is a cell id, not a BSIC : the value 1 refers to interRATCellID 1 in the MEASUREMENT CONTROL.Expect an RSSI in the report : with gsm-Carrier-RSSI set to TRUE, the current text asks the UE to include it, although this capture has none.The report is the input for the next step : the network can use it to trigger a redirection or a handover to GSM.
Reference
- 3GPP TS 25.331 v19.0.1 : clauses 8.4.2, 8.6.7.2, 8.6.7.5, 8.6.7.6, 10.3.6.33, 10.3.7.12a, 10.3.7.28, 10.3.7.30, 10.3.7.126 and 14.3.1.3, and the ASN.1 of MeasurementControl-r9-IEs, InterRATMeasurement-r9, Event3c and BSICReported in clause 11
- 3GPP TS 25.123 v19.0.0 : clause 8.1A.2.5, GSM measurements in CELL_DCH for the 1.28 Mcps option
- 3GPP TS 45.005 v19.0.0 : Table 2-2, fixed designation of ARFCN