A WCDMA UE in CELL_DCH does not decide handover by itself. The RNC configures measurements, and the UE sends a MEASUREMENT REPORT when a configured event occurs. Each event has a number and a letter, and the number tells the kind of measurement. This page lists the events of 25.331, explains how the most common one is evaluated, and shows how the events appear in RRC.

Followings are the topics to be covered in this page.

• Measurement Report Trigger Events
• How Event 1A Triggers a Report
• Events in RRC Signalling
• Reference

Measurement Report Trigger Events

Which event should the RNC configure for a given purpose? The table below lists every reporting event in 25.331 v19.0.1 clause 14, with a short description of each. The first digit groups the events by measurement type.

The digit maps directly onto the subclauses of 25.331 clause 14. Events 1x are intra-frequency events (clause 14.1), 2x inter-frequency (14.2) and 3x inter-RAT, such as GSM or E-UTRA (14.3). Events 4x report traffic volume on a transport channel (14.4) and 5A the downlink quality (14.5). Events 6x are UE internal measurements such as transmit power (14.6), and 7x are UE positioning events (14.7).

Events 1A, 1B and 1C drive soft handover in FDD. Event 1A proposes a cell to add to the active set, 1B a cell to remove, and 1C a cell to replace a weaker active cell. Event 1D reports a change of the best cell, and the RNC often uses it for the HS-DSCH serving cell change. Events 1G, 1H and 1I apply to TDD only, and 1J was added in Release 6 for cells that carry DCH but not E-DCH.

Inter-frequency and inter-RAT measurements usually need gaps in the transmission, which the Compressed Mode page covers. Events 2D and 2F are commonly used to start and stop compressed mode, and events 2B and 3A trigger the handover itself. The LTE measurement events page shows the equivalent events A1 to B2 of E-UTRA.

• 1x intra-frequency : soft handover and best cell change.
• 2x inter-frequency, 3x inter-RAT : hard handover and compressed mode control.
• 4x to 7x : traffic volume, quality, UE internal and positioning.

How Event 1A Triggers a Report

The events are not plain threshold comparisons. Event 1A compares a new cell with the whole active set, so the reporting range moves with the cells the UE already uses. This section follows event 1A through 25.331 clause 14.1.2.1, because events 1B and 1C use the same parts.

With CPICH Ec/N0 or CPICH RSCP as the measurement quantity, event 1A triggers when

10 Log M_New + CIO_New >= W x 10 Log( M₁ + ... + M_NA ) + (1 - W) x 10 Log M_Best - (R_1a - H_1a/2)

M_New is the measurement of the candidate cell and CIO_New its individual cell offset. M_i are the active set cells not forbidden to affect the reporting range, N_A is their number, and M_Best is the strongest of them. R_1a is the reporting range constant, and H_1a the hysteresis. W is set by the RNC and weights the sum of the active set against the best cell. With W = 0, the range is measured from the best cell only. With a larger W, a strong active set moves the range upward.

The equation alone does not send a report. It has to hold for the time to trigger, and the cell has to be allowed by triggering condition 2. The UE then adds the cell to the variable TRIGGERED_1A_EVENT and reports it. If the reporting interval is not 0, the UE repeats the report periodically up to the reporting amount, until the RNC adds the cell or the cell leaves the range. The reporting deactivation threshold stops event 1A reports once the active set holds more cells than that threshold.

For pathloss the sign of the comparison is reversed, and M_Best is the cell with the lowest pathloss. Event 1B uses the same equation with R_1b and the opposite sign on the hysteresis, so a cell must fall clearly below the range before it is reported as leaving it.

• Reporting range follows the active set : weighted by W between the sum and the best cell.
• R, H and CIO in dB : reporting range 0 to 14.5 dB and hysteresis 0 to 7.5 dB, in 0.5 dB steps.
• Time to trigger and periodic reporting : a report needs a stable condition, then repeats until the RNC acts.

Events in RRC Signalling

How does the event reach the UE and come back to the RNC? The RNC configures the event in the MEASUREMENT CONTROL message, and the UE names the event that fired in the MEASUREMENT REPORT. Both use the definitions below.

Following is based on 25.331 v19.0.1 (Release 19)

EventIDIntraFreq ::=				ENUMERATED {
										e1a, e1b, e1c, e1d, e1e,
										e1f, e1g, e1h, e1i, e1j,
										spare6, spare5, spare4, spare3, spare2,
										spare1 }

EventIDInterFreq ::=				ENUMERATED {
										e2a, e2b, e2c, e2d, e2e, e2f, e2g, spare1 }

EventIDInterRAT ::=					ENUMERATED {
										e3a, e3b, e3c, e3d }

The event identities above are what a MEASUREMENT REPORT carries in its event results. The intra-frequency list keeps spares after e1j, while inter-frequency ends at e2g and inter-RAT at e3d. The configuration below carries the parameters of each event. The release suffix grows as parameters were added: IntraFreqEvent-r7 is the latest intra-frequency choice, and InterFreqEvent-r12 and InterRATEvent-r12 are the latest for the other two groups.

Following is based on 25.331 v19.0.1 (Release 19)

IntraFreqEvent-r7 ::=				CHOICE {
	e1a									Event1a-r4,
	e1b									Event1b-r7,
	e1c									Event1c,
	e1d									Event1d,
	e1e									Event1e-r6,
	e1f									Event1f-r6,
	e1g									NULL,
	e1h									ThresholdUsedFrequency-r6,
	e1i									ThresholdUsedFrequency-r6,
	e1j									Event1j-r6
}

Event1a-r4 ::=						SEQUENCE {
	triggeringCondition					TriggeringCondition2,
	reportingRange						ReportingRange,
	forbiddenAffectCellList				ForbiddenAffectCellList-r4				OPTIONAL,
	w									W,
	reportDeactivationThreshold			ReportDeactivationThreshold,
	reportingAmount						ReportingAmount,
	reportingInterval					ReportingInterval
}

InterFreqEvent-r12 ::=				CHOICE {
	event2a								Event2a-r11,
	event2b								Event2b-r11,
	event2c								Event2c-r11,
	event2d								Event2d-r10,
	event2e								Event2e-r11,
	event2f								Event2f-r10,
	event2g								Event2g
}

InterRATEvent-r12 ::=				CHOICE {
	event3a								Event3a-r12,
	event3b								Event3b-r12,
	event3c								Event3c-r12,
	event3d								Event3d
}

Event1a-r4 holds the parameters of the equation in the previous section. The ASN.1 gives those parameters as integers. The tabular part of 25.331 scales them: the reporting range and the hysteresis in steps of 0.5 dB, and W in steps of 0.1.

Following is based on 25.331 v19.0.1 (Release 19)

ReportingRange ::=					INTEGER (0..29)

W ::=								INTEGER (0..20)

Hysteresis ::=						INTEGER (0..15)

TimeToTrigger ::=					ENUMERATED {
										ttt0, ttt10, ttt20, ttt40, ttt60,
										ttt80, ttt100, ttt120, ttt160,
										ttt200, ttt240, tt320, ttt640,
										ttt1280, ttt2560, ttt5000 }

TimeToTrigger is given in milliseconds, from 0 to 5000. The value name tt320 is written without the third t in the specification itself, and it means 320 ms.

• EventIDIntraFreq, EventIDInterFreq, EventIDInterRAT : the event names used in the MEASUREMENT REPORT.
• IntraFreqEvent-r7, InterFreqEvent-r12, InterRATEvent-r12 : the latest configuration choices in 25.331 v19.0.1.
• Integer IEs : scaled by 0.5 dB or 0.1 in the tabular definitions.

Reference

[1] 3GPP TS 25.331 v19.0.1 - clause 14, Specific functions: measurement reporting events; clause 11.3, Information element definitions