LTE Measurements Understanding

There are several IEs to configure measurement control in detail, which are under measConfig IE in RRC Connection Reconfiguration message. They are not clearly described with specific picture but here is what I understand.

Form the top picture, we might think of an ideal case as (neighbour) cell measured value in yellow goes up linearly. Threshold is bottom line and offset (including several kinds of sub-offset values though) and hysteresis can be applied. Entering condition in this case would mean yellow line cross over threshold (+hysteresis) with blue "x" mark. From this point, timeToTrigger and reportInterval can be applied through the blue block. Leaving condition in this case could not be applied because it never get down go threshold (-hysteresis) as grey "x" mark. If measConfig needs to be removed, it can be managed by RRC Connection Reconfiguration procedure. From the bottom one, we can think of entering and leaving condition with blue "x" marks. At least leaving condition seems like buffer in order to keep UE measurement as steady as possible and avoid cell measured value fluctuated. There are two types of reportConfig - triggerType; event and periodical.

Trigger Type - Event

Measurement control with event trigger type (triggerType IE) allows UE do measure on its own and report it if it meets the criteria as the type configured. The concept of Measurement Events can be illustrated below, which is same way network emulators use in. The allocation of each event is my intent to show same logic of events; Event A3, A4, and A5 have same logic A6, B1, and B2.

According to 3GPP specs, each event has both entering and leaving condition. Entering condition is met from blue x mark and the report activity is supposed to be kept through the blue box area. Leaving condition can be ignored in order to understand how they work even though it is marked as grey x.

Event A1 (Serving becomes better than threshold)

Entering condition: Ms > Thresh + Hys
Leaving condition: Ms < Thresh - Hys

Event A2 (Serving becomes worse than threshold)

Entering condition: Ms < Thresh - Hys
Leaving condition: Ms > Thresh + Hys

Event A3 (Neighbour becomes offset better than PCell)

Entering condition: Mn > Ms + Thresh + Hys + offset
Leaving condition: Mn < Ms + Thresh + Hys + offset

Event A4 (Neighbour becomes better than threshold)

Entering condition: Mn > Thresh + Hys - offset
Leaving condition: Mn < Thresh - Hys - offset

Event A5 (PCell becomes worse than threshold1 and neighbour becomes better than thrreshold2)

Entering condition: Ms < Thresh1 - Hys & Mn > Thresh2 + Hys - offset
Leaving condition: Ms > Thresh1 + Hys & Mn < Thresh2 - Hys - offset

Event A6 (Neighbour becomes offset better than SCell)

Entering condition: Mn > Ms + Hys + offset
Leaving condition: Mn > Ms + Hys + offset

Event B1 (InterRAT neighbour becomes better than threshold)

Entering condition: Mn > Thresh + Hys - offset
Leaving condition: Mn < Thresh - Hys - offset

Event B2 (PCell becomes worse than threshold1 and interRAT neighbour becomes better than thrreshold2)

Entering condition: Ms < Thresh1 - Hys & Mn > Thresh2 + Hys - offset
Leaving condition: Ms > Thresh1 - Hys & Mn < Thresh2 + Hys - offset

Event B1 and B2 have the same logic as Event A4 and A5 have. Event A6 has exactly same logic as Event A3 has but relation between LTE SCell and neighbour cells. Event A6 must be considered under CA (Carrier Aggregation) and neighbour LTE cell environment. You may notice Ms (Measurement result of serving) and Mn (for neighbour). Offset was described above.

IEs described above are pretty much straightforward, which can be found in TS36.331 5.5.4 Measurement report triggering. Offset for Event A3 would be offsetFreq + cellIndividualOffset (under measObjectEUTRA to serving) + offsetFreq + cellIndividualOffset (under measObjectEUTRA to neighbour) + a3offet (under reportConfigEUTRA). Offset for Event A4 and A5 would be offsetFreq + cellIndividualOffset (under measObjectEUTRA to neighbour).
In order to configure report config, these kinds of IE nodes are required below. Here are two examples.

ReportConfigToAddMod

reportConfigId: 1
reportConfig: reportConfigEUTRA (0)
reportConfigEUTRA
  triggerType: event (0)
event
eventId: eventA5 (4)
  eventA5
a5-Threshold1: threshold-RSRP (0)
threshold-RSRP: -90dBm (50)
a5-Threshold2: threshold-RSRP (0)
threshold-RSRP: -80dBm (60)
  hysteresis: 1dB (2)
  timeToTrigger: ms320 (8)
  triggerQuantity: rsrp (0)
reportQuantity: both (1)
  maxReportCells: 1
  reportInterval: ms1024 (4)
  reportAmount: r8 (3)

ReportConfigToAddMod
reportConfigId: 1
reportConfig: reportConfigEUTRA (0)
reportConfigEUTRA
  triggerType: event (0)
  event
  eventId: eventA3 (2)
eventA3
a3-Offset: 0dB (0)
.... .0.. reportOnLeave: False
  hysteresis: 1dB (2)
  timeToTrigger: ms640 (11)
  triggerQuantity: rsrp (0)
  reportQuantity: both (1)
  maxReportCells: 1
  reportInterval: ms1024 (4)
  reportAmount: r8 (0)

# References

TS36.331 5.5.4 Measurement report triggering
TS36.331 6.3.5 Measurement information elements

RSRP - Allowed Measurement Bandwidth

UE performs measurements within the allowed measurement bandwidth (allowedMeasBandwidth IE under measObject IE), which is different from system bandwidth or transmission bandwidth. It is allowed to be chosen from mbw6 through mbw100 within the system bandwidth. TS36.214 says RSRP measurement is defined within the measurement bandwidth, which is found more specifically, within 6 RB (mbw6) in TS36.133. This is also can be found as measurement bandwidth for reelection and handover between different frequency carriers in TS36.521-3 RRC conformance testing spec. This is definition of RSRP in TS36.214 5.1.1 Reference Signal Received Power.

Reference signal received power (RSRP), is defined as the linear average over the power contributions (in [W]) of the resource elements that carry cell-specific reference signals within the considered measurement frequency bandwidth.

The real reported RSRP measurement values can be mapped to dBm value below. Typically we would think of "IE value - 140 = dBm value" though.

# References

TS36.214 5.1.1 Reference Signal Received Power (RSRP)
TS36.133 8.1.2.3 E-UTRAN inter frequency measurements
TS36.133 9.1.4 RSRP Measurement Reporting Mapping

L3 Filtering

Before moving to L3 filtering, we better think of what really happens during measurement on UE side. Here is how TS36.300 explained on measurement model. UE internal layer 1 filtering at point A is actually implementation dependent and not constrained by the standard.

Whenever UE L1 does measurements, the measured values are reported to RRC layer through L3 filtering; B and C above. Measured results are input of L3 filtering and the output of this filtering is reported to RRC layer. The filter function (equation) is defined in TS36.331 and requires a filter coefficient parameter value ( filterCoefficient IE), which is delivered by measurement control over RRC Connection Reconfiguration procedure. RRC does not seem like remembering any previous reported values and just evaluating reported values with criteria. One internal measurement path reports C and another does C, which are compared each other in RRC layer if requested by measurement control over RRC Connection Reconfiguration procedure. Finally if internal reported value fulfills the criteria, RRC prepares to report it through air link, which is called Measurement Report message.