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I work for an internationally renowned terminal manufacturer and am responsible for the research and development of modem chips.
In the early days of 5G, he was responsible for the development of the terminal data service layer and the core network. Currently, he is leading the research on technical standards for 6G computing power networks.
The content of the blog mainly revolves around:
5G/6G protocol explanation
computing power network explanation (cloud computing, edge computing, end computing)
advanced C language explanation
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1. Introduction to NR measurement events
In any telecommunication technology (2G, 3G, 4G or 5G), mobility decisions are made by base stations based on measurement reports from mobile devices. To measure the signal quality of the serving cell and neighboring cells, there are various measurement types (for example, RSRP, RSRQ, SINR) and various measurement methods (for example, periodicity, event triggering).
Ideally, the base station should allow the UE to report the signal quality of the serving cell and neighboring cells, and trigger handover through a single measurement, but in actual use, the problem of ping-pong handover needs to be considered. In order to avoid this situation, the 3GPP specification proposes a set of predefined measurement reporting mechanisms performed by the UE. These predefined measurement report types are called "events". The "event" type reported by the terminal is specified by the RRC signaling message sent by the base station .
2. The "event" type defined by 3GPP
- A1 event: the signal quality of the serving cell is higher than the specified threshold;
- A2 event: The signal quality of the serving cell is lower than the specified threshold;
- A3 event: The service quality of the adjacent cell is higher than the SpCell service quality by a specified offset value;
- A4 event: The signal quality of the neighboring cell is higher than the specified threshold;
- A5 event: The signal quality of the serving cell is lower than the specified threshold, while the signal quality of the neighboring cell is higher than the specified threshold;
- A6 event: The service quality of the adjacent cell is higher than the SCell service quality by a specified offset value;
- B1 event: The signal quality of neighboring cells in different systems is higher than the specified threshold;
- B2 event: The signal quality of the PCell is lower than the specified threshold, but the signal quality of the adjacent cell of the different system is higher than the specified threshold;
If we observe these events carefully, we can classify A1~A6 as events under the same system , and B1~B2 as events between different systems . The terminal will always measure the serving cell and neighboring cells, and verify it according to the threshold or offset defined in the reporting configuration, and report the measurement amount if the conditions are met. RSRP, RSRQ, or SINR can all trigger QoS reporting events.
The following table summarizes the parameters used in the different events and their ranges:
| event | parameter | Value range in signaling | Actual value range |
|---|---|---|---|
| A1，A2，A4，A5，B1 | RSRP Threshold | 0~127 | -156~-31 dBm |
| RSRQ Threshold | 0~127 | -40~20 dB | |
| SINR threshold | 0~127 | -23~40 dB | |
| all events | Hysteresis (hysteresis value) | 0~30 | 0~15 dB |
| A3，A6 | Offset (offset value) | -30~30 | -15~15 dB |
| A3，A4，A5，A6，B1，B2 | cell-specific offset | -24~24 dB | |
| B1，B2 | LTE RSRP | 0~97 | -140~-44 dBm |
| LTE RSRQ | 0~34 | -19.5~-3 dB | |
| LTE SINR | -23~40 | -23~40 dB |
3. Introduction of each "event"
3.1 A1 event
The A1 event is triggered when the quality of the serving cell exceeds the threshold. It is typically used to cancel an ongoing handover process . This may be required if the UE moves towards the edge of the cell and triggers the handover procedure, but then returns to good coverage before the handover procedure completes.
The following figure shows the conditions for triggering and canceling:
- Ms - Hys (hysteresis value) > Thresh (triggers A1 event)
- Ms + Hys (hysteresis value) < Thresh (stop A1 event)
The meanings of the parameters used in the above formula are as follows:
- Ms: is the measurement result of the serving cell without considering any offset. For RSRP, the unit of measurement is dBm; for RSRQ and SINR, the unit of measurement is dB;
- Hys: It is the hysteresis parameter of A1 event, the unit is dB. Hys is configured through reportConfigNR , the value range is between 0 and 30, and the actual dB value can be obtained by multiplying by 0.5. For example, the configured value is 5, then Hys = 5 x 0.5 =2.5dB;
- Thresh: is the threshold of this event, its value is defined in reportConfigNR a1 -Threshold , the unit of the threshold is the same as the measured value;
3.1.1 Examples
Configure the threshold for the A1 event as -85dBm RSRP with 1 dB hysteresis. Assume that the measured RSRP of the serving cell is -89dBm.
- -89 - 1 > -85 This condition is not met, the terminal will not report the A1 event. Assuming that the UE moves to the center of the cell now, its RSRP status is improved, and the measured value is about -83dBm;
- -83 -1 > -85 This condition is met, and the terminal reports the A1 event.
3.2 A2 events
The A2 event is usually used to trigger the handover procedure when the UE moves towards the edge of the cell . The A2 event does not involve any neighbor cell measurements, so it can be used to trigger a blind handover procedure . Or it can be used to trigger measurements of neighboring cells, which can then be used for measurement-based handover procedures.
For example, after the A2 event is triggered, the gNB can configure measurement gaps and inter-frequency or inter-system measurements. This method means that the terminal only needs to complete intra-frequency/inter-frequency or inter-system measurement when the coverage condition is relatively poor and the probability of handover is high.
The following figure shows the trigger and cancel conditions:
- Ms + Hys < Thresh (triggers A2 event)
- Ms - Hys > Thresh (stop A2 event)
The meanings of the parameters used in the above formula are as follows:
- Ms: is the measurement result of the serving cell without considering any offset. For RSRP, the unit of measurement is dBm; for RSRQ and SINR, the unit of measurement is dB;
- Hys: It is the hysteresis parameter of A2 event, the unit is dB. Hys is configured through reportConfigNR , the value range is between 0 and 30, and the actual dB value can be obtained by multiplying by 0.5. For example, the configured value is 5, then Hys = 5 x 0.5 =2.5dB;
- Thresh: is the threshold of this event, its value is defined in a2-Threshold in reportConfigNR , the unit of the threshold is the same as the measured value;
3.2.1 Examples
Configure the threshold for event A2 as -75dBm RSRP with 1 dB hysteresis. Assume that the measured RSRP of the serving cell is -69dBm.
- -69 + 1 < -75 Obviously the condition is not met, and the terminal will not report the A2 event. Assuming that the UE moves to the edge of the cell now, its RSRP condition becomes worse, and the measured value is about -78dBm;
- -83 + 1 < -75 conditions are met, and the terminal reports the A2 event;
3.3 A3 events
The A3 event is triggered when a neighboring cell is better than a specific cell (SpCell) by an offset value. This particular cell may be the primary serving cell of the Primary Cell Group (MCG) or the Secondary Cell Group (SCG). The offset can be positive or negative.
This event is usually used for intra-frequency or inter-frequency handover . When the A2 event is triggered, the measurement gap can be configured to measure the inter-frequency object and the A3 event for inter-frequency handover. The A3 event provides a handover triggering mechanism based on relative measurement results , for example, when the RSRP of a neighboring cell is stronger than that of a specific cell.
The following figure shows the trigger and cancel conditions:
- Mn + Ofn + Ocn – Hys > Mp + Ofp + Ocp + Off (triggers A3 event)
- Mn + Ofn + Ocn + Hys < Mp + Ofp + Ocp + Off (stop A3 event)
The meanings of the parameters used in the above formula are as follows:
- Mn: is the measurement result of the neighboring cell, without considering any offset;
- Ofn: is the specific offset of the neighbor cell reference signal, that is, the offsetMO defined in the measObjectNR corresponding to the neighbor cell ;
- Ocn: is the specific offset of the adjacent cell, that is, the cellIndividualOffset defined in measObjectNR corresponds to the frequency of the adjacent cell, and if it is not configured for the adjacent cell, it is set to zero;
- Mp: is the measurement result of SpCell, without considering any offset;
- Ofp: is the specific offset of the SpCell measurement object, which corresponds to the offsetMO defined in the measObjectNR of the SpCell ;
- Ocp: is a SpCell-specific offset, which corresponds to the cellIndividualOffset defined in SpCell's measObjectNR , and is set to zero if it is not configured for SpCell;
- Hys: same meaning as above;
- Off: is the offset parameter of this event, defined in a3-Offset in reportConfigNR ;
- Mn, Mp: RSRP is expressed in dBm, RSRQ and SINR are expressed in dB;
3.3.1 Examples
Suppose a3-offset is set to 3dB, hys, Ofn, Ofp and Ocp are set to 0. Once the UE finds that the measured value of any neighboring cell is 3dB better than that of the serving cell, it shall report the A3 event. For example, the neighbor cell RSRP=-78dB, the serving cell RSRP=-82, here the neighbor cell is better and meets the A3 event offset, so the UE reports the A3 event to the gNB.
3.4 A4 events
When the A4 event is triggered, the service quality of the neighboring cell is higher than the defined threshold. This event can be used for handover procedures that do not depend on the coverage of the service unit . For example, in the load balancing function , it is decided to handover the UE from the serving cell according to the load condition rather than the radio link condition. In this case, the UE only needs to verify that the target cell is above a certain signal threshold and can provide sufficient coverage.
The following figure shows the trigger and cancel conditions:
- Mn + Ofn + Ocn – Hys > Thresh (triggers A4 event)
- Mn + Ofn + Ocn + Hys < Thresh (stop A4 event)
The meanings of the parameters used in the above formula are as follows:
- Mn: same meaning as above;
- Ofn: same meaning as above;
- Ocn: same meaning as above;
- Hys: same meaning as above;
- Thresh: The meaning is the same as above, which corresponds to a4-Threshold in reportConfigNR ;
3.4.1 Examples
Suppose the threshold RSRP of A4 event = -80 dBm, hys and other offsets are set to 0. Once the UE finds that the measured RSRP value of any neighboring cell is higher than -80 dBm, the UE shall report this event. For example, the RSRP measured by the neighboring cell = - 76 dBm, and the A4 event condition is met, the UE will report the event to the gNB.
3.5 A5 events
The A5 event is triggered when the quality of service of a specific cell is lower than the threshold, while the quality of service of neighboring cells is higher than the threshold. You can think of the A5 event as a combination of the A2 event and the A4 event.
This event is usually used in the same-frequency or inter-frequency handover process . After the A2 event is triggered, the measurement gap and the A5 event can be configured for inter-frequency switching. A5 events provide a handover triggering mechanism based on absolute measurements . When the signal quality of a specific cell deteriorates and it is necessary to switch to another cell that does not meet the A3 event, the A5 event can be used to trigger a time-sensitive handover .
The following figure shows the trigger and cancel conditions:
Event trigger condition:
- Mp + Hys < Thresh1
- Mn+ Ofn +Ocn – Hys > Thresh2
Event exit condition:
- Mp – Hys > Thresh1
- Mn+ Ofn +Ocn + Hys < Thresh2
The meanings of the parameters used in the above formula are as follows:
- Mp, Mn, Ofn, Ocn, Hys: same meaning as above;
- Thresh1: The meaning is the same as above, corresponding to a5-Threshold1 in reportConfigNR , and the unit is the same as Mp;
- Thresh2: The meaning is the same as above, corresponding to a5-Threshold2 in reportConfigNR , and the unit is the same as Mn;
3.5.1 Examples
Assume that a5-Threshold1 is set to RSRP= -85dBm, a5-Threshold2 is set to RSRP= - 80db, and other offsets and hysteresis are set to 0. UE Report A5 When the RSRP of the serving NR cell is lower than -85 dBm and the RSRP of the neighboring NR cell is higher than -80, both conditions need to be met at the same time. Assuming that the UE serving RSRP is -90 dBm and the neighbor cell RSRP is -78 dBm, and these two thresholds are met, then the UE can report the A5 event to the gNB.
3.6 A6 event
The A6 event is triggered when the neighbor cell is better than the secondary cell by an offset . Offsets can be positive or negative. This measurement report event is applicable to carrier aggregation , that is, there is a connection of a secondary serving cell in addition to the primary serving cell.
The following figure shows the trigger and cancel conditions:
- Mn + Ocn – Hys > Ms + Ocs + Off (triggers A6 event)
- Mn + Ocn + Hys < Ms + Ocs + Off (stop A6 event)
The meanings of the parameters used in the above formula are as follows:
- Mn, Ocn, Hys: the meaning is the same as above;
- Ms: is the measurement result of the secondary serving cell, without considering any offset;
- Ocs: is the cell-specific offset of the secondary serving cell, corresponding to the cellIndividualOffset defined in measObjectNR , if not configured, it is set to zero;
- Off: is the offset parameter, corresponding to a6-Offset in reportConfigNR ;
- For Mn and Ms, if it is RSRP, the unit is dBm, if it is RSRQ and RS-SINR, the unit is dB;
- The units of Ocn, Ocs, Hys, and Off are dB;
3.7 B1 events
The B1 event can be used in the handover process between different systems, and this process does not depend on the coverage of the serving cell . For example, in the load balancing function , the decision whether to handover the UE to LTE is based on the load situation rather than the radio link condition. In this case, the UE only needs to verify that the service quality of the target cell in other systems (such as LTE) is above a certain signal level threshold and can provide sufficient coverage.
The following figure shows the trigger and cancel conditions:
- Mn + Ofn + Ocn – Hys > Thresh (triggers B1 event)
- Mn + Ofn + Ocn + Hys < Thresh (stop B1 event)
The meanings of the parameters used in the above formula are as follows:
- Mn: Measurement results of adjacent cells in different systems, without considering any offset;
- Ofn: the frequency-specific offset corresponding to the adjacent cells of different systems, which is defined in the eutra-QOffsetRange in measObjectEUTRA ;
- Ocn: The cell-specific offset of the adjacent cell of the different system, which is defined in the cellIndividualOffset in measObjectEUTRA . If it is not defined, it is set to 0 by default;
- Hys: Hysteresis parameter, which is defined in reportConfigInterRAT ;
- Thresh: threshold value, its unit is the same as Mn , and its value is defined in b1-ThresholdEUTRA in reportConfigInterRAT ;
- Mn: Its unit can be dB or dBm, depending on the signal type of the adjacent cell of the different system to be measured;
- The unit of Ofn, Ocn, Hys is dB;
3.8 B2 events
When the signal quality of a primary serving cell is lower than a threshold, but the service quality of an adjacent cell of a different system is higher than a threshold, a B2 event is triggered. This can be used to trigger inter-system handover when the primary serving cell becomes poor.
The following figure shows the trigger and cancel conditions:
Triggering conditions:
- Mp + Hys < Thresh1
- Mn + Ofn + Ocn – Hys > Thresh2
Exit condition:
- Mp – Hys > Thresh1
- Mn + Ofn + Ocn + Hys < Thresh2
The meaning of the parameters is the same as above, so I won’t go into details
