# 5G UE Measurements and Reporting

## Summary


Measurements are essential to determine the health of any cellular system given the current configuration. Measurements help the UE and the network make decisions so that resources are managed better and ultimately quality of service is achieved. Measurements are done by both UE and the network, although this article focuses only on measurements performed by a 5G UE. 

Typically, a UE measures downlink signals while network measures uplink signals. However, it's possible for a UE to measure uplink signals sent by other UEs. 

RRC manages measurement configuration. Most measurements are executed by Layer 1, although some may be at Layer 2. RRC does filtering on Layer 1 measurements. If filtered measurements meet reporting criteria, they're reported to the network. Some measurements are reported by Layer 1 directly to the network.

While UE measures many different signals, the main ones are based on **SSB** and **CSI-RS**.

## Discussion

### What are some acronyms pertaining to 5G UE measurements?

For convenience we use these acronyms related to measurements in this article: Channel State Information (CSI), Demodulation Reference Signal (DMRS), Reference Signal (RS), Reference Signal Received Power (RSRP), Reference Signal Received Power per Branch (RSRPB), Reference Signal Received Quality (RSRQ), Received Signal Strength Indicator (RSSI), Signal-to-Noise and Interference Ratio (SINR), Synchronization Signal (SS) and Synchronization Signal Block (SSB). 


### With respect to RRC states, what measurements are done by a 5G UE?

An essential UE procedure in RRC\_IDLE is cell selection. In RRC\_IDLE and RRC\_INACTIVE, the UE can also do cell reselection. For both these procedures, UE measures RSRP and RSRQ of a cell. 

For cell reselection, if supported and enabled, UE may do **Relaxed Measurements**. This is typically useful when the UE has low mobility or not at the cell edge. 

If configured, a UE in RRC\_IDLE or RRC\_INACTIVE may collect measurements and report them later in RRC\_CONNECTED. This is called **Logged Measurements**. It's related to a feature called *Minimization of Drive Test (MDT)*. 

In RRC\_CONNECTED, the UE is configured via dedicated signalling to perform intra-frequency or inter-frequency NR measurements, or inter-RAT measurements for E-UTRA or UTRA-FDD frequencies. The network uses these to decide on carrier aggregation, dual connectivity or handover.  


### What are some basic facts about 5G UE measurements?

For downlink channel sounding, 5G NR uses two main downlink signals that a UE measures:

    + **SSB**: Transmitted with a low duty cycle on a limited bandwidth compared to LTE's Cell-Specific Reference Signals (CRS) that's sent on the entire channel bandwidth. SSB measurements are used to determine path loss and average channel quality.
    + **CSI-RS**: Used for tracking rapidly changing channel conditions to support mobility and beam management.In general, measurements in FR1 are from UE's antenna connector. In FR2, measurements are based on the combined signal from antenna elements mapped to a given receive branch. If UE is using receiver diversity, it reports the maximum value. 

Although a UE can be configured for measurements early on, measurement reports can be sent to the network only after **AS security activation**. 


### Which are the main quantities measured by a 5G UE?

We describe the main UE measurements involving SS:

    + **SS-RSRP**: Average power of resource elements carrying secondary synchronization signals. In addition, resource elements of PBCH-DMRS and CSI-RS can be included.
    + **SS-RSRPB**: Similar to SS-RSRP but for each antenna connector (FR1) or for each receive branch (FR2). Measurements can include PBCH-DMRS but not CSI-RS.
    + **SS-RSRQ**: Given N resource blocks within the measurement bandwidth, this is \(N \cdot SS{\text-}RSRP/RSSI\_{NR\,Carrier}\). Both SS-RSRP and NR carrier RSSI are measured over the same resource blocks.
    + **SS-SINR**: This is SS-RSRP over average noise-plus-interference power. The latter is measured based on RRC configuration or over the same resource elements as SS-RSRP measurement.Equivalent measurements pertaining to CSI-RS are CSI-RSRP, CSI-RSRQ and CSI-SINR. 

**RSSI** is average power over certain OFDM symbols in a measurement bandwidth corresponding to channel bandwidth. It includes co-channel serving and non-serving cells, adjacent channel interference, thermal noise, etc. 


### Which are some feature-specific measurements done by a 5G UE?

Measurements for inter-RAT include: 

    + IEEE 802.11 WLAN RSSI: for handovers to Wi-Fi
    + Reference Signal Time Difference (RSTD) for E-UTRA: relative timing difference between an E-UTRA cell and the E-UTRA reference cell
    + E-UTRA RSRP, RSRQ and RS-SINR
    + UTRA FDD CPICH RSCP, UTRA FDD carrier RSSI and UTRA FDD CPICH Ec/NoMeasurements for MR-DC include: 

    + SFN and Frame Timing Difference (SFTD): Measured between PCell and PSCell.Measurements for sidelink channels include: 

    + Sidelink RSSI
    + Sidelink Channel Occupancy Ratio (SL CR)
    + Sidelink Channel Busy Ratio (SL CBR)
    + PSBCH-RSRP, PSSCH-RSRP and PSCCH-RSRPMeasurements for UE positioning include:  

    + Timing between a E-UTRA cell and a GNSS-specific reference time
    + DL PRS-RSRP measured on Positioning Reference Signal (PRS)
    + DL Reference Signal Time Difference (RSTD) measures the relative time difference between a Transmission Point (TP) and the reference TP
    + UE Rx–Tx time difference measured per TP
    + SS Reference Signal Antenna Relative Phase (SS-RSARP)

### What are Cross Link Interference (CLI) measurements?

CLI is a problem in TDD when a base station receiving in the uplink is facing interference from another base station transmitting in the downlink. It can happen across network operators due to out-of-band emissions. CLI can be mitigated by time-synchronization across base stations, that is, they share a common clock, phase reference and frame structure. 

Even within the same operator network, CLI can occur since cell neighbours may be using different TDD DL/UL patterns. CLI can be mitigated by gNBs coordinating their configuration over Xn and F1 interfaces. 

If capable, a 5G UE measures and reports CLI-RSSI. These reports can also include SRS-RSRP measurements on Sounding Reference Signal (SRS), which are uplink signals coming from other UEs. This quantifies interference on downlink due to nearby uplink transmissions. Both CLI-RSSI and SRS-RSRP are measured within the active DL BWP. These are applicable only for RRC\_CONNECTED intra-frequency in TDD mode. 

In EN-DC and NGEN-DC, Secondary Node (SN) configures CLI measurements. In NE-DC, Master Node (MN) does it. In NR-DC, both MN and SN can do this. 


### What's the difference between 5G NR L1 and L3 measurements?

RSRP, RSRQ, SINR, and RSSI are quantities measured at L1, not L3. We use the term "L3 measurement" to imply that L3 does filtering on the values and does the final reporting. Filtering is done to remove the effect of fast fading and ignore short-term variations. Though L1 may collect measurements more often, L3 might report them at a larger configured periodicity. Thus, L3 takes a longer-term view of channel conditions. 

To avoid ping-pong behaviour and unnecessary reporting, L3 manages event-based reporting. It evaluates reporting criteria to decide if a report needs to be sent. Apart from thresholds, such criteria include hysteresis. 

But L1 reports some measurements to quickly react to changing channel conditions. Beam management is an example. These are referred to as L1-RSCP and L1-SINR. L1 reports are part of Channel State Information (CSI).  

CSI-RS measurements and L1 reporting can be periodic, semi-periodic (activated/deactivated by MAC signalling) or aperiodic (triggered by DCI signalling). L1 reporting is on PUCCH (periodic, semi-periodic) or PUSCH (aperiodic, semi-periodic). 


### What L2 measurements are reported by a 5G UE to the network?

At Layer 2, there's measurement and reporting of **UL PDCP delay** for packets during the reporting period. Delay is reported at a granularity of 0.1ms and per Data Radio Bearer (DRB). At most one measurement identity per cell group has this quantity configured for reporting. The corresponding measurement object is ignored. 

The delay is in fact queuing delay. It's the time taken to obtain uplink grant from the time packet enters PDCP from upper SAP. It's up to gNB to convert these per-DRB reports to delays at the level of QoS flows. 

For completeness, we note that many more L2 measurements are done on the network side. These are described in TS 38.314 specification. 


### What's in a typical 5G NR measurement configuration?

A 5G UE is given the following measurement details: 

    + **Measurement Objects**: Specifies what is to be measured. For NR and inter-RAT E-UTRA measurements, this may include cell-specific offsets, blacklisted cells to be ignored and whitelisted cells to consider for measurements.
    + **Reporting Configuration**: Specifies how reporting should be done. This could be periodic or event-triggered.
    + **Measurement ID**: Identifies how to report measurements of a specific object. This is a many-to-many mapping: a measurement object could have multiple reporting configurations, a reporting configuration could apply to multiple objects. A unique ID is used for each object-to-report-config association. When UE sends a MeasurementReport message, a single ID and related measurements are included in the message.
    + **Quantity Configuration**: Specifies parameters for layer 3 filtering of measurements. Only after filtering, reporting criteria are evaluated. The formula used is \(F\_n = (1–a)\*F\_{n-1} + a\*M\_n\), where \(M\) is the latest measurement, \(F\) is the filtered measurement, and \(a\) is based on configured filter coefficient.
    + **Measurement Gaps**: Periods that the UE may use to perform measurements.

### Which are the event-triggered measurements reported by 5G UE RRC?

Events are triggered based on thresholds, hysteresis and sometimes offsets. RRC specification defines the following: 

    + Event A1: Serving becomes better than threshold
    + Event A2: Serving becomes worse than threshold
    + Event A3: Neighbour becomes offset better than SpCell
    + Event A4: Neighbour becomes better than threshold
    + Event A5: SpCell becomes worse than threshold1 and neighbour becomes better than threshold2
    + Event A6: Neighbour becomes offset better than SCell
    + Event B1: Inter RAT neighbour becomes better than threshold
    + Event B2: PCell becomes worse than threshold1 and inter RAT neighbour becomes better than threshold2
    + Event I1: Interference becomes higher than threshold
    + Event C1: The NR sidelink channel busy ratio is above a threshold
    + Event C2: The NR sidelink channel busy ratio is below a thresholdOnly the serving cell is relevant for events A1/A2. For other events, consider only whitelisted cells if enabled; else consider any neighbour cell detected based on the measurement object configuration provided it's not in the blacklist. 

Events B1/B2 relate to inter-RAT. They're used in EN-DC deployments as well where they're reported via E-UTRA RRC signalling. 


### How do I interpret events used in event-triggered measurement reporting?

A1 is typically used to **cancel a handover procedure** since the UE has re-established good coverage on the serving cell. With A2, UE has poor coverage on serving cell. Since neighbour cell measurements are not available with A2, network can initiate a **blind handover**; or provide UE configuration to perform neighbour cell measurements (eg. A3 or A5). 

Events A3 and A6 contain offsets specific to a neighbour cell. Both involve relative measurements, that is, comparing one cell with another. A3 may lead to intra- or inter-frequency handover away from the Special Cell (SpCell). A6 is relevant to carrier aggregation, where a Secondary Cell (SCell) is configured. A6 may not result in a handover but instead **reconfiguration of cell groups** (MCG or SCG). 

A4 could trigger a handover but the decision is not based on radio conditions on the serving cell. It could be for other reasons such as **load balancing**. B1 is similar for the inter-RAT case. 

A5 can be seen as a combination of A2 and A4, leading to intra- or inter-frequency handover. B2 is similar for the **inter-RAT** case. 


### Which are the 3GPP specifications relevant to 5G UE measurements?

We note the following specifications: 

    + **TS 37.340**: Multi-connectivity overall description: Stage-2. Specifies measurement model for multi-connectivity operation involving E-UTRA and NR.
    + **TS 38.133**: Requirements for support of radio resource management. Specifies measurement requirements (including performance requirements), procedures and UE measurement capabilities.
    + **TS 38.215**: Physical layer measurements. Specifies measurement quantities.
    + **TS 38.300**: NR and NG-RAN overall description: Stage-2. Specifies measurement model.
    + **TS 38.304**: User Equipment (UE) procedures in idle mode and in RRC Inactive state. Specifies cell reselection measurement rules and relaxed measurements.
    + **TS 38.314**: Layer 2 measurements. Specifies mostly network requirements but section 4.3 is for UE.
    + **TS 38.331**: Radio Resource Control (RRC): protocol specification. Specifies UE reporting of measurements.
    + **TS 38.533**: User Equipment (UE) conformance specification: Radio Resource Management (RRM).Specification *TS 28.552: 5G performance measurements* is mostly about network-side measurements including network slicing. We mention it here for completeness.  

## Milestones

Dec  
2017

3GPP publishes Release 15 "early drop". In 5G RRC specification TS 38.331, version 15.0.0, measurement configurations specified include measurement objects, measurement identifies, reporting, gaps, L3 filtering, and events A1-A6. Measurements supported include SS-RSRP, SS-RSRQ, SS-CINR, CSI-RSRP, CSI-RSRQ, CSI-CINR, RSSI and more. 

Dec  
2019

In TS 38.133, version 16.2.0, these are introduced: CLI measurements, UMTS inter-RAT measurements, SRVCC-related measurements, and more. 

Jul  
2020

3GPP publishes Release 16 specifications. In 5G RRC specification TS 38.331, version 16.3.1, has a number of additions: measurement configuration for RRC\_IDLE and RRC\_INACTIVE; CLI-RSSI and SRS-RSRP reporting; sidelink and UTRA-FDD reporting; UL PDCP delay reporting; IEs `UE-MeasurementsAvailable` and `needForGapsInfoNR` as part of RRCReconfigurationComplete and RRCResumeComplete messages; and UE positioning measurements.