5GNR: Measurement and Reporting configuration

The network configure a RRC CONNECTED UE to perform measurements and UE report measurement report in accordance with the measurement configuration.

As per current version of specification, the network may configure the UE to perform the following types of measurements.

1. NR measurements.
2. Inter-RAT measurements of E-UTRA frequencies.

NR use both SSB based measurements and CSI-RS based measurements for handover. We will discuss the use case of SSB and CSI-RS based measurement further down in this article.

Likewise LTE, in NR also, the CSI-RS are used for Channel State Information (CSI) acquisition, which is important for scheduling and link adaptation (i.e. control the allocation of number of PRB, modulation scheme etc). But In NR, CSI-RS use has been broadened further and they are also used for Reference Signal Received Power (RSRP) measurements, which is important for mobility management.

Thus CSI-RS in NR is used for downlink CSI estimation, cell mobility and beam management.

Use case of CSI-RS:

1. For UEs in RRC CONNECTED state, CSI-RS can be configured in order to improve the quality of UE measurements and to provide better user eccentric mobility experience. For example, in high-frequency bands, narrow-beam CSI-RS can be configured for the UEs at the edge of the cell in order to achieve better signal-to-interference-plus-noise ratio (SINR) range and measurement accuracy. So for the UE at the cell edge its good to configure CSI-RS for better measurement accuracy.
2. The SSBs are blindly decoded, however, in order to decode the CSI-RS, the UEs must be already in connected mode, since they need to be configured in advance by the gNB.
3. SSBs generally used with wide beams and CSI-RSs with narrow ones. On one hand, wide SSBs make the system more robust to blockage, since they can propagate signals in more directions. On the other hand, narrow CSI-RSs provide better Signal to Noise Ratio (SNR) and better measurement accuracy.

Note — CSI-RS measurement configuration used for beam mobility is different from the CSI-RS measurement configuration used for cell mobility. For beam mobility and channel state information, this is configured under IE (csi-measConfig) whereas for cell level mobility this is configured under (measConfig). Refer Fig 1.

Fig 1:- CSi-MeasConfig (used for beam mobility) and measConfig (used for cell mobility)

Fig 2: Main IE’s under meas configuration

After doing the measurement on the configured reference signal (SSB/CSI-RS), UE send the measurement report to n/w provided that reporting criteria met. Below is the possible way to send the MR report to the n/w, Fig -3

Fig 3: -Possible way to send measurement report

Fig 2, MeasObjectToRemove, reportConfigToRemove and measIdToRemove is eaxact similar like to LTE, simply release the measObj ID, reportConfig ID and measID respectively.

Discussed so far — Reference signal (SSB/CSI-RS)for measurement and possible use case of reference signal.

Here further we will discuss about measObjectToAddMod and reportConfigToAddMod in the detail.

measObjectToAddMod

Fig 4: IE structure of measObjNR

1. ssbFrequency: -Indicates the frequency of the SSB block associated to this MeasObjectNR.
2. ssbSubcarrierSpacing: Subcarrier spacing of SSB block. Only the values 15 or 30 (<6GHz), 120 kHz or 240 kHz (>6GHz) are applicable.
3. SMTC (SSB based measurement timing configuration): — This is one of the important parameter. As we know the possible SSB burst periodicity is 5, 10, 20, 40, 80 and 160ms but for a UE in connected mode, it is not required to measure with this periodicity and the appropriate measurement periodicity can be configured according to the channel conditions. This would avoid unnecessary measurements and save UE power also at some extent. The SMTC window has been introduced to notify the UE about the periodicity and the timing of the SSBs that the UE must use for cell quality measurements. Refer Fig 5 ( for Cell B, SSB periodicity and SMTC periodicity is different). Meas Gap length should be greater then SMTC window length (refer fig 6). Thus n/w appropriately set the SMTC window and meas gap length based on the SSB burst periodicity for the corresponding measObject.

A UE does not major any SSB index out of the SMTC duration.

Fig 5: SSB Periodicity and SMTC periodicity

Fig 6: Relation between SMTC window and Meas Gap Length

For SMTC configuration, refer fig 7. For the exact calculation of SFN and subframe, refer (TS 38.331, 5.5.2.10).

Duration (fig 7) of the measurement window in which to receive SS/PBCH blocks. It is given in number of subframes

Fig 7: SMTC Periodicity and duration

Reference signal config: As discussed in the starting of this article, there are two reference signal which are used for the measurement, as given below

1. SSB-ConfigMobility
2. csi-rs-resourceConfigMobility

Fig 8: SSB-configMobility

SSB-ConfigMobility (ssb-ToMeasure) : The set of SS blocks to be measured within the SMTC measurement duration. The first/ leftmost bit corresponds to SS/PBCH block index 0, the second bit corresponds to SS/PBCH block index 1, and so on. Value 0 in the bitmap indicates that the corresponding SS/PBCH block is not to be measured while value 1 indicates that the corresponding SS/PBCH block is to be measured. When the field is not configured the UE measures on all SS blocks. Regardless of the value of this field, SS/PBCH block outside of the applicable smtc are not to be measured.

Example — shortBitmap (1110) -> measure SSB index 0, 1 and 2 here.

CSI-RS-resourceConfigMobility: Refer Fig (9).

1. csi-RS-CellList-Mobility: List of cells.
2. csi-rs-ResourceList-Mobility: List of CSI-RS resources for mobility.
3. csi-rs-measurementBW- For CSI-RS measurement in intra-frequency measurements, UE is expected to perform CSI measurement within its active DL BWP at the time when the measurement occurs. For inter freq measurement, BW can be wider than active BWP but it should be less than UE supported max BW. For CSI-RS based RRM, if CSI-RS numerology is different from the active DL BWP numerology, the UE is not required to receive any DL channels and signals in the active DL BWP during the duration that UE performs CSI-RS measurement in the same active DL BWP.
4. associated-SSB: The higher layer parameter associated-SSB configures the index of associated SS/PBCH block for a CSI-RS resource (A wide SSB beam may have multiple CSI-RS based narrow beam, hence concept of associated SSB is used) The associated SS/PBCH block and CSI-RS resource are from cell indicated by the high layer parameter cellId (Fig 9). For a given CSI-RS resource, if the associated SS/PBCH block is configured but not detected by the UE, UE is not required to monitor the corresponding CSI-RS resource.
5. refServCellIndex: Indicates the serving cell providing the timing reference for CSI-RS resources without associatedSSB. The field may be present only if there is at least one CSI-RS resource configured without associatedSSB. In case there is at least one CSI-RS resource configured without associatedSSB and this field is absent, the UE shall use the timing of the PCell.
6. slotConfig: Indicates the CSI-RS periodicity (in milliseconds) and for each periodicity the offset (in number of slots). As CSI-RS are UE specific reference signal, so UE must know the availability (Time/freq domain) information to measure it.

Fig 9: CSI-RS-resourceConfigMobility

What NR Intra Freq measurement: SSB centre frequency and the SCS of the serving cell and the SSB centre frequency and SCS of the neighbor cell are the same.

Further refinement is ongoing… Please share your suggestion and feedback.