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 the resources are managed better and with this, the quality of service is achieved. Measurements are done by the UE and the Network. A UE measures downlink signals while a network measures uplink signals. It is possible for a UE to measure uplink signals sent by other UE’s. RRC (Radio Resource Control) manages measurement configurations. Most measurements are executed by Layer 1, although some are at layer 2. RRC does filtering on layer 1 measurements. If the filtered measurements meet reporting criteria, they are reported to the network. Some measurements are reported by layer 1 directly to the network. The UE measures many different signals, the main ones are based on the SSB and CSI-RS.
For downlink channel sounding, 5G NR uses two main downlink signals that a UE measures
- SSB (Synchronization Signal Block): It is transmitted with a low duty cycle on a limited bandwidth as compared to LTE’s Cell-specific Reference signals (CRS) that are sent on the entire channel bandwidth. SSB measurements are used to determine path loss and average channel quality.
- CSI-RS (Channel State Information – Reference Signal): It is used for tracking rapidly changing channel conditions to support mobility and beam management.
Quantities measured by 5G UE
- SS-RSRP (Secondary Synchronization Signal- Reference Signal Received Power): It is the average power of the resource elements carrying secondary synchronization signals. In this, resource elements of PBCH-DMRS and CSI-RS can be included.
- SS-RSRPB: it is similar to SS-RSRP but for each antenna connector or for each receiver branch.
- SS-RSRQ (Secondary Synchronization Signal- Reference Signal Received Quality): it is the quality of the signal received and it is measured over the same resource block.
- SS-SINR (Secondary Synchronization Signal – Signal to Interference Noise power): it is SS-RSRP over average noise plus interference power.
- RSSI (Received Signal Strength Indicator): It is average power over certain OFDM symbols in a measurement bandwidth corresponding to the channel bandwidth. It includes co-channel serving and non-serving cells, adjacent channel interference, thermal noise, etc.
Event-triggered measurements reported by 5G UE RRC
Events are triggered based on thresholds, hysteresis, and offsets:
- Event A1: Serving becomes better than the threshold.
- Event A2: Serving becomes worse than the threshold.
- Event A3: Neighbor becomes offset better than the S cell
- Event A4: Neighbor becomes better than the threshold
- Event A5: Serving cell becomes worse than thrshold1 and neighbor becomes better than threshold 2.
- Event A6: Neighbor becomes offset better than S Cell
- Event B1: Inter-RAT neighbor becomes better than the threshold
- Event B2: P Cell becomes worse than threshold 1 and inter-RAT neighbor becomes better than threshold 2.
- Event I1: Interference becomes higher than the threshold.
- Event C1: The NR Sidelink channel busy ratio is above a threshold.
- Event C2: The NR Sidelink channel busy ratio is below the threshold.
Measurements are done by 5G UE
An important UE procedure in RRC_IDLE is the cell selection procedure. In RRC_IDLE and RRC_INACTIVE, the UE can also do cell reselection processes. For both these procedures, UE measures the RSRP and RSRQ of the cell. For Cell re-selection, the supported and enables UE may do Relaxed measurements. This could be useful when the UE has low mobility or is not located at the cell edge. If it is configured, a UE in RRC_IDLE or RRC_INACTIVE may collect measurements and report them later in RRC_CONNECTED. These are called logged measurements. It is related to a feature which is called Minimization of the Drive Test (MDT). In RRC_CONNECTED, the UE is configured via dedicated signaling to perform intra-frequency or inter-frequency NR measurements, or inter-RAT measurements for E-UTRA or UTRA-FDD frequencies. The network uses these parameters to decide on carrier aggregation, dual connectivity, or handover.
Some of the feature-specific measurements done by 5G UE:
- 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/No.
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-RSRP
5G NR Measurement configuration
- Measurement objects: It 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 configurations: It specifies how reporting should be done. This could be periodic or event-triggered.
- Measurement ID: It identifies how to report measurements of a specific object. When UE sends an MR message, a single ID and related measurements are included in the message.
- Quantity configuration: It specifies parameters for layer 3 filterings of measurements.
- Measurement gaps: periods that the UE may use to perform measurements.