Delta signaling in 5G NR

In 5G New Radio (NR), delta signaling refers to a signaling mechanism used to efficiently transmit control information between the base station (gNB) and user equipment (UE). It’s designed to reduce overhead and improve spectral efficiency in the wireless communication system. In traditional signaling, the UE and the network exchange all of the configuration parameters for each radio resource, even if the parameters have not changed since the last time they were communicated. This can lead to a significant amount of signaling overhead, especially when the UE is frequently changing cells or operating in different frequency bands.

Delta signaling involves sending only the difference (delta) between the current control information and the previously received control information, rather than sending the entire control message repeatedly. The network can only communicate to UE the changes to the configuration parameters since the last time they were communicated. This can significantly reduce the amount of signaling data that needs to be exchanged, which can improve the performance of the network and reduce latency.

During Handover from Source Cell to Target cell, if both the Source and target cell use same Configurations for certain Physical and control channels, the target cell can re-use the same configuration and modify the parameters which are different at target cell and send reconfiguration for only those parameters. This configuration change can significantly decrease the size of the RRC reconfiguration message during handover.

Delta signaling is used in a variety of scenarios in 5G NR, including:

  • Handover: When the UE moves from one cell to another, delta signaling can be used to update the UE’s configuration with the new cell’s parameters.
  • RRC resume: When the UE wakes up from an inactive state, delta signaling can be used to restore the UE’s configuration to the state it was in before it went to sleep. Delta configuration is used when UE moves from RRC_INACTIVE state to RRC_CONNCTED state. Network only sends the delta signaling that needs to be modified in which only changed parameters are signaled – to restore the configuration of a UE in the inactive state. This option is not possible for UEs in the idle state.
  • Paging: When the network needs to page the UE, delta signaling can be used to update the UE’s paging parameters.
  • Config update: When the network needs to update the UE’s configuration, delta signaling can be used to communicate the changes to the UE.

How network determines to use delta signaling ?

The network determines when to use delta signaling based on a number of factors, including:

  • The amount of change in the UE’s configuration: If the UE’s configuration has changed significantly since the last time it was communicated, the network will use full signaling to update the UE’s configuration. However, if the UE’s configuration has only changed slightly, the network may use delta signaling to update the UE’s configuration.
  • The cost of signaling: Delta signaling is more efficient than full signaling, but it also requires more processing power on the UE and the network. The network will need to weigh the cost of signaling against the benefits of using delta signaling.
  • The latency requirements: Delta signaling can introduce some latency, as the network needs to determine which parameters have changed since the last time they were communicated. The network will need to consider the latency requirements of the application when deciding whether to use delta signaling.

Lets take an example if Handover message and how delta signaling is determined

  • UE triggers Measurement Report, Measurement report can contain Multiple Cells and also include Beam Measurements of Target cells
  • gNB receives the Measurement reports and determines to perform Handover to target cell based on the measurementReport and RRM Information.
  • The source gNB sends Handover request message to Target cell, the information can include
    • Target Cell ID
    • C-RNTI of the UE in the source Gnb
    • Current RRC Reconfiguration provided to UE by Source Cell.
    • RRM-configuration including UE inactive time
    • Basic AS-configuration including antenna Info and DL Carrier Frequency
    • The current QoS flow to DRB mapping rules applied to the UE .
    • The SIB1 from source Gnb
    • The UE capabilities for different RATs,
    • PDU session related information, and can include the
    • UE reported measurement information including beam-related information if available.
    • The PDU session related information includes the slice information and QoS flow level QoS profile(s)
  • Now the Target gnb receives all this information from the source gNB, it can decide based on the information received to perform Multiple checks and determine what action is required
    • The Target cell performs an Admission control check.
    • If Network slicing was configured to the UE, the target gNB performs Slice-aware admission control.
    • If there are any PDU sessions associated with non-supported slices then the target gNB rejects such PDU Sessions.
    • The Target cell can determine if there are other cells that can be configured as Scells from the MeasurementReport to increase the data rates to UE.
    • If the MeasurementReport contains beam measurements i.e SSB index, then the Target cell can configure Contention Free RACH to the UE, If there are Multiple beam indexes, the target cell can configure CFRA resources to Multiple SSB beams.
    • If the Target cell determines that the Source Cell and Target cell uses Same configuration for data and control PHY Channels and RRM Configuration, then the target cell and indicate ‘Delta Signaling’ which configures only the parameters that needs to be modified.
  • The target gNB prepares the handover with L1/L2 and sends the HANDOVER REQUEST ACKNOWLEDGE to the source gNB, which includes a transparent container to be sent to the UE as an RRC message to perform the handover. The target gNB also indicates if a DAPS Handover is accepted.
  • For Inter-RAT Handover, the delta config is only applicable to SDAP/PDCP configuration, if the EUTRA is connected to 5GC.

How does the UE knows if delta signaling is configured in the handover message?

In the RRC Reconfiguration message network indicates whether delta signaling is being used or it is sending a new configuration for the target cell. The FullConfig IE indicates to UE if Full configuration is applicable or if delta signaling is being used.

  • The IE FullConfig is applicable for the RRCReconfiguration message for intra-system intra-RAT HO.
  • For inter-RAT HO from E-UTRA to NR-SA, fullConfig indicates whether or not delta signalling of SDAP/PDCP from source RAT is applicable.
  • FullConfig is not sent if any DAPS bearer is configured or when the RRCReconfiguration message is transmitted on SRB3.
  • In the case of NSA FullConfig is not sent if the RRCReconfiguration message for SCG is contained in another RRCConnectionReconfiguration message that is transmitted on SRB1.
  • The field is mandatory present in case of inter-system handover from E-UTRA/EPC to NR.
  • It is optionally present, Need N, during reconfiguration with sync and also in first reconfiguration after reestablishment; or
  • It is optionally present for intra-system handover from E-UTRA/5GC to NR.
  • This Field is absent for other RRCReconfiguration messages apart from mentioned above.
  • FullConfig IE is also optionally present in RRCResume message.

Reference :

3GPP TS 38.331 Radio Resource Control (RRC) Protocol specification Link