5G NR System Information Type#1 | 5G SIB 1

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In 5G NR, System Information Block #1 (SIB#1) carries information relevant when evaluating if a UE is allowed to access a cell and defines the scheduling of other system information.

It also provides radio resource configuration information that is common for all UEs and barring information required for unified access control.

Characteristics of NR System Information Type#1

  • MIB and SIB 1 are know as Minimum System Information or MSI in 5G NR
  • SIB 1 alone can be referred as Remaining Minimum System Information or RMSI
  • In 5G NR, MIB provides all the required information to UE for decoding SIB 1
  • SIB 1 is cell-specific information i.e. only valid for serving cell
  • It carries the critical information required for the UE to access the cell
  • It also includes information related to the availability and scheduling of other SIBs e.g. mapping of SIBs to System Information (SI) message, periodicity, SI-window size etc.
  • It is transmitted with a periodicity of 160 ms and have variable transmission repetition periodicity within 160 ms.
  • When operating in ENDC or NSA mode, SIB 1 transmission is not required
  • PDSCH resources for SIB 1 are allocated using a DCI Format 1_0 transmission within Common Search Space Set
  • In vRAN or Open RAN architecture, CU-DU gNB architecture, the DU is responsible for generating the
    content of both the MIB and SIB1.
  • MIB and SIB 1 forwared to CU using the gNB-DU System Information parameter structure included within either the F1AP: F1 Setup Request or F1AP: gNB DU Configuration Update message by DU

Transmission Details

  • Signalling radio bearer: Not Applicable
  • RLC-SAP: TM
  • Logical channels: BCCH
  • Transport Channel: DL-SCH
  • Physical Channel: PDSCH
  • Scrambling RNTI :  SI-RNTI
  • Direction: gNB to UE

SIB 1 Content Category

Below is the category list for content provided by SIB 1 message to UE.

  • Cell Selection Information
  • Cell Access Related Information
  • Connection Establishment Failure Control
  • SI Scheduling Information
  • Serving Cell Common Configuration
  • IMS Emergency Support Flag
  • eCall Over IMS Support Flag
  • UE -Timer and Constants
  • UAC-Barring Information
  • Use Full Resume ID Flag

Cell Selection Information

SIB 1 Cell Selection Information includes Q-RxLevMin, q-RxLevMinOffset, Q-RxLevMinSUL, Q-QualMin and q-QualMinOffset parameters.

The range for each parameters is given below.

    • Q-RxLevMin is used to indicate the minimum received RSRP level in dBm required for cell selection and re-selection in NR cell. Actual value Qrxlevmin = field value * 2  in dBm
    • q-RxLevMinOffset is an offset (in dB) that needs to be added to the signaled (not actual) value of Qrxlevmin.  If this field is absent, the UE applies a (default) value of 0 dB as an offset.
    • Q-RxLevMinSUL If the UE supports SUL frequency for this cell, Qrxlevmin (explained above) is obtained from this field. Actual value Qrxlevmin = field value * 2 [dBm].
    • Q-QualMin is used to indicate the  minimum received RSRQ level in dB  required for cell selection and re-selection in NR cell. If this field is absent, the UE applies the (default) value of negative infinity for Qqualmin which means that quality (RSRQ) check will always pass irrespective of the received signal quality.
    • q-QualMinOffset is an offset (in dB) that needs to be added to the signaled value of Qqualmin. If this field is absent, the UE applies a (default) value of 0 dB as an offset.

Cell Access Related Information

SIB 1 Cell Access Related information contains PLMN information, Tracking Area Code (TAC), RAN Area Code (RAC) and Cell Identity (Cell ID) parameters. SIB 1 can broadcast upto 12 PLMN’s cell access information.

The range for each parameters is given below.

    • trackingAreaCode indicates TAC to which the cell indicated by cellIdentity belongs to. After acquisition of SIB1, if the trackingAreaCode (per PLMN) is not present, UE considers a cell as barred.
    • ranac is used to identify a RAN area within the scope of a Tracking Area. RAN area code is used when configuring RAN Notification Area.
    • CellIdentity is used to identify a cell within a PLMN.
    • cellReservedForOtherUse parameter indicates whether the cell is reserved. The field is applicable to all configured PLMNs. If a cell is reserved, camping is not allowed except for specific UEs.

Connection Establishment Failure Control

ConnEstFailurControlis used to configure parameters for connection establishment failure control. When an RRCSetupRequest is sent by the UE, timer T300 is started and the UE considers the connection establishment attempt as failed if it doesn’t receive a response from the gNB before T300 expires.

If the timer T300 expires connEstFailCount times, then the UE starts a timer given by connEstFailOffsetValidity and reduces RSRP/RSRQ quality by connEstFailOffset parameter, when performing cell selection/re-selection.

The range for each parameters is given below.

    • connEstFailCount is number of times that the UE detects T300 expiry on the serving cell before applying connEstFailOffset
    • connEstFailOffset is the parameter Qoffsettemp used in 3GPP 38.304 for cell selection/reselection process. If this field is absent, the value of infinity shall be used for Qoffsettemp
    • connEstFailOffsetValidity is the amount of time in seconds that the UE applies connEstFailOffset before removing the offset from evaluation of the cell.

SI Scheduling Information

SIB1 SI Scheduling information provides information for other System information broadcast (SIB2 to SIB 9), thier periodicity and SI windows Length and RACH parameters. 

The range for each parameters is given below.

Serving Cell Common Configuration

SIB1 Serving Cell Common information is used to configure cell specific parameters of a UE’s serving cell . This includes Downlink frequency information, offset to point A, Initial Downlink Bandwidth part (BWP), and paging channel configuration.

It also includes RACH-ConfigCommon, PUSCH-ConfigCommon, PUCCH-ConfigCommon and timeAlignmentTimerCommon .

The range for each parameters is given below.

IMS Emergency Support Flag

SIB 1  IMS Emergency Support Flag, indicates whether the cell supports IMS emergency bearer services for UEs in limited service mode. If absent, IMS emergency call is not supported by the network in the cell for UEs in limited service mode

eCall Over IMS Support Flag

SIB 1 eCall Over IMS Support Flag indicates whether the cell supports emergency call over IMS services.

UE -Timer and Constants

SIB1 UE Timer and Constants provides, timers and constants used by the UE in RRC_CONNECTED, RRC_INACTIVE and RRC_IDLE state.

UAC-Barring Information

SIB1 UAC-Barring Information contains information about cell barring based upon Unified Access Barring (UAC). If you to more about cell access barring read this 5G NR Cell Access Control

Use Full Resume ID Flag

SIB1 Use Full Resume ID Flag parameter indicates which resume identifier and Resume request message should be used when UE wants to transit to connected state from inactive state. The UE uses full I-RNTI and RRCResumeRequest1 if the field is present, or short I-RNTI and RRCResumeRequest if the field is absent.

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