SIB1 (System Information Block Type 1) is the most critical broadcast message in 5G NR.
It gives a UE (User Equipment) the minimum information needed to access a 5G cell.

SIB1 is carried inside:

• BCCH (Broadcast Control Channel)
• Delivered via DL-SCH
• Scheduled through PDCCH using SI-RNTI

Its structure and definitions come from 3GPP TS 38.331.

🧩 High-Level Purpose of SIB1

SIB1 tells the UE:

✔ Who the network is

(PLMN identities, cell identity)

✔ How to access the network

(Access classes, barring, TAC, cell selection thresholds)

✔ How the cell is configured

(Serving cell parameters, SSB positions, PDCCH configurations)

✔ Where other SIBs can be found

(si-SchedulingInfo)

🧱 SIB1 Structure According to 3GPP

Below is the simplified and blog‑friendly structure — still aligned with TS 38.331.

1. cellSelectionInfo

Helps the UE determine if the cell is suitable.

• q-RxLevMin

Minimum required received signal level for camp‑on.

• q-RxLevMinOffset

Adjustment for specific situations (e.g., closed access).

2. cellAccessRelatedInfo

Defines operator identity and where the cell belongs.

• plmn-IdentityList

One or more PLMN IDs
Example: MCC/MNC combinations

• trackingAreaCode (TAC)

Used for mobility & registration management.

• cellIdentity

28-bit NR Cell ID

3. servingCellConfigCommon

Provides the initial common configuration for the UE.

Includes:

• Subcarrier spacing
• SSB periodicity
• PRACH config
• RACH parameters
• UL/DL ConfigCommon

A UE uses this before RRC connection is fully established.

4. ssb-PositionsInBurst

Indicates which SSB beams exist and their bitmap positions.

This is crucial for beam management.

5. pdcch-ConfigSIB1

Describes where the UE can find the PDCCH that schedules SIB1 or SI messages.

Contains:

• SearchSpace ID
• ControlResourceSet (CORESET) ID
• Periodicity

This ensures the UE checks the right location in time/frequency.

6. ims-EmergencySupport

Tells the UE if IMS emergency call support is present.

7. si-SchedulingInfo

Explains how other SIBs (like SIB2, SIB3, SIB5 etc.) are scheduled.

Includes:

• Repetition periods
• Mapping between SI messages & SIBs
• Scheduling window information

📘 SIB1 – Clean Hierarchical Structure (Blog Friendly)

SIB1
 ├── cellSelectionInfo
 │     ├── q-RxLevMin
 │     └── q-RxLevMinOffset
 │
 ├── cellAccessRelatedInfo
 │     ├── plmn-IdentityList
 │     ├── trackingAreaCode
 │     └── cellIdentity
 │
 ├── servingCellConfigCommon
 │
 ├── ssb-PositionsInBurst
 │
 ├── pdcch-ConfigSIB1
 │
 ├── ims-EmergencySupport
 │
 └── si-SchedulingInfo