What SIB1 is and how it arrives
System Information Block 1 (SIB1) is carried on PDSCH, scheduled by DCI 1_0 with SI-RNTI. It is scheduled by the DCI decoded in blind decoding: SIB1 is at RBs 24–38, symbols 2–12, of slot 2, using MCS 5 (QPSK, code rate 0.38).
SIB1 is transmitted periodically every 160 ms (default), with up to 8 repetitions within that period for coverage. A UE that misses one can catch the next repetition without waiting the full 160 ms.
SIB1 top-level structure
| Field | Our value | Purpose |
|---|---|---|
| cellSelectionInfo | q-RxLevMin = −70 | Minimum signal level to camp |
| cellAccessRelatedInfo | PLMN 244-05 | Operator identity |
| connectToOtherAMF | — | AMF selection hints |
| trackingAreaCode | 0x1F4 = 500 | TAC for registration/paging |
| cellIdentity | 0x1A2B3C4 | Unique 28-bit cell ID |
| servingCellConfigCommon | various | BWP, RACH, DL config |
| si-SchedulingInfo | SIB2–SIB9 | When/where other SIBs appear |
| ue-TimersAndConstants | T300=ms1000 | RRC procedure timers |
Field 1 — cellSelectionInfo
This is the most important field for determining whether the UE can camp on the cell. It provides the threshold values used in the S-criterion.
q-QualMin: Minimum required quality level in the cell (dB). Only applicable for RSRQ-based cell selection.
// From SIB1 for our cell: q-RxLevMin = −70 ← actual value = −70 × 2 = −140 dBm q-QualMin = −20 ← actual value = −20 dB (RSRQ) // UE measures: RSRP (measured) = −85 dBm ← reference signal received power RSRQ (measured) = −10 dB ← reference signal received quality // S-criterion (TS 38.304): Srxlev = RSRP − q-RxLevMin = −85 − (−140) = +55 dB > 0 ✓ Squal = RSRQ − q-QualMin = −10 − (−20) = +10 dB > 0 ✓ // Both positive → cell is suitable
Field 2 — cellAccessRelatedInfo
This field carries the PLMN identity list — which mobile operators this cell serves. A UE checks this against its SIM card to verify it is on a valid network.
// PLMN list (up to 12 entries): PLMN 1: MCC = 244, MNC = 05 ← Finland, Elisa PLMN 2: MCC = 244, MNC = 91 ← Elisa MVNO // UE SIM card: HPLMN: MCC = 244, MNC = 05 ← matches PLMN 1 ✓ // Additional identifiers: trackingAreaCode = 0x1F4 = 500 cellIdentity = 0x1A2B3C4 = 27,471,812 (28-bit cell ID)
Field 3 — servingCellConfigCommon
This is the largest SIB1 field. It configures the UE's initial downlink BWP and provides the complete RACH configuration — the information the UE needs to send Msg1 and actually connect to the network.
Initial Downlink BWP
// Initial DL BWP (active immediately after SIB1 decoding): locationAndBandwidth: startRB = 0 ← starts at Point A (CRB 0 = 3496.38 MHz) numRBs = 52 ← 52 × 360 kHz = 18.72 MHz subcarrierSpacing = 30 kHz ← matches subCarrierSpacingCommon in MIB // Frequency range: BWP_start = 3496.38 MHz (Point A) BWP_end = 3496.38 + 52 × 0.360 = 3515.10 MHz
RACH Configuration
The RACH configuration tells the UE exactly how to send its first uplink message to the gNB. Without this, the UE cannot connect. Key parameters:
| Parameter | Our value | Meaning |
|---|---|---|
| prach-ConfigIndex | 16 | Preamble format 0, long sequence |
| msg1-FrequencyStart | 4 RBs | PRACH frequency offset from BWP start |
| msg1-FDM | 1 | Number of PRACH occasions in frequency |
| zeroCorrelationZoneConfig | 13 | Preamble sequence guard zone |
| preambleReceivedTargetPower | −110 dBm | Target received preamble power at gNB |
| powerRampingStep | 4 dB | Power increase per RACH retry |
| ra-ResponseWindow | sl10 | UE waits up to 10 slots for Msg2 |
| preambleTransMax | 7 | Max preamble retries before failure |
Field 4 — si-SchedulingInfo
SIB1 also tells the UE how to find SIB2 through SIB9 — the other system information blocks. Each is scheduled periodically and the UE only reads them if needed.
| SIB | Content | When needed |
|---|---|---|
| SIB2 | Intra-frequency cell reselection parameters | Cell reselection in IDLE |
| SIB3 | Intra-frequency neighbour cell list | Mobility in IDLE |
| SIB4 | Inter-frequency neighbour cells | Inter-frequency reselection |
| SIB5 | Inter-RAT (LTE, WCDMA) neighbours | Multi-RAT reselection |
| SIB6/7 | ETWS earthquake/tsunami warning | Emergency broadcast |
| SIB9 | GPS/UTC time synchronisation | Timing-sensitive applications |