SIB24: The Invisible Bridge from 4G to 5G
SIB24 is the System Information Block that orchestrates LTE to 5G NR cell reselection. NR-ARFCN, priority, radio thresholds: full field analysis.
There is a basement parking garage three levels underground in La Part-Dieu, Lyon. Concrete walls, steel beams, zero GPS. An RF engineer sits in a parked van with a scanner laptop, watching a terminal stubbornly camp on Band 3 LTE while a perfectly operational 5G NR cell on n78 exists directly above. The terminal never attempts to leave 4G. It has no idea 5G is there.
The reason is a single missing element in the LTE broadcast: System Information Block 24.
SIB24 is the mechanism through which a 4G base station tells every idle terminal in its coverage area how to find 5G. When it is properly configured, terminals migrate to NR seamlessly and silently. When it is missing or misconfigured, the 5G layer becomes invisible to every device in idle mode, regardless of how strong the NR signal actually is.
What SIB24 Does and Why It Matters
SIB24 (System Information Block Type 24) is defined in 3GPP TS 36.331, the RRC specification for E-UTRA. It was introduced as part of the EN-DC (E-UTRA NR Dual Connectivity) framework and the inter-RAT cell reselection procedures from LTE to NR. Its function is precise: deliver to idle-mode terminals every parameter they need to measure, evaluate, and potentially reselect onto a 5G NR cell.
Where SIB24 Sits in the SIB Hierarchy
In LTE, SIB1 tells the terminal which other SIBs are available and when they are scheduled for broadcast. SIB24 is classified as an optional SIB. If it is not listed in the schedulingInfoList within SIB1, the terminal has no awareness that an NR layer exists.
This is the critical point. The absence of SIB24 from the eNB configuration completely prevents idle-mode reselection to 5G. The NR cell can be transmitting at full power with an excellent signal footprint, and no idle terminal will ever attempt to measure it.
What SIB24 Contains
SIB24 carries a structured set of parameters within the CarrierFreqListNR-r15 information element. Each entry in this list describes one NR carrier frequency candidate for reselection.
Each parameter drives a specific step in the terminalโs decision logic:
- NR-ARFCN 650400: identifies the NR center frequency to measure. The value 650400 maps to band n78, centered around 3.5 GHz. This is the primary TDD band deployed across Europe for 5G NR mid-band coverage.
- cellReselectionPriority 7: the reselection priority assigned to this NR frequency. LTE priorities range from 0 (lowest) to 7 (highest). A value of 7 means the NR layer takes precedence over the serving LTE layer. The terminal will attempt to reselect to NR as soon as radio conditions permit.
- q-RxLevMin -64 dBm: the minimum received signal level required for the NR cell to be considered a reselection candidate. This threshold filters out cells that are too weak to provide reliable service.
- p-MaxNR 23 dBm: the maximum transmit power the terminal is allowed to use on the NR carrier. This is the standard value for consumer-grade devices.
- subCarrierSpacingSSB kHz30: the subcarrier spacing of the NR SSB (Synchronization Signal Block). For band n78 in FR1, 30 kHz is the standard spacing per 3GPP TS 38.104 Table 5.4.3.3-1.
- t-ReselectionNR 2s: the reselection timer. The terminal must measure the NR cell above the required threshold for at least 2 consecutive seconds before triggering reselection. This prevents erratic reselections caused by transient signal spikes.
The Reselection Process Step by Step
LTE-to-NR reselection via SIB24 follows a deterministic sequence, entirely driven by the terminal in idle mode. The network makes no decision at this stage.
The UE in idle mode on LTE decodes SIB1, identifies SIB24 in the scheduling info, and decodes it at the next broadcast occasion.
The UE configures its receiver to measure NR-ARFCN 650400 with 30 kHz subcarrier spacing. It searches for SSB transmissions on this frequency.
The UE detects an NR SSB, identifies the PCI (Physical Cell Identity), and measures SS-RSRP. Real example: PCI 613, SS-RSRP = -119 dBm.
The UE computes: Srxlev = SS-RSRP - (q-RxLevMin + q-RxLevMinOffset). If Srxlev > 0, the NR cell is eligible. At -119 dBm against a -64 dBm threshold, this fails.
If the S-criterion is met AND the NR cell remains above threshold for t-ReselectionNR (2s), the UE reselects to NR. Otherwise, it stays on LTE and re-evaluates periodically.
Real-World Field Capture with HiCellTek
In the capture performed with HiCellTek on commercial cell ECI 24237064 (TAC 50437), the terminal detected PCI 613 on band n78 with an SS-RSRP of -119 dBm. This signal level sits well below the q-RxLevMin threshold of -64 dBm defined in the SIB24.
This level of protocol-layer visibility, decoded in real time directly from the Qualcomm DIAG interface on the device, exposes the raw SIB content exactly as the modem chipset receives it. Where a standard network indicator simply displays โ4Gโ or โ5Gโ, protocol decoding reveals the complete signaling mechanics: every IE, every threshold, every timer. It is the difference between observing a symptom and understanding its root cause.
The result: the terminal sees the NR cell but the S-criterion is not satisfied. It stays on LTE. This behavior is entirely correct and intentional. SIB24 does not force a blind reselection to 5G. It defines the conditions under which reselection is acceptable. A -119 dBm NR signal would not support stable service, so the terminal remains on a functional 4G layer.
This is exactly the type of result that a field engineer must know how to interpret. The 5G cell is visible but unreachable at this measurement point. The question then becomes: why is the NR signal that weak? gNodeB coverage limitation? Physical obstruction? Antenna tilt misconfiguration?
Operator Parameter Strategy and Field Impact
SIB24 parameter selection is not arbitrary. Each value reflects an operator strategy with direct consequences on user experience and network traffic distribution.
Reselection Priority as a Network Steering Tool
According to GSMA data published in 2025, 78% of European operators deploying 5G NR on band n78 configure the NR reselection priority at 7, the maximum value. This aggressive setting pushes terminals toward 5G at every opportunity, offloading the LTE layer and improving 5G usage metrics in regulatory filings.
Some operators take a more conservative approach, using priorities of 5 or 6, particularly in areas where NR coverage is patchy. A terminal that reselects to an NR cell and subsequently loses signal must fall back to LTE, generating a service interruption that the user perceives as a brief connectivity drop.
The q-RxLevMin Tradeoff
A high q-RxLevMin (closer to 0 dBm) drastically shrinks the NR reselection zone. Terminals will only reselect when very close to the gNodeB. A low threshold (below -120 dBm) allows reselection to NR cells at the extreme edge of coverage, risking degraded throughput and rapid fallback to LTE.
The -64 dBm value observed on this commercial network is moderately conservative. It ensures terminals only reselect to NR when the signal is strong enough for quality service. According to Opensignal benchmarks from January 2026, operators maintaining thresholds between -70 and -62 dBm achieve the best service continuity scores in dense urban environments.
The t-ReselectionNR Timer: Stability vs Responsiveness
Estimated LTE/NR ping-pong rate by timer value (source: 3GPP TR 36.839 modeling, extrapolated for NR)
The 2-second timer observed on this cell is a well-balanced choice. Short enough to avoid excessive delay in NR migration, long enough to filter out rapid signal fluctuations caused by multipath fading or terminal movement.
SIB24 in NSA vs SA Context
SIB24 operates exclusively in the idle-mode reselection scenario. In connected mode, it is the network that decides whether to add an NR carrier through B1/B2 measurement events configured via RRC Reconfiguration.
Non-Standalone (NSA)
In NSA deployments, SIB24 enables idle terminals to position themselves on the best available NR cell. When the terminal transitions to connected mode (voice call, data session), the serving LTE cell remains the PCell and the network may activate the NR SCG (Secondary Cell Group) through RRC reconfiguration. Idle-mode reselection via SIB24 and connected-mode SCG addition are complementary but fundamentally different mechanisms.
Standalone (SA)
In a pure 5G SA deployment, the terminal camps directly on NR. SIB24 has no role when the terminal is already on the NR layer. However, in hybrid deployments (which represent the reality of nearly every production network in 2026), SIB24 remains essential for idle-mode transition from legacy LTE layers to NR.
According to the GSMA Intelligence report from February 2026, only 23% of global 5G deployments operate in pure SA mode. The remaining 77% rely on NSA or hybrid configurations where SIB24 is a critical component of inter-technology mobility.
Field Troubleshooting: When SIB24 Fails
Back to the Lyon NOC scenario. A cluster of 47 cells sees its LTE-to-NR reselection rate drop from 83% to 12% overnight. How do you diagnose a SIB24-related failure?
Verify SIB24 Presence
The first step is confirming that SIB24 is actually being broadcast by the affected eNBs. Decode SIB1 and check whether SIB24 appears in the schedulingInfoList. If it does not, terminals have zero visibility of the NR layer in idle mode. Root causes include eNB software upgrades that reset the configuration to defaults, OSS template changes applied without validation, or parameter errors during hardware swaps.
Validate Parameter Consistency
Even when SIB24 is present, inconsistent parameters can block reselection. An NR-ARFCN that points to a frequency with no active NR cell, an unrealistic q-RxLevMin, or an excessively long t-ReselectionNR (above 7 seconds) will effectively neutralize the mechanism.
Correlate with Radio Measurements
The final level of diagnosis involves verifying that the NR cells referenced in SIB24 are actually detectable with sufficient signal strength. A targeted drive test on the indicated NR-ARFCN maps the actual NR coverage footprint and compares it against the configured q-RxLevMin threshold. Gaps between the two reveal either coverage holes that need RF optimization or threshold values that need adjustment.
The Bridge That Defines the 5G Experience
SIB24 is a quiet piece of signaling infrastructure, but its impact on the end-user experience is disproportionately large. It is the invisible bridge between two generations of radio technology. Without it, 5G NR remains deployed infrastructure that idle terminals cannot access. With it, properly configured, the 4G-to-5G transition becomes transparent, automatic, and optimized for service quality.
For the network engineer, mastering SIB24 means understanding the very first link in the 5G user experience chain. Before throughput, before latency, before slicing, there is the terminalโs ability to find the NR cell. That ability depends entirely on this system information block.
The next time an LTE-to-NR reselection KPI drops without obvious cause, start by decoding SIB24. The answer is almost always there.
What q-RxLevMin thresholds are you seeing on the networks you audit? Are they aligned with actual NR coverage, or are there gaps worth investigating?
Founder of HiCellTek. 15+ years in telecom, operator side, vendor side, field side. Building the field tool RF engineers deserve.
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