Module 5 — SIB1 & Cell Selection

S-Criterion —
Is this cell good enough to camp on?

After decoding SIB1, the UE applies the S-criterion — a pair of signal threshold checks. If both pass, the cell is suitable. If either fails, the UE rejects this cell and continues searching. This section derives the formulas, applies them to our example, and shows both passing and failing cases.

TS 38.304 §5.2.3.2 TS 38.331 §6.3.2

The S-criterion formulas

A cell is suitable for cell selection if the following condition is met:

    S_rxlev > 0   AND   S_qual > 0

where:
    S_rxlev = Q_rxlevmeas − (Q_rxlevmin + Q_{rxlevminoffset}) − P_compensation
    S_qual = Q_qualmeas − (Q_qualmin + Q_{qualminoffset})

3GPP TS 38.304, Section 5.2.3.2

Every term explained

S-criterion parameters — definitions and our values TS 38.304 §5.2.3.2, TS 38.331 §6.3.2

Parameter	Description	Source	Our value
Q_rxlevmeas	Measured RSRP (dBm)	UE measurement	−85 dBm
Q_rxlevmin	Min required RSRP from SIB1 (dBm). Actual = IE × 2	SIB1: q-RxLevMin=−70	−140 dBm
Q_{rxlevminoffset}	Additional offset, if configured	SIB1 (optional)	0 dB
P_compensation	max(P_EMAX − P_PowerClass, 0). Only non-zero if UE max power < P_EMAX	SIB1 + UE cap	0 dB
Q_qualmeas	Measured RSRQ (dB)	UE measurement	−10 dB
Q_qualmin	Min required RSRQ from SIB1 (dB)	SIB1: q-QualMin=−20	−20 dB
Q_{qualminoffset}	Additional offset, if configured	SIB1 (optional)	0 dB

Worked example — our cell (passing case)

S-criterion calculation — RSRP = −85 dBm, RSRQ = −10 dBTS 38.304 §5.2.3.2

// Srxlev:
Srxlev = Q_rxlevmeas − (Q_rxlevmin + Q_rxlevminoffset) − P_compensation
       = −85 − (−140 + 0) − 0
       = −85 + 140
       = +55 dB  > 0  ✓

// Squal:
Squal  = Q_qualmeas − (Q_qualmin + Q_qualminoffset)
       = −10 − (−20 + 0)
       = −10 + 20
       = +10 dB  > 0  ✓

// Decision:
Srxlev = +55 > 0  AND  Squal = +10 > 0
→ CELL SUITABLE ✓ — UE may camp

Interactive S-criterion — adjust RSRP to see pass/fail boundary TS 38.304 §5.2.3.2

RSRP (dBm): −85 dBm RSRQ (dB): −10 dB

Failing case — weak signal

If the UE measures RSRP = −148 dBm (very weak signal, near cell edge):

S-criterion — RSRP = −148 dBm (failing case)TS 38.304 §5.2.3.2

// Srxlev:
Srxlev = −148 − (−140) − 0 = −8 dB  < 0  ✗

// Even if Squal passes, Srxlev failure alone rejects the cell
// Decision:
→ CELL UNSUITABLE — UE continues searching

// What UE does next:
1. Mark this cell as "barred" for the measurement period
2. Continue GSCN sweep on same band
3. If all intra-frequency cells fail → try next band (n1, n28...)
4. If all bands exhausted → display "No Service"

P_compensation — why it exists

P_compensation accounts for UEs whose maximum transmit power is lower than the cell's expected power class. If a UE can only transmit at 20 dBm but the cell expects 23 dBm (P_EMAX), the uplink will be limited. The S-criterion adds a margin to ensure the UE can close the uplink link as well, not just the downlink.

In our example, P_EMAX = 23 dBm from SIB1, UE power class = 23 dBm → P_compensation = max(23−23, 0) = 0.

Hysteresis — preventing ping-pong

In practice, the measured RSRP fluctuates due to fading. A UE camping on a cell will not immediately abandon it the moment Srxlev drops below 0. Additional hysteresis and timers (T311, T310) prevent excessive reselection. These are configured in SIB2 for the camping state, not SIB1.

S-criterion — our cell result

Srxlev = −85 − (−140) = +55 dB > 0 ✓

Squal = −10 − (−20) = +10 dB > 0 ✓

Decision → Cell suitable

Next step → PLMN check and access control

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