Cell Selection and Reselection in LTE

Cell selection and reselection are foundational procedures in LTE idle mode operation. These mechanisms determine which cell a UE camps on, ensuring robust connectivity, optimal service quality, and efficient resource utilization as the UE moves or radio conditions change.

Overview: Cell Selection vs. Reselection

• Cell Selection: Performed when a UE powers on, loses coverage, or completes a PLMN search. The UE scans and selects a suitable cell to camp on.
• Cell Reselection: In idle mode, the UE continually evaluates neighboring cells. If certain criteria are met, it switches to a more favourable cell—whether intra-frequency, inter-frequency, or even inter-RAT (e.g., to UMTS/GSM).

Cell Selection Process

As per spec 36.304 there are two distinct cell selection approaches

• Initial Cell Selection — “Starting from Scratch”

Imagine your LTE phone is waking up for the very first time, or it just lost all memory of previous networks. It doesn’t know which radio channels are being used in the area, nor does it know which frequencies belong to which mobile operators.

Here’s how it works, step by step:

• Full Band Scan: The UE (User Equipment, e.g., your phone) starts by scanning all E-UTRA frequency bands it supports. This is like searching through every available FM station to find where music is playing, except here the phone is looking for LTE cells.
• Channel-by-Channel Search: On each supported carrier frequency, the UE tunes in and listens for LTE signals from nearby eNodeBs. It checks each radio channel methodically, gathering RSRP.
• Strongest Cell Hunt: For each frequency, it identifies which cell is broadcasting the strongest signal—this is usually the closest or least obstructed cell.
• Selection Criteria Check: Importantly, not every cell is eligible. The phone checks whether the cell passes a technical “suitability test” (the Srxlev and Squal criteria discussed earlier: is the signal strong and clean enough?).
• Camp On a Suitable Cell: As soon as a suitable cell is found, the UE “camps” on it—that is, it locks onto this cell and begins idle mode operation (monitoring paging, fetching SIBs, etc.).

Why is this approach needed?

This process guarantees that, even with zero information about the local network landscape, the UE can always eventually find and connect to a valid LTE cell. It may take a few seconds, but it’s a “guaranteed fresh start.”

• Stored Information Cell Selection — “Using Past Experience”

Now imagine your phone has already been in this area before—or it recently scanned some networks and remembered useful details. It’s like returning to a favourite coffee shop where you already know the Wi-Fi password.

Here’s how this works:

• Stored Frequency List: The UE keeps a record of previously detected carrier frequencies, as well as extra cell-specific parameters from past system information messages (e.g., measurement control, SIBs).
• Quick Candidate Check: Rather than starting a time-consuming full-band scan, the UE first checks these known frequencies for available LTE cells. This is much faster—especially in areas where operator frequencies rarely change.
• Suitability Test: On each stored frequency, the phone checks if a cell is present and if it meets the selection criteria (Srxlev/Squal).
• Camp If Found: If a suitable cell is found among the stored candidates, the UE camps on it immediately. If not, only then does it fall back to a full, “initial” cell selection.

Why is this approach valuable?
It speeds up reconnection after power cycling, airplane mode, or returning to a familiar location. Users experience much less waiting before regaining network access.

Cell Selection Criterion: The S-criterion

When a UE scans for cells to camp on, it uses the S-criterion as defined in 3GPP TS 36.304 to determine whether a candidate cell is suitable. This criterion ensures that the UE only connects to cells with both adequate signal strength and quality, thereby maintaining reliable coverage and user experience.

Srxlev > 0 AND Squal > 0

Srxlev = Cell selection RX level value (reflects signal strength)
Squal = Cell selection quality value (reflects signal quality)
 
 
where:

Reference: 3gpp 36.304

• Qrxlevmeas: Measured RX level of the candidate cell (usually RSRP, in dBm).
• Qrxlevmin: Minimum required RX level in the cell, broadcast in SIB1.
• Qrxlevminoffset: Offset applied only during a periodic search for a higher-priority PLMN while camped in a VPLMN (Visited PLMN).
• Pcompensation: Compensation factor for UEs that cannot reach the cell’s maximum uplink power.
• Qqualmeas: Measured signal quality (typically RSRQ) of the candidate cell, in dB.
• Qqualmin: Minimum required quality value for the cell, also in SIB1.
• Qqualminoffset: Like Qrxlevminoffset, applied during periodic searches for a higher-priority PLMN.
• PEMAX: The maximum transmit power the UE may use when transmitting on the uplink in the cell, as defined in TS 36.101.
• PPowerClass: The maximum RF output power of the UE according to its power class.

Example 1: S-criterion Fails

Assume the following values (across 6 time points as UE is approaching cell but still in poor coverage):

Time	Qrxlevmeas (dBm)	Qrxlevmin (dBm)	Srxlev (dB)	Qqualmeas (dB)	Qqualmin (dB)	Squal (dB)
1	-97	-95	-2	-15	-14	-1
2	-96	-95	-1	-14	-14	0
3	-95	-95	0	-13	-14	1
4	-94	-95	1	-12	-14	2
5	-93	-95	2	-11	-14	3
6	-92	-95	3	-10	-14	4

• At time points 1-2, both Srxlev and Squal are ≤ 0 (S-criterion fails).
• At time points 3-6, both become positive, so S-criterion starts to pass from time 3 onward.

Example 2: S-criterion Passes

Assume a UE is already in good coverage, moving closer:

Time	Qrxlevmeas (dBm)	Qrxlevmin (dBm)	Srxlev (dB)	Qqualmeas (dB)	Qqualmin (dB)	Squal (dB)
1	-88	-95	7	-7	-14	7
2	-87	-95	8	-6	-14	8
3	-86	-95	9	-5	-14	9
4	-85	-95	10	-4	-14	10
5	-84	-95	11	-3	-14	11
6	-83	-95	12	-2	-14	12

All time points have both Srxlev and Squal > 0 (always passing).

Cell Reselection in LTE

Cell reselection in LTE is the process where the UE in idle mode autonomously evaluates the quality of available cells and moves (“reselects”) to a new cell if it offers better radio conditions or higher network-assigned priority. This ensures UEs remain “always best connected,” supports seamless mobility, and balances load across the network.

LTE reselection operates based on a rich set of parameters, most of which are broadcast in SIB3 and SIBs 4/5/6 for other frequencies/RATs.

Types of Cell Reselection

Intra-frequency Reselection

• Occurs among cells on the same carrier frequency.
• Often between neighbouring eNodeBs or sectors.

Inter-frequency Reselection

• UE evaluates cells on different E-UTRA frequencies (e.g., 1800 MHz vs 2100 MHz LTE carriers).
• Typically used for load balancing, coverage enhancement, or priority-based offload.

Inter-RAT Reselection

• UE considers non-LTE (UTRAN/GERAN/CDMA) cells.
• Managed through dedicated system information (SIB5 for UTRAN/GERAN, SIB6 for CDMA).

Reselection Priorities & Mechanism

The network can assign absolute priorities for different frequencies/RATs (using SIB3/4/5). UE first considers higher-priority frequencies, only considering lower priorities if no better cells are found at higher ones.

Reselection Process (High-Level):

• Measure Serving & Neighbor Cells (RSRP/RSRQ for LTE, RSCP/EcNo for UTRAN, etc.)
• Apply S-criterion to candidate cells (must be above Qrxlevmin/Qqualmin and other thresholds)
• Rank cells using network-provided parameters.
• Apply time-to-trigger (Treselection) and hysteresis to avoid ping-ponging.
• Reselect if new cell is better for the duration of Treselection.

SIB3 – The Heart of Cell Reselection Configuration

SIB3 is the main source of reselection parameters for intra-frequency operation.

• q-Hyst: Hysteresis in dB added to serving cell rank; (Range: 0, 1, 2, 3, 4, 6, 8, 10, 12, 14 dB)
• speedStateReselectionPars: Enables mobility-based scaling of hysteresis and timers.
• mobilityStateParameters: Parameters for detecting UE speed (low/medium/high) using cell reselection counts.
• q-HystSF: Scaling factors for hysteresis based on mobility state.
• sf-Medium: Hysteresis scaling for medium speed; (Options: dB-6, dB-4, dB-2, dB0)
• sf-High: Hysteresis scaling for high speed; (Options: dB-6, dB-4, dB-2, dB0)
• s-NonIntraSearch: Threshold for triggering inter-frequency/inter-RAT search when serving cell quality is poor; (Range: -20…49 dB)
• threshServingLow: Minimum serving cell quality below which reselection is prioritized; (Range: 0…31 dB)
• cellReselectionPriority: Absolute priority of the frequency for cell reselection; (Range: 0 (lowest) to 7 (highest))
• q-RxLevMin: Minimum required RSRP in dBm for the cell to be suitable; (Range: -140…-44 dBm)
• p-Max: Maximum allowed UE TX power in the cell; (Range: 23…33 dBm, per UE power class)
• s-IntraSearch: Threshold to trigger neighbor cell measurements within the same frequency; (Range: -20…49 dB)
• allowedMeasBandwidth: Allowed measurement bandwidth in resource blocks (RBs); (Options: 6, 15, 25, 50, 75, 100 RBs)
• presenceAntennaPort1: Indicates if Antenna Port 1 is present (for MIMO, TRUE/FALSE).
• neighCellConfig: Configuration bitmap indicating support for certain neighbor cell features; (2 bits: PCI confusion, MBSFN, etc.)
• t-ReselectionEUTRA: Time (seconds) a better cell must remain suitable before reselection; (Range: 1, 2, 4, 8 s)
• t-ReselectionEUTRA-SF: Scaling factor for t-Reselection based on mobility state; (Options: 0.25, 0.5, 0.75, 1.0)

Reselection Criteria – Equations and Rules

Absolute Priority Based Reselection (Inter-Freq & IRAT)

• UE reselects to a higher-priority frequency/RAT if the candidate cell meets threshold (Srxlev > ThreshX_HighP, Squal > ThreshX_HighQ) for Treselection.
• Lower or equal priority reselection uses serving cell’s quality thresholds (threshServingLowP/Q) and additional offset/hysteresis.

Ranking-Based Reselection (Intra-Freq & Equal Priority Inter-Freq)

For serving cell and for neighbour cell:

• Qmeas: Usually RSRP average (cell’s measured signal strength)
• Qhyst: Hysteresis from SIB3
• Qoffset: Cell/frequency-specific offset

Reselection is triggered if: A candidate cell remains better ranked than the serving cell for the duration of TreselectionEUTRA (from SIB3/SIB4).

Measurement and Search Rules

• UE only measures other frequencies/RATs if the serving cell quality drops below the sNonIntraSearchP/Q thresholds.
• To save battery and signalling, measurements are avoided if serving cell is very good.

The UE only needs to measure neighbour cells for possible reselection if its current cell’s signal strength (Srxlev) or quality (Squal) falls below the configured intra-frequency search thresholds (S_IntraSearchP and S_IntraSearchQ).

• If both are above the thresholds, neighbor cell measurement can be skipped to save battery.
• If not, the UE must perform intra-frequency measurements to find a better cell.

The UE uses priority and signal thresholds to decide when to measure other LTE frequencies or other RATs (like 3G/2G):
1. Higher-priority frequencies/RATs: The UE must always measure neighbour cells on E-UTRA or inter-RAT frequencies with a higher reselection priority than its current serving cell.
2. Equal or lower-priority frequencies/RATs:
> The UE only needs to measure these if its current cell’s signal strength (Srxlev) or quality (Squal) drops below the configured S_NonIntraSearchP and S_NonIntraSearchQ thresholds.
> If both Srxlev and Squal are above those thresholds, these measurements can be skipped (to save UE battery and reduce unnecessary measurements).
 
Triggering Conditions

Reselection can be triggered by:

• Radio degradation on serving cell (e.g., signal drops below Qrxlevmin or sIntraSearchP).
• Discovery of a better neighbour (higher RSRP/priority, or lower offset, for long enough).
• SIB3/4/5/6 change in system information (parameters updated).
• UE entering a new mobility state (e.g., detected as fast moving, which may scale tReselection or Qhyst).

Scenario 1: Intra-frequency Reselection

• UE camps on Cell A, RSRP falls as user moves away.
• UE measures neighbours (Cells B, C) on same frequency.
• Cell B’s RSRP becomes consistently higher than Cell A’s + Qhyst for tReselectionEUTRA.
• UE reselects to Cell B.

Scenario 2: Inter-frequency Reselection

• UE camps on 1800 MHz cell (Priority 5), detects 2100 MHz cell (Priority 7).
• 2100 MHz cell’s RSRP > ThreshX_HighP and S-criterion satisfied.
• UE moves to 2100 MHz cell after TreselectionEUTRA.

Scenario 3: Inter-RAT Reselection

• LTE coverage fades, UE measures UTRAN (3G) neighbours from SIB5.
• 3G cell exceeds ThreshX_HighP for inter-RAT, UE moves to 3G.

References:

• 3GPP TS 36.304: Defines LTE cell selection/reselection procedures and criteria for idle mode UEs.
• 3GPP TS 36.331: Specifies the structure and content of SIB3, SIB4, and SIB5 for LTE system information broadcasting.
• 3GPP TS 36.133: Details the measurement requirements and thresholds for cell selection and reselection in LTE.
• ETSI TS 136 304: Official ETSI publication of the 3GPP TS 36.304 idle mode UE procedures for LTE.