Definition
Cell Reselection is the process in which a terminal autonomously selects a better cell based on measurement results while in idle state (RRC_IDLE) or RRC_INACTIVE, which differs from handover (network-controlled) in connected state . Reselection is based on parameters such as RSRP/RSRQ of the serving cell and neighboring cells, and frequency priority, affecting terminal residence quality, the success rate of subsequent call establishment, and power consumption. Reselection is decided autonomously by the terminal and does not require network signaling.
Reselection Criteria
- S Criterion: The serving cell must meet the Srxlev and Squal thresholds; otherwise, reselection is required.
- R Criterion: For same-frequency reselection, the neighbor cell R value = Qmeas + Qoffset, and the serving cell R value is Rs; reselection is triggered when the neighbor cell R exceeds the serving cell R continuously for Treselection.
- Priority Reselection: High-priority frequency/system can be reselected immediately (if thresholds are met), whereas low-priority requires the serving cell to be below the threshold.
- Reselection Hysteresis: Treselection prevents ping-pong reselection.
Inter-Frequency and Inter-System
The terminal performs inter-frequency (such as LTE B3/B8) or inter-system (such as LTE/5G, LTE/WCDMA) reselection based on the frequency priority and reselection parameters in the system information. Operators can guide terminals to stay on the target frequency band (such as prioritizing 5G) through priority settings. The MLB in SON can adjust cell offsets to influence reselection and achieve load balancing.
Typical Use Cases
Idle state mobility, inter-frequency/inter-system reselection, load balancing, power saving (staying on high-quality cells reduces reselection and measurement). SimtoLink's IoT SIM cards automatically perform cell reselection in mobile scenarios, and the cloud platform can monitor changes in device cell residence.
Cell reselection affects the residence quality of IoT devices in mobile scenarios. Good reselection strategies can reduce dropouts and improve call setup success rates. SimtoLink's IoT SIM cards support networks of the three major operators, and devices can automatically perform cell reselection in mobile scenarios. The SimtoLink cloud platform can monitor device location changes to provide connection quality analysis for mobile IoT scenarios.
Measurement opens up only after the serving cell falls below the S-criterion
An idle device does not compare every neighbor all the time. While the serving cell’s received level and quality are still above the thresholds in system information, intra-frequency and inter-frequency measurements can be reduced or skipped, which saves power. Once the cell falls below the S-criterion, the device must look for another cell it can camp on. If the threshold is set high, the device measures often on a cell it can still use, and the battery falls. If it is set low, the device stays in a very poor cell until the next call setup fails. To see whether reselection even has a chance, check first whether the serving cell already fails the camping condition, not only whether a neighbor is stronger.
Cell selection before the first call and reselection while moving are not the same trigger. At power-on the device looks for a cell that meets the criterion. While moving it compares neighbors on top of a cell it already camps on. Writing a failed search at boot as a reselection failure sends you to reselection hysteresis, when the real problem may be that no campable cell exists, or the card is barred from the band it is allowed to use.
Frequency priority can push the device onto a layer with worse coverage
Reselection is not only a contest of who has the higher level. System information gives each frequency a priority. As soon as a higher-priority frequency passes its own threshold, the device may leave a lower-priority cell that has a better level. Operators use this rule to push devices onto a chosen network layer. If an IoT device is pushed onto a high band that is very weak indoors, idle mode looks “camped on the target network,” and the connection fails as soon as it is set up. Put priority, threshold, and the actual level together: a lead in level with a low priority still makes the device leave.
Inter-system reselection looks at priority the same way. A module that supports only some radio standards, faced with a higher-priority frequency it does not support, stays on the current system. The log may still show that a neighbor system was measured. That is not the same as having reselected onto it. Service design cannot assume that “5G was measured” means the device is camping on 5G.
If hysteresis is too short, a meter camps back and forth between two cells
Intra-frequency reselection requires the neighbor to be better than the serving cell for a reselection time. That hysteresis blocks a brief fade. If it is too short, a device in a vehicle or swinging in the wind camps back and forth between two cells, and each change can bring a location update, so signaling outweighs service traffic. If it is too long, the device stays on a cell that is already unusable, and the next report has to finish a reselection first, with the delay piled at the start of the report. The right length should cover the typical fade duration on site, not a static laboratory bench value.
A SimtoLink IoT SIM card completes reselection in idle mode from the device’s own measurements. The network does not send a command each time. If the cloud platform records the camping cell jumping back and forth while traffic is very small, suspect reselection parameters or priority, not a suspended card. When a fixed meter is pushed by priority onto an edge cell, changing the card usually does not change the frequency priority in system information. Handle it from the installation position or from the bands the device is allowed to camp on.
A barred neighbor makes reselection fail at the boundary
System information or the subscription can bar the device from reselecting to certain cells, certain frequencies, or certain public land mobile networks. Once barred, the device cannot camp there even if the neighbor level is higher. It stays on a serving cell that has already gone poor, until that cell itself fails the camping condition, and then it searches. At a boundary the level jitters, reselection never happens, and then a whole stretch goes offline. Treating that as hysteresis that is too long leads you to change a timer that is not being used. Read first whether the neighbor is on the barred list.
Equivalent networks and a forbidden roaming list also act on an operator boundary. If an IoT SIM card is subscribed for local use only, a strong neighbor measured near the boundary may belong to a network that cannot be used, and the device refuses to reselect. A SimtoLink card is bound by the subscription here. The module enforces the barred list. If a device fixed at a border or at the edge of a campus goes offline again and again, check the allowed networks and frequencies before the reselection threshold. Moving the installation so the serving cell meets the camping condition again is sometimes more effective than loosening hysteresis.