Cell Reselection Procedure in LTE:- There are some procedure followed in cell reselection for LTE listed as below:-
1.Serving Cell Measurment
2.Cell Reselection Triggering
3. Cell Ranking
4. Cell Reselection.
We will discuss one by one.
1. Serving Cell Measurement:- UE, in Idle state, wakes up at the end of every DRX cycle to measure the signal of its serving cell (Qrxlevmeas) and calculate the received signal level (Srxlev) of the serving cell to decide whether it should stay or move to another cell.
2. Cell Reselection Triggering:- If the received signal level of the serving cell (Srxlev) is greater than the specified threshold value (s-IntraSearch for Intra Frequency and S-NonIntraSearch for Inter frequency)), the UE stays in the current serving cell. If not, it triggers a cell reselection procedure. The threshold values are delivered through SIB 3, and defined as s-IntraSearch in Release 8 and as s-IntraSearchP and s-IntraSearchQ in Release 9.
3. Cell Ranking:- The UE ranks each cell (Rs, Rn) based on the measured signal strength of the serving cell (Qmeas,s) and neighbor cells (Qmeas,n). Parameters required for cell ranking are delivered through SIBs 3 and 4. The serving cell is ranked using the hysteresis (q-Hyst) value stored in SIB 3 while the neighbor cells are ranked based on the offset (q-OffsetCell) value specified for each cell in SIB 4.
4.Cell Reselection:- After serving cell and neighbor cells are ranked, the UE decides whether the cell reselection criterion is satisfied (Rn > Rs) or not. If there are neighbor cell(s) that satisfy the criteria, the UE selects the best satisfying cell, and then reselct there. Cell reselection is performed only when the criterion is satisfied for a certain period of time (t-ReselectionEUTRA).
We can prevent too frequent cell reselection and make sure reselection is performed in proper manner by hysteresis and cell-specific offset values. In addition, we can control the q-Hyst and t-ReselectionEUTRA values by applying appropriate scaling factor (q-hystSF, t-ReselectionEUTRA-SF) depending on the traveling speed of the UE.
Below table is the parameter summary for the cell reselection.
1.Serving Cell Measurment
2.Cell Reselection Triggering
3. Cell Ranking
4. Cell Reselection.
We will discuss one by one.
1. Serving Cell Measurement:- UE, in Idle state, wakes up at the end of every DRX cycle to measure the signal of its serving cell (Qrxlevmeas) and calculate the received signal level (Srxlev) of the serving cell to decide whether it should stay or move to another cell.
2. Cell Reselection Triggering:- If the received signal level of the serving cell (Srxlev) is greater than the specified threshold value (s-IntraSearch for Intra Frequency and S-NonIntraSearch for Inter frequency)), the UE stays in the current serving cell. If not, it triggers a cell reselection procedure. The threshold values are delivered through SIB 3, and defined as s-IntraSearch in Release 8 and as s-IntraSearchP and s-IntraSearchQ in Release 9.
3. Cell Ranking:- The UE ranks each cell (Rs, Rn) based on the measured signal strength of the serving cell (Qmeas,s) and neighbor cells (Qmeas,n). Parameters required for cell ranking are delivered through SIBs 3 and 4. The serving cell is ranked using the hysteresis (q-Hyst) value stored in SIB 3 while the neighbor cells are ranked based on the offset (q-OffsetCell) value specified for each cell in SIB 4.
4.Cell Reselection:- After serving cell and neighbor cells are ranked, the UE decides whether the cell reselection criterion is satisfied (Rn > Rs) or not. If there are neighbor cell(s) that satisfy the criteria, the UE selects the best satisfying cell, and then reselct there. Cell reselection is performed only when the criterion is satisfied for a certain period of time (t-ReselectionEUTRA).
We can prevent too frequent cell reselection and make sure reselection is performed in proper manner by hysteresis and cell-specific offset values. In addition, we can control the q-Hyst and t-ReselectionEUTRA values by applying appropriate scaling factor (q-hystSF, t-ReselectionEUTRA-SF) depending on the traveling speed of the UE.
Below table is the parameter summary for the cell reselection.
| Parametrs | Description | SIB Type |
| S-Intrasearch | Value that triggers Intra frequency measurment(dB) | 3 |
| q-Hyst | Hysteresis value for Serving cell ranking | 3 |
| q-RxlevMin | Minimum Rx Level required for UE to continue on serving Cell | 3 |
| p-Max | Maximum TX power allowed for UE | 3 |
| allowedMeasBandwidth | DL BW to be measured by UE | 3 |
| t-ReselectionEUtra | Cell Reselection timer Value | 3 |
| physcellID | PCI of neighbouring cell | 4 |
| q-offsetCell | offset value for serving cell ranking | 4 |
| intrafreqblackCelllist | List of neighbour cells that are black listed for reselection | 4 |
| physcellidrange | PCI Range | 4 |
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