2. Cell Reselection: Required System Information and Criteria

Before we learn the detailed cell reselection procedure in Chapter 3, we will provide a brief overview of a cell reselection procedure in Section 2.1 first. Then, we will look into what system information is required for the procedure and the criteria for the cell reselection in Sections 2.2 and 2.3, respectively.

2.1 Overview of Cell Reselection Procedure

It is UE who is in control of cell reselection. UE obtains information needed for cell reselection (e.g. threshold values used to decide whether to measure the signal strength of neighbor cells or not, parameters used for calculating rank of the serving cell and neighbor cells, etc.) from the system information broadcasted by eNB. In case of intra-frequency cell reselection, required information is delivered through System Information Block (SIB) 3 and SIB 4, which will be discussed further in Section 2.2.

■ Cell Reselection Triggering

Serving Cell Measurement: UE, in Idle state, wakes up at the end of every DRX cycle to measure the signal of its serving cell (Q_rxlevmeas) and calculate the received signal level (Srxlev) of the serving cell to decide whether it should stay or move to another cell. Here, the UE’s transmission and reception conditions are reflected in the calculation, for example by applying minimum received signal level Q_rxlevmin, allowed maximum TX power level P_EMAX, etc., (see Section 2.2 for details).

Cell Reselection Triggering: If the received signal level of the serving cell (Srxlev) is greater than the specified threshold value (s-IntraSearch), the UE stays in the current serving cell. If not, it triggers a cell reselection procedure. The threshold value that works as triggering criterion is delivered through SIB 3, and defined as s-IntraSearch in Release 8 and as s-IntraSearchP and s-IntraSearchQ in Release 9.

UE in Idle state wakes up at the end of every DRX cycle to measure the received signal level of its serving cell (Srxlev) when it has stayed in the same location for a while. Let’s assume a UE that has camped on the same serving cell for a while because the received signal level of the cell has remained lower than the set threshold (s-IntraSearch). If it leaves the serving cell, the received signal level of the cell decreases gradually. Finally when the received signal level becomes lower than the threshold (s-IntraSearch), a cell reselection procedure is triggered. Then the UE begins to measure the signal strength of the neighbor cells (i.e. non-serving cells).  

■ Cell Reselection Criteria

Cell-Ranking Criterion: The UE ranks each cell (R_s, R_n) based on the measured signal strength of the serving cell (Q_meas,s) and neighbor cells (Q_meas,n). Parameters required for cell ranking are delivered through SIBs 3 and 4 (see Section 2.2). 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.

Cell Reselection: Once the serving cell and neighbor cells (non-serving cells) are ranked, the UE checks whether the cell reselection criterion is satisfied (R_n > R_s) or not. If there are neighbor cell(s) that satisfy the criterion, the UE selects the best satisfying cell, and then camps there. Cell reselection is performed only when the criterion is satisfied for a certain period of time (t-ReselectionEUTRA).

Mobile operators can prevent too frequent cell reselection and make sure reselection is performed in accordance with the cell status by controlling the UE’s dwelling time on the serving cell, based on the hysteresis and cell-specific offset values. In addition, they 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.

2.2 System Information

System Information (SI) refers to the information broadcasted by eNB and consists of MIB (Master Information Block) and SIBs (System Information Blocks; SIBs 1 ~ 16) [4]. MIB, SIB 1 and SIB 2 are mandatory, but the rest SIBs are optional. All SI is delivered to UE through an RRC message like MIB, SIB 1, or SI message⁴. An SI message consists of a group of SIBs (SIBs 2 ~ 16), excluding MIB and SIB 1.

UE performs a cell reselection procedure based on the SI broadcasted by eNB. MIB, SIB 1 and SIB 2 are applied to all the UEs, either in Connected state (EMM-Registered, ECM-Connected, RRC-Connected) or in Idle state (EMM-Registered, ECM-Idle, RRC-Idle). Conversely, SIBs 3 ~ 8 are only used in cell reselection by those in Idle state. Table 1 describes different types of SI and their parameters (see our One-Shot Gallery for more information about for a selected list of SIB information⁵).

Table 1. Cell Reselection-related System Information

Let’s assume two mobile operators (A and B) who operate the network as follows:

In case of Operator A, its eNB needs information relating to intra-frequency for cell reselection (eNB broadcasts SIBs 3 and 4, but not SIBs 5, 6, 7 and 8. On the other hand, Operator B’s eNB needs information relating to all three types of reselection: intra-frequency, inter-frequency, and inter-RAT (3G UTRA) (eNB broadcasts SIBs 3, 4, 5 and 6).

The SI broadcasted by eNB is commonly applied to all the UEs, but each UE may receive different type of SI depending on its capacity (e.g. Release 9 UE or Release 11 UE).

Figure 1 displays a UE camping on Cell 5 at eNB 2 along with its neighbor eNBs. Figure 2 however shows eNB 2 only, along with its mandatory and cell reselection-related SI.

Figure 2. Cell Reselection-related SIBs Broadcasted by eNB2 

The network used in Figure 1 is an LTE-only network that uses a single frequency (lteFA1) with home PLMN only. This document is about the intra-frequency cell reselection procedure. The SI related to intra-frequency cell reselection is broadcasted through SIBs 3 and 4. Table 2 below lists the parameters used in the two SIBs, but not those related to other frequency, RAT or VPLMN.⁶

Table 2. Cell Reselection Parameters (SIBs 3 and 4)

2.3 Cell Reselection Criteria

Though not within the scope of this document, below we will review briefly the criteria used in cell reselection after UE is turned on. One of the following criteria is applied (i.e. criterion (1-1) for Release 8 UE and (1-2) for Release 9 or later UE). Table 3 provides a description of parameters used in the criteria.

■ Cell Selection Criteria

Table 3. Cell Selection Criteria Parameters (TS 36.304 [5])

During the initial cell selection, a cell must have the cell RX level (Srxlev) greater than the sum of Q_rxlevmin and P_Compensation to be a serving cell. The Srxlev must be greater than Q_rxlevmin because, in order for UE to correctly receive messages from its serving cell, the measured signal strength must be at least greater than Q_rxlevmin. UE’s Tx power (P_PowerClass) lower than the value allowed in the cell (P_EMAX) will result in greater P_Compensation value, making it hard to select the cell. When selecting a cell, UE’s transmission and reception conditions are considered.

In case of Release 9, in addition to Srxlev, Squal is added as a cell selection criterion. Q_rxlevmeas is the cell’s Reference Signal Received Power (RSRP) while Q_qualmeas is the cell’s Reference Signal Received Quality (RSRQ). Compared to RSRP which only indicates the strength of received signal, RSRQ provides more accurate information for radio link quality because it indicates the signal to interference and noise ratio (SINR).

■ Cell Reselection Triggering

UE camping on the serving cell may continue to camp on there without having to measure other cells if the serving cell’s RX level fulfills the criterion (2-1) for Release 8 UE or (2-2) for Release 9 or later UE.    

In the above inequalities, values of S_intraSearch, S_intraSearchP and S_intraSearchP are given by SIB 3 (see Table 2). If the serving cell does not fulfill either of the foregoing criteria ((2-1) or (2-2)), i.e. if it fulfills the criterion (3-1) in case of Release 8 UE or (3-2) in case of Release 9 or later UE below, the UE begins measuring the neighbor cells for reselection.

Cell Reselection Triggering (Neighbor Cell Measurement Triggering)

■ Cell Reselection Criteria

Cell Ranking Criterion

If the measured Srxlev of the serving cell fulfills the foregoing criterion ((3-1) or (3-2)), the UE begins to measure the neighbor cell’s RSRP. Then based on the resulting measurements, it ranks all the cells by applying the criteria (4-1) and (4-2) below [5]. Table 4 shows the parameters used in cell ranking.

Table 4. Cell Reselection Criteria Parameters (TS 36.304 [5])

Cell Reselection

If there are multiple neighbor cells that fulfill the criterion (5) below, the UE selects the best cell and camps on it.

As seen in the foregoing criteria (4-1) and (4-2), R_s and R_n are calculated using different criteria. The greater Q_hyst and Qoffset values are, the longer the UE can stay in the serving cell.