LTE Cell Camping and Selection Procedure

Interviews, LTE, RF Basics, Tech Fundas

Cell Selection and Re-selection:

After selecting a PLMN, a UE selects a cell in the PLMN to camp on. After camping on the cell, the UE monitors system information, performs measurements on the serving cell and neighboring cells based on measurement rules, and selects a better cell to camp on based on cell re-selection criteria. The UE performs cell selection or re-selection using the selection- or re-selection-related parameters from the system information or from the RRC Connection Release message received from eNodeB. Figure below shows the cell selection and cell re-selection procedures.

Cell Search

Cell searching is a procedure in which a UE achieves time and frequency synchronization with a cell, obtains the physical cell identifier (PCI), and learns the signal quality and other information about the cell based on the PCI. A UE performs cell search on all frequencies before selecting or re-selecting to a cell. In an LTE system, two types of synchronization signals for cell search: primary synchronization signals (PSS) and secondary synchronization signals (SSS). The cell search procedure is as follows:

  • A UE monitors the PSS to acquire slot synchronization accurate to within 5 ms, and to identify the cell number
  • The UE monitors the SSS to acquire frame synchronization and to identify the cell group number.
  • The UE calculate the PCI by using (3 x cell group number + cell number) .
  • The UE monitors the Downlink reference signal (RS) to determine the signal quality in the cell.
  • The UE reads information on the broadcast channel (BCH) and decode Master Information Block (MIB) to acquire  detailed about the cell.

Cell Selection

When a UE transitions from connected mode to idle mode or after it has selected a PLMN, the UE must select a cell to camp on. When the UE transitions from connected mode to idle mode:

  • UE first attempts to select the most recent cell it has camped on in connected mode, or it selects a Suitable Cell on a frequency that is allocated through the RRC Connection Release message.
  • If such a cell is not available, the UE attempts to find a Suitable Cell by performing the Stored Information Cell Selection procedure.
  • If the UE fails to find a Suitable Cell, the UE performs the Initial Cell Selection procedure.

Stored Information Cell Selection

The Stored Information Cell Selection procedure requires stored information related to carrier frequencies and cell parameters. The information is retrieved from previous measurement control information elements or from pre-existing SI messages. This information can help speed up cell selection. The Stored Information Cell Selection procedure is as follows:

  • The UE searches known carrier frequencies for a Suitable Cell.
  • If the UE finds a Suitable Cell, it selects that cell to camp on.
  • If the UE fails to find a Suitable Cell, it initiates the Initial Cell Selection procedure

Initial Cell Selection

The Initial Cell Selection procedure does not require that the UE know in advance which frequencies are E-UTRA carrier frequencies. Here UE scans all carrier frequencies on the supported E-UTRAN bands to find a Suitable Cell. At each carrier frequency, UE searches for the strongest cell only. If the UE finds a Suitable Cell, it selects that cell to camp on. If the UE fails to find a Suitable Cell, it selects an Acceptable Cell to camp on.

Cell Selection Criteria

During cell selection, UE selects an E-UTRAN cell that meets cell selection criteria. UE selects an E-UTRAN cell to camp on when both Srxlev and Squal are greater than zero.

The variables in the preceding formulas are described as follows:

  • Qrxlevmeas is the measured cell Rx signal level (RSRP value) of the cell
  • Qrxlevmin is the minimum cell Rx signal level. It is broadcast in SIB1
  • Qrxlevminoffset is the offset to Qrxlevmin. It is broadcast in SIB1 a. The UE camping on a cell in a VPLMN considers this offset parameter when it attempts to camp on a cell in a higher-priority PLMN during periodic cell reselection. The cell in the higher-priority PLMN delivers the value of this parameter to the UE for the Srxlev evaluation.
  • Pcompensation is the result of the function: Max (PMax – UE Maximum Output Power, 0)
  • PMax is the maximum transmit power that the UE can apply to uplink transmission and it is broadcast in SIB1
  • UE Maximum Output Power is the maximum output power that the UE can physically achieve. It is not configurable on the network side.
  • Qqualmeas is the measured cell Rx signal quality (RSRQ value) of the cell.
  • Qqualmin is the minimum cell Rx signal quality required for the cell to become a Suitable Cell and it is broadcast in SIB1
  • Qqualminoffset is the offset to Qqualmin. It is broadcast in SIB1 . UE camping on a cell in a VPLMN considers this offset parameter when it attempts to camp on a cell in a higher-priority PLMN during periodic cell reselection. The cell in the higher-priority PLMN delivers the value of this parameter to the UE for the Squal evaluation.

Neighboring Cell Priority 

After camping on a cell, a UE may attempt to re-select to and then camp on another cell. Before cell re-selection, the UE is required to measure the signal quality level of neighboring cells. The UE then re-selects to a neighboring cell by using a specific cell re-selection criterion according to the priority of the neighboring cell.

During measurement and implementation of cell re-selection, the UE needs to obtain the priorities of neighboring frequencies. Note that frequencies of different RATs must be configured with different priorities. During the evaluation for re-selection to an intra-frequency neighboring cell, the UE ignores the priority of the target frequency because cells working on the same frequency have the same priority. The priorities for E-UTRAN frequencies and inter-RAT frequencies include common priorities and dedicated priorities.

Common Priority

  • The priority of the serving frequency is specified and broadcast in SIB 3 .
  • The priority of the non-serving E-UTRA frequency is specified and broadcast in SIB5. A maximum of 16 non-serving E-UTRA frequencies can be broadcast.
  • The priority of the neighboring UTRAN frequency is specified and broadcast in SIB6. A maximum of 16 neighboring UTRAN frequencies can be broadcast.
  • The priority of the neighboring GERAN frequency is specified and broadcast in SIB7. A maximum of 16 neighboring GERAN frequency groups can be broadcast.
  • The priority of a neighboring CDMA2000 frequency is specified broadcast in SIB8.

Dedicated Priority

During release of UE radio resources, the IdleModeMobilityControlInfo IE in the RRC Connection Release message provides the radio resources for each UE. A maximum of 16 non-serving E-UTRA frequencies, 16 neighboring UTRAN frequencies, and 16 neighboring GERAN frequency groups can be delivered for each UE. Cell re-selection dedicated priorities include dedicated SPID priorities and operator-specific dedicated priorities.

UE capability-based frequency Filtering

  • If the eNodeB has obtained UE capabilities, it filters out frequencies that UEs do not support based on systems supported by UEs and UE frequency capabilities.

Target PLMN-based Filtering

  • If all home PLMNs of external cells under the frequency are not included in the target PLMN list, the eNodeB filters out the frequency.
  • If no external cells are configured on the frequency (no PLMN information of external cells), the eNodeB fails to obtain PLMN attributes of the frequency and filters out the frequency.

Blacklisted Cell Identifying

During cell re-selection, UEs are not allowed to measure or re-select to the blacklisted cells of the serving cell. Blacklisted cells are classified into intra-frequency and inter-frequency blacklisted cells. UEs identify blacklisted cells by PCIs.

  • Information about intra-frequency blacklisted cells is delivered in IntraFreqBlackCellList of the SIB4 and is excluded from IntraFreqNeighCellList of the SIB4.
  • Information about inter-frequency blacklisted cells is delivered in InterFreqBlackCellList of the SIB5 and is excluded from InterFreqNeighCellList of the SIB5.