RF Wireless World

LTE UE cell selection

Cell selection allows a LTE compliant UE to camp on a cell. There are three types of cell selection:

•  Initial cell selection
•  Stored information cell selection
•  Cell selection when leaving RRC connected mode
All these types are depicted in the figure.

Initial Cell selection: It does not require any prior knowledge. The UE scans all RF carriers while searching for a suitable cell.

Stored Information cell selection:uses RF carrier and cell parameter informations to identify suitable cell. Initial cell selection procedure is initiated when stored type fails.

The cell selection when leaving RRC CONNECTED mode applies to UE making transition from RRC CONNECTED to RRC IDLE mode. The eNodeB can direct UE towards a specific RF carrier by including 'redirected carrier information' within RRC CONNECTION RELEASE MESSAGE; otherwise UE is free to search any RF carrier.

All the three types of cell selection techniques involve UE searching for a suitable cell. A suitable cell is defined as a cell which will have following characteristics.

•  Belongs to the selected PLMN, registered PLMN or an equivalent PLMN
•  It is not barred.
•  Belongs to at least one tracking area which is not forbidden
•  satisfies cell selection criteria

The cell selection criteria is defined as follows:

S_rxlev > 0
Where,
S_rxlev = Q_rxlevmeas - (Q_rxlevmin+Q_{rxlevminoffset}) - P_compensation

Q_rxlevmeas = RSRP measured by UE in dBm
Q_rxlevmin = min. required RSRP signalled within SIB-1
Q_{rxlevminoffset} = usually included in SIB-1. If not included than value of zero is set.
P_compensation Defined as MAX(P_EMAX - P_UMAX, 0)

Priorities between RF carriers and different RATs are not used during cell selection.

REFERENCE: 3GPP TS 36.304

LTE UE cell reselection

UE is free to do cell reselection either the serving cell RF carrier strength is poor or suitable high strength neighbour cell is available in the same RAT or different RAT. RAT stands for Radio Access Technology. It can be GSM, UMTS or LTE.

UE only perform cell reselection evaluation for LTE RF carriers and inter-RAT RF carriers which are defined within the SI and for which the UE has a priority. Blacklisted cells are excluded from list of candidate cells for cell reselection.

Absolute priorities can be allocated to RF carriers belonging to both LTE and other RATs.

REFERENCE: 3GPP TS 36.304, 3GPP TS 36.331

LTE RELATED LINKS

Difference between SC-FDMA and OFDMA
LTE Cyclic Delay Diversity
LTE eNodeB Physical Layer Measurements
LTE EPC Network Inerfaces
LTE UE Physical Layer Measurements
LTE RSRP vs RSRQ
LTE Bearer types
LTE Signalling radio bearers
LTE QoS
LTE EPS mobility management states
LTE EPS connection management states

RF and Wireless Terminologies