How do YOU select does your phone select an AT&T cell ?

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If you have purchased a new LTE phone(UE) from an AT&T showroom. You walk out of the store all excited. You power on the phone. The map to go from no connection at all to your UE selecting AT&T connection to a cell works in 3 simple steps

  1. LTE Network Search
  2. PLMN Selection
  3. Cell Selection

 

What is Cell Selection ?

Cell selection is a procedure which allows UE to camp on to  a cell. So that UE can make an actual RF connection to the network, after camping on to the cell. During cell selection off course UE , will look for a cell with better RF coverage. However , the better RF coverage is computed mathematically as shown in Selection criteria at the end below.

cell_selection

Conditions for camping on a Cell

UE will always prefer to camp onto a suitable cell. Therefore , it will always keep on looking for a suitable cell. If it cannot find one, then it will go and camp on an acceptable cell. The question which comes to mind is. What the heck is a suitable cell ?

 

Suitable Cell:

A suitable cell is defined as a cell which

  • belong to selected PLMN/ registered PLMN or Equivalent PLMN
  • is not barred
  • belongs to at least one Tracking Area which is not forbidden
  • satisfies the cell selection criteria

 

 

Types of Cell Selection

There are 3 types of cell selection

  1. Initial Cell Selection
  2. Stored Information Cell Selection
  3. Cell selection when leaving RRC Connected mode

 

 

initial_cell_selection

Cell Selection type: Initial Cell Selection

 

stored_info_cell_selection

If the UE was already connected, and for example went out of coverage. On the return back into good coverage Stored Information cell selection will take place

 

 

If UE is leaving from RRC Connected Mode to RRC Idle Mode, then cell selection will take place like this

If the UE is leaving from RRC Connected Mode to RRC Idle Mode, then cell selection will take place like this

 

 

Criteria for Cell Selection

In general Cell Selection criteria is based on finding a better RF coverage cell for the UE. The coverage  strength and quality of a cell in LTE is measured generally on :

  • Reference Signal Received Power (RSRP)
  • Reference Signal Received Quality (RSRQ)

 

S-Criteria based on RSRP (Cell Selection Criteria)

Cell Selection Criteria(also know as the S-criteria) is defined as

s_criteria

 

The above criteria is based on RSRP

 

Parameters for Cell Selection

Qrxlevmeas = RSRP measured by the UE in dBm

Qrxlevmin = minimum required RSRP signaled within System Information Block 1 (SIB 1)

Qrxlevminoffset = measurement offset which is only applied when a cell is evaluated for cell selection as a result of periodic search for higher priority PLMN while camped on a Visited PLMN. Qrxlevminoffset is optionally included within SIB 1 . A value of 0 dB is applied if Qrxlevminoffset is excluded from SIB 1.

Pcompensation = Defined as MAX(P emax  – P umax, 0 ) where P emax is the maximum allowed uplink transmit power within the cell, and  P umax is the maximum transmit power capability of the UE. If the UE transmit power capability is less than the maximum allowed transmit power , Pcompensation makes cell selection criteria more difficult to achieve . This avoids UE camping on cells for which they have insufficient transmit power to access i.e., towards cell edge

 

S-Criteria based on RSRQ (Cell Selection Criteria)

The newer version of cell selection criteria(S criteria ) according to release 9 of 3GPP specifications add a quality check to the cell selection criteria given as:

 

s_criteria_rsrq

 

For any questions, comments or criticism, please comment below.

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