LTE : Selection Reselection Criteria

 

Cell Reselection is a kind of mechanism to change cell after UE is camped on a cell and stay in IDLE mode. This is to let UE get connected to cell which has the best condition among all the cells to which the UE is allowed to camp on. But UE does not change cells randomly, it uses a set of pretty complicated criteria and algorithms for this reselection process. The details of these criteria and algorithms will be described later but the high level guideline is as follows :

• First Level Criteria :  Absolute Priority
• Second Level Criteria : Radio Link Quality
• Third Level Criteria : Cell Accessibility

When you power on UE, usually it goes on with the following sequence.

i) Cell Detection/Search

ii) Cell Selection

iii) RACH and Completion of Registration

iv) < In IDLE >

v) Keep measuring RSRP/RSRQ for the cell it registered

vi) if RSRP/RSRQ is very poor or undetectable, go to step i) for finding other cell

  if RSRP/RSRQ is measurable at least, it evaluate < Cell Reselection Criteria >

  perform Cell Reselection if the criteria is met stay in the current cell if the criteria is not met

Step vi) is the most complicated process during the idle stage and in this page I will mostly deal with < Cell Reselection Criteria >. Understanding this criteria is the most important thing in implementing and testing Cell Reselection.

Cell Selection Criterion

For a cell to be selected, they should meet a certain criteria as follows.

The cell selection criterion S is fulfilled when:

Srxlev > 0 AND Squal > 0

When UE is in HPLMN

Squal = Qqualmeas – Qqualmin > 0

Srxlev = Qrxlevmeas – (Qrxlevmin + QrxlevminOffset) – Pcompensation > 0

 

When UE is in VPLMN

Squal = Qqualmeas – (Qqualmin + QqualminOffset) > 0

Srxlev = Qrxlevmeas –(Qrxlevmin + QrxlevminOffset) – Pcompensation > 0

​

Note: Out of the variables used in the equation, only Qrxlevmeas and Qqualmeas is the value UE really measures when it turns ON and most of the other parameters are broadcasted in SIB1.

Where:

The previous section described how initial cell selection will work and the difference between LTE FDD and TD-LTE. However, only when specific criteria are fulfilled is the UE allowed to camp on that cell. These criteria for cell selection as well as cell reselection for LTE are specified in [3]. It is further illustrated by a description of the two procedures: In the initial cell selection procedure, as described in the previous sections, no knowledge about RF channels carrying an E-UTRA signal is available at the UE. In that case the UE scans the supported E-UTRA frequency bands to find a suitable cell. Only the cell with the strongest signal per carrier will be selected by the UE. The second procedure relies on information about carrier frequencies and optionally cell parameters received and stored from previously-detected cells. If no suitable cell is found using the stored information the UE starts with the initial cell selection procedure. S is the criterion defined to decide if the cell is still suitable . This criterion is fulfilled when the cell selection receive level is  is computed based on the Equation below:



    is the measured receive level value for this cell, i.e. the Reference Signal Received Power (RSRP).  
        This measured value is the linear average over the power of the resource elements that carry the cell specific 
        reference signals over the considered measurement bandwidth. Consequently, it depends on the configured 
        signal bandwidth. In the case of receiver diversity configured for the UE, the reported value will be equivalent 
        to the linear average of the power values of all diversity branches.
        
         is the minimum required receive level in this cell, given in dBm. This value is signaled as 
        Q_rxLevmin by higher layers as part of the System Information Block Type 1 (SIB Type 1). Q_rxLevmin is           
        calculated based on the value provided within the information element (-70 and -22) multiplied with factor 2 in  
        dBm.


        is an offset to Q_rxlevmin that is only taken into account as a result of a periodic search for a  
        higher priority PLMN while camped normally in a Visitor PLMN (VPLMN). This offset is based on the  
        information element provided within the SIB Type 1, taking integer values between (1…8) also multiplied by a 
        factor of 2 in dB. This gives a wider range by keeping the number of bit transmitting this information. The 
        offset is defined to avoid “ping-pong” between different PLMNs. If it is not available then Q_{rxlevminoffset} is
        assumed to be 0 dB.


        is a maximum function as shown in Equation 5. Whatever parameter is higher, 
        P_EMAX- P_UMAX  or 0, is the value used for P_Compensation. P_EMAX [dBm] is the maximum power a UE is           
        allowed to use in this cell, whereas P_UMAX [dBm] is the  maximum transmit power of an UE according to the        
        power class the UE  belongs too. At the moment only one power class is defined for LTE, which corresponds                    
        to Power Class 3 in WCDMA that specifies +23 dBm.  P_EMAX is defined by higher layers and corresponds to  
       the parameter P-_MAX defined in [2]. Based on this relationship, P_EMAX can take values between -30 to +33 
       dBm. Only  when P_EMAX > +23 dBm P_Compensation is it considered when calculating S_rxlev. The P-_MAX 
       information element (IE) is part of SIB Type 1 as well as in the "RadioResourceConfigCommon" IE, which 
       is part of the SIB Type 2.

As explained above, all parameters except for Q_rxlevmeas are provided via system information. In a real network a UE will receive several cells perhaps from different network operators. The UE only knows after reading the SIB Type 1 if this cell belongs to its operator’s network (PLMN Identity). First the UE will look for the strongest cell per carrier, then for the PLMN identity by decoding the SIB Type 1 to decide if this PLMN is a suitable identity. Afterwards it will compute the S criterion and decide for a suitable cell or not.