Cell Selection and Reselection Parameters in WCDMA – Idle Mode Behaviour

1. 2G → 3G cell reselection
2. 3G cell reselection:
   • 3G → 3G
   • 3G → 2G.

This article will analyse in detail idle mode behaviour. First, cell reselection from 2G to 3G
will be discussed. Next, 3G cell reselection will be reviewed, consisting of both 3G to 3G cell reselection and 3G to 2G cell reselection.

2G → 3G Cell Reselection Parameters.

The graph above shows the main parameters involved in cell reselection from 2G to 3G: Qsearch_l, FDD_Qoffset and FDD_Qmin. We assume that the UE is camping in idle mode on a 2G cell, and that the RSSI of this serving cell is going down. At the same time we assume a 3G neighbour is available from which the CPICH RSCP is getting better and better. At a certain moment the reselection from the 2G serving cell to the 3G neighbour cell will be performed.

Initially the RSSI of the serving 2G cell is quite good, and the UE is happy and does not feel the need to measure. But at a certain moment the RSSI of the 2G serving cell drops below the parameter Qsearch_I. From this moment the UE is not that happy anymore and starts to measure.

Once the UE starts to measure an offset is applied to the 2G serving cell: FDD_Qoffset, which is the second parameter involved. This offset can be both positive or negative. The 2G serving cell with the offset applied is now compared against the 3G neighbour. At a certain moment the 3G neighbour is getting better than the 2G serving cell with the offset applied.

At this moment a timer of 5s is started. If this condition is fulfilled during the complete timer duration, then the reselection to the JG neighbour cell can be performed, at least if one additional condition is fulfilled: the CPICH Ec/No of the 3G neighbour cell should be better than FDD_Qmin, which is the third parameter involved.

2G → 3G cell reselection – Qsearch_I.

The mapping tables above show the different parameter values for Qsearch_I. The first table is used to start 3G measurement if the RSSI of the 2G serving cell drops below a specific threshold. The second table is used to start 3G measurements if the RSSI of the 2G serving cell exceeds a specific threshold.

Several examples are given in the above to illustrate this principle. A quite special parameter value can also be chosen for Qsearch_t: 7 or always. This means that 3G neighbours are always measured independent of the GSM serving cell RSSI. This special parameter value is used by the majority of operators.

2G → 3G cell reselection – FDD_Qoffset.

The mapping tables above show the different parameter values for FDD_Qoffset. The first table is used to apply a negative offset to the GSM serving cell. The second table is used to apply a positive offset to the serving 2G cell. Several examples are given in the above to illustrate this principle.

A quite special parameter value can also be chosen for FDD_Qoffset: O or always. This means that the 3G neighbour is always better than the GSM serving cell. This special parameter value is used by the majority of operators.

2G → 3G cell reselection – FDD_Qmin

The mapping tables above show the different parameter values for FDD_Qmin. Both the old and new tables are shown. Several examples are given in the picture above to illustrate this principle. The new mapping table was introduced to allow operators to use higher values for FDD_Qmin.

1. 3G cell is only a suitable candidate for reselection if also the S-criterion is fullfilled.
2. Defines whether a 3G cell is a suitable candidate for reselection.
3. Two conditions to fulfull.
   • Squal = Qqualmeas – Qqualmin > 0.
   • Srxlev = Qrxlevmeas – Qrxlevmin (- P_compensation) > 0
4. A(handover) relation needs to be defined between the concerned cells before the 2G – → 3G celreselection is enabled.
5. The 2G → 3G cell reselection parameters can be read from 2G system information message.

Before a reselection can be performed to a 3G cell, this cell needs to fulfil in addition the S-­criterion. This criterion defines whether a 3G cell is a suitable candidate for reselection or not. Two conditions need to be fulfilled: first of all the cell needs to have a minimum quality, and secondly the cell needs sufficiently strong RSCP.

A very typical value for Qqualmin is -18db. Also important to keep in mind is that a (handover) relation needs to be defined between the concerned cells before a reselection is possible between them. All 2G to 3G cell reselection parameters can be found on the 2G system information in idle mode.

Read More: 3G Cell Reselection Procedure Overview.