LTE Idle Mode Mobility in One Diagram

Concept of Events in Idle mode mobility, role of Cell Edge in mobility and hence an introduction of a new quantity Srxlev.


qRxLevMin defines the Cell Edge, can be configured at cell level, usually depends on signal characteristics of a particular band.


Srxlev is a quantity measured from Cell Edge towards Base Station and is Zero at cell edge while highest next to the base station. That may be the reason that all the thresholds in idle mode are relative to Cell Edge (qRxLevMin) and correspondingly Ue makes a decisions when it crosses any of those thresholds.


Pcomensation is a penulty for low powered Ues (Reducing the Cell Size for low Powered Ues with respect to the configured power)


Intrafreq reselection (A3 based) is better cell reselection means reselecting the target cell when Srxlev of target cell becomes better than Srxlev of serving cell.


Interfreq reselection to low/equal priority cell (A5 based) needs to meet two conditions A51 & A52, means reselecting the target cell when Srxlev of serving cell goes below threshServingLow (A51) and that of target goes above threshXlow (A52).


Interfreq reselection to high priority cell (A4 based) needs to meet only one condition, means when Srxlev of target cell goes above threshXhigh.


If the Ue is not at highest priority (7) cell, it continues to look for highest priority cell periodically.


When Srxlev of serving cell goes below SintraSearch And SnonintraSearch thresholds, intrafreq and interfreq towards low/equal priority target cell measurements are triggered respectively.


In connected mode, as we all know, the mobility (from one cell to another) is controlled by base station and assisted by Ue (by sending Measurement Reports).


In Idle mode, it's just the opposite of that, the mobility (from one cell to another) is controlled by Ue and assisted by base station (by sending SIBs).


In idle mode to get a little bit more control over Ue's mobility in interfreq environment (multi-carrier sites), the concept of cell reselection priority is introduced. It's my own visualization to understand these few things, less talked about.


Srxlev = Qrxlevmeas - qRxLevMin 


Note: Often Rel9 is mentioned in literature because Squal is not recognized by UEs < Rel9