Cell reselection — Ranking, priorities and hysteresis — how a camped device moves
The device — identity, state, mobility
Where it sits
What it is
What it is. How a device that is already camped decides to move to another cell. TS 38.304 §5.2.4. It is the idle-mode counterpart of handover — and the essential difference is that nobody orders it.
Handover against reselection:
| Handover | Reselection | |
|---|---|---|
| Device state | CM-CONNECTED / RRC_CONNECTED | RRC_IDLE or RRC_INACTIVE |
| Decided by | the network, from reported measurements | the device, from broadcast rules |
| Signalling | measurement reports, then a reconfiguration order | none |
| Cost | real | almost nothing |
An idle device is not in a conversation with anyone. There is no connection over which to report measurements or receive an order — so the network broadcasts the rules in system information and each device applies them itself. Millions of devices change cell continuously and the network never hears about it.
The two mechanisms, §5.2.4:
Frequency priorities. Every carrier frequency carries a priority, broadcast (or given to the device individually). A higher-priority frequency is reselected whenever a cell on it is merely good enough — above a threshold, for a period. A lower-priority one is reselected only when the current frequency has become bad. This is how an operator steers idle devices onto a preferred band without talking to any of them.
Ranking, within equal priority. Cells on the same or equal-priority frequencies are ranked by the
R criterion, Rs for the serving cell and Rn for a neighbour, using RSRP plus per-cell offsets
and hysteresis.
What stops it oscillating. Two devices matter here and both are in the criterion: hysteresis
(Qhyst) makes the serving cell count for more than it measures, so a neighbour must be better by a
margin rather than merely better; and a timer (TreselectionRRC) requires the neighbour to stay
better for a period. Between two cells of nearly equal strength — on a boundary, at a window — a
device without both would ping-pong, and every reselection means re-reading system information and
possibly a registration if the
tracking area changed.
Measurement is skipped when it is not needed. §5.2.4.2 lets the device stop measuring neighbours
entirely while the serving cell is comfortably above SIntraSearchP and SIntraSearchQ. A phone with
good signal does not scan — which is a substantial part of why idle standby time is measured in days.
And it scales its own timers by speed. §5.2.4.3 defines mobility states — normal, medium, high — inferred by the device from how many reselections it has recently made, with scaling rules that shorten the timers when it is evidently moving fast. The device infers its own speed from its own history and adapts.
Read on
This concept is read as part of one argument in ref-ue-mobility, alongside the rest of its group.
Before this concept, the hierarchy says to learn the following — the full chain, in order: