In GSM there was a dedicated channel for periodically reporting the controller (BSC) with the current situation faced by UE. But in 3G, the measurements can be set to Periodic or Event-triggered. Almost unanmously, all operators decided for Event-triggered measurements.
The following categories of measurement events are defined for UTRA FDD
- 1-series (event 1A to 1F):
- Intra-frequency events
- 2-series (event 2A to 2F):
- inter-frequency events
- 3-series (event 3A to 3D):
- inter-RAT events
- 4-series (event 4A and 4B):
- traffic volume events
- 5-series (event 5A):
- quality event
- 6-series (event 6A to 6G):
- UE internal events
- 7-series (event 7A to 7C):
- UE positioning events
From all the above events, the most commonly discussed are the Series 1 Events because they lead to start and stop of Soft Handover. Event 1A, 1B and 1C are the king of all events.
- 1A:
- A Primary CPICH enters the reporting range.
- 1B
- A primary CPICH leaves the reporting range. Similar concept as in 1A.
- 1C:
- A non-active primary CPICH becomes better than an active primary CPICH
- 1D:
- Change of best cell
- 1E:
- A Primary CPICH becomes better than an absolute threshold
- 1F:
- A Primary CPICH becomes worse than an absolute threshold
More about these can be read in the Chapter 5 of book under Radio Resource Management Chapter.