CSFB priorities

Hi Experts,
In CSFB scenario I have 2 priorities, the higher one for 2100 3G and the lower priority for 900 GSM.
If the radio conditions for the lower priority (900 GSM) are better than the high priority frequency (2100 UMTS)… then CSFB will be preformed to GSM or UMTS?

It always look for high prio Freq first.
If cell not available then go to second prio.

Even if the radio conditions are much better for the lower priority?

Yes prio it will look first.

It depend on the CSFB strategy: measurement-based or redirection.
If redirection is used, enb will redirect csfb UE to UMTS or GSM based on higher prioity RAT/frequency. Each time of redirection, enb only deliver 1 frequency. If enb deliver UMTS frequency but actually UE can not measure any UMTS signal, UE may try to search any other frequency band to request service.
If measurement-based or PS handover is used, UE will send measurement result to enb, enb select target cells for UE.

Using B1 event for measurement based.

There can be measurement based redirection as well, which might take longer than blind redirection and Handover.
For MO csfb, UE can search for other frequencies or RAT, but in case of MT call, those might fail if redirected arfcn not found

I never hear about measurement-based redirection.
But as my thinking, redirection means that: no measurement.

Redirection is kind of measurement based it needs threshold of csfb and if not meeting it fails in execution so there is csfb protect to timer too which holds for some time.
As per its recommended value.

The strategy is measurement based but I noticed that even if the lower priority frequency has better radio conditions the us always select the higher priority frequency.

Measurement based Redirection involves the UE completing measurements from the target layer prior to receiving the redirection command ,This helps to improve reliability in terms of ensuring that the target layer has coverage.