Just to keep everybody on their toes it seems the wise people at 3GPP decided to introduce mobility from 3G CELL_FACH to LTE from release 11 onwards. As the figure above shows (TS 36.331), that all important arrow from CELL_FACH to E-UTRA RRC_IDLE has been introduced. Why it was not included in the first place is still a mystery to me as from a UE complexity point of view I wouldn't have thought it would be that difficult. From a RAN point of view it just re-uses the FACH measurement occasion concept defined since release 99 that allows a UE to search for inter frequency and IRAT neighbours during specific time intervals when it knows the RAN will not send it any data on the SCCPCH.
In addition to the above, SIB19 on 3G has some changes as the operator can choose if all EARFCNs will be searched for or only the higher priority ones (if applicable).
Although CELL_FACH can be thought of as a transient state it is possible that a UE can stay there long depending on RNC inactivity timers and keep alive periodicity from applications on the UE. With this change then the possibility of being "stuck" in 3G is minimised.
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