l Setup Time, Transition to Target RAT RRC Release with Redirect There are two basic methods for directing the UE to the target system in CSFB call setup: RRC release with redirection and PS handover. These methods can be either blind or measurement based, depending on the vendor implementation. The blind redirect to the target RAT can be considered the baseline method. It is easy to configure and requires very little planning effort, since only the carrier frequency (or BCCH frequencies in case of 2G) of the target RAT needs to be known. With blind redirect the transition time from LTE to 3G for a non-stationary UE is around 2 seconds, based on field VoLTE Optimization measurements. On the other hand, after initial access to target RAT the UE may cache¹ the target RAT system information, and in subsequent CSFB calls a stationary UE does not have to re-read the whole system information from air interface. Reading pre-stored SIBs from UE memory reduces the time to access a 3G target cell to about half (less than 1.5 seconds typical). This makes the simple CSFB with blind redirect an attractive initial solution in terms of deployment complexity versus call setup time performance. The time to read system information in target RAT can be removed by providing the required system information in RRC release. If the RRC release is preceded by target RAT measurements then it is possible to provide the exact target cell system information to the UE. This completely eliminates the time to read system information, and in case of 3G the UE is able to access the target RAT in less than a second (around 0.6 seconds typical). On the other hand, the optional measurements and RIM signalling to fetch the system information from target RAT BSC/RNC prior to RRC Release consumes some of the call setup time gain. Blind redirect with system information is also possible, as 3GPP Release 9 specification allows sending system information of up to 32 2G cells and up to 16 3G cells. The system information could even be regularly updated via 