urn time from 3G to LTE after CSFB call finishes, with cell reselection vendor-proprietary triggers, for example, serving cell received level and data buffer fill factor. In this case the return time depends on the time to measure LTE target cell. Comparison of Return Time from 3G to LTE after a CSFB Call Figure 14.9 shows an example of the measured empirical distribution of the return time from 3G to LTE, via cell reselection as well as blind and measurement-based redirect, obtained from drive test measurements. The return time is defined as time difference of disconnect message received from MSS in 3G to Tracking Area Update Complete message sent by UE in LTE. For the redirect methods the time to return depends heavily on the type of triggers and measurement thresholds implemented in RNC. Therefore, the results should be considered examples only. For basic idle mode reselection in 90% of cases the UE returns to LTE in less than 18 sec- onds. Since the reselection is possible only in idle and CELL_PCH mode, UE data activity as well as DCH-to-FACH and FACH-to-PCH inactivity timers will have impact on the results. In the measured network slow high-priority search measurement schedule was used (Spriori- tysearch1 = 0 and Sprioritysearch2 = 0). For blind redirect back to LTE, the transition is the fastest method to return back to LTE, assuming that there is good enough LTE coverage. The average time is about 2 seconds, but it can be seen that in a few percent of cases the redirect is not triggered quickly, or the UE has problems accessing LTE or completing the TAU procedure. In the measurement-based redirect the UE returns back to LTE in less than 7 seconds in half of the cases. In about 10% of the cases the return takes more than 10 seconds. Measurement VoLTE Optimization Migrate from dual radio CDMA + Provide faster call setup time LTE devices to LTE-only devices Enhance voice quality with wideband Take benefit of low band LTE for codec extended coverage Enable simultaneous voice and LTE Improve spectral efficienc