LTE to target RAT with redirect without target RAT measure- ments (ESR = Extended Service Request) implementation, there may also be additional delay if up-to-date system information needs to be retrieved over RIM signalling from BSC or RNC in the target system. The transition from LTE to 2G can also be either blind or preceded by measurements. The 2G measurements require both RSSI measurement and BSIC decoding and therefore are slow compared to 3G measurements; with several BCCH frequencies in the measurement list the typical time from measurement command to first measurement report is between 1 and 2 seconds, which can be considered too long for the CSFB call setup. Transition time to target RAT with PS handover CSFB with PS CSFB with PS CSFB with PS CSFB with PS CSFB with blind PS handover to 3G, handover to 3G, handover to 3G, 1x1 handover to 3G, 1x1 handover to 3G 2x16 neighbours 1x16 neighbours neighbours neighbours (ttt=0 ms) (ttt=256 ms) (ttt=256 ms) (ttt=256 ms) ESR to measurement configuration measurement configuration to measurement report measurement report to handover command handover command to handover complete in target RAT handover complete to initial NAS message Figure 14.4 Transition time from LTE to 3G with PS handover. The notation 2x16 means that neighbour list has a total of 32 neighbours on two 3G frequencies, 16 neighbours per frequency LTE Small Cell Optimization The transition time with PS handover depends on the time to obtain first valid measurement of the target system. For 3G target and time to trigger of 0 ms the best achievable measurement time is 50-100 ms, although this assumes that the 3G neighbour list has only few neighbours SO that UE is able to perform the neighbour search step during one measurement gap. Time to trigger will directly add to this time. In Figure 14.4 measured transition time for different neighbour list lengths and times to trigger are shown. The time to first measurement report is strongly dependent on the neighbour list length. In the extreme example of o