y Provide evolution from voice to rich Prepare evolution to LTE-only next-generation IMS services deployments Figure 14.10 Motivations for VoLTE deployments configuration and thresholds to trigger the redirect play a role in the return time. The criteria for triggering blind and measurement-based redirect are RNC vendor dependent and not specified by 3GPP. Voice over LTE There are a number of motivations for migrating from CS voice to VoLTE by the mobile operators. The main motivations are shown in Figure 14.10. VoLTE can enhance voice quality with wideband codec. VoLTE also provides faster call setup time and offers simultaneous LTE data together with the voice call. The data capability allows to introduce new IMS services. VoLTE can also make the device architecture simpler for CDMA operators when the dual radio implementation can be avoided. VoLTE also improves the spectral efficiency compared to CS voice. VoLTE can also enhance network coverage if LTE can use lower frequency with more favourable propagation. Migration from CSFB to IMS-based VoIP solution, that is, VoLTE, is typically done in several steps starting from CSFB only, then combining of CSFB and VoLTE and finally full VoLTE where all voice calls (inter-operator, inter-network and roaming) are fully utilizing VoLTE. SRVCC enables handover from IMS-based VoIP to CS speech when the UE is running out of LTE coverage. Also reverse SRVCC (SRVCC based on release 11) enables handover from CS speech back to IMS-based VoIP. Simplified VoLTE system architecture based on [3] is presented in Figure 14.11. 14.4.1 Setup Success Rate and Drop Rate The voice call performance in 2G and 3G networks has been extremely good and therefore the expectation for VoLTE performance in LTE networks is set very high. The share of VoLTE calls given as ratio of quality of service class identifier (QCI) one E-RAB setup attempts to all QCIE-RAB setup attempts, in Figure 14.12, shows that the amount of VoLTE calls is still very small for three example networks depending on the VoLTE