 Ringing Ringing 200 OK 200 OK Answer Voice or video session Figure 14.14 Simplified VoLTE call setup signalling. Based on GSMA IR.92 LTE Small Cell Optimization Example 1 QCI1 Example 2 QCI1 Example 1 all QCIs Example 2 all QCIs Figure 14.15 E-RAB drop rate for QCI1 and all QCIs Figure 14.13 shows the E-RAB setup success rate for QCI1 and for all QCIs. The QCI1 E-RAB setup success rate is slightly worse than E-RAB setup success rate for all QCIs and this difference is mostly caused by the fact that there are SO much less QCI1 setups compared to QCIs that the weight of setups attempted under poor radio conditions is increased and therefore the success rate is worse. However, it should be noted that the RRC connection reconfiguration process (for QCI1 E-RAB setup) optimization was not completed in this example network yet. Drop Rate The QCI1 E-RAB drop rate is slightly higher compared to all QCIs as seen from Figure 14.15. As explained in Chapter 12 the mobility performance in terms of late handovers and handover success rate in general has a high correlation with the E-RAB drop rate. This correlation is even stronger for QCI1 E-RAB due to the much longer session time (excluding any inactivity time) as shown in Figure 14.16 and therefore much more handovers per E-RAB. The more the handovers per E-RAB the higher the probability for mobility-related problems and mute calls (due to RRC re-establishment) and dropped calls. Therefore, the optimization actions related to the mobility performance (handover success rate and too late handovers) as discussed in Chapter 12 are essential for VoLTE end-user experienced performance optimization. 14.4.2 TTI Bundling and RLC Segmentation VoLTE uplink coverage can be improved by using the feature called TTI bundling. The idea is to repeat the same transmission in four consecutive TTIs which enables to transmit four times more energy. The operation is illustrated in Figure 14.17. The voice packets from AMR codec arrive every 20 ms and can be transmitted in single 1-ms TTI without T