tion PUSCH HARQ BLER (%) Figure 14.31 Relative (to PUSCH HARQ BLER = 0% MOS) POLQA MOS versus. PUSCH HARQ BLER. Used AMR codec bitrate = 23.85 kbps and target block error ratio of transport blocks for first transmission = 10% An example case of laboratory measurements shown in Figure 14.34 without TTI bundling shows that UL BLER starts increasing rapidly at RSRP of -114 dBm. Used fading channel here is enhanced pedestrian A (EPA) with 3 km/h UE velocity and with 3 dB reference signal boosting. Used AMR codec bitrate = 12.65 kbps and target block error ratio of transport blocks for first transmission = 10%; 2 X 15 W was used over 10 MHz band. The MOS value is maintained up to RSRP level of -114 dBm after which the increasing jitter and packet loss start to impact negatively on the MOS values as shown in Figure 14.35. PDSCH HARQ BLER (%) Figure 14.32 Relative (to PDSCH HARQ BLER = 0% MOS) POLQA MOS versus. PDSCH HARQ BLER. Used AMR codec bitrate = 23.85 kbps and target block error ratio of transport blocks for first transmission = 10% VoLTE Optimization RTP packet loss rate (%) Figure 14.33 Relative (to RTP packet loss rate = 0%) POLQA MOS versus RTP packet loss ratio. Used AMR codec bitrate = 23.85 kbps and target block error ratio of transport blocks for first transmission = Figure 14.34 Laboratory measurements of RSRP and PUSCH BLER without TTI bundling Figure 14.35 Laboratory measurements of RSRP and POLQA MOS without TTI bundling LTE Small Cell Optimization As shown in Figures 14.34 and 14.35 the VoLTE call quality degrades rapidly in poor radio conditions. In order to maintain the VoLTE call quality in poor coverage conditions TTI bundling or RLC segmentation can be used (as explained in Section 14.4.2) and hence coverage can be extended. However, in case the UE is at the LTE coverage edge the VoLTE call needs to be handed over to the other radio access technology (RAT). This is explained in the next section. 14.5 Single Radio Voice Call Continuity When the terminal runs out of LTE coverage area and maintaini