TI bundling. TTI bundling gives four times more transmission time which improves the link budget. The average power over 20 ms increases by 6 dB with TTI bundling when the peak power during the TTI transmission hits its maximum value. VoLTE Optimization Average session time per E-RAB (QCI1) (s) Average session time per E-RAB (non-GBR) (s) Figure 14.16 Average session duration per E-RAB for QCI1 and for non-GBR QCIs based on 15- minute samples over 24-hour period The simulated benefit of TTI bundling in the uplink base station sensitivity is shown in Table 14.3. The gain of TTI bundling is approximately 4 dB. The simulation takes into account the maximum delay budget for VoLTE which is assumed to be 50 ms for one radio leg. Maximum six retransmissions are allowed without TTI bundling and three retransmissions with TTI bundling. The measured benefit of TTI bundling is shown in Figure 14.18. The measurement is done in weak signal area where RSRP is below -120 dBm and UE transmits with full power. The uplink BLER without TTI bundling is very high above 70% which leads to bad voice quality. The uplink BLER drops to below 10% with TTI bundling providing a major improvement in the voice quality. Another solution to improve the uplink coverage is segmentation of voice packets into smaller units. Figure 14.19 illustrates the segmentation of Radio Link Control (RLC) Service Data Unit (SDU) into four smaller Protocol Data Units (PDU). Each smaller PDU is sent separately over the air interface. The packet segmentation enables to increase the uplink transmission time to boost the coverage. The concept is shown in Figure 14.20. The drawback of segmentation is the increase in the overhead. The voice packet of AMR 12.65 kbps including 20 ms No TTIbundling With TTIbundling Figure 14.17 TTI bundling operation LTE Small Cell Optimization Table 14.3 Simulated base station sensitivity for VoLTE with and without TTI bundling Without TTI bundling With TTI bundling Number of bundled TTIs Max number of retransmissions Base station sensit