ivity (dBm) -120.6 -124.4 compressed IP header is 35 bytes. Each RLC packet needs additionally RLC header of 3 bytes and Cyclic Redundancy Check (CRC) of 3 bytes. The overhead without segmentation is (3+3)/35 = 17%. The overhead increases to (3+3)/(35/4) = 69% which will eat large part of the benefit from segmentation. Therefore, TTI bundling should preferably be used to improve the uplink coverage rather than packet segmentation and simultaneous use of TTI bundling and packet segmentation should be avoided. 14.4.3 Semi-persistent Scheduling VoLTE packets can be transmitted with dynamic scheduling where every packet is scheduled separately. The dynamic scheduling has the benefit that it allows smart scheduling solutions in the frequency domain and it allows fast link adaptation based on the CQI feedback from UE. The challenge with dynamic scheduling is that PDCCH signalling capacity is required for the allocation of every voice packet arriving with 20-ms period. An alternative solution is semi-persistent scheduling (SPS) where eNodeB pre-assigns resources for each VoLTE connection SO that no PDCCH signalling is needed. SPS is practical for VoLTE traffic since eNodeB knows that VoLTE packets will arrive every 20 ms. The limitation with SPS is that the resources in the frequency domain, the modulation and the coding schemes are fixed and cannot be modified based on CQI reporting. These two scheduling principles are shown in Figure 14.21. Without TTI bundling With TTI bundling Figure 14.18 Uplink BLER in weak signal area with and without TTI bundling VoLTE Optimization RLC SDU Figure 14.19 Packet segmentation principle 20 ms No TTI bundling With TTI bundling 16 ms With TTI bundling and packet segmentation Figure 14.20 TTI bundling and packet segmentation operation Dynamic scheduling Semi-persistent scheduling (((c)())) (((c)())) eNodeB eNodeB CQI report SPS configuration Allocation on PDCCH Data on PDSCH Data on PDSCH CQI report Allocation on PDCCH Data on PDSCH Data on PDSCH Figure 14.21 Dynamic and semi-persistent