ion transfer INVITE, the SRVCC cannot be cancelled and call continued in LTE when UE returns back to EPC. This is due to MME sending PS-to-CS cancel notification to SRVCC MSS and SRVCC MSS will release towards ATCF/SCC AS with SIP BYE the session transfer leg (access transfer leg) with incorrect cause code and call is released. This cancellation procedure is improved in 3GPP release 11 as specified in section 12.3.3.4 in [10] and once implemented the SRVCC can be cancelled and call can return back to LTE. This 3GPP procedure enhancement LTE Small Cell Optimization Dropped call VoLTE dropped call probability due to SRVCC probability LTERF Figure 14.42 Probability of dropping a call due to LTE RF Quality and due to SRVCC as a function of RF Quality in LTE can improve the SRVCC success rates and ease the optimization of SRVCC measurements and triggering thresholds in LTE. Another SRVCC procedure improvement introduced in 3GPP release 10 is the alert phase SRVCC which means that it is possible to trigger the SRVCC to target system during the VoLTE call setup phase. The alert phase SRVCC can be triggered during the VoLTE setup signalling phase, see Figure 14.14. This feature once implemented in terminal and in IMS improves the voice call setup success rate from end-user point of view and can also lease the SRVCC triggering thresholds optimization. Figure 14.43 shows the probability of SRVCC in an example network. The SRVCC prob- ability was reduced from 7% to below 3% by network optimization. More than 1 million SRVCC attempts are included in this graph. Percentage of VoLTE calls using SRVCC 3-month period Figure 14.43 Percentage of VoLTE calls using SRVCC VoLTE Optimization 14.6 Summary In LTE the voice calls can be done either by CS fall back, that is, moving the voice call, before the actual setup, to target system, for example, WCDMA, and complete the signalling in the target system or by VoLTE, that is, running the voice over the LTE radio. CSFB requires no new core network element, that is, IMS is not needed, bu