Voice-over-LTE (VoLTE) may require better LTE Reference Signal Received Power (RSRP) compared to data service, i.e. while the LTE radio signal may be good enough for pure data session, it may not be reliable enough for VoLTE services. In scenarios where the radio network is dimensioned for data services, eNB may trigger SRVCC handover to CS, e.g. when the UE falls into marginal or weak LTE coverage or when an EPS bearer with QCI-1 reliability is not sufficiently met. In certain network dimensioning, the VoLTE coverage border may be a function of the selected codec and its selected configuration, its rate and mode adaptation, and potentially the applied application layer redundancy, as well as the required QoS of the VoLTE bearer. In these cases, legacy RAN might unnecessarily hand over fairly good VoLTE calls to 2G/3G CS via SRVCC HO, because it is unaware of the robustness of the selected codec. Radio Resource Management functions could potentially avoid unnecessary SRVCC HOs, if appropriate information is made available. TS 26.114 (which is used as basis for the GSMA IR.92 VoLTE profile) includes several tools for increased robustness of speech calls with initial selection of Codecs and their Configuration and in-call dynamic rate and mode adaptation and maybe application layer full redundancy. EVS, especially the EVS Channel Aware mode, demonstrates higher robustness against transmission errors than AMR and AMR-WB codecs by application-layer partial redundancy. The present document investigates possible solutions to maintain voice quality on LTE as high as possible and by that avoiding or at least delaying SRVCC as much as possible and by that minimize the negative impact on user experience for VoLTE subscribers in areas with weak LTE coverage. The recommendations and conclusions of this document were used to develop the Coverage and Handoff Enhancements for Multimedia (CHEM) feature which extended beyond SRVCC and LTE to general handoffs. Aside from inter-domain handoffs, this added intra-domain handoffs that also includes intra-system and inter-system handoffs for any type of media. Normative and informative annexes for this are specified in TS 26.114.

The present document provides a study on the enhanced VoLTE performance (eVoLP). The study focuses on:

• Guidelines or requirements to ensure that MTSI clients send requests to adapt to robust modes of codec operation when necessary. This study may require investigating performance results for different conditions and adaptation procedures.
• Mechanisms to indicate at setup a terminal's ability to send adaptation triggers (e.g. to adapt to the most robust codec mode).
• Evaluate the impact of proprietary client implementations of Packet-Loss Concealment and Jitter Buffer Management (JBM) on having different Max PLR and potential mechanisms to indicate this to the network.