Although commercially launched in 2014, to date only about 30% of LTE networks around the world provide VoLTE voice services. VoLTE improves the mobile voice experience substantially. The improvement is most noticeable in lower call set up times. Voice Quality Mean Opinion Score (MOS) will increase and VoLTE allows for adaptation by all IP 5G networks. Migrating voice services from 2/3G to 4G LTE facilitates sunset strategies for legacy technologies, thereby freeing frequency bands for more spectrum-efficient 4G and 5G technologies.

Implementation of VoLTE not only requires the installation of an IP Multimedia Subsystem (IMS) node, but it also requires optimisation of the LTE Radio Access network (RAN). The biggest challenge for VoLTE implementation lies in maintaining the call success rates. Intermittent 4G coverage can cause higher drop rates in comparison to Circuit Switched Fallback (CSFB) legacy voice calls. The quality of the LTE RAN design will therefore have a big impact on the VoLTE user experience.

VoLTE Coverage

VoLTE coverage and VoLTE quality may be further improved by application of  RAN features like Robust Header Compression (RoHC) (Uplink gain up to 2.5 dB), Transmission Time Interval (TTI) Bundling (Uplink gain up to 3.5 dB ), VoLTE voice rate control (Uplink gain up to 1 dB) and/or VoLTE coordinated multipoint transmission and reception (CoMP) (Uplink gain up to 2 dB).

VoLTE Mobility

The Single Radio Voice Call Continuity (SRVCC) feature should be used to assure the continuation of a VoLTE call when a VoLTE user moves into or out of LTE coverage. SRVCC is necessary when LTE coverage is not contiguous, for example in cases of indoor or tunnel LTE coverage.

VoLTE Capacity

Increase in LTE data and/or VoLTE traffic results in a deteriorated signal-to-interference-plus-noise ratio (SINR) distribution and hence reduced network capacity and service quality. Applying optimisation and performance enhancing RAN features will improve SINR distribution and hence improved network capacity and service quality.

Simulated RAN quality -DL SINR distribution

VoLTE RAN Capacity Planning is highly correlated with the quality of the RF design. When for example applying a 12,65 kbps AMR-WB VoLTE codec, a 20 ms speech frame leads to 253 voice data bits/20 ms. With overhead this is 733 bits/20ms. When applying RoHC the overhead may be reduced to 300 voice data bits/20ms. Now suppose a 10 MHz bandwidth, which allows for 50 PRBs per 1 ms TTI or 168 REs with 120 REs for voice data. Each RE carries 1 OFDM symbol with 2, 4, or 6 coded bits according to respectively QPSK, 16 QAM or 64 QAM modulation.

Applied DL SINR distribution in RAN network

At substandard SINR cell edge conditions, the connection quality might exhibit a CQI of 1, which results in a transmission capacity of 17 voice bits per PRB. 300 bits/20ms are needed to be transmitted, which means that 300/17 ~ 17 PRBs are required to carry one VoLTE conversation. Since there are 20 x 50 PRBs/20 ms available, a total of 20 x 50/17= 58 simultaneous VoLTE calls can be made if full system capacity was reserved for VoLTE. For a grade of service of 0,01% this would translate into 35 Erlangs.

VoLTE capacity as a function of RAN network quality

Omnitele assists operators and regulators with VoLTE implementation and verification

Omnitele has assisted various operators in implementing VoLTE in their network. LTE RAN coverage, VoLTE Mobility and VoLTE RAN capacity were designed and optimised. In addition, Omnitele has carried out VoLTE Interoperability Testing and VoLTE performance verification. The Omnitele team has specialised in VoLTE implementation by delivering the following services:

  • Design and optimisation of LTE RAN coverage and quality, including indoor design.
  • Design and verification of VoLTE enhancing RAN features (e.g. Robust Header Compression, TTI Bundling, VoLTE voice rate control, VoLTE Coordinated Multi-Point (CoMP), etc.
  • Verification of VoLTE mobility through SRVCC to 3G UTRAN and SRVCC to 2G GERAN test cases.
  • Verification of VoLTE service quality KPI values (Call Set-up times, Call Success Rates, MOS scores, etc.) by means of OSS counters and/or drive-testing throughout the VoLTE service area.
  • Execution of VoLTE Terminal Certification tests needed to qualify the network for use of smartphone devices of selected vendors.

With VoLTE, you will benefit from the full advantage of voice integration with multimedia IP services. You are ready for high quality voice incorporation into 5G/IoT. But make sure that your RAN will not become the limiting factor for successful VoLTE operation.

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