In early 2015, the first operators enabled Voice over LTE (VoLTE) for their customers in Europe. This requires LTE coverage, a VoLTE-capable phone and – at least in the first months – a dedicated subscription.
Today, LTE coverage is widely available and state-of-the-art devices support VoLTE natively. Now is the time to answer some questions: Does VoLTE improve telephony for normal end customers? Do they benefit from a better quality of experience (QoE)?
The higher capacity of LTE networks to carry voice calls is an initial improvement, for sure. Although a customer may not experience it directly, the capacity of cells in legacy technologies, especially in downtown areas during business hours, is close to reaching its limit. With LTE carrying voice calls, this situation relaxes.
How then is the improved quality directly experienced? This is a valid question, especially in view of the latest enhancements made in 3G and even 2G. In these technologies, HD-voice and transcoder free operation are also regularly used today. Both enable higher speech quality in voice calls. What should be better in VoLTE? A look at today’s networks, based on Germany-wide drive and walk tests, offers some insight.
During the test campaign with the Rohde & Schwarz mobile network testing benchmarking solutions Benchmarker II (in combination with the Vehicle Roof Box hosting up the probes) and Freerider III, almost 16,000 voice calls were performed between two VoLTE-capable devices. All calls were made as long-distance calls from one region to another, with both phones always registered on the same network.
90% of calls in HD-voice quality
First, we have to state that across all operators 67% of these calls were established as pure VoLTE (LTE to LTE) calls; for the remaining third, one or both sides were handed down from LTE to 3G by CSFB (Circuit Switch Fallback) or were pure 3G/2G calls. With 3G/2G also supporting HD-voice, it must be noted that in fact far more than 90% of all calls were performed in HD-voice. This marks a significant improvement in HD-voice availability compared to similar results from previous measurements.
Yet, it makes sense to look a little bit closer, as there are two clear advantages of using VoLTE: one is the time it takes to set up a call and get connected. When measured directly on the phone, the connection time (from pressing the ‘Call’ button to being connected) decreases from almost 10 seconds for a CSFB or 3G/2G call to about 6 seconds for an LTE to LTE call.
VoLTE measurements demonstrate perceptible advantages
The second advantage is speech quality. Even though it is always called HD-voice, there are still differences. In both 3G/2G calls and LTE to 3G/2G calls, the bit rate of the AMR wideband encoder is limited to 12.65 kbit/s, already enabling a very good audio quality. In pure VoLTE (LTE to LTE) calls, however, there is no restriction, and the AMR wideband encoder can use its maximum rate of 23.85 kbit/s. The advantage is clearly perceptible and also measurable; the MOS value under perfect conditions improves by about 0.2 from about 4.0 to 4.2 on a five-point scale.
However, the next step in speech quality evolvement has already been taken: the use of the new EVS (Enhanced Voice Services) codec. It will provide even more audio bandwidth in mobile bit rates and promises exceptional gain in audio quality. First networks are now ready for EVS, and initial mobile devices have been delivered. In a few months, let’s have a look how speech quality further evolves with the use of this new technology.