VoLTE MOS Score: Meaning, Thresholds, and Voice Quality Diagnostics

Key takeaway: VoLTE MOS is measured on a 1-5 scale. A score above 3.5 is acceptable, above 4.0 is good, and above 4.3 is excellent. The theoretical maximum in AMR-WB is about 4.5. Main degradation causes: RSRP < -110 dBm, SINR < 5 dB, jitter > 30 ms, or packet loss > 1%.

The MOS (Mean Opinion Score) is the universal metric for perceived voice quality on mobile networks. On VoLTE (Voice over LTE) networks, it objectively quantifies user experience during a call. But how should you interpret this score? What thresholds are acceptable? And how do you diagnose degradation?

What is MOS?

The Mean Opinion Score is a 1-to-5 scale that rates perceived voice quality:

MOS Score	Quality	Description
4.3 - 5.0	Excellent	HD quality, no perceptible degradation
3.5 - 4.3	Good	Slight imperfections, fully usable
2.5 - 3.5	Fair	Audible degradation, listening effort required
1.5 - 2.5	Poor	Difficult communication, frequent dropouts
1.0 - 1.5	Unusable	Communication impossible

With VoLTE using the AMR-WB (Wideband) codec, the theoretical maximum MOS is approximately 4.5. In practice, a well-optimized network maintains an average MOS between 3.8 and 4.3.

MOS Measurement Algorithms

VisQOL (Google)

VisQOL (Virtual Speech Quality Objective Listener) is an open-source algorithm developed by Google. It uses machine learning to predict MOS from the audio signal. This is the algorithm used by HiCellTek for real-time field MOS measurement.

Advantages: free, open-source, real-time capable, strong correlation with subjective evaluations.

POLQA (ITU-T P.863)

POLQA (Perceptual Objective Listening Quality Analysis) is the ITU-T reference standard for voice quality measurement. It succeeds PESQ (P.862).

Advantages: internationally recognized standard, maximum accuracy. Disadvantages: expensive commercial license (~15,000 EUR), requires reference signal.

Operator MOS Thresholds

Operators typically define contractual KPIs for VoLTE MOS:

Operator type	Minimum MOS	Target MOS
Tier-1 (Europe/US)	3.5	> 4.0
Tier-2 (Africa/ME)	3.0	> 3.5 — see our African operator case study for real-world examples
Regulator (e.g., ARCEP)	3.0	Not defined
Indoor acceptance (SSV)	3.5	> 3.8

Common Causes of MOS Degradation

Radio Network

• Low RSRP (< -110 dBm): poor coverage, codec downgrade
• Low SINR (< 5 dB): interference, retransmissions, packet loss
• Poor handover configuration: audio gaps during cell transitions
• RAN congestion: insufficient scheduling for VoLTE bearers

IP Transport

• High jitter (> 30 ms): RTP inter-packet delay variation
• Packet loss (> 1%): gaps in audio stream
• High latency (> 200 ms one-way): difficult conversation, echo

IMS Configuration

• Suboptimal codec: AMR-NB instead of AMR-WB loses HD quality
• SPS misconfiguration: Semi-Persistent Scheduling introducing jitter
• Missing TTI bundling at cell edge causing packet loss

Field Diagnostics with HiCellTek

The HiCellTek suite automatically correlates MOS with radio KPIs in real time:

1. L3 Capture: decoding RRC and NAS messages for handovers, reconfigurations, and bearer issues
2. RF KPIs: RSRP, RSRQ, SINR measured simultaneously
3. MOS Score: continuously calculated via VisQOL during the call
4. Automatic correlation: root cause identification (radio vs transport vs IMS)

Best Practices for Field MOS Measurement

• Measure during busy hours (10am-12pm, 2-5pm)
• Use calls of 60 seconds minimum for representative MOS
• Repeat each measurement 3 times minimum per test point
• Compare with a reference terminal on the same network
• Export traces in HLOG or QMDL format for post-processing

MOS remains the most meaningful indicator for business teams and regulators. Mastering its interpretation and diagnostics is essential for any network engineer working on VoLTE voice quality.