An Overview of MCX Measurement Needs

Clément Lauriat   Monday, 25th August 2025

Earlier this year, I had the opportunity to attend Critical Communications World (CCW) in Brussels as an exhibitor with my company.

I particularly appreciate smaller conferences like this because they bring together diverse stakeholders around a single, focused topic.

I was able to interact with mobile device manufacturers, application developers, operators, and project managers, as well as attend several presentations and discussions on the current challenges, standardization progress, and open questions in the field.

As a company specializing in testing and measuring voice, mobile networks, and service performance, Opale Systems is naturally positioned to support the validation of ETSI, 3GPP, and ITU standards in the domain of Mission Critical Services (MCX).

Moreover, by joining the Global Certification Forum (GCF), Opale Systems is proud to actively participate in the definition of future certification criteria.
This article does not aim to describe the detailed work of these groups, but rather to provide an overview of the key measurement needs already established by 3GPP and ETSI for latency and audio quality KPIs in MCX services.

Latency: A Fundamental KPI

Legacy critical communications were based on dedicated radio systems. In contrast, MCX enables migration to high-speed 4G/5G networks, supporting information exchanges between agents and control centers, agent-to-agent communications, group calls, and inter-group communications.

An MCX terminal can belong to multiple call groups, while 5G opens the door to advanced use cases such as:

• Agent and team management
• 4K video transmission
• Drone integration
• Augmented reality devices
• Post-incident review and analysis

All of these applications share one fundamental requirement: latency control—the time it takes for information to travel from point A to point B.

Why is latency so critical?

For field agents — whether medical teams, firefighters, public safety officers, or military units — reaction time is often the difference between control and crisis, between safety and danger.
Traditionally, response relied on the observation, instinct, and experience of field agents.

Now, new broadband services such as video, sensors, and IoT data multiply the available sources of information. But while these add many “eyes and ears,” the decision-making capacity (the brain) does not multiply accordingly.

This means that responsiveness is only possible if the information is transmitted within controlled, predictable timeframes.

 3GPP Latency KPIs for MCX Voice (TS 22.179)

3GPP has defined four key latency KPIs for MCX voice communications:
• KPI1: PTT Access Time – Latency between pressing the Push-to-Talk (PTT) button and receiving the grant signal. (Measured on sender side).
• KPI2: End-to-End PTT Access Time – Time for the application to place a call or join a group plus KPI1. (Measured on sender side).
• KPI3: Mouth-to-Ear Latency – Time difference between the start of audio transmission and reception. (Measured on receiver side).
• KPI4: Late Call Entry Time – Time taken for a user joining a group to start receiving audio. (Measured on receiver side).

 Quality vs. Intelligibility

 These are two related but distinct aspects:

• Quality: Listening comfort (clarity, absence of artifacts, background noise, clipping, sufficient volume, etc.)
• Intelligibility: The degree to which speech is understandable to the listener

Quality Assessment: POLQA (ITU-T P.863)
3GPP and ETSI rely on POLQA (Perceptual Objective Listening Quality Analysis), defined in ITU-T P.863, as the standard for assessing voice quality.
POLQA outputs a Mean Opinion Score (MOS) on a scale from 1 to 5, predicting the experience of human listeners. It does this by comparing a reference sample with a degraded test sample after passing through the system under test (SUT).
POLQA has been validated against large-scale databases of human assessments (P.800 methodology) and remains the most effective reference-based predictive model available

Intelligibility Assessment: Beyond Legacy Models
While predictive models exist, many are outdated and not adapted to the broadband capabilities of MCX communications.
A more relevant approach is the Modified Rhyme Test (MRT), commonly used in military transmission assessments and hearing aid validation.

• Listeners are asked to identify a target word from a group of six similar-sounding words (e.g., mat, pat, cat, bat, cac, bat).
• Tests are conducted under varying conditions (optimal, noisy, compressed, lossy, etc.).
• Results across a representative panel are used to calculate the system’s intelligibility index.

The NIST ABC-MRT method automates this by combining MRT logic with Articulated Band Correlation analysis. Results show:

• 95.4% correlation with human MRT tests across multiple conditions (Pearson correlation)
• Low prediction error (RMSE = 0.066), demonstrating that objective prediction is feasible and reliable for intelligibility testing.

Measurement Methods
Whether measuring 3GPP KPIs or ABC-MRT intelligibility, the testing philosophy remains the same.

A robust test setup should provide:

1. Agent/operator emulation
2. Endpoint-based testing
3. Accurate time synchronization for latency measurement
4. Fullband audio support for QoE analysis
5. Noise simulation for realistic conditions
6. Push-to-Talk triggering capability

For example, Opale Systems’ DSLA platform, driven by the MultiDSLA application server, connects to mobiles or control center consoles to:

• Trigger PTT
• Generate and record audio
• Measure POLQA MOS
• Measure latencies (KPI1–KPI4)
• Conduct intelligibility tests (ABC-MRT) under noisy conditions

All metrics are collected both locally and remotely, with options for monitoring, reporting, and alerting.

Conclusion

While KPI requirements for video and data services are still under definition, the voice scope is well established—though refinements are still possible.
By using solutions like Opale Systems’ MultiDSLA, it is already possible to measure and validate:
• Latency KPIs
• Voice quality (MOS via POLQA)
• Speech intelligibility (ABC-MRT)