White Paper on MMS UWB Operation in IEEE 802.15.4ab
2025-09-29
Ultra-Wideband (UWB) technology has become a leading solution for fine-ranging applications such as indoor positioning, asset tracking, and secure access. However, conventional IEEE 802.15.4z-based UWB ranging still suffers from inherent limitations: strict regulatory power limiting the total link budget of ranging, unstable performance in non-line-of-sight conditions, and multipath interference from environments. As a result, UWB's effective ranging distance is often restricted to a few meters, with degraded stability in real-world deployments.
These challenges are particularly serious in automotive UWB digital key use cases, where reliable passive entry demands accurate detection of user's distance from the car under diverse scenarios. In practice, some typical issues of UWB digital key severely impact user experience, including:
- ranging stability degrading sharply when digital keys are carried in back pockets or bags
- complex environments such as underground parking introducing false first-path detection due to heavy multipath reflection
- UWB anchors inside vehicles facing severe attenuation from metal body structures and glass
To overcome these limitations, IEEE 802.15.4ab (in progress) introduces Multi-Millisecond (MMS) UWB operation, in which MMS UWB ranging fragments exchange across multiple milliseconds, enabling accumulation of the channel impulse response (CIR) over multiple milliseconds. This significantly improves the link budget of UWB ranging, extends the reliable ranging distance, and enhances multipath resilience. The standard defines three MMS UWB modes:
- Narrowband-Assisted (NBA) MMS UWB: NBA MMS UWB utilizes a narrowband O-QPSK PHY for MMS control and report phases, and the UWB channel for the MMS ranging phase, which can achieve full MMS gain but requires extra narrowband hardware and new spectrum. This mode also needs to consider the coexistence mechanism with other wireless technologies (e.g., WIFI).
- UWB-Driven MMS UWB: This mode runs the MMS control, ranging, and report phases purely over the UWB channel, simplifying design but constrained by the link budget of control and reporting.
- Out-of-Band (OOB) MMS UWB: This mode offloads MMS control and report phases to external wireless connections, most commonly Bluetooth Low Energy (BLE), while dedicating the UWB channel exclusively to exchanging MMS ranging fragments.
Among these, BLE-assisted MMS emerges with great potential as a practical approach for digital key enhancement. It matches NBA MMS in full MMS link budget gain without new hardware cost or spectrum requirements, avoids coexistence issues, and leverages the globally harmonized ecosystem of BLE already embedded in smartphones and vehicles.
This white paper presents the principles, modes, and performance analysis of MMS UWB operation in IEEE 802.15.4ab (in progress), with emphasis on its application in automotive digital keys. By extending the effective ranging distance and reliability of UWB through MMS UWB operation, the next generation of automotive UWB digital keys can deliver a secure, consistent, and user-friendly experience, while accelerating global deployment.
Keywords:
UWB, MMS, IEEE 802.15.4ab, BLE-assisted MMS, NBA MMS, UWB-driven MMS, Digital Key, Channel Sounding, CIR, RSF, RIF
