Mari: scaling BLE to 1,000 robots

What it is

Mari (Micro-Robot Access Radio Infrastructure) is a custom wireless link layer that scales Bluetooth Low Energy (BLE) to 100 nodes per gateway. It runs baremetal C on Nordic nRF52/nRF53 chips and replaces the whole BLE stack above the radio: connection setup, scheduling, association, handover. The result is a purpose-built radio fabric for dense, real-time, mobile IoT.

What I built

I designed and implemented Mari end-to-end as Lead Research Engineer at Inria Paris:

Slot frame and TSCH scheduler — TDMA + frequency hopping across 40 BLE channels, 1.7 ms slots
Multi-gateway architecture — capacity-aware gateway selection, parallel RSSI monitoring, sub 50 ms handover with hysteresis
Join / association protocol — IDLE → SCANNING → SYNCED → JOINING → JOINED, with backoff
Firmware — baremetal C on Nordic nRF52833 / nRF52840 / nRF5340
Marilib — Python host library for messaging and telemetry, enabling integration with tools for OTAP on the 1,000 DotBot testbed
First author on the journal paper and the EWSN demo

Why it’s hard

At hundreds of robots, off-the-shelf BLE simply doesn’t fit:

BLE GATT is limited to roughly 7–32 connections per host
WiFi collapses under collisions in dense, mobile scenarios
6TiSCH / SmartMesh don’t meet mobility with sub-second latency requirements

Mari rebuilds everything above the BLE radio: a deterministic TSCH scheduler, a join state machine, and a multi-gateway design - all using only 20 kB of RAM.

Numbers

100 nodes per gateway, several gateways per testbed
<0.3 s end-to-end latency, >90% packet delivery ratio, <50 ms handovers
Deployed at 700+ robots in the DotBots Testbed
🏆 Best Demo Award, EWSN 2025 — a 120-device live demo
426 commits, 6 releases (latest v0.8.0, Jan 2026) - open source

Publications

Mari: Responsive Wireless Communication for Low-Power Large-Scale Robot Swarms. G. Fedrecheski, Y. Gao, F. Maksimovic, A. Abadie, M. Vučinić, T. Chang, T. Watteyne. Ad Hoc Networks Journal, to appear. [Preprint]
Demo: Mari Allows Connecting Large Scale Robot Swarms over BLE using TSCH with Fast Handovers and Multiple Independent Gateways. G. Fedrecheski, A. Abadie, M. Vučinić, F. Maksimovic, S. Alvarado-Marin, T. Watteyne. EWSN 2025, Leuven, Belgium. 🏆 Best Demo Award. [PDF]