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A Duty-Cycle-Efficient Synchronization Protocol for Slotted-Aloha in LoRaWAN

Amavi Dossa, El Mehdi Amhoud

TL;DR

The paper tackles the scalability bottleneck of LoRaWAN in Massive IoT by enabling Slotted-Aloha for class A devices, which requires precise synchronization without excessive downlink duty-cycle. It proposes a dynamic synchronization protocol built on a two-guard-interval slot design and a single network-server time reference, with remaining-time signaling encoded in downlink frames (2 bytes) and integrated via a dedicated FPort. The core contributions include a novel slot structure, algorithms for the Network Server and end-device, and a LoRaWAN integration strategy demonstrated through experiments showing improved duty-cycle efficiency and reliable per-device resynchronization. This work enables practical Slotted-Aloha deployment in LoRaWAN networks, offering scalable synchronization with minimal hardware changes and signaling overhead.

Abstract

In the current context of massive IoT, the Pure-Aloha scheme used in LoRaWAN is reaching its limit, and Slotted-Aloha is being considered as an alternative, as it offers twice Pure-Aloha's packet success rate. It however requires synchronization across the nodes. In this paper, we propose a new slot structure adapted to devices with low quality clock, and a duty-cycle efficient synchronization protocol for LoRaWAN class A devices with the lowest overhead to date. We discuss the conditions of its integration into LoRaWAN. The experimental results confirm that it succeeds in tracking each device's synchronization state, identifying the exact moment they desynchronize and resynchronizing them. The proposed protocol is also proven to be more duty-cycle efficient than existing fixed-rate synchronization solutions.

A Duty-Cycle-Efficient Synchronization Protocol for Slotted-Aloha in LoRaWAN

TL;DR

The paper tackles the scalability bottleneck of LoRaWAN in Massive IoT by enabling Slotted-Aloha for class A devices, which requires precise synchronization without excessive downlink duty-cycle. It proposes a dynamic synchronization protocol built on a two-guard-interval slot design and a single network-server time reference, with remaining-time signaling encoded in downlink frames (2 bytes) and integrated via a dedicated FPort. The core contributions include a novel slot structure, algorithms for the Network Server and end-device, and a LoRaWAN integration strategy demonstrated through experiments showing improved duty-cycle efficiency and reliable per-device resynchronization. This work enables practical Slotted-Aloha deployment in LoRaWAN networks, offering scalable synchronization with minimal hardware changes and signaling overhead.

Abstract

In the current context of massive IoT, the Pure-Aloha scheme used in LoRaWAN is reaching its limit, and Slotted-Aloha is being considered as an alternative, as it offers twice Pure-Aloha's packet success rate. It however requires synchronization across the nodes. In this paper, we propose a new slot structure adapted to devices with low quality clock, and a duty-cycle efficient synchronization protocol for LoRaWAN class A devices with the lowest overhead to date. We discuss the conditions of its integration into LoRaWAN. The experimental results confirm that it succeeds in tracking each device's synchronization state, identifying the exact moment they desynchronize and resynchronizing them. The proposed protocol is also proven to be more duty-cycle efficient than existing fixed-rate synchronization solutions.
Paper Structure (10 sections, 9 equations, 6 figures, 4 tables, 2 algorithms)

This paper contains 10 sections, 9 equations, 6 figures, 4 tables, 2 algorithms.

Table of Contents

  1. Introduction
  2. LoRaWAN Overview and Problem Statement
  3. Dynamic Synchronization Protocol Design
  4. Slot Structure Design
  5. Synchronization and Monitoring Protocol
  6. Algorithm 1: Network Server
  7. Algorithm 2: end-device
  8. Protocol Integration into LoRaWAN
  9. Experimental Results
  10. Conclusion

Figures (6)

  • Figure 1: LoRaWAN Star of Stars topology
  • Figure 2: Slotted-Aloha: Virtual double confidence intervals
  • Figure 3: Apparent drift effect on slot position
  • Figure 4: Test bench equipment: A National Instruments USRP-2920 SDR as Gateway; An Adafruit Feather M0 and a TTGO Esp32 LoRa are used as end-devices; The network server was running on a 6-cores 12-threads 4.0 GHz laptop.
  • Figure 5: Protocol performance: devices synchronization and monitoring
  • ...and 1 more figures