Efficient Communication and Data Aggregation in Battery-Free Internet-of-Things
Authors:   , Liu Gaosheng 
Published: January 2025

The Internet of Things (IoT) is a fundamental technology for industrial automation that has been extensively explored and deployed.
Yet, existing IoT systems pose significant sustainability issues due to the batteries required to power IoT devices: Batteries require significant maintenance efforts to recharge/replace them in large-scale deployments, and they contain hazardous chemicals that can pollute the environment if not recycled properly.
As an alternative, battery-free IoT has gained traction by eliminating the battery and powering the device with energy directly harvested from the environment.
While improving sustainability, battery-free IoT brings extra complexity caused by intermittency. Battery-free IoT devices work intermittently since the harvested energy is typically insufficient to sustain the device's operations continuously.
Many studies have focused on intermittent computing, aiming to ensure the forward progress of program execution on a single battery-free IoT device.
However, intermittency also imposes significant challenges to communication, which has been under-explored so far.

This dissertation presents our systematic study on how to perform efficient communication and data aggregation among battery-free IoT devices with intermittency. We organize the study into four parts, each offering a thorough investigation under a specific environmental condition. First, we present RICS, a routing scheme for battery-free IoT where all battery-free devices have static, homogeneous charging time. RICS employs linear probing~(deterministic working time synchronization) for device discovery and introduces a novel pendulum-based data forwarding mechanism to accelerate data transmission across multiple hops. In addition, RICS implements an optimal topology construction mechanism for data aggregation. Second, we introduce Swift, a routing scheme for battery-free IoT when battery-free devices have static but heterogeneous charging time. To speed up device discovery, Swift features a number-theory-based synchronization protocol exploiting the Linear Congruential Generator (LCG) properties. Swift further enhances the pendulum-based data forwarding and topology construction mechanisms to address the challenges brought by charging time heterogeneity. Third, we consider arbitrary charging conditions and present Pulsar, an all-to-all communication protocol for battery-free IoT. Pulsar is based on the insight that the little certainty provided by a battery-powered IoT device (as the coordinator) among battery-free ones can largely simplify the communication protocol design. Pulsar adopts linear probing for device-coordinator discovery and employs scheduled slots to implement device-device communication patterns. Finally, we present FreeBeacon, a general-purpose communication protocol for battery-free IoT. FreeBeacon further enhances Pulsar with a more robust design for device-coordinator discovery when facing device power failures and supports different data aggregation~(communication) patterns.

Overall, to address the challenges brought by intermittency, we propose solutions for enabling efficient communication and data aggregation in battery-free IoT under varying energy conditions and communication patterns. Based on these explorations, we also synthesize a list of takeaways that will be useful to researchers and practitioners in the general field of sustainable IoT.