Batteryless sensor nodes, powered solely by energy harvesting, are a promising alternative to battery-powered sensor nodes. However, energy harvesting rates being very low, unreliable, and time-varying, nodes cannot sustain a continuous operation, making them intermittently powered. As a result, these nodes incur unpredictable wakeup times due to continuously varying off-times. To perform tasks like distributed sensing, time synchronization, and communication for intermittently powered nodes, timely execution and robust coordination is vital. To achieve robust coordination, nodes must guarantee to be ON at a coordinated target time regardless of harvesting variations. To ensure robust coordination of on-times, we propose PAIL, a novel hardware-software approach where the hardware component enforces a constant off-time, with small variations. The software component dynamically corrects for those variations. We fabricated a PAIL sensor node and validated its ability to discover neighboring nodes. Using extensive simulations calibrated from experimental measurements, we observe that PAIL maintains nearly 99.3% coordination at steady state. In the context of pairwise communication, we show that leveraging PAIL coordination, nodes improve packet tail latency by 99%.

Wireless sensor nodes powered solely by energy-harvesting show promise in enabling truly pervasive, long-duration sensing by avoiding the fragility, cost, and maintenance limitations of batteries. Unfortunately, since the amount of energy harvested is often significantly less than the active consumption of the device, these devices operate intermittently with limited control of when they are on and how long they are off. Such uncontrollable intermittency first poses a challenge for traditional time synchronization error metrics, since nodes cannot reliably communicate time values at known intervals, resulting in the illusion that nodes are out-of-sync. Second, long duration off-times can exceed the inherent timing limit of persistent clocks that intermittent nodes rely on to measure off-times. Bursty groups of long off-times can cause traditional time synchronization mechanisms to re-converge slowly, incurring significant periods of high error in which nodes are effectively out-of-sync. In this paper, we define the meaning of a shared sense of time for intermittently-powered nodes, and propose two intermittency-aware synchronization error metrics. We then propose an intermittency-resilient time synchronization mechanism, called Levee, that exhibits more rapid re-convergence after losing time and a 2.12× reduction in maximum time synchronization error for a 48-hour period.

Two classes of intermittency have emerged in the batteryless intermittent research community: hard intermittency and soft inter-mittency. While conceptually similar, these two intermittencies represent very different approaches to the intermittency problem. In this position paper, we examine these two intermittencies in detail. We discuss the tradeoffs and evaluate the performance potential of the two intermittencies in the context of communication between intermittent nodes. Finally, we conclude that both types of intermittency have merits under different conditions and application requirements, and we argue for greater understanding of how these two disparate classes of intermittencies may interact and coexist within a single network.

Batteryless energy-harvesting sensor nodes can operate indefinitely, but if the harvesting rate is too low, they must operate intermittently. Intermittent operation imposes various challenges upon the system. One of the least-studied is communication–if nodes are unpowered for long, unpredictable periods of time, how can they reliably communicate with each other? In prior work, we proposed the concept of lifecycle management protocols (LMPs) to mitigate this issue and enable wireless communication directly between intermittent sensor nodes using active radios. In this paper, we propose a design framework for a class of LMPs. We then provide analytical models for the delay and throughput of two-node communication using this framework. Finally, we implement this framework on hardware and validate our models in an experimental setting. To the best of our knowledge, this is the first design framework for, and implementation of, protocols for enabling and improving general-purpose communication between intermittent sensor nodes using active radios.

Nodes in batteryless sensor networks operate intermittently, making tasks such as node-to-node communication and coordinated computation extremely challenging. Adding to this challenge, a node typically has little control over its intermittency. Therefore, in this paper, we introduce a new class of protocols, which we call lifecycle management protocols (LMPs), to better control and manage the intermittency of batteryless nodes. These protocols may be designed and optimized for a particular task; here, we propose and evaluate a set of LMPs designed to enable direct communication between intermittent batteryless sensor nodes with active radios.

Batteryless sensing devices powered solely by ambient energy sources are expected to operate in an intermittent manner, since they do not have a predictable, or even continuous, energy supply. When such an intermittent system is powered off, it cannot keep track of time using conventional means. However, a continuous sense of time is critical for any system running real-time or time-sensitive applications. In this paper, we present HARC (Heterogeneous Array of Redundant Persistent Clocks), a novel solution to the problem of timekeeping for batteryless, intermittently-powered systems. HARC uses a heterogeneous, redundant array of capacitor-based persistent clocks that each decay in parallel, but at different rates, to provide variation-resilient high accuracy over a wide range of power off-times. We demonstrate the feasibility and effectiveness of HARC using experimental evaluations on a HARC prototype, and trace-based simulations of HARC-supported communication directly between two devices intermittently-powered by RF harvesting.

The restriction of access to software systems is more important than ever. For example, critical data is increasingly being stored on web services that are accessible from anywhere in the world. Yet most primary authentication methods are still largely based on passwords, which are vulnerable to various attacks such as phishing scams and keyloggers. Advanced methods of behavior-based authentication exist, but most are designed for a specific area or system and are not generally applicable. In this paper, we propose a generic continuous authentication scheme for software systems, which supplements existing authentication schemes and works as an auxiliary layer to provide additional protection against impostors. The kernel of our scheme is a novel monitoring engine that detects impostors in real-time based on behavior and context information. We evaluate our scheme on a dataset consisting of real users' historical records provided by our industrial partner, and the results demonstrate that our approach achieves a high classification accuracy with only a short delay in detection, allowing for real-time, continuous authentication.

This paper presents RIVER-MAC, a very efficient receiver-initiated asynchronously-duty-cycled medium access control (MAC) protocol for IoT devices. The key innovations of RIVER-MAC include (1) a CCA-based rendezvous to reduce idle listening for the sender node by an order of magnitude, and (2) a beacon train-based collision resolution scheme to reduce contention between receiver nodes, a previously-overlooked issue in receiver-initiated MAC protocol design. We have implemented RIVER-MAC in Contiki OS, and used extensive Cooja simulations to demonstrate its high performance compared to RI-MAC (a classic receiver-initiated protocol), as well as ContikiMAC (a state-of-the-art sender-initiated asynchronously duty-cycled MAC protocol) in our tested scenarios. We also have used analytic studies to show that RIVER-MAC yields a comparable performance with a wakeup radio-based scheme, an emerging alternative to duty-cycled MAC protocols for IoT devices.

The technology of low-power wireless sensor networks (WSNs) needs to become more flexible to cater to emerging data-driven applications and the Internet of Things. For example, WSNs need to look beyond the traditional many-to-one data collection traffic model and begin to support multicast communications. However, efficient multicasting in WSNs is challenging. In this paper, we propose to apply the concept of opportunistic forwarding to create an opportunistic multicast framework for duty-cycled WSNs. Our framework allows for any node to directly and efficiently multicast to any subset of known potential destinations. We propose several variations of schemes to operate within this framework. We evaluate our framework and the proposed schemes using simulations.

Thunder localization provides an important solution to lightning location systems. This paper designs a smartphone- based thunder localization system, ThunderLoc. The key idea is to turn the localization problem into search problem in Hamming space by collecting the dual-microphone data of smartphones via crowdsensing mechanism. We utilized the TDOA of dual- microphone integrated in smartphone. After the quantization with a bit for the TDOA measurement from the smartphone nodes, thunder localization is performed by minimizing the Hamming distance between the measured binary sequence and the binary vectors in a database. Evaluation results demonstrate that ThunderLoc can effectively localize the virtual thunder with good robustness.

Energy-efficient and robust-time synchronization is crucial for industrial Internet of things (IIoT). Some energy-efficient time synchronization schemes that achieve high accuracy have been proposed recently. However, some unsynchronized nodes namely isolated nodes exist in the schemes. To deal with the problem, this paper presents R-Sync, a robust time synchronization scheme for IIoT. We use a pulling timer to pull isolated nodes into synchronized networks whose initial value is set according to level of spanning tree. Then, another timer is set up to select backbone node and its initial value is related to the distance to parent node. Moreover, we do experiments based on simulation tool NS-2 and testbed based on wireless hardware nodes. The experimental results show that our approach makes all the nodes get synchronized and gets the better performance in terms of accuracy and energy consumption, compared with three existing time synchronization algorithms TPSN, GPA, STETS.

As wind energy continues to expand to new frontiers in terms of the location, number, and size of wind turbines, the industry has begun to seek smarter operations and management solutions. Wireless sensing nodes could provide a low-cost platform to support a variety of applications designed to reduce the levelized cost of energy and increase the safety of wind turbines. However, a wireless sensor node deployed on a wind turbine blade would have an extremely limited energy supply. To combat this limitation, we present BladeMAC, a new MAC-layer protocol designed for sensor nodes deployed on rotating wind turbine blades. BladeMAC overcomes a unique cyclical channel problem to allow a sensor node attached to a rotating blade to opportunistically and efficiently offload its data to a sink node attached to the turbine tower. We have implemented and evaluated BladeMAC using Contiki OS and the Cooja simulation tool. We present results showing that BladeMAC effectively deals with the cyclical channel problem at a wide range of data arrival intervals, and that BladeMAC is insensitive to rotation speed and rotation speed fluctuations.

Barrier coverage is a natural application of sensor networks in which sensors are deployed to detect intruders or protect crucial resources. In this paper, we consider a hybrid sensor network with a two-phase deployment, in which less-expensive static sensors are first randomly deployed in an area, and then more-expensive mobile sensors are deployed to fill coverage gaps. We use a probabilistic model to take into account the practical constraints of detection probability and false positives. We propose an iterative scheme that finds a sensor deployment strategy that minimizes the total sensor cost. Our scheme makes use of a graph transformation and includes speed-up strategies. We present simulation results that verify the correctness of the proposed scheme and demonstrate the effectiveness of the speed-up strategies.

Barrier coverage is a fundamental application for wireless sensor networks. In this paper, we consider a practical probabilistic sensing model and propose an iterative scheme, called BaCo, to provide strong barrier coverage under this model, with the objective of minimizing the number of active sensors. Moreover, we build the barrier under practical constraints of minimum detection probability and maximum false alarm probability. We use simulations to show that BaCo converges quickly and achieves better results than previous work while also bounding the system false alarm probability.

This paper presents DiVA, a novel hybrid range-free and range-based acoustic source localization scheme that uses an ad-hoc network of microphone sensor nodes to produce an accurate estimate of the source's location in the presence of various real-world challenges. DiVA uses range-free pairwise comparisons of sound detection timestamps between local Voronoi neighbors to identify the node closest to the acoustic source, which then estimates the source's location using a constrained range-based method. Through simulation and experimental evaluations, DiVA is shown to be accurate and highly robust, making it practical for real-world applications.

This paper presents DiVA, a new acoustic source localization scheme that uses an ad-hoc network of microphone sensor nodes to produce an accurate estimate of the source's location. DiVA uses pairwise comparisons of sound detection timestamps between local Voronoi neighbors to identify the node closest to the acoustic source and then estimates the source's location. The scheme improves on the state of the art by effectively dealing with anchor nodes' position error, time stamp measurement error and time synchronization error in real world conditions. Through simulation and experimental evaluations, DiVA is shown to be more robust than existing solutions under different error conditions.

Recent efforts in applying anycast techniques to duty-cycled wireless sensor networks have shown promising results in terms of reduced delay and energy consumption. This paper further increases the energy savings by introducing EDAD, an energy-centric cross-layer data collection protocol designed for anycast communications in asynchronously duty-cycled wireless sensor networks. EDAD uses a new anycast routing metric, EEP, that minimizes the expected energy consumed along the path of a packet and automatically adapts to network settings. Simulation results show that EDAD consumes less energy than similar existing protocols, while maintaining a comparable delay and high delivery rate.

Barrier coverage is an important application of sensor networks. This paper studies how to build a strong barrier with mobile sensors in which the maximum moving distance of sensors is minimized. Our work differs from others in the way the y-coordinate of the barrier is determined. We optimize the y-coordinate of the barrier instead of fixing it a priori. An efficient algorithm is proposed, in which the search space of the y-coordinate of the barrier is first discretized and then searched over iteratively. In the theoretical worst case, O(N4) iterations may be needed to find the optimal barrier location, where N is the number of sensors, but in practice, our algorithm requires less than O(N2) iterations, as confirmed in simulation.