Wouter van Kleunen

Industry: using dynamic WSNs in smart logistics for fruits and pharmacy

In this paper we describe a smart way to apply dynamic wireless sensor networks (WSN) in logistics. Especially in the temperature controlled supply chain (cold chain), perishable goods like fruits and pharmaceuticals greatly benefit from real-time quality monitoring during storage and transport in order to avoid quality degradation and spoilage. In our system, wireless sensor nodes called SmartPoints monitor the environmental conditions and generate alarms when specific events are detected. Additionally, they calculate the remaining shelf life of the perishable goods they travel with. When there is an Internet-connected WSN available during travel, the shelf-life prediction and associated alarms are directly sent to a back-end server. Alternatively they are logged on the SmartPoints and flushed upon arrival, such that the remaining shelf-life and alarms are immediately clear and a full history will be available later. Our dynamic WSN supports a number of protocols that enable support for the dynamic processes in logistic processes. The Ambient middleware supports real-time monitoring and remote maintenance across the Internet via wired and mobile wireless network access technologies. Additionally, the middleware offers easy integration with third-party applications. Ambient Studio utilizes the middleware for remote WSN configuration and monitoring.

Underwater localization by combining Time-of-Flight and Direction-of-Arrival

In this paper we present a combined Time-of-Flight (ToF) and Direction-of-Arrival (DoA) localization approach suitable for shallow underwater monitoring applications such as harbor monitoring. Our localization approach combines one-way ranging and DoA estimation to calculate both position and time-synchronization of the blind-node. We will show that using this localization approach, we are able to reduce the number of reference nodes required to perform localization. By combining ToF and DoA, our approach is also capable of tracking and positioning of sound sources under water. We evaluate our approach through both simulation and underwater experiments in a ten meter deep dive-center (which has many similarities with our target application in terms of depth and reflection). Measurements taken at the dive-center show that this environment is highly reflective and resembles a shallow water harbor environment. Positioning results using the measured Time-of-Arrival (ToA) and DoA indicate that the DoA approach outperforms the ToF approach in our setup. Investigation of the DoA and ToF measurement error distributions, however, indicate the ToF-based localization approach has a higher precision. Shown is that both ToF and DoA and the combined approach achieve sub-meter positional accuracy in the test environment. Using the error distributions derived from the measurement in the dive-center, we run simulations of the same setup. Results from the simulation indicate ToF is more accurate than DoA positioning. Also in simulation all approaches achieve sub-meter accuracy.

aLS-Coop-Loc: cooperative combined localization and time-synchronization in underwater acoustic networks

Traditionally, underwater localization and time-synchronization are performed separately. This, however, requires two-way ranging between nodes to determine propagation delays resulting in high power consumption and communication overhead. One-way ranging can be used by using a combined time-synchronization and localization approach. While such an approach exists for non-cooperative networks, to the best of our knowledge no such approach exist for cooperative networks. A cooperative approach has significant benefits in terms of number of reference nodes required, flexibility of reference nodes, and accuracy of localization and time-synchronization. Therefore, in this paper we propose a cooperative combined localization and time-synchronization for underwater acoustic networks. We show our approach requires less communication and improves energy-efficiency of the ranging measurement phase, compared to existing Multi-Dimensional Scaling (MDS) approaches using two-way ranging or prior time-synchronization. Using simulation we evaluate the localization and time-synchronization accuracy of our approach and compare it with existing MDS approaches and a non-cooperative approach. Simulations shows that our cooperative approach outperforms non-cooperative approaches in terms of accuracy of localization and time-synchronization and is able to perform localization with fewer reference nodes. We also show that our approach outperforms MDS with prior time-synchronization in terms of accuracy.

Proteus II: design and evaluation of an integrated power-efficient underwater sensor node

We describe the design and evaluation of an integrated low-cost underwater sensor node designed for reconfigurability, allowing continuous operation on a relatively small rechargeable battery for one month. The node uses a host CPU for the network protocols and processing sensor data and a separate CPU performs signal processing for the ultrasonic acoustic software-defined Modulator/Demodulator (MODEM). A Frequency Shift Keying- (FSK-) based modulation scheme with configurable symbol rates, Hamming error correction, and Time-of-Arrival (ToA) estimation for underwater positioning is implemented. The onboard sensors, an accelerometer and a temperature sensor, can be used to measure basic environmental parameters; additional internal and external sensors are supported through industry-standard interfaces (I2C, SPI, and RS232) and an Analog to Digital Converter (ADC) for analog peripherals. A 433 MHz radio can be used when the node is deployed at the surface. Tests were performed to validate the low-power operation. Moreover the acoustic communication range and performance and ToA capabilities were evaluated. Results show that the node achieves the one-month lifetime, is able to perform communication in highly reflective environments, and performs ToA estimation with an accuracy of about 1-2 meters.

MDS-Mac: A Scheduled MAC for Localization, Time-Synchronisation and Communication in Underwater Acoustic Networks

In this paper we describe a design for an underwater MAC protocol which combines localization, time-synchronisation and communication. This protocol is designed for small-scale clustered networks in which all nodes are able to communicate with each other. We consider an integrated design of localization, time-synchronisation and communication important because scheduled communication, localization and time-synchronisation require estimation of propagation delays between nodes to operate accurately and efficiently. For localization we use multidimensional scaling because it does not require the use of reference nodes. Our MAC design consists of two phases, i.e. an unscheduled coordination phase and a communication phase. During the first phase, propagation delays are estimated, relative positions are calculated using multidimensional scaling, and timesynchronisation is performed. During the communication phase, sensor-data is transmitted using scheduled communication. Using simulation we evaluate the feasibility of such a design. By measuring the time required for the coordination phase at different modulation rates, we derive the required modulation rate for acceptable coordination phase times.

Experiments with aLS-Coop-Loc cooperative combined localization and time-synchronization

Performing real world experiments with underwater communication is difficult and time-consuming. Input for evaluation of localization and time-synchronization derived from experiments is not readily available. Using real-world experiments we evaluate the performance of our cooperative combined localization and time-synchronization approach called aLS-Coop-Loc and a non-cooperative approach. We perform experiments using the SeaSTAR Proteus node and a Commercial Off-the-Shelf (COTS) node from Kongsberg Maritime at a lake and at Strindfjorden in Norway. These experiments provide realistic insight into ranging performance in real-world environments. Evaluation shows that the cooperative approach outperforms non-cooperative approaches in terms of accuracy of localization and time-synchronization. aLS-Coop-Loc provides about about 2% to 34% better position accuracy and 50% improved time-synchronization.

Calibration-Free Signal-Strength Localization Using Product-Moment Correlation

Localization, a process of determining the position of a blind node, can be used in various applications. Signal-strength localization provides a low-cost and lowpower solution to positioning. Signal-strength positioning approaches using fingerprinting or calibrated approaches require a time-consuming calibration phase. Existing selfcalibrating approaches, which do not require a priori calibration, use a least-squares fitting model to determine both the position of the blind node as well as the optimal environmental parameters. In this paper, we propose an approach using the ProductMoment correlation between the measured signal strength and the estimated signal strengths. Such approach does not require estimation of the environmental parameters or prior calibration and outperforms existing self-calibrating least-squares approaches. We compare our approach to existing least-squares calibration-free positioning approaches. Moreover, we look at the Cramer-Rao Bound (CRB) of signal-strength localization and using simulations we show that the product-moment correlation outperforms leastsquares approaches and follows the CRB closely. Simulation and evaluation using a real-world experiment dataset show the product-moment approach significantly outperforms least-squares approaches. The productmoment approach follows the CRB much more closely and achieves up to twice more accurate positions in certain scenarios. When the error ratio increases and the number of reference positions stays fixed at 6, the productmoment approach scores 20% more accurate positions. In the cooperative localization scenario, the product-moment correlation performs 40% better.

Simplified scheduling for underwater acoustic networks

The acoustic propagation speed under water poses significant challenges to the design of underwater sensor networks and their medium access control protocols. Similar to the air, scheduling transmissions under water has significant impact on throughput, energy consumption, and reliability. In this paper we present an extended set of simplified scheduling constraints which allows easy scheduling of underwater acoustic communication. We also present two algorithms for scheduling communications, i.e. a centralized scheduling approach and a distributed scheduling approach. The centralized approach achieves the highest throughput while the distributed approach aims to minimize the computation and communication overhead. We further show how the centralized scheduling approach can be extended with transmission dependencies to reduce the end-to-end delay of packets. We evaluate the performance of the centralized and distributed scheduling approaches using simulation. The centralized approach outperforms the distributed approach in terms of throughput, however we also show the distributed approach has significant benefits in terms of communication and computational overhead required to setup the schedule. We propose a novel way of estimating the performance of scheduling approaches using the ratio of modulation time and propagation delay. We show the performance is largely dictated by this ratio, although the number of links to be scheduled also has a minor impact on the performance.