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The realization of efficient, robust, and adaptable applications for the emergent Internet of Underwater Things enables the sustainable and effective conservation and exploitation of our oceans and waterways. Recent advances have fo- cused on Orthogonal Frequency-Division Multiplexing (OFDM) physical layers for supporting applications requiring high data rates and swift adaptation to changing underwater conditions. This prompts the need of tools for testing new OFDM-enabled underwater solutions. To this aim, this paper presents the implementation and evaluation of an OFDM-based physical layer module for the popular underwater network simulator DESERT. We aim at modeling the flexibility of the software-defined acoustic SEANet modem by realizing OFDM features that can vary in time, including the number and the selection of subcarriers and their modulation on a per-transmission basis. We demonstrate the usage of the proposed module through the DESERT-based simulation of three simple OFDM-enabled cross-layer MAC protocols in underwater acoustic networks of different sizes. The diverse and detailed set of results are obtained by using our physical layer module simply and swiftly. Our results also confirm the advantages of using the OFDM technology in solutions for underwater networking in challenging environments. 

This paper discusses asynchronous distributed inference in object tracking. Unlike many studies, which assume that the delay in communication between partial estimators and the central station is negligible, our study focuses on the problem of asynchronous distributed inference in the presence of delays. We introduce an efficient data fusion method for combining the distributed estimates, where delay in communications is not negligible. To overcome the delay, predictions are made for the state of the system based on the most current available information from partial estimators. Simulation results show the efficacy of the methods proposed. 

The present work details a novel approach to increase the transmitting sensitivity of piezoelectric micromachined ultrasonic transducer arrays and performing the direct modulation of digital information on the same device. The direct modulation system can reach 3× higher signal-to-noise ratio level and 3× higher communication range (from 6.2 cm boosted to 18.6 cm) when compared to more traditional continuous wave drive at the same energy consumption levels. When compared for the same transmission performance, the direct modulation consumes 80% less energy compared to the continues wave. The increased performance is achieved with a switching circuit that allows to generate a short high-AC voltage on the ultrasonic array, by using an LC tank and a bipolar junction transistor, starting with a low-DC voltage, making it CMOS-compatible. Since the modulation signal can directly be formed by the transmitted bits (on/off keying encoding) this also serve as the modulation for the data itself, hence direct modulation. The working principle of the circuit is described, optimization is performed relative to several circuital parameters and a high-performance experimental application is demonstrated. 

We consider a network of distributed underwater sensors whose task is to monitor the movement of objects across an area. The sensors measure the strength of signals emanated by the objects and convey the measurements to the local fusion centers. Multiple fusion centers are deployed to cover an arbitrarily large area. The fusion centers communicate with each other to achieve consensus on the estimated locations of the moving objects. We introduce two efficient methods for data fusion of distributed partial estimates when delay in communication is not negligible. We concentrate on the minimum mean squared error (MMSE) global estimator, and evaluate the performance of these fusion methods in the context of multiple-object tracking via extended Kalman filtering. Numerical results show the superior performance compared to the case when delay is ignored. 

Underwater wireless communication and network- ing are becoming key enablers of a number of critical marine and underwater applications. Experimentation is underway, in controlled environments as well as at sea, that concerns the deployment of several underwater devices providing wireless communication capabilities to sensors of different nature. Con- trolling the deployment at sea of these devices, remotely and efficiently, is paramount for enabling expedite testing of hardware and protocol development. To address this need, this paper presents the design, development, and testing of a Smart Buoy for real-time remote access to underwater devices and for provision of power and extended computational capabilities. Experimental results are shown concerning the time needed to connect with the Smart Buoy, the power consumption of its operations, and the energy harvesting intake (via solar panels) in time. We also investigate the buoy lifetime when powered by solar panels and supporting acoustic modems over varying traffic scenarios. 

The present work reports on the novel implementation of a miniaturized receiver for underwater networking merging a Piezoelectric Micromachined Ultrasonic Transducer (PMUT) array and signal conditioning circuitry in a single, packaged device. Tests in both a large water tank and a pool demonstrated that the system can attain large enough Signal-to-Noise Ratio (SNR) for communication at distances beyond two meters. An actual communication test, implementing an Orthogonal Frequency Division Multiplexing (OFDM) scheme, was used to characterize the performance of the link in terms of Bit Error Rate (BER) vs SNR. In comparison to previous work demonstrating high-data rate communication for intra-body links and acoustic duplexing, this implementation allows for significantly larger distances of transmission, while addressing the signal conditioning and submersible packaging needs for underwater conditions, thus enabling PMUT arrays for operating as complete underwater communication receivers.