Beatrice Tomasi

Underwater acoustic data communication is an enabling technology for operations such as environmental monitoring, exploitation of ocean resources, and autonomous inspection of offshore infrastructure. The only viable technique for subsea telemetry over longer distances is by encoding the information in sound waves. However, the influence of the ocean environment on sound propagation can be harsh, resulting in a poor performance of protocols designed for terrestrial radio-frequency communications. This justifies active research on physical-layer algorithms and network protocols that are robust to acoustic propagation in the oceans. Factors that hamper progress are the lack of accepted standard test channels and hard-to-obtain field data. A majority of academic institutions cannot afford at-sea experimentation, resorting to simplified channel models to test new protocols. In order to make field data accessible to a wider audience, this paper presents a dataset of hydrophone recordings of communication and channel probe waveforms collected in an enclosed fjord environment. The SFI Smart Ocean Dataset for Acoustic Communications (SODAC) allows users to test communication algorithms on in situ data, with opportunities to publish reproducible results that set a realistic benchmark for other researchers.

EMSO ERIC is a unique European distributed marine Research Infrastructure dedicated to the observation and study of the deep ocean in the long term in fixed regional areas. It provides different services of which access to its infrastructure by external users -engineers, scientists and researchers-, working both in the public and private sectors. The aim of this service, called physical access, is to facilitate access to instrumented platforms deployed at different sites across the European seas, from the seabed to the surface, in order to perform experiments in geosciences and engineering in real ocean conditions. Depending on the logistics and availability of each site, users may deploy their own platforms, instruments, systems or technologies to be tested by the existing equipment that, in this case, can provide reference measurements. Users may also deploy their own systems on the existing EMSO platforms, either in standalone mode or connected to them, receiving power and, in some cases, being able to transmit data by satellite or by cable, depending on the site. Projects requiring the use of several EMSO sites are also accepted. The host EMSO Regional Facility provides logistics and technical support in order to deploy and recover the systems, access the data and it may also offer training and co-development. EMSO ERIC launches the physical access call on a yearly basis and evaluates the received project proposals every two months. Access is free of charge and funding is available for travel, consumables, shipping, operations and hardware adaptations needed to run the project. Since 2022, when the first call was launched, ten projects with varied topics have been funded and are in different phases of execution.

Underwater unmanned vehicles are complementary with human presence and manned vehicles for deeper and more complex environments. An autonomous underwater vechicle (AUV) has automation and long-range capacity compared to a cable-guided remotely operated vehicle (ROV). Navigation of AUVs is challenging due to the high absorption of radio-frequency signals underwater and the absence of a global navigation satellite system (GNSS). As a result, most navigation algorithms rely on inertial and acoustic signals; precise localization is then costly in addition to being independent from acoustic data communication. The purpose of this paper is to propose and analyze the performance of a novel low-cost simultaneous communication and localization algorithm. The considered scenario consists of an AUV that acoustically sends sensor or status data to a single fixed beacon. By estimating the Doppler shift and the range from this data exchange, the algorithm can provide a location estimate of the AUV. Using a robust state estimator, we analyze the algorithm over a survey path used for AUV mission planning both in numerical simulations and at-sea experiments.

This paper describes the experimental methodology followed to measure the effective rate of horizontal underwater optical communications in Trondheimsfjord at the end of the winter time in 2024. We also provide the measurements of water properties for optical attenuation, absorption, and solar irradiance in the same area. Measured effective rates in real conditions were of the order of a few Mbps at distances up to 30 m and at depths greater than 60 m.

This article presents a study of the Multiuser Multiple-Input Multiple-Output (MU-MIMO) Method for the multiple access of multiuser mobile underwater acoustic communications. Orthogonal multiple access schemes, such as time-division multiple access (TDMA), frequency-division multiple access, or code-division multiple access, suffer from limited data rate due to an orthogonal sharing of communication resources. In an MU-MIMO system, each user is viewed by the receiver as an input of a conventional multiple-input multiple-output (MIMO) channel using spatial multiplexing. The receiver, having multiple hydrophones, can then exploit the spatial diversity to decode the input streams of each user through successive cancellation of interstream interference. However, unlike traditional MIMO decoding, in a mobile context, each user can have its own motion-induced Doppler scale factor that needs to be efficiently treated at the receive side. First, a communication system is presented in detail, where Doppler scale domain equalization is proposed to cope with multiple mobile users having each their own Doppler scale factor. Then, a data rate performance evaluation of MU-MIMO shows its advantages compared to typical orthogonal multiple access schemes. Performance evaluation is carried out for a two-user configuration in terms of the theoretical achievable data rates computed on channels sounded at-sea and by the evaluation of effective rates obtained through channel replay of the full modulation–demodulation stages on the same sounded channels. Achievable rates of MU-MIMO with successive interference cancellation are compared with single user and TDMA, set as upper and lower performance bounds, respectively. For the studied configuration, results show that a data rate gain of 64% over TDMA is expected in MU-MIMO. Finally comparisons with the effective rates are provided for both the static and mobile users. They reveal a gain of the same level as that expected in theory at signal-to-noise ratio (SNR) above 12 dB for static users. The price to pay is a robustness loss of MU-MIMO at low SNR. In the mobile scenario, results show an additional loss in robustness compared to the static MU-MIMO case, so that the equivalence with the theoretical performance is obtained at SNR above 15 dB.

The objective of this study is to evaluate Grassmannian constellations combined with a spread spectrum multiple access scheme for underwater acoustic mobile multiple access communication systems. These communication systems enable the coordination of a fleet of Autonomous Underwater Vehicles (AUVs) from a surface or bottom control unit, e.g., a boat. Due to its robustness against phase rotation, the demodulator of Grassmannian constellations uses non-coherent detection, and the main advantage of such modulation lies in the spectrum efficiency gain with respect to conventional differential modulation. The communication system under study in this paper consists of (i), at the transmitter side, a Grassmannian modulation used in an orthogonal spread spectrum multiple access scheme called Multiuser Hyperbolic Frequency Modulation (MU-HFM) and (ii), at the receiver side, a non-coherent array decoder. The modulation and demodulation are presented as well as the considered spreading sequences. Finally, performances of the proposed transmission scheme are evaluated over replayed underwater acoustic channel responses collected at sea by a multi-sensor acoustic acquisition system.

Non-orthogonal multiple access (NOMA) schemes have been proved to increase the multiple-access achievable rate with respect to orthogonal multiple access (OMA). In this paper we propose a novel communication system that combines multi-level coded modulation and polar codes in a NOMA scenario. Computational complexity decreases with the proposed scheme with respect to state-of-the-art solutions. We also highlight the trade-off between error rate performance and computational complexity.

Channel state information (CSI) acquisition is a significant bottleneck in the design of Massive MIMO wireless systems, due to the length of the training sequences required to distinguish the antennas (in the downlink) and the users (for the uplink where a given spectral resource can be shared by a large number of users). In this article, we focus on the downlink CSI estimation case. Considering the presence of spatial correlation at the base transceiver station (BTS) side, and assuming that the per-user channel statistics are known, we seek to exploit this correlation to minimize the length of the pilot sequences. We introduce a scheme relying on non- orthogonal pilot sequences and feedback from the user terminal (UT), which enables the BTS to estimate all downlink channels. Thanks to the relaxed orthogonality assumption on the pilots, the length of the obtained pilot sequences can be strictly lower than the number of antennas at the BTS, while the CSI estimation error is kept arbitrarily small. We introduce two algorithms to dynamically design the required pilot sequences, analyze and validate the performance of the proposed CSI estimation method through numerical simulations using a realistic scenario based on the one-ring channel model.

To meet the goal of ten-fold increase in spectral efficiency, multiuser multiple-input-multiple-output (MU-MIMO) techniques capable of achieving high spatial multiplexing gains are expected to be an essential component of fifth-generation (5G) radio access systems. This increase in multiplexing gain, made possible by equipping base stations with a large number of antennas, entails a proportional increase in channel state information (CSI) acquisition overhead. This article addresses the problem of reducing this CSI overhead by optimizing the amount of time-frequency resources allocated for channel training purposes while not affecting the quality of the associated channel estimate. First we show that in MU-MIMO, adapting pilot symbol density in the time-frequency grid should be performed both on a per resource block (RB) basis and on the basis of groups of users sharing similar channel conditions. Next, we propose a practical scheme that can perform grouping based per-RB pilot pattern adaptation. Finally, we evaluate using both analytical and numerical results the gain in spectral efficiency that can be achieved using this scheme as compared to conventional MU-MIMO systems that use fixed pilot patterns.

Maximum Likelihood Sequence Detectors (MLSD) have been largely used to mitigate the Chromatic Dispersion (CD) in Intensity Modulation/Direct Detection (IM/DD) optical communication systems. For practical applications, the high complexity of the receivers remains an important issue. In this paper, we analyze the design of MLSD-based receivers using the Orthogonal Volterra Kernel Model for IM/DD optical communication systems in Metro Optical applications. We discuss the impact in complexity and performance of the main parameters of the model and provide three design options for the MLSD-based receiver. Finally we provide numerical simulations showing the Bit Error Rate (BER) performances of the three considered designs for both On-Off Keying (OOK) and higher order Pulse Amplitude Modulation (PAM).