Filippo Campagnaro

On the feasibility of fully wireless remote control for underwater vehicles

In this paper, we explore the possibility of controlling a Remotely Operated Vehicle (ROV) via a fully wireless control channel. As a first step, we review the expected bit rate offered by optical, acoustic as well as radio-frequency underwater communication technologies, as a function of the distance between the transmitter and the receiver. We then discuss the ROV data transfer requirements and discuss which ones can be supported by a given technology at a given distance. Finally, we simulate the performance of the system during missions of interest, and conclude by discussing the effectiveness of wireless control methods for ROVs.

Implementation of a multi-modal acoustic-optical underwater network protocol stack

We describe the implementation of a protocol stack for multi-modal underwater networks, where multiple physical layer technologies are available to each node. This condition implies greater flexibility, by allowing each node to decide how to serve specific transmissions and traffic classes depending on system, quality-of-service, and application-specific policies implemented in a controller. In this paper, we describe the implementation details of an acoustic/optical multi-modal underwater network stack through the DESERT Underwater framework. Our system services various traffic types with different quality-of-service demands, and allows mobility. Extensive numerical results show that our multi-modal system offers greater throughput, robustness to mobility and traffic types, and much lower service delay.

Simulation of a Multimodal Wireless Remote Control System for Underwater Vehicles

While the design of reliable and enduring remotely operated vehicles for underwater operations is currently a hot research area, there are not many options to control these systems in real time using wireless telemetry. In particular, a reliable, fully wireless control method that exploits state-of-the-art underwater wireless communication technologies is still lacking. In this paper, we consider the design and engineering of one such multimodal control system comprising optical and acoustic underwater communications, and characterize its performance under different configurations of the communication protocol stack. Compared to previous work, we introduce a proactive mechanism to switch among the components of the multimodal system by means of a signaling mechanism that requires negligible overhead. Our results suggest that a multimodal wireless control system can provide satisfactory control performance by supplying different levels of interaction with the vehicle, depending on the technology in use, and by reliably and timely switching between the available communication technologies.

A Handshake-Based Protocol Exploiting the Near-Far Effect in Underwater Acoustic Networks

We propose a scheme that opportunistically exploits received power diversity across different packets to both favor spatial reuse in underwater acoustic networks and limit the exposed terminal problem. Since the power attenuation in the underwater acoustic channel is large, such power differences are common. This situation, also referred to as the near-far effect and often considered a problem, is converted into a resource by means of multipacket reception (MPR). Yet, even without MPR capability our scheme offers performance benefits. Our scheme is simple, lightweight and distributed, and can be easily implemented over any protocol based on carrier-sense multiple access with collision avoidance. Our results show that higher throughput and lower transmission delay are achieved compared to a benchmark channel access protocol. Our results are validated and demonstrated in a lake experiment. To allow reproducibility, the implementation of our scheme is publicly available.

Routing in multi-modal underwater networks: A throughput-optimal approach

While acoustic signals are still the main communication means under water, other technologies are being developed, e.g., based on optical and radio-frequency electro-magnetic waves. Each technology has its own advantages and drawbacks to trade off, e.g., communication range vs. bit rate. Recently, new approaches are emerging to leverage the advantages of several underwater technologies by incorporating them in a multi-modal communication system. In this paper, we address a fundamental part of these multi-modal systems by proposing a novel routing protocol for networks of multi-modal nodes. Our protocol makes distributed optimal and fair decisions about the per-link flow, prevents bottlenecks, and allows simultaneous transmissions using multiple technologies in order to advance a packet towards its destination. We analyze the performance of our protocol via model-based simulations and compare it to benchmark results. Our results show that our protocol successfully leverages all technologies to deliver data, even in the presence of imperfect topology information.

Design and evaluation of a low-cost acoustic chamber for underwater networking experiments

Testing acoustic equipment before sea experiments is a necessary step, which usually requires large and expensive facilities. In this paper, we present the design guidelines, structure and details of a small-scale, low-cost acoustic chamber for in-lab testing of underwater acoustic networks. The chamber has been assembled with the objective to be of low cost and limited size: therefore, its installation fits small university laboratories that cannot afford large testing pools. The chamber was designed to mitigate the extreme multi-path which, in a small chamber, makes communications unreliable. Considering this challenge, our chamber includes a phono-absorbing coating on the walls and floor, to be optionally complemented by a panel of the same coating material, to be installed at the water surface level. After providing the details of several phono-absorbing materials to motivate our specific choice, we carry out a number of transmission experiment with EvoLogics modems, proving that our design substantially reduces the severe multi-path and thereby improves the communications quality.

Full Reconfiguration of Underwater Acoustic Networks through Low-Level Physical Layer Access

Underwater acoustic communications experiments often involve custom implementations of schemes and protocols for the physical and data link layers. However, most commercial modems focus on providing reliable or optimized communication links, rather than on allowing low-level reconfiguration or reprogramming of modulation and coding schemes. As a result, the physical layer is typically provided as a closed, non-reprogrammable black box, accessible by the user only through a specific interface. While software-defined modems would be the ultimate solution to overcome this issue, having access to the symbols transmitted by the modems using a proprietary modulation format already opens up a number of research opportunities, e.g., aimed at the cross-layer design and optimization of channel coding schemes and communication protocols. In this paper, we take the latter approach. We consider the commercial EvoLogics modem, driven by a custom firmware version that bypasses the channel coding methods applied by the modem, and allows the user to set the transmit bit rate to any desired value within a given set. This makes it possible to evaluate different coding schemes in the presence of different bit rates. Our results show that the custom firmware offers sufficient flexibility to test different configurations of the coding schemes and bit rates, by providing direct access both to correctly decoded and to corrupted symbols, which can be separated at the receiver for further processing. In addition, we show that the DESERT Underwater framework can also leverage the same flexibility by employing low-level physical layer access in more complex networking experiments.

Multimodal Underwater Networks: Recent Advances and a Look Ahead

The recent uptake of non-acoustic underwater transmission systems suggests that in the near future it will be common for underwater devices to incorporate different physical communication technologies. Such devices are typically described as multimodal. They seek flexibility by compensating for the shortcomings of a given technology through the advantages of another. For example, a system encompassing acoustic and optical communication systems can provide long-range, low-bit rate communications, while enabling faster data transfer at very short range. As the development of non-acoustic underwater communications is taking momentum, so is the research on how to optimally exploit the multimodal communications capabilities in different scenarios. This paper presents a survey of past and recent work on this topic, covering the development both of the communication technologies and of the networking schemes and protocols for multimodal networks. As an example of the opportunities offered by multimodal communications, we discuss two different case studies. We conclude with an outlook on likely future developments for multimodal communications.

On the Relationship Between the Underwater Acoustic and Optical Channels

Wireless transmissions in water are mostly carried out via long-range (but low-rate) underwater acoustic communications, or short-range (but high-rate) underwater optical communications. In this paper, we are interested in finding out whether a statistical relationship exists between underwater acoustics and optics. Besides the theoretical interest of such a relationship, predicting the quality of the optical link through acoustics is also relevant in the context of a multimodal system with both acoustics and optics. Our study is based on a large data set acquired during the NATO ALOMEX 2015 expedition. During this experiment, we simultaneously measured several characteristics of the acoustic and optical links at multiple locations, reflecting a diversity of sea environments. Our results show a strong correlation between the properties of the acoustic link and the reliability of optical communications. This correlation makes it possible to predict the state of the underwater optical link at a certain depth and range. Due to the complexity of the acoustic and optical channels, we could not find the source of this correlation. This paper is, therefore, aimed to stimulate a theoretical study of the mutual properties of underwater acoustic and optical communication links. For reproducibility, we share the processed data from the experiment.

Leveraging the Near–Far Effect for Improved Spatial-Reuse Scheduling in Underwater Acoustic Networks

We present a spatial reuse resource allocation scheme for underwater acoustic networks that organizes communications so as to avoid destructive collisions. One prime source of collisions in underwater acoustic networks is the so called near–far effect, where a node located farther from the receiver is jammed by a closer node. While common practice considers such a situation a challenge, in this paper we consider it a resource, and use it to increase the network throughput of spatial-reuse time-division multiple access. Our algorithm serves two types of communications: 1) contention-free and 2) opportunistic. Our objective is to maximize the time slot allocation, while guaranteeing a minimum per-node packet transmission rate. The result is an increase in the number of contention-free packets received, and a decrease in the scheduling delay of opportunistic packets. Numerical results show that, at a slight cost in terms of fairness, our scheduling solutions achieve higher throughput and lower transmission delay than benchmark spatial-reuse scheduling protocols. These results are verified in a field experiment conducted in the Garda Lake, Italy, where we demonstrated our solution using off-the-shelf acoustic modems. To allow the reproducibility of our results, we publish the implementation of our proposed algorithm.