Abstract
In this paper we assess the influence of the conductivity of the medium on the radiation characteristics and input impedance of a loop antenna designed for underwater communications. The initial study is based on simulation of the antenna characteristics using FEKO electromagnetic (EM) software. Additionally, an equivalent circuit model of the antenna is also obtained and simulation of the input impedance on ADS software shows good agreement with the EM simulator. It is found that the radiation pattern, of the loop antenna changes significantly with the conductivity of the medium, from freshwater to seawater. The loop antennas were built with baluns and tested in freshwater, where the insertion loss between two identical antennas and the radiation pattern of one of them were also measured. The results show good agreement with the simulations.

Abstract
The radiation characteristics of a dipole antenna when immersed in both fresh and seawater are assessed through simulation and experimental work. Simulations show that the antenna's bandwidth and radiation pattern change with the properties of the medium where it is placed, namely the conductivity of the medium. Two dipoles antennas with current baluns were built and tested experimentally in freshwater. The tests included the measurement of the insertion loss between two identical dipole antennas and their radiation pattern. The results obtained show a good agreement between the simulation and experimental results.

Abstract
Radio Frequency (RF) communications su.er high attenuation underwater, limiting the range of standard IEEE 802.11 networks to a few centimeters underwater. The usage of custom RF solutions at lower frequencies to increase range entails high development costs, and proprietary hardware. This paper evaluates the performance of cost-e.ective, mass-market IEEE 802.11 networks underwater at di.erent frequencies in the sub-GHz bands using Software Defined Radio platforms. Optimizations to the gr-ieee802.11 implementation, together with custom designed antennas, allowed the testing of frequencies in the 70-700 MHz range, in a large-scale freshwater tank, and in real-world, seawater conditions in Tagus river estuary. Results show a frequency-dependent communications range up to 5 m in freshwater and 1.8 m in seawater, with bitrates up to 550 kbit/s and delay under 45 ms. This proves the feasibility of IEEE 802.11 networks in the sub-GHz bands for applications such as video streaming from an autonomous underwater vehicle and underwater sensors. Copyright 2015 ACM.

Abstract
The demand for broadband underwater communications is being pushed by the increasing use of Autonomous Underwater Vehicles (AUV) in underwater missions. IEEE 802.11, already used in AUV for above water communications, can also be employed underwater to enable cost-effective, high bandwidth, short range communications. However, the high RF attenuation underwater induces high variations of received power, which may affect the performance of existing rate adaptation algorithms designed for over-the-air networks. This paper evaluates the most relevant state-of-the-art IEEE 802.11 rate adaptation algorithms in underwater environment. Simulation results show the AARF algorithm outperforms the widely used Minstrel algorithm, as well as the CARA, RRAA and ONOE algorithms. Copyright 2015 ACM.

Abstract
The usage of Autonomous Underwater Vehicles (AUVs), Remotely Operated Vehicles (ROVs), and sensors in surveillance, maintenance and inspection of underwater facilities is increasing the need for broadband, cost-e ective communications solutions. Current solutions, mainly based on acoustic communications, enable long ranges but provide low bitrates and have high communication delays. Despite its strong attenuation underwater, RF is envisioned as a technology to enable broadband, short-range communications. We present an ns-3 and an experimental evaluation of IEEE 802.11 networks in freshwater at 700 MHz, 2.4 GHz and 5 GHz frequency bands. Evaluation results con rm the accuracy of the new developed ns-3 underwater RF propagation model and show that IEEE 802.11 networks are feasible for broadband, short range underwater communications, with range and throughput exceeding 2 m and 100 Mbit/s, respectively.

Abstract
Current maritime communications rely on expensive or proprietary technology, such as satellite, WiMAX, and narrowband HF radios. Broadband communications are limited to the near shore 3G/4G coverage provided by mobile operators. The usage of unlicensed and IEEE 802.11 networks may provide ship owners a low-cost broadband access to the Internet offshore, enabling real-time navigation applications and voice/video communications, while increasing safety onboard. Also, they can support underwater communications by acting as a bridge between shore and devices operating underwater. In this paper we present a performance evaluation of IEEE 802.11n networks in the 5.8 GHz band in a maritime environment. A point-to-point link was established between a fishing ship and shore. From our tests, communication links up to 7 km at 1 Mbit/s are possible, showing the advantage of using long range IEEE 802.11 links for broadband maritime communications.