# Underwater Electromagnetic Sensor Networks--Part I: Link Characterization +
## 1 Introduction  
Reliable underwater communications systems are required for many applications like tactical surveillance, pollution and environmental monitoring, oceanography research, among others. The latter ones are all related to the study of climate change, oceans and ecosystems. Valuable monitoring techniques would be extremely helpful to analyse constant variation of marine characteristics and human impact on these ecosystems. For example, the process of marine defaunation[1], which climate change is accelerating, requires efficient underwater monitoring. This is where underwater wireless sensor networks play an important role, as a solution to this problem, making possible communication between sensing nodes and periodic data retrieval. However, the underwater scenario is a really harsh environment and different wireless technologies have to be examined. Acoustic communications are a standard for marine underwater communications and optical communications are proving to be useful in some underwater scenarios. Electromagnetic propagation has been studied for decades but just a few technologies have made it through. Acoustic systems are widely used for long distances in deep waters, but they have some drawbacks, especially in shallow waters, due to time-varying multipath propagation, Doppler shifting and Doppler spread [2]. Moreover, its bandwidth is limited to 5-7 KHz providing a low data rate. By contrast, optical technology can reach huge data rates (>1 Gbps) but is limited to short-range communications and clear waters [3]. Therefore, EM solutions are being re-evaluated for their use in shallow waters, where it can be useful to deploy a sensor network. Although EM waves suffer high attenuation in sea water, a series of advantages are offered, for instance, considerable bandwidth, fast propagation, immunity to turbidity, no multipath or Doppler propagation problems, and inexpensive and proven EM circuitry and antennas. It is widely accepted that the underwater acoustic channel is one of the most difficult types of communication media in use nowadays and high power systems have a negative impact on marine fauna [4]. In addition, optical technology is limited by water turbidity and ambient light noise, especially in coastal areas. Consequently, EM propagation has an increasing interest and is being studied by several research groups all over the world, hoping to establish it as a real alternative [5; 6; 7]. It is important to remark that different approaches in underwater radiocommunications systems lead to a wide range of results in literature. When analysing the state-of-the-art, there are five important aspects to highlight: antennas used, frequency bands, power transmitted, distances achieved and transmitter-receiver depths. In some cases, real condition experiments have been carried out. In others, simulations or theoretical approaches have been conducted, and, in a few cases, a combination of both is encountered. Regarding antennas, loop antennas and different types of dipoles are usually tested by research groups, but no agreement has been found on which is the best choice. Dipoles with and without insulated sections are examined in [7; 8; 9], whilst others carry out experiments with loop antennas [7]. Nevertheless, magnetic loop antennas seem to have a certain acceptance. According to [PERSON] and [PERSON] [10], propagation over long distances at frequencies of MHz is feasible because water conductivity decreases at certain frequencies, yet this has not be proven experimentally. Accordingly, [PERSON] and [PERSON] [7] showed the possibility of communicating at 1 MHz over 90 m in a real sea water experiment, but no additional measurements have been published to validate these results in sea water. Instead, their research line was changed to fresh water in the last papers. Dipoles and loop antennas were tested. However, the latter seemed to be more successful.