When higher data rates are needed, we should make use of radio frequency (RF) methods, which are able to reach communication data rates of up to 100 Mb/s in very short distances, apart from presenting substantial immunity from the environmental features. In acoustic and ultrasonic communications, researchers usually work on varying the type of modulation and communication protocol, in order to minimize the effects of reflections, and on achieving as high a communication data rate as possible. Although acoustic communication is a proven technology, it presents some main drawbacks, like the low data rate (0 b/s to 20 kb/s), which is limited by some factors, such as low carrier frequency, strong reflections and attenuation when the communication is performed near the surface, as well as poor performance in turbid water with large particles, sensitivity to varying environmental characteristics and the salinity. Moreover, they are the most used methods, since they are able to reach large distances (over 20 km ). Systems based on acoustic waves are less sensitive to fine particles suspended in the water and to the water turbidity, than the optical waves. However a strong backscattering is caused by suspended particles and they are affected by the turbidity of the water, so they are not good options for long distances. Systems based on optical communication are able to reach very high propagation speeds. Each technique has its own characteristics, with its benefits and drawbacks, mainly due to the chemical characteristics and physical constraints of the medium. Underwater communication research is primarily focused on the use of optical signals, electromagnetic signals and the propagation of acoustic and ultrasonic signals. On one hand, the main research lines are based on increasing the distance and bandwidth, and, on the other hand, the attempt to reduce the energy consumption of underwater devices, with the aim of increasing the network lifetime. Nowadays, there is extensive ongoing research activity relating to underwater communications and underwater sensor networks.

It can be used for precision monitoring in applications such as contaminated ecosystems or for device communicate at high depth. Although our proposal provides short communication distances, it provides high data transfer rates. Finally, we compare our communication system proposal with the existing systems. Based on our measurements, we provide some experimental models of underwater communication in fresh water using EM waves in the 2.4 GHz ISM frequency band. The performed tests show the maximum distance between sensors, the number of lost packets and the average round trip time. In this paper, we show our wireless sensor node deployment and its performance obtained from a real scenario and measures taken for different frequencies, modulations and data transfer rates. In these cases, while most researchers focus their efforts on increasing the data rate for low frequencies, we propose the use of the 2.4 GHz ISM frequency band in these special cases.

In some cases, wireless sensors must be placed quite close to each other in order to take more accurate measurements from the water while having high communication bandwidth. Moreover, there are many problems inherent to the medium such as reflections, refraction, energy dispersion, etc., that greatly degrade communication between devices. One of the main problems in underwater communications is the low data rate available due to the use of low frequencies.

Army antenna and wave propagation ppt