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Acoustic Communications

To obtain the require information using the underwater subsea systems the machenacism that is used underwater and the sensor nodes and vehicles should possess self-configurabel capabilities. As like they must be coordinating their operation among them selfs by exchanging configuration or location and movement information, and should transfer the observation data to the onshore station. So we go for the underwater acoustic system which consists of number of sensors and vehicles self-organisation in an autonomous network which could adapt to the required characteristics of the ocean environment.

One of the most used typical physical layer technology in uunderwater communication is acoustic communication.as the electromagnetic subsea systems fail to propagate at higher frequency and could only propagate at lower frequency like (30-300 Hz) wich would still require large anteena and high transmission power.but were as in optical systems they do not suffer from such high attenuation but they also suffer from the scattering of the signal.and also they require high precision in pointing the narrowlaser beams so most of the underwater systems are acoustic systems .( http://www.ece.gatech.edu/research/labs/bwn/UWASN/index.html)

Underwater acoustic communications are mainly influenced by path loss, noise, multi-path, Doppler spread, and high and variable propagation delay. All the above factors determine the temporal and spatial variability of the acoustic channel, and make the available bandwidth of the Underwater Acoustic channel (UW-A) limited and dramatically dependent on both range and frequency. Long-range systems that operate over several tens of kilometres may have a bandwidth of only a few kHz, while a short-range system operating over several tens of meters may have more than a hundred kHz bandwidth. In both cases these factors lead to low bit rate. Underwater acoustic communication links can be classified according to their range as very long, long, medium, short, and very short links. Acoustic links are also roughly classified as vertical and horizontal, according to the direction of the sound ray.

Factors that influence acoustic communications:-

* Path loss

1. Attenuation.

Is mainly due to absorption due to conversion of acoustic energy into heat, which increases with distance and frequency. It is also caused by scattering and reverberation, refraction, and dispersion .Water depth plays a key role in determining the attenuation.

2. Geometric Spreading

. This refers to the spreading of sound energy as a result of the expansion of the wave fronts. It increases with the propagation distance and is independent of frequency. There are two common kinds of geometric spreading: spherical (Omni-directional point source), and cylindrical (horizontal radiation only).

* Noise

1. Man made noise.

This is mainly caused by machinery noise and shipping activity especially in areas encumbered with heavy vessel traffic.

2. Ambient Noise.

Is related to hydrodynamics like movement of water including tides, current, storms, wind, rain and seismic and biological phenomena.

* Multi-path

1. Multi-path propagation may be responsible for severe degradation of the acoustic communication signal, since it generates Inter-Symbol Interference (ISI).

2. The multi-path geometry depends on the link configuration. Vertical channels are characterized by little time dispersion, whereas horizontal channels may have extremely long multi-path spreads.

3. The extent of the spreading is a strong function of depth and the distance between transmitter and receiver.

* High delay and delay variance

1. The propagation speed in the UW-A channel is five orders of magnitude lower than in the radio channel. This large propagation delay (0.67 s/km) can reduce the throughput of the system considerably.

2. The very high delay variance is even more harmful for efficient protocol design, as it prevents from accurately estimating the round trip time (RTT), which is the key parameter for many common communication protocols.

* Doppler spread

1. The Doppler frequency spread can be significant in UW-A channels, causing degradation in the performance of digital communications: transmissions at a high data rate cause many adjacent symbols to interfere at the receiver, requiring sophisticated signal processing to deal with the generated ISI.

2. The Doppler spreading generates: i) a simple frequency translation, which is relatively easy for a receiver to compensate for; ii) a continuous spreading of frequencies, which constitutes a non-shifted signal, which is more difficult for a receiver to compensate for.

3. If a channel has a Doppler spread with bandwidth B and a signal has symbol duration T, then there are approximately BT uncorrelated samples of its complex envelope. When BT is much less than unity, the channel is said to be under spread and the effects of the Doppler fading can be ignored, while, if greater than unity, it is overspread.

Most of the described factors are caused by the chemical-physical properties of the water medium such as temperature, salinity and density, and by their spatial-temporal variations. These variations, together with the wave guide nature of the channel, cause the acoustic channel to be temporally and spatially variable. In particular, the horizontal channel is by far more rapidly varying than the vertical channel, in both deep and shallow water.

Internal architecture of an underwater sensor node:-

Advantages of this acoustic approach are:-

* No direction limitations the sound signal could be transmitted in all the directions

* The high spped of data transfer from transmitter to receiver

* Not effected by pollutants, oxidation, and roughness as well

* Acoustic is mainly used for long distance propagation

This approach has the following disadvantages:-

* Real time monitoring is not possible:- The recorded data cannot be accessed until the instruments are recovered which may even take several months in the process

* If failure or misconfiguration occur it may not be possible to detect them before the instruments are recovered this would lead to complete failure of the system

* The amount of data that can be stored would be limited depending upon the memory device installed in the system

* Low frequency due to i2r looses

* The physical setup of the acoustic system would huge

* The bandwidth of the channel is limited or short bandwidth