Underwater channel Underwater channel modellingmodelling

By: Supriya S. Ankushe Under Guidance of

Dr.D.D.Doye

INDEX:

IntroductionChannel modelling Additive noise channel model Deficiency in current communication Acoustic communication channel modellingOptical communication channel modellingApplicationsReferences

Introduction:

Underwater is a very challenging medium for communication,due to some of its limitations.

In recent years, underwater communication technologies have progressed rapidly ,with the development of acoustic modulation and demodulation , optical modulation and demodulation.

Underwater communication networks are the important means to achieve marine monitoring ,data acquisition and strategic communications.

Technique of sending and receiving message below water.

Most commonly employed using hydrophones.

Channel Modelling :

In order to evaluate the effectiveness of a given channel some coding and processing techniques must be applied before construction.

Such analysis reduces the cost of developing a complex system by reducing the amount of hardware .

Basic additive noise channel model:

Mathematically given by:y(t) = x(t) h(t) + n(t) ⊗

Where, x(t) is the input sequence ,h(t)is the impulse response and n(t) is the additive noise. Y(t) is the output sequence.The symbol denotes the convolution.⊗

. Electromagnetic waves can not travel over longer distances in seawater.

Radio waves propagate under water at extremely low frequencies (30Hz-300Hz) & require large antennas and high transmission power.

Optical waves do not suffer much attenuation but are affected by scattering.

Acoustic waves are the single best solution for communicating Under water.

Deficiency in current communication :

Underwater channel models:

1. Acoustic channel model

2. Optical channel model

Acoustic Communication

Science which deals with the study of all mechanical waves in solid ,liquid and gas including topics such as ultrasound, sound or vibration.

It is a branch of physics concerned with properties of sound ,it may include absorption, control, production and transmission of sound.

Underwater Acoustics is the study of propagation of sound in water & interaction of mechanical waves that constitute with water & its boundaries.

Typical frequencies associated with Underwater Acoustics are 10Hz to 1MHz.

The propagation of sound in the ocean at frequencies lower than 10 Hz is not possible.Frequencies above 1 MHz are rarely used because they are absorbed very quickly.

Underwater Acoustics is also known as HYDROACOUSTICS.

About Acoustic communication:

Basic acoustic communication model :

It is used to transmit data through underwater.

Converts digital data into special underwater sound signals.

These signals are then received by a second acoustic modem and converted back into digital data.

Can be used for underwater telemetry, diver communications and underwater monitoring .

This technology is important because it provides an accurate and efficient means to send and receive data in underwater.

Role of Acoustic modem :

Acoustic channel modelling:

1.Loss modelling: The acoustic energy of a sound wave is lost due to scattering and absorption phenomenon.

a)Absorption: It does not depend only on the distance between the transmitter and receiver but also on the signal frequency. The signal frequency determines the absorption loss which occurs because of the transfer of acoustic energy into heat.

b)Path loss: Path loss that occurs in an underwater acoustic channel over a distance L for a Signal of frequency f is given by equation as

Where, A0 is a unit-normalizing constant, k is the spreading factora(f) is the absorption coefficient

c)Scattering: Scattering is a mechanism for loss, interference and fluctuation. A rough sea surface causes attenuation of the mean acoustic field propagating in the ocean waveguide. The attenuation increases with increasing frequency.

Loss due to the wave scattering in the surface is given by:

It is based on the Gaussian normal distribution function for the surface displacement variable. Here, k denotes wave number, h denotes the RMS height of the particle, phi is the angle of collision to the normal surface, R is the pressure reflection for the normal

Advantages of acoustic communication:

1.Low loss in the Water 2.Range of communication can be very long

Limitations of acoustic communication :

1. Limited bandwidth available 2. Multipath propogation 3. Propogation loss 4. The diversity of an underwater channel due to time invariant nature of underwater channel 5. Can only handle a relatively low bit rate 6. The acoustic wave speed in water is slow (1500 m/s), which results in latency in communication.

UW optical communications is a rapidly growing area of research, Compared to acoustics, optics can provide orders of magnitude more bandwidth (megabits per second to gigabits per second) for high-speed data transfer over short ranges.

Optical wireless communication is possible in water, especially in the blue/green light wavelengths because it suffers less attenuation in water compared to other colors.

Underwater wireless Optical communication

Optical Technology :

Transmitter devices:

LEDs and laser diodes(LDs) are a popular choice among hardware designers due to their size, cost, and ease of use. solid state lasers can be used for high power densities.

Receiver devices:

Large-aperture, high-efficiency, high-gain, and high-speed photodetectors are desirable.Recently, high-speed (1 GHz) photomultiplier tubes(PMTs),avalanche photodiodes (APDs), or hybrid designs with high gain ( 10 ) and large apertures ( 25 mm) have become commercially available.

Advantages of optical communication:

1.Higher bit rate 2.Longer bandwidth 3.Propogates faster in water

Limitations of optical channel :

1.Absorption

The absorption phenomenon is due to both the background media which is sea water and the suspended particulates in sea water.

In clear water, such as in the deep open ocean, absorption is the dominant source of loss. Links operating in these environments are typically characterized as “photon limited.”

2.Scattering :

First, it attenuates the transmitted signal reducing signal to noise ratio. Second, it creates the inter-symbol-interference (ISI) effect corrupting the signal waveform.

In turbid waters, such as in the littoral, coastal, and near-shore regions, scattering is the dominant source of attenuation. Links operating in these environments are classified as “dispersion limited.”

Two types of dispersion exist :

1.Spatial dispersion: which spreads and attenuates the transmitted laser beam in space.

2. Temporal dispersion: The path-length differences between scattered photons, or between scattered and nons cattered photons, result in a temporal dispersion.

Modeling the wireless under water optical communication :

1.Using extinction factor:It is the reduction of the intensity of light as a function of optical distance.Extinction factor formula:

μσ =ασ (gas )+ sσ (gas )+ασ (aerosol)+sσ (aerosol)

In general, the extinction coefficient μσ includes both the absorption coefficient ασ and the scattering coefficient sσ, of both the gas and the aerosols present in the gas:

Advantages:

Can capture only the effect of signal to-noise ratio on the communication channel. Disadvantages:

Dispersion effect is not considered.

2.Using Radiative transfer equation:

The RTE is a differential equation describing radiance. RTE states that a beam of light loses energy through divergence and extinction(including both absorption and scattering away from the beam) and gains energy from light sources in the medium and scattering directed towards the beam .

Vector radiative transfer (VRT) theory which can capture both the attenuation and multiple scattering effects also takes into account the polarization effect.

The specific intensity of radiation is the energy flux per unit time, unit frequency, unit solid angle and unit area normal to the direction of propagation.

The radiative transfer equation states that the specific intensity of radiation Iσ during its propagation in a medium is subject to losses due to extinction and to gains due to emission.

Mathematically RTE is given by:

Where, x is the co-ordinate along the optical path μσ is the extinction coefficient ρ is the mass density jσ is the emission coefficient per unit mass.

The important parameter for the vector radiative transfer equation is the Mueller matrix which describes the scattering characteristics of the channel.

To calculate the Mueller matrix,the characteristics of scattering particles which are the index of refraction, the particle density, and the particle distribution are required.

In general,Muller matrix is given by:

S = [ S1 S2 S3 S4 ]

where the submatrices S1 , S2 ,S3 , and S4 are given by

Mueller matrix

Solutions to the RTE generally fall into two categories:

1.Numerical or analytical: Numerical techniques, while highly accurate, tend to be computationally complex.

2.Analytical techniques: These are fast and far less computationally complex.

Applications:

Marine monitoring

Data acquisition

Underwater data links can be combined with satellite data links to provide data in real-time from instruments on the seafloor to scientists ashore.

Can be used to provide early warnings of tsunamis generated by undersea earthquakes. Pressure sensors that are deployed on the seafloor can detect tsunamis.

U.S. National Oceanic and Atmospheric Administration (NOAA) Deep-ocean Assessment and Reporting of Tsunamis (DART) program has installed bottom pressure sensors near regions with a history of tsunami generation, to measure waves as they spread.

Future scope:

1.Hybrid technology ,combination of acoustic and optical channel model must be developed so as to improve the performance.

2. Additionally, pollution control, climate recording, ocean monitoring (for prediction of natural disturbances) and detection of objects on the ocean floor are other areas that could benefit from enhanced underwater communications.  

Conclusion:

Despite much development in this area of the underwater wireless communication, Despite much development in this area of the underwater wireless communication, the main objective is to overcome the present limitations and implement advanced the main objective is to overcome the present limitations and implement advanced technology for oceanographic research and cope up with the environmental effects technology for oceanographic research and cope up with the environmental effects on the noise performance of underwater systems to compete with the future on the noise performance of underwater systems to compete with the future challenges like effective transmission of audio and video signals etc.challenges like effective transmission of audio and video signals etc.

References:

[1]Sermsak Jaruwatanadilok, Member, IEEE, “Underwater Wireless Optical CommunicationChannel Modeling and Performance Evaluation using Vector Radiative Transfer Theory”, IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 26, NO. 9, DECEMBER 2008

[2] Chantri Polprasert, Member, IEEE, James A. Ritcey, Fellow, IEEE, and Milica Stojanovic, Fellow, IEEE, “ Capacity of OFDM Systems Over Fading Underwater Acoustic Channels”, IEEE JOURNAL OF OCEANIC ENGINEERING, VOL. 36, NO. 4, OCTOBER 2011.

[3] “ UComms: A Conference and Workshop on Underwater Communications, ChannelModeling, and Validation”, IEEE JOURNAL OF OCEANIC ENGINEERING, VOL. 38, NO. 4, OCTOBER 2013 603.

[4] Henry S. Dol, Mathieu E. G. D. Colin, Michael A. Ainslie, Paul A. van Walree, Member, IEEE, and Jeroen Janmaat, “ Simulation of an Underwater Acoustic Communication Channel Characterized byWind-Generated Surface Waves and Bubbles”, IEEE JOURNAL OF OCEANIC ENGINEERING, VOL. 38, NO. 4, OCTOBER 2013