underwater acoustic propogation channels


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underwater acoustic propogation channels

  1. 1. Propagation and Scattering Effects in Underwater Acoustic Communication Channel SEMINAR PRESENTATION ON SUBMITTED BY SHUDHANSHU SINGH 1104331044 EC 6TH SEMESTER Underwater Acoustic Communication Channel 1
  2. 2. UNDER THE GUIDANCE OF PROF. J.P. SAINI Underwater Acoustic Communication Channel 2
  3. 3. CONTENT  Traditional approach for ocean bottom monitering  Sound as a wireless medium  BW limitations  Variations in speed of sound  Multipath Propogation  Noise  Scattering  Propogation speed  Signal Processing  Underwater Applications  Challenges Underwater Acoustic Communication Channel 3
  4. 4. Traditional approach for ocean- bottom monitoring Deploy underwater sensors to record data during the monitoring mission, and then recover the instruments. Disadvantages : • Real time monitoring is not possible. • No interaction is possible between onshore control systems and the monitoring instruments. • If failures or misconfigurations occur, it may not be possible to detect them before the instruments are recovered. • The amount of data that can be recorded during the monitoring mission by every sensor is limited by the capacity of the onboard storage devices (memories, hard disks, etc). Underwater Acoustic Communication Channel 4
  5. 5. Use sound as the wireless communication medium • Radio waves propagate at long distances through conductive sea water only at extra low frequencies (30-300 Hz), which require large antennae and high transmission power. • Optical waves do not suffer from such high attenuation but are affected by scattering. Moreover, transmission of optical signals requires high precision in pointing the narrow laser beams. Underwater Acoustic Communication Channel 5
  6. 6. UNDERWATER ACOUSTIC COMMUNICATION SYSTEM Underwater Acoustic Communication Channel 6
  7. 7. BANDWIDTH LIMITATIONS Absorption coefficient increases rapidly with frequency: fundamental bandwidth limitation. Only very low frequencies propagate over long distances Underwater Acoustic Communication Channel 7
  8. 8. depth c surface layer (mixing) const. temperature (except under ice) main thermocline temperature decreases rapidly deep ocean constant temperature (4 deg. C) pressure increases Sound speed increases with temperature, pressure, salinity. continental shelf (~100 m) continental slice continental rise abyssal plain land sea surf shallow deep Variations in speed of sound Underwater Acoustic Communication Channel 8
  9. 9. • Multipath structure depends on the channel geometry, signal frequency, sound speed profile. •Models are used to obtain a more accurate prediction of the signal strength. • Ray model provides insight into the mechanisms of multipath formation: deep water — ray bending shallow water — reflections from bottom. Multipath propagation Underwater Acoustic Communication Channel 9
  10. 10. Underwater Acoustic Communication Channel 10 Mechanisms of multipath formation • Deep water: a ray, launched at some angle, bends towards the region of lower sound speed (Snell’s law). • Continuous application of Snell’s law  ray diagram (trace). tx distancec Rays bend repeatedly towards the depth at which the sound speed is minimal.
  11. 11. Underwater Acoustic Communication Channel 11 Shallow water: reflections at surface have little loss; reflection loss at bottom depends on the type (sand,rock, etc.), angle of incidence, frequency. tx rx Multipath gets attenuated because of repeated reflection loss, increased path length.
  12. 12. NOISE Ambient (open sea) •turbulence •shipping •surface •thermal Site-specific: •man-made •biological (e.g., shrimp) •ice cracking, rain •seismic events Underwater Acoustic Communication Channel 12
  13. 13. SCATTERING Water surface Fish shoaling Bubbles Underwater Acoustic Communication Channel 13
  14. 14. Underwater Acoustic Communication Channel 14 Nominal: c=1500 m/s (compare to 3 x 108 m/s) Two types of problems: • Motion-induced Doppler distortion (v~ few m/s for an AUV) • Long propagation delay. Propagation speed tt(1 v/c) ff(1±v/c) DOPPLER EFFECT
  15. 15. Underwater Acoustic Communication Channel 15 • Bandwidth-efficient modulation (PSK, QAM) • Phase-coherent detection • Synchronization • Equalization • Multichannel combining Signal processing
  16. 16. Underwater Acoustic Communication Channel 16 inp. K com- biner forward forward + _ decision feedback adaptation algorithm inp.1 inp.2 data out sync. filter coefficients training data data est.
  17. 17. Challenges • Battery power is limited and usually batteries can not be recharged because solar energy cannot be exploited. • The available bandwidth is severely limited. • Channel characteristics, including long and variable propagation delays, multi-path and fading problems. • High bit error rates. • Underwater sensors are prone to failures because of fouling, corrosion, etc. • A unique feature of underwater networks is that the environment is constantly mobile, naturally causing the node passive mobility. • The ocean can be as deep as 10 km. Underwater Acoustic Communication Channel 17
  18. 18. Underwater applications  Seismic monitoring,  Pollution monitoring,  Ocean currents monitoring,  Equipment monitoring and control,  Autonomous Underwater Vehicles (AUV). To make these applications viable, there is a need to enable underwater communications among underwater devices. Underwater Acoustic Communication Channel 18
  19. 19. REFRENCES  Paul A. van Walree, Member, IEEE, “Propagation and Scattering Effects in Underwater Acoustic Communication Channels,” IEEE JOURNAL OF OCEANIC ENGINEERING, VOL.38, NO.4, OCTOBER 2013  Kalangi Pullarao Prasanth, Modelling and Simulation of an Underwater Acoustic Communication Channel, THESIS, January 2013  Thomas J. Hayward and T. C. Yang, Underwater Acoustic Communication Channel Capacity: A Simulation Study, Naval Research Laboratory, Washington, DC 20375  Milica Stojanovic, Northeastern University, Underwater Acoustic Communication Channels: Propagation Models and Statistical Characterizatio, James Preisig, Woods Hole Oceanographic Institution Underwater Acoustic Communication Channel 19
  20. 20. THANK YOU Underwater Acoustic Communication Channel 20