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1 Prof. Brandt-Pearce Lecture 7 Underwater, Inter-Satellite and Satellite-to-Underwater Optical Communications Optical Wireless Communications.

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Presentation on theme: "1 Prof. Brandt-Pearce Lecture 7 Underwater, Inter-Satellite and Satellite-to-Underwater Optical Communications Optical Wireless Communications."— Presentation transcript:

1 1 Prof. Brandt-Pearce Lecture 7 Underwater, Inter-Satellite and Satellite-to-Underwater Optical Communications Optical Wireless Communications

2 2 Outline  Underwater Optical Communications Introduction Underwater Channel Challenges  Inter-Satellite Optical Communications  Satellite-to-Underwater Optical Communications

3 3  Modeling the channel is the first step in UW communications  The channel is completely different from other FSO systems  The transmitter and receiver can be very similar to aforementioned FSO systems  Ocean water has widely varying optical properties depending on location, time of day, organic and inorganic content, as well as temporal variations such as turbulence and surface motion.  To construct an optical link it is important to understand these properties. Underwater (UW) Optical Communications

4 4  The physical properties of water is important in modeling the channel UW Channel  Ocean water vary both geographically and vertically with depth  Geographically it changes from the deep blue ocean to littoral waters near land  Vertically, the amount of light that is received from the sun is used to classify the type of water.  The water depth also determines the background radiation from sun light

5 5  The topmost layer is called the euphotic zone and is defined by how deeply photosynthetic life can be found UW Channel  Below this zone is the disphotic zone (1 km deep): the light is too faint to support photosynthesis.  From the lower boundary of this zone and extending all the way to the bottom is the aphotic zone, where no light ever passes and animals have evolved to take advantage of other sources of food.

6 6  The various water types are divided into two categories: oceanic (blue water) and coastal waters (littoral zone). UW Channel  The oceanic group is subdivided into 3 groups: Type I-III  types I: extremely pure ocean water  type II: turbid tropical-subtropical water  type III: mid-latitude water  The coastal group are subdivided into Types 1 through 9  1-9: coastal waters of increasing turbidity

7 7  Absorption, elastic and inelastic scattering:  Absorption: a w = absorption of pure water a o c = specific absorption of chlorophyll a y = specific of yellow substance (acids) UW Channel

8 8  The spectral transmittance for various water types UW Channel

9 9 Absorption in UW Channel  Pure seawater is absorptive except around a 400nm-500nm window, the blue-green region of the visible light spectrum BlueGreen

10 10 Absorption in Natural Water “Absorption and scattering of light in natural waters” Vladimir I. Haltrin

11 11 Scattering in UW Channel  Scattering in pure seawater is larger for shorter wavelengths

12 12  UW can be implemented in three different forms Line-of sight (LOS) Reflective Non-line-of-sight (NLOS) UW Link Geometries  LOS: the transmitter directs the light beam in the direction of the receiver  Reflective: Receiver receives the signal after reflection from sea surface  NLOS: The power is received via scattering from particles inside water

13 13 UW Link Geometries: LOS

14 14 UW Link Geometries: Reflective

15 15  For reflective communications the receiver and transmitter need to be close to sea surface  It also requires some angle constraints; the transmitter and receiver distance have to be large compared to their depth  Hence in some situations nor LOS nor reflective communications can be used  Non-line-of-sight (NLOS) communications is the option that would be interesting for these cases. UW Link Geometries: NLOS  It is very similar to UV NLOS communications except the wavelength  The transmitted optical signal is scattered in different directions because of molecules, particles and air bubbles

16 16  Inter-symbol interference (ISI) The power scattered inside water cause dispersion on the transmitted signal Not only the first order scattering is large, higher order scatterings are also have considerable effect on the received signal The broadened pulses cause ISI ISI effect can be severe since the scattering is strong for UW Challenges of UW Communications  Background Light Since the operation wavelength is in visible range, the background radiation is strong for links that are close to surface  Scintillation and Beam Wander Because of strong turbulences, the scintillation and beam wander effect is large The channel is not reliable unless a wide transmittance angle is used

17 17 Impulse Response of UW Communications Low scatteringMedium scattering High scattering J. Li, et. al., “Channel capacity study of underwater wireless optical communications links based on Monte Carlo simulation”, Journal of Optics, J. Opt. 14 (2012) (7pp)

18 18  Modulation Techniques Modulation techniques with high-spectral efficiencies are desired Spectral encoding modulations can only be done in blue to green range Non-coherent or differentially phase encoded modulations are preferred: OOK, PPM, DPSK  Applications Submarine communications Underwater sensor networks UW Communications

19 19 Outline  Underwater Optical Communications Introduction Underwater Channel Challenges  Inter-Satellite Optical Communications  Satellite-to-Underwater Optical Communications

20 20  Optical communication is needed for connecting satellites to each other since it can provide Tb/s links  Weight of the optical system that can be mounted on satellite is limited  Lasers are used as sources because higher directivity of the optical beam allows higher data/power efficiency (more Mbps for each Watt of power)  It requires highly accurate pointing acquisition and tracking Inter-Satellite Optical Communications

21 21  Data relay (like the Tracking and Data Relay Satellites, TDRS, that served the Space Shuttle) (Mbps from a LEO/GEO satellite or aircraft to earth via another GEO satellite)  For broadband links (multi-Gigabit over thousands of km) (in Telecom Constellations among S/C in LEO/MEO/GEO)  For Space Science Links (Mbps or Kbps over millions of km) (between Lagrange Points of Interplanetary Space to Earth Stations or GEO) Applications

22 22  First Generation of terminals were in nm band- ASK(PPM)-Direct Detection  Second Generation were in 1064 nm BPSK, Coherent Detection  1550nm, ASK, Direct Detection has been studied and demonstrated on ground Technologies

23 23  Challenges:  Galactic cosmic rays  Solar wind high energy particles  Magnetically trapped charged particles dependant on solar activity  Thermal variations  Advantages  No turbulence  No multipath effect  No fading Challenges and Advantages

24 24  Pointing and tracking is the most important consideration  Due to the relative motion of the stations, an active mechanism is required to maintain optical alignment  Cooperative optical beam tracking is a viable solution in which each station employs the optical beam of the other station as a guide to point its own beam toward the other Pointing and Tracking

25 25  Transceiver structure  The stations continually measure the arrival direction of their impinging optical beams using a position-sensitive photodetector  In short range applications with negligible light propagation delay, the station transmit their optical beam along this measured direction  For a large propagation delay, the optical beams must be transmitted within a certain angle with respect to the instantaneous LOS Cooperative Optical Beam Tracking

26 26  Communication from satellite to submarine has always been a problem  This is because water is a good absorber of electromagnetic waves  Exceptions are VLF and blue-green optical waves  With VLF the depth of penetration is few tens of meters Satellite-to-Underwater Optical Communications


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