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Optical Wireless Communications
Prof. Brandt-Pearce Lecture 7 Underwater, Inter-Satellite and Satellite-to-Underwater Optical Communications
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Outline Underwater Optical Communications
Introduction Underwater Channel Challenges Inter-Satellite Optical Communications Satellite-to-Underwater Optical Communications
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Underwater (UW) Optical Communications
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.
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UW Channel The physical properties of water is important in modeling the 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
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UW Channel The topmost layer is called the euphotic zone and is defined by how deeply photosynthetic life can be found 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.
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UW Channel The various water types are divided into two categories: oceanic (blue water) and coastal waters (littoral zone). 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
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UW Channel Absorption, elastic and inelastic scattering: Absorption:
aw = absorption of pure water aoc = specific absorption of chlorophyll ay = specific of yellow substance (acids)
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UW Channel The spectral transmittance for various water types
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Absorption in UW Channel
Pure seawater is absorptive except around a 400nm-500nm window, the blue-green region of the visible light spectrum Blue Green
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Absorption in Natural Water
βAbsorption and scattering of light in natural watersβ Vladimir I. Haltrin
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Scattering in UW Channel
Scattering in pure seawater is larger for shorter wavelengths
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UW Link Geometries UW can be implemented in three different forms
Line-of sight (LOS) Reflective Non-line-of-sight (NLOS) 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
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UW Link Geometries: LOS
The optical signal reaching the receiver is obtained by multiplying the transmitter power, telescope gain, and losses and is given by π π : average transmitter optical power π π : optical efο¬ciency of the transmitter π π
:optical efο¬ciency of the receiver d: perpendicular distance between the transmitter and the receiver π:angle between the perpendicular to the receiver plane and the transmitter-receiver trajectory π΄ π
ππ : receiver aperture area π 0 : laser beam divergence angle
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UW Link Geometries: Reflective
The UW reflective optical communications uses total internal reflection to transmit signal to the receiver In some communication scenarios the line of sight is not available In this case, the laser transmitter emits a cone of light, deο¬ned by inner and outer angles π πππ and π πππ₯ in the upward direction π π : angles of incidence π π‘ : angles of transmission Since the refractive index of air is lower than that of water, total internal reο¬ection can be achieved above a critical incidence angle π π = arcsin π πππ π π€ππ‘ππ
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UW Link Geometries: NLOS
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. 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
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Challenges of UW Communications
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 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
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Impulse Response of UW Communications
Low scattering Medium scattering J. Li, et. al., βChannel capacity study of underwater wireless optical communications links based on Monte Carlo simulationβ , Journal of Optics, J. Opt. 14 (2012) (7pp) High scattering
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UW Communications Modulation Techniques Applications
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
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Outline Underwater Optical Communications
Introduction Underwater Channel Challenges Inter-Satellite Optical Communications Satellite-to-Underwater Optical Communications
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Inter-Satellite Optical Communications
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
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Applications 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)
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Technologies 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
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Challenges and Advantages
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
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Pointing and Tracking 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
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Cooperative Optical Beam Tracking
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
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Satellite-to-Underwater Optical Communications
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
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