Optical Wireless Communications

Optical Wireless
Prof. Brandt-Pearce
Lecture 7
Underwater, Inter-Satellite and
Satellite-to-Underwater Optical
 Underwater Optical Communications
 Introduction
 Underwater Channel
 Challenges
 Inter-Satellite Optical Communications
 Satellite-to-Underwater Optical Communications
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
UW Channel
 The physical properties of water is important in modeling the
 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
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.
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
UW Channel
 Absorption, elastic and inelastic scattering:
 Absorption:
aw = absorption of pure water
aoc = specific absorption of chlorophyll
ay = specific of yellow substance (acids)
UW Channel
 The spectral transmittance for various water types
Absorption in UW Channel
 Pure seawater is absorptive except around a 400nm-500nm
window, the blue-green region of the visible light spectrum
Absorption in
Natural Water
“Absorption and scattering of light
in natural waters”
Vladimir I. Haltrin
Scattering in UW Channel
 Scattering in pure seawater is larger for shorter wavelengths
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
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
 : average transmitter optical power
  : optical efficiency of the transmitter
 :optical efficiency of the receiver
d: perpendicular distance between the transmitter and the receiver
:angle between the perpendicular to the receiver plane and the transmitterreceiver trajectory
  : receiver aperture area
 0 : laser beam divergence angle
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, defined by inner
and outer angles  and  in the upward direction
  : angles of incidence
  : angles of transmission
 Since the refractive index of air is lower than that of water, total internal
reflection can be achieved above a critical incidence angle

  = arcsin

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
 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
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
Impulse Response of UW Communications
Low scattering
High scattering
Medium scattering
J. Li, et. al., “Channel capacity
study of underwater wireless
optical communications links
simulation” , Journal of Optics,
J. Opt. 14 (2012) 015403 (7pp)
UW Communications
 Modulation Techniques
 Modulation techniques with high-spectral efficiencies are desired
 Spectral encoding modulations can only be done in blue to green
 Non-coherent or differentially phase encoded modulations are
preferred: OOK, PPM, DPSK
 Applications
 Submarine communications
 Underwater sensor networks
 Underwater Optical Communications
 Introduction
 Underwater Channel
 Challenges
 Inter-Satellite Optical Communications
 Satellite-to-Underwater Optical Communications
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
 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
 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
(between Lagrange Points of Interplanetary Space to Earth Stations
or GEO)
 First Generation of terminals were in 800-850 nm band-
ASK(PPM)-Direct Detection
 Second Generation were in 1064 nm BPSK, Coherent
 1550nm, ASK, Direct Detection has been studied and
demonstrated on ground
Challenges and Advantages
 Challenges:
 Galactic cosmic rays
 Solar wind high energy particles
 Magnetically trapped charged particles dependant on solar
 Thermal variations
 Advantages
 No turbulence
 No multipath effect
 No fading
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
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
Satellite-to-Underwater Optical
 Communication from satellite to submarine has always been a
 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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