Optical Wireless
An Overview
Chintan Shah
cmshah@cse.buffalo.edu
December 9, 2004
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Outline
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Introduction
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What is Optical Wireless?
Applications
Transmitter and Receiver
Topologies
Challenges and Limitations
Topology Control and Routing
Conclusion
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What is Optical Wireless?
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Optical Wireless a.k.a. Free Space
Optics (FSO) refers to the transmission
of modulated light beams through the
atmosphere to obtain broadband
communication
Line-of-sight technology
Uses lasers/LEDs to generate coherent
light beams
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What is Optical Wireless?
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Data rates of up to 2.5 Gbps at
distances of up to 4km available in
commercial products
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Last Mile problem
Connecting the user directly to
the backbone high speed fiber
optic network is known as the
Last Mile problem
 FSO as the low cost bridging
technology
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More Applications
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Allows quick Metro network extensions
Interconnecting local-area network
segments spread across separate
buildings (Enterprise connectivity)
Fiber backup
Interconnecting base stations in cellular
systems
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Transmitter
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FSO uses the same transmitter
technology as used by Fiber Optics
Laser/LED as coherent light source
Wavelengths centered around
850nm and 1550nm widely used
Telescope and lens for aiming light
beam to the receiver
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Safety while
using Lasers
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Eye Safety
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Classifies light sources depending on the amount of power they emit
650 nm
(visible)
880 nm
(infrared)
1310 nm
(infrared)
1550 nm
(infrared)
Class 1
Up to 0.2 mW
Up to 0.5 mW
Up to 8.8 mW
Up to 10 mW
Class 2
0.2-1 mW
N/A
N/A
N/A
Class 3A
1-5 mW
0.5-2.5 mW
8.8-45 mW
10-50 mW
Class 3B
5-500 mW
2.5-500 mW
45-500 mW
50-500 mW
Table1: Laser safety classification for point-source emitter
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Class 1 eye safety requirement for lasers used indoors
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Array of LEDs are used
Class 3B eye safety requirement for laser used outdoors
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1550 nm lasers are generally chosen for this purpose
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Receiver
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Photodiode with large active area
Narrowband infrared filters to reduce
noise due to ambient light
Receivers with high gain
Bootstrap receivers using PIN diode and
avalanche photodiode (APD) used
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Simplified Transceiver Diagram
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Point-to-Multipoint Topology
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Point-to-Point Topology
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Ring with Spurs Topology
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Mesh Topology
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Typical Topology in a Metro
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Challenges
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Physical Obstruction
Atmospheric Losses
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Free space loss
Clear air absorption
Weather conditions (Fog, rain, snow, etc.)
Scattering
Scintillation
Building Sway and Seismic activity
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Physical Obstruction
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Construction crane or flying bird comes in
path of light beam temporarily
Solution:
 Receiver can recognize temporary loss of
connection
 In packet-switched networks such shortduration interruptions can be handled by
higher layers using packet retransmission
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Free space loss
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Proportion of transmitted
power arriving at the
receiver
Occurs due to slightly
diverging beam
Solution:
 High receiver gain and large receiver aperture
 Accurate pointing
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Clear Air Absorption
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Equivalent to absorption loss in optical fibers
Wavelength dependent
Low-loss at wavelengths ~850nm, ~1300nm
and ~1550nm
Hence these wavelengths are used for
transmission
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Weather Conditions
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Adverse atmospheric conditions increase Bit Error
Rate (BER) of an FSO system
Fog causes maximum attenuation
Water droplets in fog modify light characteristics or
completely hinder the passage of light
Attenuation due to fog is known as Mie scattering
Solution:
 Increasing transmitter power to maximum allowable
 Shorten link length to be between 200-500m
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Scattering
Caused by collision of
wavelength with particles in
atmosphere
 Causes deviation of light beam
 Less power at receiver
 Significant for long range
communication
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Scintillation
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Caused due to different refractive indices of small air
pockets at different temperatures along beam path
Air pockets act as prisms and lenses causing
refraction of beam
Optical signal scatters preferentially by small angles
in the direction of propagation
Distorts the wavefront of received optical signal
causing ‘image dancing’
Best observed by the simmering of horizon on a hot
day
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Scintillation (cont…)
Solution:
 Large receiver diameter to cope with
image dancing
 Spatial diversity: Sending same
information from several laser
transmitters mounted in same housing
 Not significant for links < 200m apart,
so shorten link length
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Building Sway and Seismic activity
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Movements of buildings upsets transmitter-receiver
alignment
Solution:
 Use slightly divergent beam
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Divergence of 3-6 milliradians will have diameter of 3-6 m
after traveling 1km
Low cost
Active tracking
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Feedback mechanism to continuously align transmitterreceiver lenses
Facilitates accelerated installation, but expensive
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Empirical Design Principles
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Use lasers ~850 nm for short distances and
~1550 nm for long distance communication
with maximum allowable power
Slightly divergent beam
Large receiver aperture
Link length between 200-1000m in case of
adverse weather conditions
Use multi-beam system
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Limitations of FSO Technology
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Requires line-of-sight
Limited range (max ~8km)
Unreliable bandwidth availability
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BER depends on weather conditions
Accurate alignment of transmitterreceiver necessary
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Topology Control and Routing
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Given:
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Virtual topology: List of backbone nodes and
potential links, Directed Graph G = (V, E)
Number of interfaces a node can have
Traffic profile of aggregate traffic demands
between different source destination pairs
Required:
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Optimal topology for maximizing the throughput
from the traffic profile, i.e. subgraph G’ = (V, E’)
so that interface and capacity constraints are met
and network has maximum throughput
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Solution Strategy
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The algorithm consists of two parts:
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Offline phase
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It computes the sub-graph
Gives the routes and bandwidth reservation for
every ingress-egress pair in the traffic profile
Online phase
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Uses the topology computed in offline phase to
exercise admission control
Routes individual flows
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Solution Strategy (cont)
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Task of finding sub-graph that will maximize throughput while
restricting degree of each vertex is computationally prohibitive
Hence, Rollout algorithm is used to obtain near optimal solution
The order in which the traffic demands are considered for link
formation decides the throughput of the system
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Basic Rollout Algorithm
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General method for obtaining an
improved policy for a Markov decision
process starting with a base heuristic
policy
One step look ahead policy, with the
optimal cost-to-go approximated by the
cost-to-go of the base policy
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Basic Rollout Algorithm (Math)
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Consider problem: Maximize G(u) over
set of feasible solutions U and each
solution consist of N components u =
(u1, u2, …, uN)
The base-heuristic algorithm (H)
extends a partial solution (u1, u2, …,
uk), (k < N) to a complete solution (u1,
u2, …, uN)
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Basic Rollout Algorithm (Math)
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Thus, H(u1, u2, …, uk) = G (u1, u2, …,
uN )
The rollout algorithm (R) takes a partial
solution (u1, u2, …, un-1) and extends it
by one component. Thus,
R(u1, u2, …, un-1) = (u1, u2, …, un)
where un is chosen so as to maximize
H(u1, u2, …, un)
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Path Computation
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Find k paths for each entry in traffic profile
i = 0, d0 = 0,
d = aggregate demand for this ingress-egress pair
Repeat following until we cannot find a path or whole demand is
routed or i = k
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Find a path using constrained shortest path first (CSPF) that
accommodates (d-di)/(k-i), bandwidth and finalize links temporarily
Constraints are limited transmitter-receiver interfaces and limited
link capacity
Route as much bandwidth of this demand on this route, call it di+1,
Decrement link capacity by di+1, and i = i + 1
This algorithm routes whatever we can on these paths
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Base Heuristic
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Partial topology by routing demands
(t1,…,tn) is formed
The base heuristic routes the remaining
demands in decreasing order of
magnitude
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Index Rollout Algorithm
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Suppose demands (t1,…,tn) have been
routed
For all possible next candidate
demands, throughput is calculated
using base heuristic
tn+1 is chosen as the one that produces
maximum throughput when base
heuristic is used to route remaining
demands
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Comments
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Let:
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Computational Complexity:
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N = # of nodes
M = # of communicating ingress-egress pairs
k = # of paths calculated for each communicating
pair
Offline phase: O(kM3N2), for constant number of
communicating ingress-egress nodes
Online phase: O(k)
The base heuristic is such that the rollout
works at least as good as the heuristic
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Conclusion
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This presentation gave an overview of Optical
Wireless technology
We started with applications of FSO to provide
motivation for its study
Transmitter and receiver designs were discussed
We looked at the challenges faced by this technology
and techniques to deal with them
Finally had a brief look at the problem of Topology
Control and routing of Bandwidth Guaranteed flows
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References
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D.J.T.Heatley, D.R.Wisely, I.Neild and P.Cochrane, “Optical wireless:
The story so far”, IEEE Communications Magazine 36(12) (1998)
72-82
H.A.Willebrand and B.S.Ghuman, “Fiber Optics Without Fiber”, IEEE
Spectrum Magazine, August 2001, pp 40-45.
A.Kashyap, M.K.Khandani, K.Lee, M.Shayman, “Profile-Based
Topology Control and Routing of Bandwidth-Guaranteed Flows in
Wireless Optical Backbone Networks”, University of Maryland
http://www.freespaceoptics.org/
http://http://www.fsona.com/
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