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Outline Introduction
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? 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?
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
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 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
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
Class 1 eye safety requirement for lasers used indoors Array of LEDs are used
Class 3B eye safety requirement for laser used outdoors 1550 nm lasers are generally chosen for this purpose
Classifies light sources depending on the amount of power they emit
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Receiver
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 Physical Obstruction Atmospheric Losses
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 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 short-
duration interruptions can be handled by higher layers using packet retransmission
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Free space loss 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 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 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 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 Movements of buildings upsets transmitter-
receiver alignment
Solution: Use slightly divergent beam
Divergence of 3-6 milliradians will have diameter of 3-6 m after traveling 1km
Low cost Active tracking
Feedback mechanism to continuously align transmitter- receiver lenses
Facilitates accelerated installation, but expensive
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Empirical Design Principles 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
Requires line-of-sight Limited range (max ~8km) Unreliable bandwidth availability
BER depends on weather conditions Accurate alignment of transmitter-
receiver necessary
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Topology Control and Routing Given:
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:
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
The algorithm consists of two parts: Offline phase
It computes the sub-graph Gives the routes and bandwidth reservation
for every ingress-egress pair in the traffic profile
Online phase Uses the topology computed in offline phase
to exercise admission control Routes individual flows
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Solution Strategy (cont)
The order in which the traffic demands are considered for link formation decides the throughput of the system
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
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Basic Rollout Algorithm
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)
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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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)
Basic Rollout Algorithm (Math)
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Path Computation 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 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
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 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 Let:
N = # of nodes M = # of communicating ingress-egress pairs k = # of paths calculated for each
communicating pair Computational Complexity:
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 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 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/