Capacity Scaling in FreeSpaceOptical Mobile AdHoc Networks

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Capacity Scaling in Free-Space-Optical Mobile
Ad-Hoc Networks
Mehmet Bilgi University of Nevada, Reno
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Agenda

• RF and FSO Basics
• FSO Propagation Model
• FSO in Literature
• Mobility Model and Alignment
• Simulation Results
• Conclusions
• Future Work

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RF and FSO Illustration
Receiver
Transmitter
Receiver
Directional FSO antenna
Transmitter
Omni-directional RF antenna

• Different natures of two technologies
  omni-directional and directional

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RF Saturation

• A well-known fact RF suffers from frequency
  saturation and RF-MANETs do not scale well
• vn as n is increased 1
• Linear scalability can be achieved with
  hierarchical cooperative MIMO 2 imposing
  constraints on topology and mobility pattern
• Omni-directional nature of the frequency
  propagation causes
• Channel is a broadcast medium, overhearing
• Security problems
• Increased power consumption to reach a given
  range
• End-to-end per-node throughput vanishes
  approaches to zero as more nodes are added
• 1 Gupta, P. Kumar, P.R. , The capacity of
  wireless networks, IEEE Transactions on
  Information Theory, 00
• 2 Ozgur et al., Hierarchical Cooperation Achieves
  Optimal Capacity Scaling in Ad Hoc Networks, IEEE
  Transactions on Information Theory, 06

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Fiber Optical Solutions

• As of 2003
• Only 5 of buildings have fiber connections
• 75 of these buildings are within 1 mile range
  of fiber
• Laying fiber to every house and business is
  costly and takes a long time
• Considered as sunk cost no way to recover
• Purchase land to lay fiber
• Digging ground
• Maintenance of fiber cable is hard
• Modulation hardware is sensitive and expensive
• ISPs are uneager to deploy aggressively because
  of initial costs
• They are deploying gradually
• Attempts existed in near past
• California, Denver, Florida (before 2000)
• 1 Source 02-146 ExParte FCC WTB Filing by Cisco
  Systems, May 16, 2003

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FSO Advantages

• Materials cheap LEDs or VCSELs with
  Photo-Detectors, commercially available, lt1 for
  a transceiver pair
• Small (1mm2), low weight (lt1gm)
• Amenable to dense integration (1000 transceivers
  possible in 1 sq ft)
• Reliable (10 years lifetime)
• Consume low power (100 microwatts for 10-100 Mbp)
• Can be modulated at high speeds (1 GHz for
  LEDs/VCSELs and higher for lasers)
• Offer highly directional beams for spatial
  reuse/security
• Propagation medium is free-space instead of
  fiber, no dedicated medium
• No license costs for bandwidth, operate at
  near-infrared wavelengths

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FSO Disadvantages

• FSO requires clear line-of-sight (LOS)
• Maintaining LOS is hard even with slight mobility
• Node often looses its connectivity intermittent
  connectivity
• Loss of connectivity is different than RFs
  channel fading
• Investigated the effects of intermittent
  connectivity on higher layers
• Especially TCP

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FSO Propagation Model

• Atmospheric attenuation, geometric spread and
  obstacles contribute to BER
• Atmospheric attenuation
• Absorption and scattering of the laser light
  photons by the different aerosols and gaseous
  molecules in the atmosphere
• Mainly driven by fog, size of the water vapor
  particles are close to near-infrared wavelength
• Braggs Law 1
• s is the attenuation coefficient, defined by Mie
  scattering
• V is the atmospheric visibility, q is the size
  distribution of the scattering particles whose
  value is dependent on the visibility
• 1 H. Willebrand and B. S. Ghuman. Free Space
  Optics. Sams Pubs, 2001. 1st Edition.

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FSO Propagation Model

• Geometric spread is a function of
• transmitter radius ?,
• the radius of the receiver ?,
• divergence angle of the transmitter ?,
• the distance between the transmitting node and
  receiving node R 1

1 H. Willebrand and B. S. Ghuman. Free Space
Optics. Sams Pubs, 2001. 1st Edition.
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FSO Literature High Speed

• Terrestrial last-mile applications
• Roof-top deployments
• Metropolitan / downtown areas
• Point-to-point high speed links
• Use high-powered laser light sources
• Use additional beams to handle swaying of
  buildings
• Gimbals for tracking the beam
• Limited spatial reuse
• Some indoor applications with diffuse optics
  (more on this later)

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FSO Literature High Speed

• Free-Space-Optical Interconnects
• Inside the large computers to eliminate latency
• Short distances(1-10s cm)
• Remedy vibrations in the environment
• Use backup beams, misalignment detectors
• Expensive, highly-sensitive tracking instruments
• Hybrid FSO/RF applications
• Consider FSO as a back-bone technology
• No one expects pure-FSO MANETs
• Single optical beam
• No effort to increase the coverage of FSO via
  spatial reuse
• Deep space communications
• 1 M. Naruse et al., Real-Time Active Alignment
  Demonstration for Free-Space Optical
  Interconnections, IEEE Photonics Tech. Letters,
  Nov. 2001

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FSO Literature

• Mobile FSO Communications
• Indoor, single room using diffuse optics
• Suitable for small distances
• Outdoor (roof-top and space) studies focus on
  swaying and vibration
• Scanning, tracking via beam steering using
  gimbals, mechanical auto-tracking
• Instruments are slow and expensive
• We propose electronical steering methods
• Effects of directional communication on higher
  layers
• Choudhury et al. worked on RF directionality,
  directional MAC
• Traditional flooding based routing algorithms are
  effected badly
• Directionality must be used for localization also
  (future work)

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Mobility Model

• Design an antenna with FSO transceivers to
• Exploit directionality and spatial reuse
• Target mobility
• Multi-element antenna using commerciallyavailable
  components
• Disconnections will still occur
• But with a reduced amount
• Recoverable with special techniques
  (auto-alignment circuit)
• Our work FSO in MANET context with mobility

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Mobility Model in NS-2

• No network simulator has FSO simulation
  capabilities
• Each transceiver keeps track of its alignments
• A table based implementation
• Alignment timers

• Example scenario
• 2 nodes with 8 interfaces each
• Node-B has relative mobility w.r.t. Node-A
• Observe the changes in alignment tables of 2
  different transceivers in two nodes

Node-B in Pos-1
Alignment tables in interface 5 of node B and
interface 1 of node A
Alignment tables in interface 4 of node B and
interface 8 of node A
Node-B in Pos-2
Alignment tables in interface 3 of node B and
interface 7 of node A
Node-B in Pos-3
Node-A
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Mobility Experiment

• Train looses and re-gains its alignment in a
  short amount of time intermittent connectivity
• Measured light intensity shows the connection
  profile
• Complete disruption of the underlying physical
  link different than RF fading
• Auto-alignment circuitry
• Monitors the light intensity in all interfaces
• Handles auto hand-off among different
  transceivers
• Initiates the search phase
• Search Phase
• When misaligned, an interfaces sends out a search
  signal (pre-determined bit sequence), freq of
  search signal
• Waits for reception
• When senses a search signal, responds it
• Interfaces restore the data transmission phase
• We want to observe TCP behaviour over FSO-MANETs

Misaligned
Aligned
Received Light Intensity from the moving train
Aligned
Misaligned
Detector Threshold
Denser packing will allow fewer interruptions
(and smaller buffering), but more handoffs.
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Simulations

• 49 nodes in a 7 x 7 grid
• Every node establishes an FTPsession to every
  other node 49x48 flows
• 4 interfaces per node, each with its own MAC
• 3000 sec simulation time
• Divergence angle 200 mrad
• Per-flow throughputs are depicted
• Random waypoint algorithm, conservative mobility
• IEEE 802.11 MAC limitation (20 Mbps)

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Stationary RF and FSO Comparison
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Stationary RF and FSO Comparison
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Mobile FSO TCP is adversely affected
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Mobile RF and FSO Comparison
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Node Density Effect

• Fixed power
• 49 nodes
• Increase the separation b/w nodes and the area
• Keep the source transmit power same
• Adjusted power
• 49 nodes
• Increase the separation b/w nodes and the area
• Adjust the source transmit power so that they can
  reach increased distance

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Node Density with Fixed Power
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Node Density with Adjusted Power
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Mobile UDP Results
UDP and TCP mobile throughput comparison
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Conclusions

• FSO MANETs are possible and provides significant
  benefit via spatial reuse
• Mobility affects TCP performance severely
• RF and FSO are complementary to each other
  coverage throughput

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Future Work

• Introduce buffers at LL and/or Network Layer
• Group concept
• Directional MAC
• Effect of search signal sending frequency

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Questions