Multi-Element Free-Space-Optical (FSO) Spherical Structures with Intermittent Connectivity Patterns

1
Multi-Element Free-Space-Optical (FSO)
SphericalStructures with Intermittent
Connectivity Patterns

• Mehmet Bilgi
• University of Nevada, Reno

2
Agenda

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

3
RF and FSO Illustration
Transmitter
Receiver
Omni-directional RF antenna
Directional FSO antenna

• Different natures of two technologies
  omni-directional and directional

4
RF Saturation

• A well-known fact RF suffers from frequency
  saturation and RF-MANETs do not scale well
• sqrt(n) as n is increased 1
• Linear scalability can be achieved with
  hierarchical cooperative MIMO 2
• Omni-directional nature of the frequency
  propagation causes
• Channel is a broadcast medium, overhearing
• 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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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

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FSO Propagation Model

• Atmospheric attenuation, geometric spread and
  obstacles contribute to BER
• Atmospheric attn. is mainly driven by fog, size
  of the water vapor particles are close to
  near-infrared wavelength

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Free-Space-Optical Communication

• Roof-top deployments in metropolitan area,
  point-to-point links via powerful lasers
• Indoor mobility with diffuse optics (10s of
  meters)
• Interconnects in short distances(1-10s cm)
• Previous work on swaying and vibration of
  buildings to tolerate disruptions
• Use gimbals, expensive tracking instruments,
  backup beams
• They do not target mobility
• Our work FSO in MANET context mobility
• Advantage spatial reuse with directional
  antennas, optical speeds, commercially available
  components, easy deployment
• Disadvantage requires clear line-of-sight,
  obstacles, mobility is a challenge

• Traditional roof-top FSO deployment

• Multi-element optical
• antenna design
• Honeycombed
• arrays of
• directional
• transceivers

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FSO-MANET gtgt Spheres gtgt Mobility

• As the mobility is introduced alignment becomes
  a challenge
• 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
• We investigated the effect of intermittent
  connectivity on higher layers especially TCP

Misaligned Aligned
Detector Threshold
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FSO-MANET gtgt Alignment re-establishment
Misaligned

• Interfaces periodically send out search signals
  (bit sequence 11010101011)
• Respond to search signals, inverse the bit
  sequence
• Restore data transfer

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FSO-MANET gtgt 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

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Mobility and Alignment Impl. In NS-2
Node-B Position - 1
Node-B Position - 2
Alignment Table of A-1 B-4 B-5 B-6

• Alignment Table of A-7
• B-4
• B-5
• B-6

• Alignment Table of A-8
• B-4
• B-5
• B-6

Node-A
Node-B Position - 3
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Simulations gtgt Mobile
Mobility Effect in FSO. TCP is adversely effected.
Random waypoint algorithm, conservative mobility
Both performs poorly in a larger area when power
is not adjusted accordingly
RF performs better when power is
adjusted, Uncovered regions causes FSOs
loss RFs power consumption is way bigger than
FSOs
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FSO-MANET gtgt Conclusions and Future Work

• FSO MANETs are doable and provides significant
  benefit via spatial reuse
• Mobility effects TCP performance severely
• RF and FSO complementary coverage throughput
• Introduce buffers at LL and/or Network Layer
• Directional MAC

15
Questions

• Thank you!
• Questions?