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Mehmet Bilgi Department ofComputer Science and Engineering
Capacity Scaling in Free-Space-Optical
Mobile Ad-Hoc Networks
Mehmet Bilgi University of Nevada, Reno
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Mehmet Bilgi Department ofComputer Science and Engineering
Agenda
RF and FSO Basics FSO Propagation Model FSO in Literature Mobility Model and Alignment Simulation Results Conclusions Future Work
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Mehmet Bilgi Department ofComputer Science and Engineering
RF and FSO Illustration
Different natures of two technologies: omni-directional and directional
Omni-directional RF antenna
Directional FSO antenna
TransmitterReceiver
Transmitter
Receiver
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Mehmet Bilgi Department ofComputer Science and Engineering
A well-known fact: RF suffers from frequency saturation and RF-MANETs do not scale well √n 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
RF Saturation
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Mehmet Bilgi Department ofComputer Science and Engineering
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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Mehmet Bilgi Department ofComputer Science and Engineering
FSO Advantages Materials: cheap LEDs or VCSELs with Photo-Detectors, commercially available,
<$1 for a transceiver pair Small (~1mm2), low weight (<1gm) 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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Mehmet Bilgi Department ofComputer Science and Engineering
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 RF’s channel fading Investigated the effects of intermittent connectivity on higher layers:
Especially TCP
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Mehmet Bilgi Department ofComputer Science and Engineering
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
Bragg’s Law [1]:
σ 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.
RL eA log10
q
V
550
91.3
kmVV
kmVkm
kmV
q
6,585.0
506,3.1
50,6.1
3/1
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Mehmet Bilgi Department ofComputer Science and Engineering
FSO Propagation Model Geometric spread is a function of
transmitter radius γ,
the radius of the receiver ς,
divergence angle of the transmitter θ,
the distance between the transmitting node and receiving node R [1]:
2
200log10
R
AG
Rmax (receiver radius)
Maximum range (our approximate model: “triangle + half-circle”) Maximum range
(Lambertian model)
Coverage AreaUncovered Area
R
Error in the approximate model
FSO Transmitter (e.g. LED)
FSO Receiver (e.g. PD)
Geom
etrical Spread of the B
eam
1 H. Willebrand and B. S. Ghuman. Free Space Optics. Sams Pubs, 2001. 1st Edition.
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Mehmet Bilgi Department ofComputer Science and Engineering
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)
Traditional roof-top FSO deployment
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Mehmet Bilgi Department ofComputer Science and Engineering
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
FSO Literature – High Speed
Interconnect with misalignment detector [1]
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Mehmet Bilgi Department ofComputer Science and Engineering
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)
FSO Literature
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Mehmet Bilgi Department ofComputer Science and Engineering
Mobility Model Design an antenna with FSO transceivers to
Exploit directionality and spatial reuse Target mobility Multi-element antenna using commercially
available 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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4
5
6
7
8
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10
11
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1314
15
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Multi-element optical
antenna design:
Honeycombed
arrays of
directional
transceivers
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Mehmet Bilgi Department ofComputer Science and Engineering
Mobility Model in NS-2
B-5 (Pos-1)
A 2
A 1
A 8
A-1
B 4
B 5
B 6
A-8
B 3
B 4
B 5
No network simulator has FSO simulation capabilities Each transceiver keeps track of its alignments
A table based implementation Alignment timers
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3
4
7
8
65
12
3
4
7
8
65
Node-A
Node-Bin Pos-1
Node-Bin Pos-2
Node-Bin Pos-3
A-7
B 2
B 3
B 4
B-3 (Pos-3)
A 8
A 7
A 6
B-4 (Pos-2)
A 1
A 8
A 7
Alignment tables in interface 5 ofnode B and interface 1 of node A
Alignment tables in interface 4 ofnode B and interface 8 of node A
Alignment tables in interface 3 ofnode B and interface 7 of node A
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
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Mehmet Bilgi Department ofComputer Science and Engineering
Mobility Experiment
0
10
20
30
40
50
60
70
0
11
17
23
33
40
.5
51
.5
65
72
79
88
.5
97
.5
10
5
11
2
12
1
12
8
Angular Position of the Train (degree)
Lig
ht In
te
ns
ity
(lu
x)
Misaligned
Aligned
Denser packing will allow fewer interruptions (and smaller buffering), but more handoffs.
Received Light Intensity from the moving train
DetectorThreshold
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
M UX
LOS op-am p& filter
M UX
LOSop-am p& filter
M UX
LOSop-am p& filter
M UX
LOS op-am p& filter
MUXD ata
S ink
4-To-2
Priority
Enc
ode
r
0
1
2
3
0
1
PD
LED
PD
LED
PD
LED
PD
LED
MU X
LOSop-am p& filter
MU X
LOSop-am p& filter
MU X
LOSop-am p& filter
MU X
LOSop-am p& filter
DE
MU
X
4-T
o-2
Pri
ority
Enc
oder
0
1
2
3
0
1
D ataSource
LED
PD
LED
PD
LED
PD
LED
PD
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Mehmet Bilgi Department ofComputer Science and Engineering
Simulations 49 nodes in a 7 x 7 grid Every node establishes an FTP
session 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)
210 meters
210
met
ers
30 m
eter
s
30 meters
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Mehmet Bilgi Department ofComputer Science and Engineering
Stationary RF and FSO Comparison
RF and FSO comparison in stationary case, no mobility
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Mehmet Bilgi Department ofComputer Science and Engineering
Stationary RF and FSO Comparison
00.05
0.10.15
0.20.25
0.30.35
Thro
ughp
ut
(KB/
s)
Number of Interfaces
RF and FSO comparison with different number of interfaces
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Mehmet Bilgi Department ofComputer Science and Engineering
Mobile FSO: TCP is adversely affected
Mobility Effect in FSO. TCP is adversely effected.
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Mehmet Bilgi Department ofComputer Science and Engineering
Mobile RF and FSO Comparison
RF/FSO comparison w.r.t. Speed
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Mehmet Bilgi Department ofComputer Science and Engineering
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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Mehmet Bilgi Department ofComputer Science and Engineering
Node Density with Fixed Power
Both performs poorly in a larger area when power is not adjusted accordingly
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Mehmet Bilgi Department ofComputer Science and Engineering
Node Density with Adjusted Power
RF performs better when power is adjusted,
Uncovered regions causes FSO’s loss
RF’s power consumption is way bigger than FSO’s
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Mehmet Bilgi Department ofComputer Science and Engineering
Mobile UDP Results
0
0.2
0.4
0.6
0.8
1
1.2
1.4
4 9
Number of Nodes
Th
rou
gh
pu
t (K
B/s
)
TCP
UDP
UDP and TCP mobile throughput comparison
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Mehmet Bilgi Department ofComputer Science and Engineering
Conclusions
FSO MANETs are possible and provides significant benefit via spatial reuse
Mobility affects TCP performance severelyRF and FSO are complementary to each
other; coverage + throughput
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Mehmet Bilgi Department ofComputer Science and Engineering
Future Work
Introduce buffers at LL and/or Network Layer Group concept Directional MAC Effect of search signal sending frequency