Transcript Document 7648536

Capacity Scaling in Free-Space-Optical
Mobile Ad-Hoc Networks
Mehmet Bilgi
University of Nevada, Reno
Mehmet Bilgi
1
Department of
Computer Science and Engineering
Agenda

RF and FSO Basics

FSO Propagation Model

FSO in Literature

Mobility Model and Alignment

Simulation Results

Conclusions

Future Work
Mehmet Bilgi
2
Department of
Computer Science and Engineering
RF and FSO Illustration
Receiver
Transmitter
Receiver
Directional FSO antenna
Transmitter
Omni-directional RF antenna
Different natures of two technologies: omni-directional and directional
Mehmet Bilgi
3
Department of
Computer Science and Engineering
RF Saturation

A well-known fact: RF suffers from frequency saturation and RFMANETs do not scale well



Omni-directional nature of the frequency propagation causes:




√n as n is increased [1]
Linear scalability can be achieved with hierarchical cooperative MIMO [2]
imposing constraints on topology and mobility pattern
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
Mehmet Bilgi
4
Department of
Computer 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
Mehmet Bilgi
5
Department of
Computer 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
Mehmet Bilgi
6
Department of
Computer 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
Mehmet Bilgi
7
Department of
Computer 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:

AL  10 log e R
3.91   



V  550 


q
V is the atmospheric visibility, q is the size distribution of the scattering particles whose value
is dependent on the visibility
1.6, V  50km

q  1.3, 6km  V  50km
0.585V 1/ 3 , V  6km

1 H. Willebrand and B. S. Ghuman. Free Space Optics. Sams Pubs, 2001. 1st Edition.
Mehmet Bilgi
8
Department of
Computer 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]:



AG  10 log 



200
R



2
Error in the approximate model
FSO
Transmitter
(e.g. LED)

(receiver radius)
Rmax
FSO Receiver
(e.g. PD)
R
Maximum range
(our approximate model: “triangle + half-circle”)
Geometrical
Spread of the Beam
Uncovered Area
Coverage Area
Maximum range
(Lambertian model)
1 H. Willebrand and B. S. Ghuman. Free Space Optics. Sams Pubs, 2001. 1st Edition.
Mehmet Bilgi
9
Department of
Computer 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
Traditional roof-top FSO deployment

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)
Mehmet Bilgi
10
Department of
Computer Science and Engineering
FSO Literature – High Speed

Free-Space-Optical Interconnects






Hybrid FSO/RF applications





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
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
Interconnect with misalignment detector [1]
Deep space communications
1 M. Naruse et al., Real-Time Active Alignment Demonstration for Free-Space Optical Interconnections, IEEE Photonics Tech. Letters,
Nov. 2001
Mehmet Bilgi
11
Department of
Computer Science and Engineering
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 autotracking

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)
Mehmet Bilgi
12
Department of
Computer 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)
1
Multi-element optical
antenna design:
Honeycombed
3
arrays of
directional
transceivers
4
10
2
11
15
16
8
14
13
7
5

Our work: FSO in MANET context with mobility
Mehmet Bilgi
13
9
12
6
Department of
Computer Science and Engineering
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:


B-5
(Pos-1)
A
2
A
1
A
8
2

1
8
Node-B
7
in Pos-1
3
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
6
4
5
Alignment tables in interface 5 of
node B and interface
1 of node A
B-4
(Pos-2)
Node-B
in Pos-2
A-7
A-8
A-1
BB
3
42
BB
4
53
BB
5
64
2
3
A
1
A
8
A
7
B-3
(Pos-3)
1
8
Node-A 7
Alignment tables in interface 4 of
node B and interface 8 of node A
Node-B
in Pos-3
A
8
A
7
A
6
Alignment tables in interface 3 of
node B and interface 7 of node A
6
4
5
Mehmet Bilgi
14
Department of
Computer Science and Engineering
Mobility Experiment
Misaligned
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:
Received Light Intensity from the moving train
70
60
Monitors
the light intensity in all interfaces
Handles
auto hand-off among different transceivers
Aligned
50
Misaligned
40
30
Detector
Threshold
20
10
Initiates
the search phase
Search Phase:

128
121
112
105
97.5
88.5
79
72
65
51.5
40.5
33
23
17
11
0
0
Angular Position of the Train (degree)
When
misaligned, an interfaces sends out a search
signal (pre-determined bit sequence), freq of search signal
Denser packing will allow fewer interruptions (and smaller buffering),
but more handoffs.
LOS
op-amp
& filter
2
3
MUX
LOS
op-amp
& filter
LED
PD
PD
LED
LED
PD
op-amp
& filter
LOS
LOS
op-amp
& filter
MUX
LOS
MUX
op-amp
& filter
PD
LED
LED
PD
PD
LED
LED
PD
0
1
2
MUX
op-amp
& filter
for reception
When
senses a search signal, responds it
1
Interfaces
LOS
MUX
op-amp
& filter
3
Waits
0
X U ME D
MUX
Data
Sink
PD
LED
redoc nE
1
ytiroi rP 2-oT -4
1
4-To-2 Priority
Encoder
0
0
MUX
Light Intensity (lux)
Aligned

restore the data transmission phase
Data
Source
LOS
MUX
op-amp
& filter
LOS
MUX
We want to observe TCP behaviour over FSOMANETs

Mehmet Bilgi
15
Department of
Computer 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)
Mehmet Bilgi
16
30 meters

210 meters
210 meters
30 meters
Department of
Computer Science and Engineering
Stationary RF and FSO Comparison
RF and FSO comparison in stationary
case, no mobility
Mehmet Bilgi
17
Department of
Computer Science and Engineering
Throughput
(KB/s)
Stationary RF and FSO Comparison
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
O
FS
O
FS
O
FS
O
FS
O
FS
RF
0
-2
6
-1
2
-1
-8
-4
Number of Interfaces
RF and FSO comparison with different
number of interfaces
Mehmet Bilgi
18
Department of
Computer Science and Engineering
Mobile FSO: TCP is adversely affected
Mobility Effect in FSO. TCP is adversely effected.
Mehmet Bilgi
19
Department of
Computer Science and Engineering
Mobile RF and FSO Comparison
RF/FSO comparison w.r.t. Speed
Mehmet Bilgi
20
Department of
Computer 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
Mehmet Bilgi
21
Department of
Computer Science and Engineering
Node Density with Fixed Power
Both performs poorly in a larger area when power is not adjusted
accordingly
Mehmet Bilgi
22
Department of
Computer 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
Mehmet Bilgi
23
Department of
Computer Science and Engineering
Mobile UDP Results
1.4
Throughput (KB/s)
1.2
1
0.8
TCP
0.6
UDP
0.4
0.2
0
4
9
Number of Nodes
UDP and TCP mobile throughput comparison
Mehmet Bilgi
24
Department of
Computer Science and Engineering
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
Mehmet Bilgi
25
Department of
Computer Science and Engineering
Future Work




Introduce buffers at LL and/or Network Layer
Group concept
Directional MAC
Effect of search signal sending frequency
Mehmet Bilgi
26
Department of
Computer Science and Engineering
Questions
Mehmet Bilgi
27
Department of
Computer Science and Engineering