Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 1 : shiv rpi Towards Multi-Hop Free-Space-Optical (FSO) Mesh Networks and MANETs: Low-Cost Building.

Shivkumar KalyanaramanRensselaer Polytechnic Institute

1 : “shiv rpi”

Towards Multi-Hop Free-Space-Optical (FSO) Mesh Networks and MANETs:

Low-Cost Building Blocks

Shiv [email protected]

: “shiv rpi”

<…or how to communicate w/ your laser pointer …>


2 : “shiv rpi”

Students and Collaborators

Jayasri Akella (PhD) Murat Yuksel (post-doc, now at Univ. Nevada, Reno) Bow-Nan Cheng (PhD) David Partyka (MS) Chang Liu (MS)

Prof. Partha Dutta (optoelectronic devices) Prof. Mona Hella (RF/photonic circuits)


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Scope of Talk

Understanding and overcoming limitations of FSO

Error correction to Improve multi-hop link performance

Use of directionality concept in the network layer: routing and localization

schemesPHY

Data-link

Line-Of-Node Localization

Multiple Element Antennas

Geographic Routing Auto-

3D-LOS Alignment

Node Localization

2-D Multiple ElementFSO Antennas

Error Correction Schemes

Orthogonal Rendezvous Routing

Network


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Free Space Optical (FSO) Communications Open spectrum: 2.4GHz, 5.8GHz, 60GHz,

> 300 GHz Lots of open spectrum up in the optical

regime!

Data transfer through atmosphere OOK Modulated light pulses.

Line of sight “optical wireless” technology. Visible to near infrared regions.

Currently terrestrial point-to-point links bridging connectivity gaps between buildings in a metro area medical imaging disaster recovery

DoD use of FSO: Satellite communications DARPA ORCL project: air-to-ground,

air-to-air, air-to-satellite

802.11a/g, 802.16e,Cellular (2G/3G)


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FSO vs RF: Directional Antenna Sizes: 2.4 Ghz, 5.8 Ghz

Dual Band 802.11a/b/g Directional antennas

5.8 Ghz 802.11aDirectional antennas

2.4 Ghz 802.11bPringles Can antennas


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FSO Trans-receivers: Much Smaller!

Higher frequency: smaller antennas Small size => Can pack in 2-d array and 3-d structures ! Increasing use of HBLEDs in solid state lighting: can leverage

low cost devices.

2-d Array of LEDs

Transreceivers: LED +PD(packed on a 3d sphere)


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Elementary FSO: sending multi-channel music

Audio Mixing: Tabletop laboratory systems used for

propagating music via multiple channels through free space


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Why Free Space Optical Communication?

FSO potential: Multi-Gbps System capacity Spatial re-use/minimal interference Suitable form factors (power, size and cost) Quick and easy installation.

If interference-limited, then attractive for the last mile access or home networking where LOS exists.

If power-limited, then attractive for sensor networks: much lower-power vs RF

Challenges: FSO Needs line-of-sight (LOS) alignment Poor performance in adverse weather

conditions: reliability How to seamlessly integrate and leverage FSO

in the context of multi-hop networks?

From LightPointe Optical Wireless Inc.


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Apps: Opportunistic Links & Networks

Expensive sat-com links for most urgent data, and delay-tolerant links to offload delay-tolerant data:

DARPA ORCL program is already looking at some of this

Opportunistic linksto cell towers.Flying over oceans…

Opportunistic linksAir-to-air or air-satellite


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FSO Advantages High-brightness LEDs (HBLEDs) and VCSELs are very low cost and

highly reliable components 35-65 cents a piece, and $2-$5 per transceiver package + up to 10 years

lifetime Amenable to high density integration (eg: VCSEL arrays)

Very low power consumption 4-5 orders of magnitude improvement in energy/bit compared to RF,

e.g. 100 microwatts for 10-100 Mbps.

Huge spatial reuse => multiple parallel channels for huge bandwidth increases due to spectral efficiency Not interference limited, unlike RF

More Secure: Highly directional + small size & weight => low probability of interception (LPI)


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FSO Issues/Disadvantages Limited range (no waveguide, unlike fiber optics) Need line-of-sight (LOS)

Any obstruction or poor weather (fog, sandstorms, heavy rain/snow) can increase BER in a bursty manner

Bigger issue: Need tight LOS alignment over long distances: Directional antenna on steroids! LOS alignment must be changed/maintained with mobility or sway!

~1km

Received powerSpatial profile: ~ Gaussian drop off


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Geometric Attenuation due to Beam Spread

22

)(2

0)( ZY

eIYI

)(cos)( 0 nII

• Divergence of light beam is primary cause for geometric attenuation.

• When an energy detector is used, only a fraction of transmitted power is received.

θ

R

SAT SAR

SourceReceiver

Laser

LED


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Typical FSO Communication System

Digital DataON-OFF Keyed Light Pulses

Transmitter (Laser/VCSEL/LED)

Receiver(Photo Diode/ Transistor)

Light beam is “directional”

(-) Line-of-sight is always needed between the transceivers.

(+) Spatial re-use, diversity, and neighbor position estimation.


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Elementary FSO System: Block Diagram

1. LED Module

2. Collimating Lens

3. External Magneto-Optic Modulator

4. Pulsed Light

5. Focusing Lens

6. Detector Unit

2 3 5 61

4


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Link Design Issues

2 3 5 61

4

LEDs Attenuation Photodetector


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LEDs• Output Optical Power

IP24.1

• Output Optical Spectral Width

• P — Output Optical Power • — wavelength • I — Input Electrical Current

Output Optical Power is dependent upon the choice of wavelength.Longer wavelengths are also more safer to humans, but room-

temperature devices don’t exist.


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Photodetector Responsivity

Responsivity is dependent upon the choice of wavelength


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Atmospheric Windows

Optical Loss is dependent upon the choice of wavelength.

Future devices

1.55um: today’s devices


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Error Probability over Single Hop


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Link Budget

PRC = PTX –Llens– LGS – Latt

• PRC — Output Optical Power in transmitter • PTX — Received Optical Power in receiver • Llens — Optical Loss Due to Lens Used in transmitter and receiver• LGS — Optical Loss Due to Geometrical Spreading in the propagation distance• Latt — Optical Loss Due to attenuation in atmosphere

Bottom Line: Trying to Achieve Greater Distance and ReliabilityWith a Single FSO Hop is Tough!

Change the game: Use shorter hops, multi-hops, low-cost BBs, and engineer reliability by using diversity at higher layers


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3d & 2d Designs: Alignment & Capacity

3-d Spheres: LOS detection through the use of 3-d spherical FSO Antennas

LOS

Node 1 Node 2

…Node 1 Node 2

Repeater 1 Repeater 2 Repeater N-1

DD/N

2d Array: 1cm2 LED/PIN => 1000 pairs in 1ft x 1ft square structure MultiGbps capacity possible, with different color LEDs (simple static WDM).


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3-d Spheres for Auto-Alignment

LED

PhotoDetector

Micro Mirror

Spherical Antenna

Optical Transmitter/Receiver Unit

Initial 3-d FSO prototypes with auto-alignment circuitry

Design of 3-d FSO antennas:

Honeycomb (tesselated) arrays of transceivers

Auto-alignment Process: Step 1: Search Phase (pilot pulses) Step 2: Data Transfer Phase


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3d-Sphere Auto-Alignment Circuit (cont’d)

E.g.: 4-circuit block diagram


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3d Spheres: Mobility Tests

Misaligned Aligned

Prior work obtained mobility in FSO for indoor using diffuse optics technology: [Barry, J.R; Al-Ghamdi, A.G.]

Limited power of a single source that is being diffused into all the directions. Suitable for small distances (typically 10s of meters), but not suitable for longer distances.

Our approach can scale to longer, outdoor distances and consumes less power.


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3d Spheres: Mobility Contd

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

Inte

ns

ity

(lu

x)

DetectorThreshold

Not aligned

Aligned

• Denser packing will allow fewer interruptions (and smaller buffering), but more handoffs…

• Even w/ buffering: becomes a “disruption”-tolerant/lossy networking problem over multiple hops.

Received Light Intensity from the moving train.


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Toy Train Experiment Contd.

tA : Time duration of alignment

θ: Divergence angle of LED.

D: Circuit delay

Ω: Train's angular speed

φ: Angular separation between transceivers on sphere.

2

2 DtA


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FSO Node Designs Important node design questions:

How good the node can be in terms of coverage or range?

How many transceivers can/should be placed on the nodes?

Do the placement patterns of transceivers matter?

Goal: maximize capacity

Tradeoff: interference vs. angles vs. packaging density

Various factors: Visibility – weather conditions source power and receiver sensitivity angles of devices – small angles are costlier packaging density

Goal: maximize coverage

Tradeoff: interference vs. angles vs. packaging density


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2-D Arrays: Increased Capacity

Consider transmission from transceiver T0 on array A (TA

0) to transceiver T0 on array B (TB0).

The cone not only covers intended receiver TB0 , but also

TB1 , TB

2 , TB4 , TB

7 .

Parameters: d: distance between arrays θ: divergence angle ρ: Package density

tandr


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Array Designs : Helical Vs Uniform Transceiver Placement

Helical array design gives more capacity for a given range and transceiver parameters due to reduced inter-channel interference.


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Inter-channel Interference & Capacity w/ OOK

Interference occurs when a subset of these potential interferers transmit when TA0

is transmitting. Probability that such an event occurs gives error probability due to crosstalk.

where p0 is probability(ZERO transmitted).

0

)1(0 )1(

222

pppSep

Sep

Y

r

j

jjYe

0

1

0

1

1-pe

pe

1

X Y

BAC capacity: )().(max 00)(

eexp

pHpppHC


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Uniform Array layout: Uncoded, Per-Channel capacity drops quickly with Package density


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Helical Array layout: Channel capacity drops slowly with Package density


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OOC (Optical Orthogonal Codes) can further improve the capacity between arrays.

Two OOCs with weight 4 and length 32. Each transceiver uses a unique code similar to CDMA wireless users in a cell.


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FSO Arrays and Space-Time Diversity

Per-Link: Code over Time and Across Multiple Spatial Channels Per-Hop

Per-Path Across a network: Build a virtual link composed of several FSO hops, and possibly perform FEC coding and mapping across multiple routed-paths.

Link 1 Link 2 Link 4Link 3


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Multi-hop Channel Model

For small errors Pe <10e-2 , the channel is approximated as:

N

iieP

1

1 1

0

1

0

N

iieP

1

N

iieP

1

1

N

iieP

1

N

iieP

1

1 1

0

1

0

1

0

1

0

1

0

1

0

1

0

1

0

1

0

1

0

N

iieP

1

N

iieP

1

1

N

iieP

1

Visibility is modeled as a two Gaussians for clear and adverse weather.


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Bit Error Rate versus Number of Hops

Assume fixed e2e rangethat is split up into hops(2.5km)

most gains with a few hops(~500m/hop)


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Multi-Hop Error Distribution: more concentratedB

ER

dis

trib

utio

n


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Multi-Hop Offers Robustness to Weather

Number of Hops

Mean BER Mean BER Variance Variance

1 1.5e-3 0.27 0.02 0.1176

5 9e-27 0.005 8e-50 0.0045

Multi-hop significantly outperforms single hop

Clear WeatherClear Weather Adverse Weather Adverse Weather


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Using Multi-directional Communications @ Layer 3

Multi-directional Antennas

Tessellated FSO Transceivers


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FSO-Meshes: Localization

FSO-based localization system with granular tessellation of transceivers

Granular tessellation allows accurate detection of angle of

arrival.

RF triangulation: needs THREE neighbors

FSO localization: needs ONE neighbor


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FSO Localization Problem

Before localization After localization

FLA

(0,0)

(x5, y5)

(x6, y6) (x7, y7)

(x2, y2)

(x4, y4)

(x3, y3)

(x9, y9)

(x8,, y8)

(x11, y11)

(x10, y10)


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FSO-Meshes: Orthogonal Rendezvous Routing

Orthogonal/Directional Routing using FSO nodes

The source and destination sends probe packets at North-South and East-West directions based on their local sense of direction.

Rendezvous point

Essentially choosing random orthogonal directions in the plane for dissemination and discovery.


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ORRP vs Geo-Routing

L3: Geographic Routing using Node IDs

(eg. GPSR, TBF etc.)

L2: ID to Location Mapping

(eg. HDT, GLS etc.)

L1: Node Localization

ORRP

N/A

Classification of Research Issues in Position-based Schemes


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Void Navigation & Deviation Correction

Basic ExampleVOID Navigation/Sparse

Networks Example

VoidS R

min(+4 = + m = +3

min(+46 = + 4m = +2

min(+4 = + m = 0

min(+44 = + 4m = +2

min(+44 = + 4m = +3

min(+46 = + 4m = +3


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ORRP: Reachability Analysis

P{unreachable} =

P{intersections not in rectangle}

4 Possible Intersection Points


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Path Stretch Analysis

Average Stretch for various topologies

• Square Topology – 1.255• Circular Topology – 1.15• 25 X 4 Rectangular – 3.24• Expected Stretch – 1.125


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State Complexity Analysis

GPSR DSDV XYLS ORRP

Node State O(1) O(n2) O(n3/2) O(n3/2)

Reachability High High 100% High (99%)

Name Resolution O(n log n) O(1) O(1) O(1)

Invariants Geography None Global Comp. Local Comp.

Notes:

• ORRP scales with Order N3/2

• ORRP states are fairly evenly distributed – no single pt of failure


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Summary FSO has interesting/complementary properties w.r.t. RF wireless Single Hop Issues: LEDs, PDs, Transmittance Windows Building Blocks:

3-d Sphere: LOS Auto-alignment, Coverage 2-d Array: Capacity, Co-channel interference due to geometric spread

Helical Designs and Orthogonal Coding mitigates interference Low-cost Multi-hop FSO Networks:

Simple OEO Repeaters, Error correction at electronic hops

Use of directional PHY property at higher layers: Localization Routing: orthogonal rendezvous routing

Low stretch, high connectivity, O(N1.5) state complexity

Future work on multi-path routing, Wifi backup, coded-multiple parallel channels, WDM for capacity etc Dual-mode systems for opportunistic V2V links (vehicular ad-hoc) Extensions of our PHY and L3 mechanisms for higher mobility.


49 : “shiv rpi”

Thanks !

: “shiv rpi”

Papers, PPTs, Audio talks:

Ps: downloadable VIDEOS of all my networking courses available freely at the above web site


50 : “shiv rpi”

Reliability through Diversity at Higher Layers

Standard technique: code across diversity modes and use degrees of freedom efficiently

Diversity ModesContinuous: Time, Frequency, Space ... Discrete: Code, Antenna, Paths, Routes …

ChannelPerformance


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Erasure Coding

Data = K

FEC (N-K)

Block Size (N)

RS(N,K) >= K of Nreceived

Lossy Network

Recover K data packets!


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Packets

Fragments

Data fragmentsPer-packet FEC fragments

Random-linear coded (RLC) FEC Fragments, coded across subsets of data/fec fragments in

window

HARQ Window(eg: window = 2 original

data pkts)

FSO sub-channelmm-wave RF subchannel

Opportunistic mapping

Interleaved RLC Fragments

Lossy, variable bit-ratesub-channels

Pkt 1 recovered w/o RLC (5 fragments received)

Pkt 2 needs RLC (only 4 fragments received). But, 5 RLC fragments, with pkt 1 fragments can recover these 4 missing fragments

Fragments suffer bursty loss: data, FEC and RLC fragments lost


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Hybrid FSO/RF-Mesh and MANETS Vision

Legacy RF MANETS

•802.1x with omni-directional RF antennas

•High-power, Interference limited

•Low bandwidth – typically the bottleneck link on a path

•Error-prone, Disruptions

•Less secure – very vulnerable to interception

Spatial reuse and angular diversity in nodesElectronic auto-alignment (auto-configuration)Optical auto-configuration (switching, routing)Low-power and highly secureInterdisciplinary, cross-layer design

Bringing optical communications and RF ad-hoc networking together…

High bandwidthLow powerDirectional – secure,Not i/f limited

Free-Space-Optical Communications

Mobile Ad-Hoc Networking

Hybrid Free-Space-Optical/RF Mobile Ad-Hoc Networks

Mobile communicationAuto-configuration

RF Communications

High reliability


54 : “shiv rpi”

3-d Sphere Node Design Parameters

Case 1: No overlap, C=L

θ

θ

R

r

R

φ

R tanθ

R tanθ

τ

ρ

Transceiver

Maximum possible range

Half lobe area

Interference area

Not covered area

Case 2: Overlap, C=L-I


55 : “shiv rpi”

Sphere: Analysis Figures

Max communication range (m) for optimal node designs given P = 32mWatts, = 170.1mRad.

Lollipop design!

On top of towers..

Installed to

ceilings, may be as

lamps..

Reasonable coverage possible:For P=32mWatts, coverage as high as: 0.7 km2 (adverse)2.10km2 (normal)3.24km2 (clear)

~500mpracticalwith cheapLEDs

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 1 : shiv rpi Towards Multi-Hop Free-Space-Optical (FSO) Mesh Networks and MANETs: Low-Cost Building.

Documents

q routing

reliability q

q visible

q fso potential

q los alignment

sight los q

q directional antenna

q data transfer