Towards Multi-Hop Free-Space-Optical (FSO)
Mesh Networks and MANETs:
Low-Cost Building Blocks
<…or how to communicate w/ your laser pointer …>
Shiv Kalyanaraman
shivkuma@ecse.rpi.edu
: “shiv rpi”
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
1
: “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)

Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
2
: “shiv rpi”
Scope of Talk

Understanding and
overcoming limitations of
FSO
Orthogonal
Geographic
Routing
Auto
Node
Localization
Rendezvous
Routing
Network

Error correction to Improve
multi-hop link performance

Use of directionality
concept in the network
layer:
 routing and localization PHY
schemes
Data-link
Error Correction Schemes
2-D
Multiple
Node
Localization Line
-Of3D-LOS
Element
Alignment
Multiple
Element Antennas
FSO Antennas
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
3
: “shiv rpi”
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)
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
4
: “shiv rpi”
FSO vs RF: Directional Antenna Sizes: 2.4 Ghz, 5.8 Ghz
2.4 Ghz 802.11b
Pringles Can antennas
Dual Band 802.11a/b/g
Directional antennas
5.8 Ghz 802.11a
Directional antennas
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
5
: “shiv rpi”
FSO Trans-receivers: Much Smaller!
2-d Array of LEDs



Transreceivers: LED +PD
(packed on a 3d sphere)
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.
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
6
: “shiv rpi”
Elementary FSO: sending multi-channel music
Audio Mixing: Tabletop laboratory systems used for
propagating music via multiple channels through free space
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
7
: “shiv rpi”
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.
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
8
: “shiv rpi”
Apps: Opportunistic Links & Networks
Opportunistic links
Air-to-air or air-satellite
Flying over oceans…
Opportunistic links
to cell towers.
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
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
9
: “shiv rpi”
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)
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
10
: “shiv rpi”
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!
Received power
Spatial profile:
~ Gaussian drop off
~1km
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
11
: “shiv rpi”
Geometric Attenuation due to Beam Spread
• 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
SAT
SAR
Receiver
Source
Laser
I (Y )  I 0e
2Y
2
R
 ( Z )2
LED
I ( )  I 0 cosn ( )
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
12
: “shiv rpi”
Typical FSO Communication System
Receiver
(Photo Diode/ Transistor)
Transmitter
(Laser/VCSEL/LED)
Digital Data
ON-OFF Keyed Light Pulses
Light beam is “directional”
(-) Line-of-sight is always needed between the transceivers.
(+) Spatial re-use, diversity, and neighbor position estimation.
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
13
: “shiv rpi”
Elementary FSO System: Block Diagram
4
1
2
3
5
1.
2.
3.
4.
5.
6.
6
LED Module
Collimating Lens
External Magneto-Optic Modulator
Pulsed Light
Focusing Lens
Detector Unit
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
14
: “shiv rpi”
Link Design Issues
4
1
2
LEDs
5
3
Attenuation
6
Photodetector
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
15
: “shiv rpi”
• Output Optical Power
P
LEDs
1.24

• P — Output Optical Power
•  — wavelength
• I — Input Electrical Current
I
Output Optical Power is dependent upon the choice of wavelength.
Longer wavelengths are also more safer to humans, but roomtemperature devices don’t exist.
• Output Optical Spectral Width
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
16
: “shiv rpi”
Photodetector Responsivity
Responsivity is dependent upon the choice of wavelength
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
17
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Atmospheric Windows
Future
devices
1.55um: today’s devices
Optical Loss is dependent upon the choice of wavelength.
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
18
: “shiv rpi”
Error Probability over Single Hop
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
19
: “shiv rpi”
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 Reliability
With 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
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
20
: “shiv rpi”
3d & 2d Designs: Alignment & Capacity
LOS
3-d Spheres: LOS detection through the use
of 3-d spherical FSO Antennas
Node 1
Node 2
D
D/N
…
Node 1
Repeater 2
Repeater 1
Node 2
Repeater N-1
2d Array: 1cm2 LED/PIN => 1000 pairs in 1ft x 1ft square structure
 MultiGbps capacity possible, with different color LEDs (simple static WDM).
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute

21
: “shiv rpi”
3-d Spheres for Auto-Alignment
LED
Micro Mirror
PhotoDetector
Spherical
Antenna
Initial 3-d FSO prototypes with autoalignment circuitry
Optical Transmitter/Receiver
Unit
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
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
22
: “shiv rpi”
3d-Sphere Auto-Alignment Circuit (cont’d)

E.g.: 4-circuit block diagram
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
23
: “shiv rpi”
3d Spheres: Mobility Tests
Misaligned


Aligned
Prior work obtained mobility in FSO for indoor using diffuse optics technology: [Barry, J.R; AlGhamdi, 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.
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
24
: “shiv rpi”
3d Spheres: Mobility Contd
Received Light Intensity from the moving train.
Light Intensity (lux)
70
Aligned
60
Not aligned
50
40
30
Detector
Threshold
20
10
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)
• 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.
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
25
: “shiv rpi”
Toy Train Experiment Contd.
tA 
2  D
2  

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.
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
26
: “shiv rpi”
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
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. Tradeoff: interference vs. angles vs. packaging density
packaging density
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
27
: “shiv rpi”
2-D Arrays: Increased Capacity

Consider transmission from transceiver T0 on array A
(TA0) to transceiver T0 on array B (TB0).

The cone not only covers intended receiver TB0 , but
also TB1 , TB2 , TB4 , TB7 .

Parameters:



d: distance between arrays
θ: divergence angle
ρ: Package density
r  d tan 
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
28
: “shiv rpi”
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.
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
29
: “shiv rpi”
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.


pe  (1 
 r 


 YSep 


YSep2 j 2  ( j 1) 2
0
p

) p0
j
where p0 is probability(ZERO transmitted).
BAC capacity: C  max H ( p0 . pe )  p0 H ( pe )
p( x)
0
1-pe
pe
X
1
1
Rensselaer Polytechnic Institute
0
Y
1
Shivkumar Kalyanaraman
30
: “shiv rpi”
Uniform Array layout: Uncoded, Per-Channel capacity drops
quickly with Package density
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
31
: “shiv rpi”
Helical Array layout: Channel capacity drops slowly with Package density
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
32
: “shiv rpi”
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.
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
33
: “shiv rpi”
FSO Arrays and Space-Time Diversity
Link 1
Link 3
Link 2
Link 4
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.
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
34
: “shiv rpi”
Multi-hop Channel Model
For small errors Pe <10e-2 , the channel is approximated as:
N
 1  P 
1
ie
i 1
1
N
P
i 1
0
N
P
ie
i 1
ie
N
 1  P 
ie
i 1
0
Visibility is modeled as a two Gaussians for clear and adverse weather.
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
35
: “shiv rpi”
Bit Error Rate versus Number of Hops
Assume fixed e2e range
that is split up into hops
(2.5km)
most gains
with a few hops
(~500m/hop)
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
36
: “shiv rpi”
BER distribution
Multi-Hop Error Distribution: more concentrated
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
37
: “shiv rpi”
Multi-Hop Offers Robustness to Weather
Multi-hop significantly outperforms single hop
Number of
Hops
Mean BER
Clear Weather
Mean BER
Variance
Variance
Adverse Weather
Clear Weather
Adverse Weather
1
1.5e-3
0.27
0.02
0.1176
5
9e-27
0.005
8e-50
0.0045
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
38
: “shiv rpi”
Using Multi-directional Communications @ Layer 3
Tessellated FSO Transceivers
Multi-directional Antennas
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
39
: “shiv rpi”
FSO-Meshes: Localization
RF triangulation: needs THREE neighbors
Granular tessellation allows accurate
detection of angle of arrival.
FSO localization: needs ONE neighbor
FSO-based localization system with granular tessellation of transceivers
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
40
: “shiv rpi”
FSO Localization Problem
(x6, y6)
(x7, y7)
(x11, y11)
(x10, y10)
(x9, y9)
(x4, y4)
(x5, y5)
FLA
(x8,, y8)
(0,0)
(x3, y3)
(x2, y2)
After localization
Shivkumar Kalyanaraman
Before localization
Rensselaer Polytechnic Institute
41
: “shiv rpi”
FSO-Meshes: Orthogonal Rendezvous Routing
Rendezvous point
The source and destination sends
probe packets at North-South and
East-West directions based on their
local sense of direction.
Orthogonal/Directional Routing using FSO nodes
Essentially choosing random orthogonal directions in
the plane for dissemination and discovery.
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
42
: “shiv rpi”
ORRP vs Geo-Routing
Classification of Research Issues in Position-based Schemes
L3: Geographic Routing using Node IDs
(eg. GPSR, TBF etc.)
L2: ID to Location Mapping
(eg. HDT, GLS etc.)
ORRP
N/A
L1: Node Localization
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
43
: “shiv rpi”
Void Navigation & Deviation Correction
Basic Example
VOID Navigation/Sparse
Networks Example
  min(+4t, 4t   min(+4t, 4t
 = g +   4t
 = g +   4t
m = +2
m = +3
  min(+4t, 6t
  min(+4t, 6t
 = g +   4t
 = g +   4t
m = +2
m = +3
Void
S
  min(+4t, 0
=g+
m=0
R
  min(+4t, 0
=g+
m = +3
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
44
: “shiv rpi”
ORRP: Reachability Analysis
P{unreachable} =
P{intersections not in
rectangle}
4 Possible Intersection Points
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
45
: “shiv rpi”
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
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
46
: “shiv rpi”
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
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
47
: “shiv rpi”
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.
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
48
: “shiv rpi”
Thanks !
Papers, PPTs, Audio talks:
: “shiv rpi”
Ps: downloadable VIDEOS of all my networking courses
available freely at the above web site
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
49
: “shiv rpi”
Reliability through Diversity at Higher Layers
Channel
Performance
Diversity Modes
Continuous: Time, Frequency, Space ...
Discrete: Code, Antenna, Paths, Routes …

Standard technique: code across diversity modes and use
degrees of freedom efficiently
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
50
: “shiv rpi”
Erasure Coding
>= K of N
received
RS(N,K)
Recover K
data packets!
FEC (N-K)
Block
Size
(N)
Lossy Network
Data = K
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
51
: “shiv rpi”
HARQ Window
(eg: window = 2 original
data pkts)
Packets
Fragments
Per-packet Data fragments
FEC fragments
Random-linear coded (RLC) FEC Fragments,
coded across subsets of data/fec fragments in
window
Interleaved RLC
Fragments
Opportunistic mapping
FSO sub-channel
mm-wave RF
subchannel
Lossy, variable bit-rate
sub-channels
Fragments suffer bursty
loss: data, FEC and RLC
fragments lost
Pkt 1 recovered w/o
RLC (5 fragments
Rensselaer Polytechnic Institute
received)
Pkt 2 needs RLC (only 4 fragments
received). But, 5 RLC fragments, with pkt 1
fragments can recover these 4 missing
fragments
Shivkumar Kalyanaraman
52
: “shiv rpi”
Hybrid FSO/RF-Mesh and MANETS Vision
RF
Communications
High reliability
Free-Space-Optical
Communications
High bandwidth
Low power
Directional – secure,
Not i/f limited
•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
Mobile Ad-Hoc
Networking
Mobile communication
Auto-configuration
Hybrid Free-SpaceOptical/RF Mobile
Ad-Hoc Networks
Spatial reuse and angular diversity in nodes
Electronic auto-alignment (auto-configuration)
Optical auto-configuration (switching, routing)
Low-power and highly secure
Interdisciplinary, cross-layer design
Bringing optical communications and RF adhoc networking together…
Legacy RF MANETS
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
53
: “shiv rpi”
3-d Sphere Node Design Parameters
Maximum
possible
range
R tanθ
R tanθ
R
R
Half lobe area
θ
θ
Interference
area
Transceiver
ρ
Not covered area
τ
r
φ
Case 1: No overlap, C=L
Case 2: Overlap, C=L-I
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
54
: “shiv rpi”
Sphere: Analysis Figures
Lollipop
design!
Max communication range (m) for optimal node designs
given P = 32mWatts,  = 170.1mRad.
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)
~500m
practical
with cheap
LEDs
On top of
towers..
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
55
: “shiv rpi”