1. 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
: “shiv rpi”

2. 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)

3. 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 schemes
Orthogonal Rendezvous Routing
Network
Geographic Routing
Node Localization
Auto -
Data-link
Error Correction Schemes
2-D Multiple
Element
FSO Antennas
Node Localization
Line - Of -
3D-LOS
Alignment
FSO and RF technologies complement each other, so a hybrid is a powerful combination
Figure: physical layer, data link layer
PHY
Multiple Element Antennas

4. 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
High speed physical layer technology
Point-to-point, Disaster recovery, considered for mobile base station backhaul
802.11a/g,
802.16e,
Cellular (2G/3G)

5. FSO vs RF: Directional Antenna Sizes: 2.4 Ghz, 5.8 Ghz
2.4 Ghz b
Pringles Can antennas
Dual Band a/b/g
Directional antennas
5.8 Ghz a
Directional antennas

6. FSO Trans-receivers: Much Smaller!
Transreceivers: LED +PD
(packed on a 3d sphere)
2-d Array of LEDs
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.

7. Elementary FSO: sending multi-channel music
Audio Mixing: Tabletop laboratory systems used for
propagating music via multiple channels through free space

8. 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?
No or minimum interference,
Cost, power, size form factors -- suitable for sensor networks.
From LightPointe Optical Wireless Inc.

9. Apps: Opportunistic Links & Networks
Air-to-air or air-satellite
Opportunistic links
to cell towers.
Flying over oceans…
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

10. 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 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)

11. 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

12. 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 R
SAT
SAR
Source
Receiver
Laser
Figure font too small
Aperture spelling
LED

13. Typical FSO Communication System
Digital Data
ON-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.
Explain diversity

14. Elementary FSO System: Block Diagram2 3 5 6 1 4
LED Module
Collimating Lens
External Magneto-Optic Modulator
Pulsed Light
Focusing Lens
Detector Unit

15. Link Design Issues 2 3 5 6 1 4
LEDs
Attenuation
Photodetector

16. Output Optical Power is dependent upon the choice of wavelength.
LEDs
Output Optical Power
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.
Output Optical Spectral Width

17. Photodetector Responsivity
Responsivity is dependent upon the choice of wavelength

18. Atmospheric Windows Future devices 1.55um: today’s devices
Optical Loss is dependent upon the choice of wavelength.

19. Error Probability over Single Hop

20. 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

21. 3d & 2d Designs: Alignment & Capacity
LOS
3-d Spheres: LOS detection through the use of 3-d spherical FSO Antennas
Node 1
Node 2 …
Repeater 1
Repeater 2
Repeater N-1 D
D/N
2d Array: 1cm2 LED/PIN => 1000 pairs in 1ft x 1ft square structure
MultiGbps capacity possible, with different color LEDs (simple static WDM).

22. 3-d Spheres for Auto-Alignment
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

23. 3d-Sphere Auto-Alignment Circuit (cont’d)
E.g.: 4-circuit block diagram

24. 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.

25. 3d Spheres: Mobility Contd
Received Light Intensity from the moving train.
Detector
Threshold
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.

26. 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.

27. FSO Node Designs Various factors: Visibility – weather conditions
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?
Various factors:
Visibility – weather conditions
source power and receiver sensitivity
angles of devices – small angles are costlier
packaging density
May go earlier..
Goal: maximize capacity
Tradeoff: interference vs. angles vs. packaging density
Goal: maximize coverage
Tradeoff: interference vs. angles vs. packaging density

28. 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
Two such identical arrays face each other to facilitate communication between the corresponding optical transceivers on the arrays.
each of the transceivers on the array is supposed to communicate only with the corresponding transceiver on the opposite array.
But because of the finite transceiver angle, the light signals transmitted will diverge by the time they reach the opposite array and they are not only received by the corresponding transceiver on the opposite array, but also by its neighboring transceivers, causing interference.

29. 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.

30. 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).
BAC capacity: 1
1-pe pe X Y

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

32. Helical Array layout: Channel capacity drops slowly with Package density

33. 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.

34. FSO Arrays and Space-Time Diversity
Link 1
Link 2
Link 4
Link 3
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.

35. Multi-hop Channel Model
For small errors Pe <10e-2 , the channel is approximated as:
Redraw fig 1
Check 10e-2
Visibility is modeled as a two Gaussians for clear and adverse weather.

36. 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)
Add line

37. Multi-Hop Error Distribution: more concentrated
BER distribution
Fix title and info

38. Multi-Hop Offers Robustness to Weather
Multi-hop significantly outperforms single hop
Number of Hops
Mean BER
Variance 1
1.5e-3
0.27
0.02
0.1176 5
9e-27
0.005
8e-50
0.0045
Clear Weather
Adverse Weather
Clear Weather
Adverse Weather
Mean error in one box, Variance in one box

39. Using Multi-directional Communications @ Layer 3
Tessellated FSO Transceivers
Multi-directional Antennas

40. FSO-Meshes: Localization
Granular tessellation allows accurate detection of angle of arrival.
RF triangulation: needs THREE neighbors
FSO localization: needs ONE neighbor
FSO-based localization system with granular tessellation of transceivers

41. FSO Localization Problem
(0,0)
(x5, y5)
(x6, y6)
(x7, y7)
(x2, y2)
(x4, y4)
(x3, y3)
(x9, y9)
(x8,, y8)
(x11, y11)
(x10, y10)
FLA
FLA is undefined!
Before localization
After localization

42. 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.

43. L3: Geographic Routing using Node IDs L2: ID to Location Mapping
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.)
L1: Node Localization
ORRP
N/A

44. Void Navigation & Deviation Correction
Basic Example
VOID Navigation/Sparse
Networks Example
Void S R
= min(+4t, 0)
= g + p
m = +3
= min(+4t, +6t)
= g + p - 4t
m = +2
m = 0
= min(+4t, +4t)

45. ORRP: Reachability Analysis
P{unreachable} =
P{intersections not in rectangle}
4 Possible Intersection Points

46. 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

47. State Complexity Analysis
GPSR
DSDV
XYLS
ORRP
Node State
O(1)
O(n2)
O(n3/2)
Reachability
High
100%
High (99%)
Name Resolution
O(n log n)
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

48. 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. Thanks ! : “shiv rpi” Papers, PPTs, Audio talks:
Ps: downloadable VIDEOS of all my networking courses
available freely at the above web site

50. Reliability through Diversity at Higher Layers
Diversity Modes
Continuous: Time, Frequency, Space ...
Discrete: Code, Antenna, Paths, Routes …
Channel
Performance
Standard technique: code across diversity modes and use degrees of freedom efficiently

51. Erasure Coding RS(N,K) Recover K data packets! >= K of N received
Lossy Network
Data = K
FEC (N-K)
Block
Size
(N)
RS(N,K)

52. HARQ Window (eg: window = 2 original data pkts) Opportunistic mapping
Packets
Fragments
Data fragments
Per-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-channel
mm-wave RF
subchannel
Opportunistic mapping
Interleaved RLC Fragments
Lossy, variable bit-rate
sub-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

53. Hybrid FSO/RF-Mesh and MANETS Vision
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 ad-hoc networking together…
High bandwidth
Low power
Directional – secure,
Not i/f limited
Free-Space-Optical
Communications
Mobile Ad-Hoc
Networking
Hybrid Free-Space-Optical/RF Mobile Ad-Hoc Networks
Mobile communication
Auto-configuration RF
High reliability
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

54. 3-d Sphere Node Design Parametersθ R r φ
R tanθ τ ρ
Transceiver
Maximum possible range
Half lobe area
Interference area
Not covered area
Case 1: No overlap, C=L
Case 2: Overlap, C=L-I

55. Sphere: Analysis Figures
Lollipop design!
Max communication range (m) for optimal node designs given P = 32mWatts,  = 170.1mRad.
Reasonable coverage possible:
For P=32mWatts, coverage as high as: 0.7 km2 (adverse)
2.10km2 (normal)
3.24km2 (clear)
Installed to ceilings, may be as lamps..
Also can be connected to array interference.. Key message is the relationship between interference and coverage, opotimal packaging..
Information retreival problem..
~500m
practical
with cheap
LEDs
On top of towers..