Towards Multi-Hop Free-Space-Optical (FSO) Mesh Networks and MANETs: Low-Cost Building Blocks

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 shivkuma@ecse.rpi.edu : “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 Geographic Routing Auto - Network Node Localization Data-link Error Correction Schemes Node Localization Line - Of - 2-D Multiple Element FSO Antennas 3D-LOS Alignment 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 802.11a/g, 802.16e, Cellular (2G/3G)

5. 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

6. 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. Transreceivers: LED +PD (packed on a 3d sphere) 2-d Array of LEDs

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? From LightPointe Optical Wireless Inc.

9. 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 links Air-to-air or air-satellite Opportunistic links to cell towers. Flying over oceans…

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 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)

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. θ SAT SAR Receiver Source R 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. Laser LED

13. Receiver (Photo Diode/ Transistor) Transmitter (Laser/VCSEL/LED) ON-OFF Keyed Light Pulses Digital Data Light beam is “directional” (-) Line-of-sight is always needed between the transceivers. (+) Spatial re-use, diversity, and neighbor position estimation. Typical FSO Communication System

14. 2 3 4 5 1 6 Elementary FSO System: Block Diagram LED Module Collimating Lens External Magneto-Optic Modulator Pulsed Light Focusing Lens Detector Unit

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

16. 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. Future devices 1.55um: today’s devices Atmospheric Windows 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. LOS Node 1 Node 2 D D/N … Node 2 Node 1 Repeater 2 Repeater N-1 Repeater 1 3d & 2d Designs: Alignment & Capacity 3-d Spheres: LOS detection through the use of 3-d spherical FSO Antennas • 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 • Auto-alignment Process: • Step 1: Search Phase (pilot pulses) • Step 2: Data Transfer Phase Initial 3-d FSO prototypes with auto-alignment circuitry Design of 3-d FSO antennas: Honeycomb (tesselated) arrays of transceivers

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

24. 3d Spheres: Mobility Tests • 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. Misaligned Aligned

25. Aligned Not aligned Detector Threshold 3d Spheres: Mobility Contd Received Light Intensity from the moving train. • 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. 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. Toy Train Experiment Contd.

27. FSO Node Designs • Various factors: • Visibility – weather conditions • source power and receiver sensitivity • angles of devices – small angles are costlier • packaging density • 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 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

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. 1-pe 0 0 pe Y X 1 1 1 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:

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. Link 4 Link 3 Link 2 Link 1 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.

35. Multi-hop Channel Model For small errors Pe <10e-2, the channel is approximated as: 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)

37. Multi-Hop Error Distribution: more concentrated BER distribution

38. Multi-Hop Offers Robustness to Weather Multi-hop significantly outperforms single hop Clear Weather Clear Weather Adverse Weather Adverse Weather

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

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

41. (x6, y6) (x7, y7) (x11, y11) (x10, y10) (x9, y9) (x4, y4) (x5, y5) (x8,, y8) (0,0) (x3, y3) (x2, y2) FSO Localization Problem FLA After localization Before 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. ORRP vs Geo-Routing Classification of Research Issues in Position-based Schemes

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

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

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