Mehmet Bilgi University of Nevada, Reno

1. Mehmet Bilgi University of Nevada, Reno Multi-Element Free-Space-Optical (FSO) SphericalStructures with Intermittent Connectivity Patterns Mehmet Bilgi

2. Mehmet Bilgi Agenda • RF and FSO Basics • FSO Propagation Model • FSO in Literature • Mobility Model and Alignment • Simulations • Conclusions • Future Work

3. Mehmet Bilgi RF and FSO Illustration Different natures of two technologies: omni-directional and directional Transmitter Receiver Omni-directional RF antenna Directional FSO antenna

4. Mehmet Bilgi RF Saturation • A well-known fact: RF suffers from frequency saturation and RF-MANETs do not scale well • sqrt(n) as n is increased [1] • Linear scalability can be achieved with hierarchical cooperative MIMO [2] • Omni-directional nature of the frequency propagation causes: • Channel is a broadcast medium, overhearing • 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

5. Mehmet Bilgi 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

6. Mehmet Bilgi Error in the approximate model Uncovered Area (i.e. P+AL+ AG < S) Coverage Area (i.e. P+AL+ AG > S) FSO Transmitter (e.g. LED)  Geometrical Spread of the Beam Rmax (receiver radius) R FSO Receiver (e.g. PD) Maximum range (Lambertian model) Maximum range (our approximate model: “triangle + half-circle”) FSO Disadvantages • FSO requires clear line-of-sight

7. Mehmet Bilgi FSO Propagation Model • Atmospheric attenuation, geometric spread and obstacles contribute to BER • Atmospheric attn. is mainly driven by fog, size of the water vapor particles are close to near-infrared wavelength

8. Free-Space-Optical Communication • Roof-top deployments in metropolitan area, point-to-point links via powerful lasers • Indoor mobility with diffuse optics (~10s of meters) • Interconnects in short distances(1-10s cm) • Previous work on swaying and vibration of buildings to tolerate disruptions • Use gimbals, expensive tracking instruments, backup beams • They do not target mobility • Our work: FSO in MANET context: mobility • Advantage: spatial reuse with directional antennas, optical speeds, commercially available components, easy deployment • Disadvantage: requires clear line-of-sight, obstacles, mobility is a challenge Traditional roof-top FSO deployment Multi-element optical antenna design: Honeycombed arrays of directional transceivers Mehmet Bilgi

9. Mehmet Bilgi Received Light Intensity from the moving train. Aligned Misaligned Denser packing will allow fewer interruptions (and smaller buffering), but more handoffs… FSO-MANET >> Spheres >> Mobility • As the mobility is introduced: alignment becomes a challenge • 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 • We investigated the effect of intermittent connectivity on higher layers: especially TCP Misaligned Aligned Detector Threshold

10. Mehmet Bilgi Aligned FSO-MANET >> Alignment re-establishment • Interfaces periodically send out search signals (bit sequence 11010101011) • Respond to search signals, inverse the bit sequence • Restore data transfer Misaligned

11. Mehmet Bilgi 210 meters 30 meters 210 meters 30 meters RF and FSO comparison in stationary case, no mobility RF and FSO comparison with different number of interfaces FSO-MANET >> Simulations • 49 nodes in a 7 x 7 grid • Every node establishes an FTPsession 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

12. Mehmet Bilgi 1 1 2 2 8 8 3 3 7 7 6 6 4 4 5 5 Mobility and Alignment Impl. In NS-2 Node-B Position - 1 Node-B Position - 2 Alignment Table of A-1: B-4 B-5 B-6 Alignment Table of A-7: • B-4 • B-5 • B-6 Alignment Table of A-8: • B-4 • B-5 • B-6 Node-A Node-B Position - 3

13. Mehmet Bilgi RF/FSO comparison w.r.t. Speed Simulations >> Mobile Mobility Effect in FSO. TCP is adversely effected. Random waypoint algorithm, conservative mobility Both performs poorly in a larger area when power is not adjusted accordingly RF performs better when power is adjusted, Uncovered regions causes FSO’s loss RF’s power consumption is way bigger than FSO’s

14. Mehmet Bilgi FSO-MANET >> Conclusions and Future Work • FSO MANETs are doable and provides significant benefit via spatial reuse • Mobility effects TCP performance severely • RF and FSO complementary; coverage + throughput • Introduce buffers at LL and/or Network Layer • Directional MAC

15. Mehmet Bilgi Questions • Thank you! • Questions?