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Mehmet Bilgi University of Nevada, Reno
Multi-Element Free-Space-Optical (FSO) Spherical Structures with Intermittent Connectivity Patterns Mehmet Bilgi University of Nevada, Reno Mehmet Bilgi
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Agenda RF and FSO Basics FSO Propagation Model FSO in Literature
Mobility Model and Alignment Simulations Conclusions Future Work Mehmet Bilgi
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RF and FSO Illustration
Transmitter Receiver Omni-directional RF antenna Directional FSO antenna Different natures of two technologies: omni-directional and directional Mehmet Bilgi
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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 Mehmet Bilgi
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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 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 Mehmet Bilgi
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Geometrical Spread of the Beam
FSO Disadvantages FSO requires clear line-of-sight Rmax (receiver radius) Maximum range (our approximate model: “triangle + half-circle”) Maximum range (Lambertian model) Coverage Area (i.e. P+AL+ AG > S) Uncovered Area (i.e. P+AL+ AG < S) R Error in the approximate model FSO Transmitter (e.g. LED) FSO Receiver (e.g. PD) Geometrical Spread of the Beam Mehmet Bilgi
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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 Mehmet Bilgi
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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
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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 Misaligned Aligned Denser packing will allow fewer interruptions (and smaller buffering), but more handoffs… Received Light Intensity from the moving train. Detector Threshold 21/09/2018 Mehmet Bilgi
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FSO-MANET >> Alignment re-establishment
Aligned Misaligned Interfaces periodically send out search signals (bit sequence ) Respond to search signals, inverse the bit sequence Restore data transfer 21/09/2018 Mehmet Bilgi
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FSO-MANET >> Simulations
210 meters 30 meters 49 nodes in a 7 x 7 grid Every node establishes an FTP session 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 Also can be connected to array interference.. Key message is the relationship between interference and coverage, opotimal packaging.. Information retreival problem.. RF and FSO comparison with different number of interfaces RF and FSO comparison in stationary case, no mobility 21/09/2018 Mehmet Bilgi
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Mobility and Alignment Impl. In NS-2
1 2 3 4 7 8 6 5 Mobility and Alignment Impl. In NS-2 Node-B Position - 1 Node-B Position - 2 Also can be connected to array interference.. Key message is the relationship between interference and coverage, opotimal packaging.. Information retreival problem.. 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 1 2 3 4 7 8 6 5 Node-B Position - 3 21/09/2018 Mehmet Bilgi
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Simulations >> Mobile
RF/FSO comparison w.r.t. Speed Mobility Effect in FSO. TCP is adversely effected. Random waypoint algorithm, conservative mobility Also can be connected to array interference.. Key message is the relationship between interference and coverage, opotimal packaging.. Information retreival problem.. 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 21/09/2018 Mehmet Bilgi
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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 Also can be connected to array interference.. Key message is the relationship between interference and coverage, opotimal packaging.. Information retreival problem.. 21/09/2018 Mehmet Bilgi
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Questions Thank you! Questions? Mehmet Bilgi
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