1. 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

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

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

4. 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

5. 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

6. 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

7. 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

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. 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

10. 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

11. 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

12. Mobility and Alignment Impl. In NS-21 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

13. 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

14. 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

15. Questions
Thank you!
Questions?
Mehmet Bilgi