1. Mehmet Bilgi and Murat Yuksel
Throughput Characteristics of Free-Space-Optical Mobile Ad-hoc Networks
Mehmet Bilgi and Murat Yuksel
Computer Science and Engineering
University of Nevada – Reno
Project Website:

2. Collaborators Faculty: Students:
Murat Yuksel Univ. of Nevada, Reno
Mona Hella Rensselaer Polytechnic Institute
Students:
Abdullah Sevincer (M.S.), UNR
Mehmet Bilgi (Ph.D.), UNR
Michelle Ramirez (B.S.), UNR

3. Outline Motivation & Vision FSO Simulation Modules
FSO Propagation
LOS Alignment Protocol
Validation Simulations
Throughput Simulations
Summary and Conclusions

4. Wireless: Spectrum Constraints
Source: Chris Ramming/DARPA: CBMANETS overview

5. Dense Deployment: No Help Beyond a Point
As we add more RF nodes, per-node throughput diminishes
Dense deployment of many omni-directional antennas increase interference
sqrt(N) as N increases (Gupta, Kumar, Tran. on Inf. Theo )‏
Can become linear with hierarchical cooperative MIMO imposing constraints on topology and mobility pattern (Ozgur et al., Tran. on Inf. Theo. 2006)‏
None is able to totally eliminate the scaling problem
The RF spectrum is getting saturated.. We need alternative communication spectrum resources.

6. Free-Space-Optical (FSO): open spectrum
Open spectrum: 2.4GHz, 5.8GHz, 60GHz, > 300 GHz
Lots of open spectrum up in the optical regime!
FSO usage:
point-to-point links
interconnects
indoor infrared communications
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)‏

7. Optical Wireless: Commodity components
LEDs…
VCSELs…
IrDAs…
Lasers…
Many FSO components are very low cost and available for mass production.

8. Free-Space-Optical (FSO) Communications Mobile Ad-Hoc Networking
FSO-MANETs Vision
Free-Space-Optical (FSO) Communications
Mobile Ad-Hoc Networking
High bandwidth
Low power
Dense spatial reuse
License-free band of
operation
Mobile communication
Auto-configuration
Free-Space-Optical
Ad Hoc Networks
Spatial reuse and angular diversity in nodes
Low power and secure
Electronic auto-alignment
Optical auto-configuration (switching, routing)‏

9. Can we leverage these benefits while solving the issues?
Optical Wireless: Why?
Positive points:
More Secure: Highly directional + small size & weight => low probability of interception (LPI)‏
High-brightness LEDs (HBLEDs) are very low cost and highly reliable components
35-65 cents a piece, and $2-$5 per transceiver package + upto 10 years lifetime
Very low power consumption (100 microwatts for Mbps!)‏
Even lower power for 1-10 Mbps
4-5 orders of magnitude improvement in energy/bit compared to RF
Huge spatial reuse => multiple parallel channels for huge bandwidth increases due to spectral efficiency
Issues:
Need line-of-sight (LOS); and alignment of LOS
Can we leverage these benefits while solving the issues?

10. FSO Issues/Disadvantages
Limited range (no waveguide, unlike fiber optics)‏
Need line-of-sight (LOS)‏
Any obstruction or poor weather (fog, heavy rain/snow) can increase BER in a bursty manner
Bigger issue: Need tight LOS alignment:
LOS alignment must be changed/maintained with mobility or sway!
Effects of relative distance and mobility
Received power
Spatial profile:
~ Gaussian drop off

11. FSO Modules: Alignment Protocol
Goal: Provide an FSO link with “seamless” alignment
Steer the data transmission among the transceivers as the nodes move with respect to each other
Need a 3-way handshake among the transceivers to assure a bidirectional alignment

12. FSO Modules: Alignment Protocol
Send “search” frames periodically
need an “alignment timer”
Receive data frames only after alignment is established
might still get wrong or erroneous frames – leave them to the higher layers
Discard
Discard
Recv(SYN | SYN_ACK |
DATA)‏
Recv(ACK, j)‏
Recv(ACK | DATA)‏
Not Aligned
Sending SYN
Recv(SYN, i)‏
Sending
SYN_ACK
Target Node = i
Start
Recv(ACK, i)‏
Alignment Timer
Timeout
Recv(SYN_ACK, i)‏
Recv(SYN, i)‏
Sending
ACK
Target Node = i
Aligned
Target Node = i
Process
Data
Recv(DATA, i)‏
Recv(SYN_ACK |
ACK)‏
Recv(DATA, j)‏
Recv(DATA, i)‏
Recv(SYN | SYN_ACK | ACK)‏
Recv(DATA, j)‏
Discard
Discard
State diagram of LOS alignment protocol

13. FSO Modules: Alignment Protocol
Maintain “alignment lists” to keep track of which transceiver is aligned with which neighbor

14. FSO Modules: Propagation & Interference
FSO Propagation
Geometric Attenuation
divergence angle
receiver’s surface
Atmospheric Attenuation
visibility
FSO Interference
Must consider the FSO signals coming from other nodes too

15. FSO Simulations in NS-2
How good/bad the transport performance will be if we have FSO nodes with
mobility
multiple transceivers?
Needed to add several things to NS-2
multi-transceiver nodes
LOS alignment protocol
FSO propagation
obstacles

16. TCP Throughput over FSO-MANETs
Performed several simulations..

17. FSO Simulations in NS-2
Propagation validation

18. FSO Simulations in NS-2
Propagation validation

19. FSO Simulations in NS-2
Propagation validation

20. FSO Simulations in NS-2
Mobility is a major problem for throughput scaling!
Nodes with wider divergence angle transceivers perform better due to resemblance to RF.

21. FSO Simulations in NS-2

22. FSO Simulations in NS-2

23. Summary & Future Work
Contributed multi-transceiver simulation modules for free-space-optical communication.
Accurate simulation of multi-transceiver FSO structures reveals differences with RF in TCP behavior.
Intermittent connectivity pattern requires re- consideration of network layers to enable cross-layer buffering.

24. Thank you! THE END Acknowledgments
This work was supported by the U.S. National Science Foundation under awards and and DARPA under contract W31P4Q-08-C-0080