Throughput Characteristics of Free-Space-Optical Mobile Ad-hoc Networks

1. Mehmet Bilgi and Murat Yuksel {mbilgi,yuksem}@cse.unr.edu Computer Science and Engineering University of Nevada – Reno Project Website: http://www.cse.unr.edu/~yuksem/fso-manet.htm Throughput Characteristics of Free-Space-Optical Mobile Ad-hoc Networks

2. Collaborators • Faculty: • Murat Yuksel (yuksem@cse.unr.edu), Univ. of Nevada, Reno • Mona Hella (hellam@ecse.rpi.edu), Rensselaer Polytechnic Institute • Students: • Abdullah Sevincer (asev@cse.unr.edu) (M.S.), UNR • Mehmet Bilgi (mbilgi@cse.unr.edu) (Ph.D.), UNR • Michelle Ramirez (beemyladybug1@yahoo.com) (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. 2000)‏ • 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 Ad Hoc Networks 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 • Spatial reuse and angular diversity in nodes • Low power and secure • Electronic auto-alignment • Optical auto-configuration (switching, routing)‏

9. 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 10-100 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 Sending SYN_ACK Target Node = i Recv(SYN, 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(DATA, i)‏ Recv(SYN_ACK | ACK)‏ Recv(DATA, j)‏ 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. THE END Thank you! Acknowledgments This work was supported by the U.S. National Science Foundation under awards 0721452 and 0721612 and DARPA under contract W31P4Q-08-C-0080