Mehmet Bilgi and Murat Yuksel Throughput Characteristics of Free-Space-Optical Mobile Ad-hoc Networks 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

Collaborators Faculty: Students: 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

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

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

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.

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)‏

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

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)‏

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 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?

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

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

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

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

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

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

TCP Throughput over FSO-MANETs Performed several simulations..

FSO Simulations in NS-2 Propagation validation

FSO Simulations in NS-2 Propagation validation

FSO Simulations in NS-2 Propagation validation

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.

FSO Simulations in NS-2

FSO Simulations in NS-2

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.

Thank you! THE END 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