IEEE WCNC 2014, Istanbul, Turkey Effective Transceiver Selection for Mobile Multi-Directional Free-Space-Optical Modules Abdullah Sevincer and Murat Yuksel Department of Computer Science and Engineering University of Nevada, Reno Project Website http://www.cse.unr.edu/~yuksem/fso-manet.htm IEEE WCNC 2014, Istanbul, Turkey

Collaborators & Sponsors Faculty Mona Hella (hellam@ecse.rpi.edu), Rensselaer Polytechnic Institute Nezih Pala (palan@fiu.edu), Florida International University Students Mahmudur Khan (mrk1611@gmail.com) (Ph.D.), UNR Prabath Palathingal (ppalathingal@cse.unr.edu) (M.S.), UNR Alumnus Abdullah Sevincer (asev@cse.unr.edu) (Ph.D.), Intel Behrooz Nakhkoob (nakhkb@rpi.edu) (Ph.D.), RPI Mehmet Bilgi (mbilgi@cse.unr.edu) (Ph.D.), Microsoft Michelle Ramirez (beemyladybug1@yahoo.com) (B.S.), US Air Force 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

Motivation Free-space-optical (FSO) communication has the potential to serve as a complementary technology to RF for the future wireless networking. Multi-element spherical modules covered with multiple highly directional FSO transceivers has been shown to work well to handle mobility for FSO communication. Reducing the modules’ energy consumption becomes a crucial issue for the FSO modules with many transceivers. Need to find an efficient algorithm to reduce power consumption while satisfying throughput performance.

Wireless Capacity – NOW! Scary trends in mobile wireless demand 2+ times increase per year since 2007. “18-fold by 2016!” Cisco, February 2012. “More than 80% is landing on WiFi”, http://Wefi.com -- excluding iPhone! Opportunistic networking is well accepted by the users!

Free-Space-Optical (FSO): open spectrum 2.4GHz, 5.8GHz, 60GHz, > 300 GHz FSO usage: point-to-point links interconnects indoor infrared communications DoD use of FSO: Satellite communications AirForce RIKA; DARPA THOR, ORCL, ORCA: air-to-ground, air-to-air, air-to-satellite 802.11a/g, 802.16e, Cellular (2G/3G)

Optical Wireless: Commodity components Digital Data ON-OFF Keyed Light Pulses Transmitter (Laser/VCSEL/LED) Receiver (Photo Diode/ Transistor) IrDAs… Lasers… LEDs… VCSELs… Many FSO components are very low cost and available for mass production.

Optical Wireless: Why? More Secure: Highly directional => low probability of interception Small size and weight: Dense packaging is possible Very low cost and reliable components, e.g. HBLEDs 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 over RF Huge spatial reuse => multiple parallel channels for huge bandwidth increases

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 maintained with mobility or sway! Effects of relative distance and mobility Received power Spatial profile: ~ Gaussian drop off Can we leverage FSO’s benefits while solving the issues?

FSO Modules: Spherical Designs How to handle mobility under LOS alignment requirement? Mobile FSO = Directionality + Angular Diversity + Electronic Steering Multi-transceiver spherical FSO designs. Need a distributed protocol for this! A B Bidirectional LOS ACM/Springer WINET 2009

FSO Modules: Spherical Designs Need to autonomously manage (electronically steer) multiple data transmissions to leverage spatial reuse. A B C

FSO Modules: Alignment Protocol LOS Alignment Process: Step 1: Search Phase Step 2: Data Transfer Phase 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 LOS Network Layer Link Layer PHY Alignment Protocol

FSO Modules: Alignment Protocol Alignment Lists to steer transmissions with over multiple alignments updated at every search interval C 1 21 NextHop. Interface Local B.7 19 A.6 11 NextHop. Interface Local B.5 17 A.6 11 A 1 B 1 21 NextHop. Interface Local A.11 11 C.19 7 21 NextHop. Interface Local B.9 14 C.11 6 NextHop. Interface Local B.11 11 C.11 6 NextHop. Interface Local A.14 9 C.17 5

Transceiver Selection Problem Brute force: send search signals at every transceiver What happens if there are many transceivers? Too frequent searches (i.e., too many active transceivers) Energy consumption Controller scalability Too infrequent searches May not detect the FSO communication opportunities Brute force alignment protocol cannot scale to 100s of transceivers..

Transceiver Selection Problem Recap goals: Discover opportunities to establish LOS with neighbors Maintain the existing links Insights: Movements of neighbors project to spatially correlated (or close) transceivers Significant number of transceivers are not relevant

Transceiver Selection Discover: need to explore Send search from as many transceivers as possible Maintain: need to exploit Send search only from the transceivers close to the ones where an alignment already exists Single Mode: discover and maintain at the same time-scale Two Mode: discover at larger time-scales How to keep the network throughput high (FSO links stay on) via minimal search messaging overhead?

Simulation Setup ns-2 simulations of TCP traffic Parameter Name Default Value Number of nodes  49 Number of flows  49x48 Visibility  6 km Number of interfaces  8, 16, 24 Mobility  1 m/s Simulation time  3000 s Transmission range & separation of nodes  30 m Area  210 x 210 m Node radius  20 cm Divergence angle  0.5 radian Photo detector diameter  5 cm LED diameter  0.5 cm ns-2 simulations of TCP traffic Compare throughput for single vs. two mode operations.. Discovery 0.03, 0.15, 0.3, 0.5, 4, and 8 seconds Maintenance 0.03, 0.15, and 0.3 seconds Search from the previous and next (+/- 1 )transceivers

Order of magnitude fewer searches in the two-mode alignment Results Order of magnitude fewer searches in the two-mode alignment

Similar throughput with 10 times fewer search messages Results Similar throughput with 10 times fewer search messages

Similar throughput with 10 times fewer search messages Results Similar throughput with 10 times fewer search messages

Summary and Future Work Messaging overhead of LOS alignment protocol can be controlled with simple tricks 10 fold savings are possible by a two-mode design Immediate future work: Discovery could be improved: use randomization More study of the mobility rate Longer term future work: A theoretical framework to formulate the tradeoffs Applicability to directional RF antennas

Questions?

Seems Inescapable by the Internet Wireless nodes will soon dominate the Internet. Currently ~1B nodes, including wireline. Urgent response to the exploding wireless demand is a necessity.

What is RF and FSO Cellular is Full: wireless airwaves are full”, CNN, Feb 21, 2012.

Dense Deployment: No Help Beyond a Point As we add more RF nodes, per-node throughput diminishes Dense deployment of many omni-directional antennas increases interference sqrt(N) as N increases (Gupta, Kumar, 2000) Can become linear with hierarchical cooperative MIMO imposing constraints on topology and mobility pattern (Ozgur et al., 2006) None is able to totally eliminate the scaling problem The RF spectrum is getting saturated.. We need alternative communication spectrum resources for opportunistic usage.

Opportunistic Wireless Channel Characteristics Complements always-on, lower-rate, higher coverage wireless. Available Capacity Optical wireless opportunistic channel RF opportunistic channel Basic RF channel Delay-tolerant applications (e.g. email, FTP, Video-on-demand) Basic RF channel (e.g. 2G, 3G, 4G/WiMAX) Real-time, interactive applications (e.g. chat, VoIP) Time This talk will consider a specific PHY technology, free-space-optics (FSO), that could be useful in this context, and its implications on higher-layer protocols.

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 Prototype: Transceiver Picture of transceivers. 3-D optical antenna design.

FSO Prototype: Mobility Experiment A B Node-A Node-B ~19Kb/s ~20m frame size 50B alignment timer 500ms ACM Mobicom 2010 ACM Mobisys 2010 IEEE ICC 2010

Literature Multiple elements/transceivers in FSO communication in interconnects which communicate over very short distances. [1] Focus: Mechanical steering or beam steering [2][3] Optical flow assignment has not been considered to manage multiple different data flows among transceivers during an on going transmission The main issues: interference (or cross-talk) between adjacent transceivers due to finite divergence of the light beam Misalignment due to vibration. M. Naruse and S. Yamamoto and M. Ishikawa. Real-time active alignment demonstration for free-space optical interconnections. IEEE Photonics Technology Letters, 13:1257–1259, November 2001. [1] Bisaillon and D. F. Brosseau and T. Yamamoto and M. Mony and E. Bernier and D. Goodwill and D. V. Plant and A. G. Kirk. Free-space optical link with spatial redundancy for misalignment tolerance. IEEE Photonics Technology Letters, 14:242–244, February 2002. [2] Canon, 2012. http://www.usa.canon.com/cusa/home [3]