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Effective Transceiver Selection for Mobile Multi-Directional Free-Space-Optical Modules Abdullah Sevincer and Murat Yuksel Department of Computer Science.

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Presentation on theme: "Effective Transceiver Selection for Mobile Multi-Directional Free-Space-Optical Modules Abdullah Sevincer and Murat Yuksel Department of Computer Science."— Presentation transcript:

1 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 IEEE WCNC 2014, Istanbul, Turkey

2 Collaborators & Sponsors Faculty – Mona Hella (, Rensselaer Polytechnic – Nezih Pala (, Florida International Students – Mahmudur Khan ( (Ph.D.), – Prabath Palathingal ( (M.S.), Alumnus – Abdullah Sevincer ( (Ph.D.), – Behrooz Nakhkoob ( (Ph.D.), – Mehmet Bilgi ( (Ph.D.), – Michelle Ramirez ( (B.S.), US Air 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 2

3 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. 3

4 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”, -- excluding iPhone! Opportunistic networking is well accepted by the users! 4

5 Free-Space-Optical (FSO): open spectrum 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) 5

6 Optical Wireless: Commodity components Many FSO components are very low cost and available for mass production. Lasers… LEDs… VCSELs… IrDAs… Digital Data ON-OFF Keyed Light Pulses Transmitter (Laser/VCSEL/LED) Receiver (Photo Diode/ Transistor) 6

7 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 7

8 Received power Spatial profile: ~ Gaussian drop off 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 Can we leverage FSO’s benefits while solving the issues? 8

9 FSO Modules: Spherical Designs How to handle mobility under LOS alignment requirement? Mobile FSO = Directionality + Angular Diversity + Electronic Steering A B Bidirectional LOS Need a distributed protocol for this! Multi-transceiver spherical FSO designs. ACM/Springer WINET 2009 9

10 FSO Modules: Spherical Designs Need to autonomously manage (electronically steer) multiple data transmissions to leverage spatial reuse. A B C 10

11 LOS Alignment Process: Step 1: Search Phase Step 2: Data Transfer Phase 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 LOS Network Layer Link Layer PHY Alignment Protocol 11

12 FSO Modules: Alignment Protocol Alignment Lists – to steer transmissions with over multiple alignments – updated at every search interval A NextHop. Interface Local Interface B.1111 C.116 1 21 B 1 C 1 NextHop. Interface Local Interface B.719 A.611 NextHop. Interface Local Interface A.1111 C.197 NextHop. Interface Local Interface A.149 C.175 NextHop. Interface Local Interface B.914 C.116 NextHop. Interface Local Interface B.517 A.611 12

13 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.. 13

14 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 14

15 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? 15

16 Simulation Setup 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 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 16

17 Results Order of magnitude fewer searches in the two-mode alignment 17

18 Results Similar throughput with 10 times fewer search messages 18

19 Results Similar throughput with 10 times fewer search messages 19

20 20 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

21 Questions? 21

22 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. 22

23 What is RF and FSO Cellular is Full: wireless airwaves are full”, CNN, Feb 21, 2012. 23

24 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. 24

25 Opportunistic Wireless Channel Characteristics Basic RF channel (e.g. 2G, 3G, 4G/WiMAX) Available Capacity Time Optical wireless opportunistic channel RF opportunistic channel Basic RF channel Real-time, interactive applications (e.g. chat, VoIP) Delay-tolerant applications (e.g. email, FTP, Video-on-demand) 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. Complements always-on, lower- rate, higher coverage wireless. 25

26 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 State diagram of LOS alignment protocol Sending SYN_ACK Target Node = i Recv(ACK, i ) Recv(SYN | SYN_ACK | DATA) Recv(ACK, j ) Discard Not Aligned Sending SYN Recv(SYN_ACK, i ) Recv(SYN, i ) Start Recv(ACK | DATA) Discard Aligned Target Node = i Recv(SYN_ACK | ACK) Recv(DATA, j ) Recv(DATA, i ) Discard Sending ACK Target Node = i Recv(DATA, i ) Recv(SYN | SYN_ACK | ACK) Recv(DATA, j ) Discard Process Data Recv(SYN, i ) Alignment Timer Timeout 26

27 FSO Prototype: Transceiver Picture of transceivers. 3-D optical antenna design. 27

28 FSO Prototype: Mobility Experiment Node-A Node-B A B ~19Kb/s ~20m frame size 50B alignment timer 500ms ACM Mobicom 2010 ACM Mobisys 2010 IEEE ICC 2010 28

29 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. [3] 29

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