Autonomous Alignment of Free-Space-Optical-Links Between UAVs Mahmudur Khan and Murat Yuksel Department of Computer Science and Engineering University of Nevada, Reno mahmudurk@nevada.unr.edu, yuksem@unr.edu 11/13/2018 University of Nevada, Reno

Free-Space-Optical Communication FSOC Transmitter (Laser Diodes, LEDs, VCSELs) Receiver (Photodetector) Channel (Free Space) Signal (Optical) 11/13/2018 University of Nevada, Reno

University of Nevada, Reno Advantages of FSOC Potential complement of RF Uses the unlicensed optical spectrum Works in conditions unfavorable for RF Low probability of interception and detection. Uses same technology as fiber optic communications Reach modulation speeds up to 10 Gbps Cost efficient 11/13/2018 University of Nevada, Reno

University of Nevada, Reno Difficulties of FSOC Maintenance of Line of Sight Transmitter and Receiver must be aligned No obstacles Vulnerable against mobility Loss of LOS 11/13/2018 University of Nevada, Reno

Problem Statement and Assumptions Two UAVs/Quadcopters Mobile and completely autonomous Equipped with Inertial Measurement Unit (IMU) Two mechanically steerable hemispherical head/arm Optical transceivers RF unavailable, GPS-free environment In band LOS alignment using only FSOC 11/13/2018 University of Nevada, Reno

Maintaining FSO Link Between Two Autonomous Mobiles Two main stages: Detection of LOS and establishment of an FSO link Maintaining the FSO link 11/13/2018 University of Nevada, Reno

University of Nevada, Reno Discovery Use GPS, IMU etc. Perform 3-way handshake using RF to establish FSO link Exchange information about: Position Direction Speed Orientation of head SYN_ACK SYN Data Data ACK 11/13/2018 University of Nevada, Reno

Maintaining the Established FSO Link Setting Up The Angular Velocity of head/arm Rotation (clockwise or counter-clockwise, up or down) 11/13/2018 University of Nevada, Reno

Setting Up The Angular Velocity of head/arm Case 1: One Node stationary, another moving a2, b2, c2 Z-axis Y-axis (West) x a2, b2, c1 Rotational Angle [Xxy, Xz] Xz = |θz’- θz| a1, b1, c1 φz x’ y a2, b2, c0 x’ a1, b1, c0 z z’ θz’ y’ θz a2, b2, 0 Xxy x’ φxy a2, b1, 0 a1, b1, 0 0, 0, c0 z’ θxy X-axis (North) 0, 0, 0 11/13/2018 University of Nevada, Reno

University of Nevada, Reno Z-axis a2, b2, c2 Y-axis (West) φxy2 Case 2: Both Node Mobile φz2 x2' x1 x2 x1’ φz1 a2’, b2’, c2’ x y’ a1, b1, c1 Xz p2, q2, r2 y z x2 a1, b1, r1 x’ a2’, b2’, r1 y’ x2' Xxy z’ θz’ φz2 θz a2, b2, 0 x1’ φxy1 0, 0, r1 a2’, b2’, 0 p2, q2, 0 a1, b1, 0 z’ Xxy φxy2 θxy X-axis (North) 0, 0, 0 11/13/2018 University of Nevada, Reno

University of Nevada, Reno Rotation (Clockwise/ Counter-Clockwise, Up/Down) a2, b2, c2 Y-axis (West) Z-axis θxy1 < θxy2 : CCW θxy1 > θxy2 : CW θxy1 = θxy2 : None θz1 < θz2 : Up θz1 > θz2 : Down θz1 = θz2 : None a1, b1, c1 θz2 p2, q2, r2 θz1 0, 0, r1 θxy2 θxy1 0, 0, 0 X-axis (North) 11/13/2018 University of Nevada, Reno

Exchange Protocols to Maintain the Link Maximum Angle of Deviation (Protocol A) Minimum SNR (Protocol B) 11/13/2018 University of Nevada, Reno

Maximum Angle of Deviation (Protocol A) Deviation from normal (θd) Divergence Angle (θ)  = θd/θ Time period of information exchange tx 11/13/2018 University of Nevada, Reno

University of Nevada, Reno  <= max Angular velocity unchanged  > max Recalculate and update angular velocity Overhead, Nrec = Number of recalculations 11/13/2018 University of Nevada, Reno

Minimum SNR (Protocol B) SNRa > SNRb  = SNRrecv - SNRmin Every tx it is checked if  <  min If  <  min angle of rotation (∠Xxy, ∠Xz) is recalculated SNRa SNRb 11/13/2018 University of Nevada, Reno

University of Nevada, Reno Link Accuracy If  > 1 or  < 0 (checked every 1ms) Counted as a miss % Link down time = (miss/total_count)*100 11/13/2018 University of Nevada, Reno

Simulation Scenarios and Assumptions MATLAB Laser Transmitters - Node speed up to 25m/s Maximum distance covered by transmitter = 2.5km θ = 2mrad, 2.25mrad and 2.5mrad LED Transmitters - Node speed up to 5m/s Maximum distance covered by transmitter = 100m θ = 3o, 5o and 7.5o 11/13/2018 University of Nevada, Reno

Simulation Scenarios and Assumptions max = 0.25, 0.50 and 0.75 (Maximum Deviation Protocol) min = 2dB, 4dB and 6dB (Minimum SNR Protocol) Initial position and flying directions of UAVs chosen randomly 11/13/2018 University of Nevada, Reno

Exponentially Decreasing Overhead Link accuracy deteriorates with increase in tx Overhead decreases exponentially 11/13/2018 University of Nevada, Reno

Find a balance between accuracy and overhead so Find maximum tx for maintaining a given accuracy 11/13/2018 University of Nevada, Reno

Find max tx Start i=i+1, tmin = tx(i-2), tmax = tx(i-1), tx(i) = (tmin+tmax)/2 i=1, tx(i) = 1ms, tmin =0, tmax = 0 Run simulation and find accuracy Run simulation and find accuracy no |Accuracy - 95%| > error ? no Accuracy > 95% ? no i=1 ? yes yes Accuracy < 95% ? i=i+1, tx(i) = 2*tx(i-1) yes yes no tmax = tx(i) tmin = tx(i) Return tx(i) i=i+1, tx(i) = (tmin+tmax)/2 End 11/13/2018 University of Nevada, Reno

Maximum Deviation Protocol with Laser Larger θ -> Larger tx Better link maintenance requires smaller tx 11/13/2018 University of Nevada, Reno

Maximum Deviation Protocol with LED Smaller αmax -> Higher tx 11/13/2018 University of Nevada, Reno

Computation Overhead Larger divergence angle reduces overhead Higher min -> Higher tx -> Smaller Overhead 11/13/2018 University of Nevada, Reno

University of Nevada, Reno Summary Maintaining longer % link up time requires Smaller tx : 80% accuracy requires tx: 1.04s 95% accuracy requires tx: 288ms Smaller αmax / Higher min and larger θ (fixed desired % link up time): Larger tx Smaller Overhead 11/13/2018 University of Nevada, Reno

University of Nevada, Reno Future Work Consider UAVs moving on curves Consider effect of atmospheric turbulence and vibration Perform real test bed experiments Multiple transceivers – electronic steering 11/13/2018 University of Nevada, Reno

University of Nevada, Reno Thanks! Questions? 11/13/2018 University of Nevada, Reno

Rotation (Clockwise/ Counter-Clockwise, Up/Down) Western Quadrants Eastern Quadrants Condition Rotation θxy1 < θxy2 CCW CW θxy1 > θxy2 θxy1 = θxy2 None θz1 < θz2 Up θz1 > θz2 Down θz1 = θz2 11/13/2018 University of Nevada, Reno

Model for Transceiver Coverage Triangle Half-Circle Divergence Angle (θ) Rtanθ Deviation from normal (θd) Rmax Follows the Lambertian law Negligible error6 6) M. Yuksel, J. Akella, S. Kalyanaraman, and P. Dutta, “Free-space-optical mo- bile ad hoc networks: Auto-congurable building blocks," Wireless Networks, vol. 15, no. 3, pp. 295-312, April 2009. 11/13/2018 University of Nevada, Reno

Some Related Work Maintaining an FSO link between two autonomous mobiles1. FSO communication between two UGVs LED transceivers This paper: FSOC between two UAVs/Quadcopters Laser and LED 1) M. Khan, M. Yuksel, “Wireless Communications and Networking Conference (WCNC), 2014 IEEE, pp. 3154-3159. IEEE, 2014. 11/13/2018 University of Nevada, Reno

Some Related Work FSO communication between two hovering UASs2. Stationary Aerial System We consider: Mobile Nodes 2) A. Kaadan, D. Zhou, H. H. Refai, and P. G. LoPresti, “Modeling of aerial-to-aerial short-distance free space optical links," in Integrated Communications, Navigation and Surveillance Conference (ICNS), 2013. IEEE, 2013, pp. 1-12. 11/13/2018 University of Nevada, Reno

Some Related Work Establishment and maintenance of a free-space optical-communication link among nearby balloons 3,4. Link maintained using GPS, RF, camera FSO used for transferring data only We use FSO for both: Link maintenance Transfer data R. W. DeVaul, E. Teller, C. L. Bie, and J. Weaver, “Establishing optical-communication lock with nearby balloon," 2012, US Patent App. 13/346,645. R. DeVaul, E. Teller, C. Bie, and J. Weaver, “Using predicted movement to maintain optical-communication lock with nearby balloon," 2013, US Patent App. 14/108,542. 11/13/2018 University of Nevada, Reno

Some Related Work Free Space Laser Communication by Facebook5 Aquila: Solar powered unmanned plane Ground Station to UAV using RF UAVs connected through free space laser communication Laser transmitting at 10Gbps 5) https://www.facebook.com/zuck/videos/10102274951725301/ 11/13/2018 University of Nevada, Reno

University of Nevada, Reno Thank You! 11/13/2018 University of Nevada, Reno

University of Nevada, Reno Protocol B & Laser Higher min -> Higher tx -> Smaller Overhead Larger θ -> Higher tx -> Smaller Overhead 11/13/2018 University of Nevada, Reno

University of Nevada, Reno Protocol A & LED Smaller αmax -> Higher tx -> Smaller Overhead Larger θ -> Higher tx -> Smaller Overhead 11/13/2018 University of Nevada, Reno

Protocol B & LED Higher min -> Higher tx -> Smaller Overhead Larger θ -> Higher tx -> Smaller Overhead 11/13/2018 University of Nevada, Reno

University of Nevada, Reno Appendix 11/13/2018 University of Nevada, Reno

University of Nevada, Reno 700nanometers (frequency 430 THz) to 1 mm (300 GHz) 11/13/2018 University of Nevada, Reno