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

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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

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

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

21. 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

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

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

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

25. 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

26. 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

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

28. 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

29. 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 , April 2009.
11/13/2018
University of Nevada, Reno

30. 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 IEEE, 2014.
11/13/2018
University of Nevada, Reno

31. 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), IEEE, 2013, pp
11/13/2018
University of Nevada, Reno

32. 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

33. 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)
11/13/2018
University of Nevada, Reno

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

35. 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

36. 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

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

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

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