Fiber optics has been a winning player in the game of high-speed communication and data transfer in cable connections. Yet, in free space RF has been the repeated choice of communication medium of the space industry. Investigating the benefits of free space optical communication over radio frequency is worthwhile. An increase in science data rate return capabilities could occur if optical communication is implemented. Optical communication systems also provide efficiencies in power, mass, and volume over RF systems1. Optical communication systems have been demonstrated from a satellite in orbit with the moon to earth, and resulted in the highest data rates ever seen through space (622Mbps)2. Because of these benefits, optical communication is far superior to RF. The HiDRA (High Data Rate Architecture) project is researching Passive Misalignment Mitigation of Dynamic Free Apace Optical Communication Links. The goal of this effort is to enable gigabit per second transmission of data in short range dynamic links (less than 100 meters). In practice this would enhance data rates between sites on the International Space Station with minimal size, weight, and power requirements. This paper will focus on an autonomous code and a hardware setup that will be used to fulfill the next step in the research being conducted. The free space optical communications pointing downfalls will be investigated. This was achieved by creating 5 python programs and a top-level code to automate this test

Heldman, Christopher

NASA Technical Reports Server

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Infrared Free Space Communication 
The autonomous testing of free space infrared communication 
 
 
Christopher Heldman 
University of Akron, Akron Ohio 44325 
 
 
I - Introduction and Motivation 
 
    Fiber optics has been a winning player in the game of high-speed communication and data transfer in cable connections. Yet, 
in free space RF has been the repeated choice of communication medium of the space industry. Investigating the benefits of free 
space optical communication over radio frequency is worthwhile. An increase in science data rate return capabilities could occur 
if optical communication is implemented. Optical communication systems also provide efficiencies in power, mass, and volume 
over RF systems1. Optical communication systems have been demonstrated from a satellite in orbit with the moon to earth, and 
resulted in the highest data rates ever seen through space (622Mbps)2. Because of these benefits, optical communication is far 
superior to RF. 
The HiDRA (High Data Rate Architecture) project is researching Passive Misalignment Mitigation of Dynamic Free Apace 
Optical Communication Links.  The goal of this effort is to enable gigabit per second transmission of data in short range dynamic 
links (less than 100 meters). In practice this would enhance data rates between sites on the International Space Station with 
minimal size, weight, and power requirements. This paper will focus on an autonomous code and a hardware setup that will be 
used to fulfill the next step in the research being conducted. The free space optical communications pointing downfalls will be 
investigated.  This was achieved by creating 5 python programs and a top-level code to automate this test. 
 
II – Automated Testing Methodology to Determine Misalignment Effects 
A laboratory set up was created to transmit an infrared data beam over a distance of 20 meters. This is used to model the travel 
distance of the communications link on the ISS. The 20 meter link simulated the distance between the EXPRESS Logistics 
Carrier and the main cabin of the ISS where the technology would be implemented. The nodes at the main cabin and the Expedite 
the Processing of Experiments to the Space Station (EXPRESS) Logistics Carrier move relative to each other and this causes 
misalignment in the system. An Anritsu BERTwave was used to characterize the received data from the IR Optical link as the 
misalignment conditions that could be seen on the ISS were simulated. The lab set up hardware included a transmitter and receiver 
board, linear and angular motors (KDC101) to change the transmission misalignment, a BERTwave to send continuous 
https://ntrs.nasa.gov/search.jsp?R=20170009201 2019-08-29T23:30:59+00:00Z
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pseudorandom data and obtain bit error rate measurements, and an Aardvark to read the I2C power data from the receiver board.  
This setup is shown below in Figure 1. 
 
Figure 1, Lab Setup for Experiment 
 
Table 1, Lab Hardware Description 
Thorlabs KDC 101 A motor controller that can interface with multiple types of mechanical movement modules and 
programed to exicute various movement commands. 
Anritsu BERTwave A bit error rate measuring device, sends a pseudorandom set of data at a given speed and reads the 
number of bit errors that occur. 
Aardvark A device to read I2C off of the Finisar board, data is in Hexadecimal and is converted to a float to 
read of the dBm power received and sent.   
FBD-1027 A circuit board that transmits the free space optical data laser and receives data. The board 
interfaces with the aardvark and BERTwave to take measurements on the data being sent.  
 
An automated testing program was created for the purpose of taking analytical data and repeated tests. The automated function 
of the test will enable obtaining much more accurate data than compared to data taken by hand. This is due to the ability to 
accurately move the motors for misalignment and take repeated power and bit error rate measurements. Implementing a fully 
automated test will also reduce operation time of the test greatly compared to human interface with the devices. Another added 
benefit of an automated code to implement the experiment is that it can be modified slightly to be applied to similar test setups.   
The test code was created as follows: A program was created to interface with a motor controller to move the motor by a specific 
amount. This enabled the ability to change the angle and lateral position of the transmitter.  Code was created to manipulate I2C 
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data from an Aardvark into power dB values, which enabled obtaining the power of the received signal. A control code was 
designed to control functions of a BERTwave error detector in a way that allowed the program to run until a specific confidence 
level of the bit error rate was achieved.  
 
Table 2, Control Software Module Description 
Motor Controller Code This code interfaces with KDC Motors and moves them a specified distance dependent on a 
value obtained from the TopLevelCode. Also it performs operations on the motor such as 
homing to the 0 position. This code returns the end position of the move operated to the top 
level code. 
Aardvark Program This code interfaces with Aardvark Hardware and read hexadecimal values. It then converts 
these values into receive and transmit power of the free space optical link. This code returns 
the transmit power and receive power to the top level code. 
BERTwave Program This code interfaces with the BERTwave and runs the data across the optical link until a 
specific confidence level is reached. It returns the bit error rate and the confidence level of the 
measurement achieved. 
TopLevelCode This code interface and controls operation of all other codes. It prints experimental data to a 
comma separated file and makes decisions on control of the codes dependent on the returned 
values. 
 
For the purpose of running automated tests, all of these programs are encompassed in a top level misalignment code that called 
each of these programs to run in the following operation: Move the angle of transmission to boar sight, take the power received 
in dBm at the current location, move the motor in the positive direction until a 0.5 dB change in receive power occurs, run the 
BERTwave data acquisition program and send data until a bit error rate received is at a 95% confidence level, take the power in 
dBm received at this new location for hysteresis correction (redundancy), and move the motor again. This program continues to 
run until the maximum angle before loss of signal occurs, then the program continues the same operation but moves the motor 
in the negative direction until loss of signal occurs, the program then runs back past its original position. Data is recorded 
continuously and printed to a CSV file. The operation of the code is shown below in figure 2.  
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Figure 2, Top-level Code operation 
 
The BERTwave Data Acquisition program was used to obtain the bit error rate data for each position of the motor. The code was 
imported to the top-level code and run for each position as shown above.  The code functioned as follows: The BERTwave would 
send data for a short period and read the error count to determine if the link can be closed. If the link is closed, then the BERTwave 
begins a testing measurement. The BERTwave runs for a second and then reads back the bit error rate, the total number of errors 
that occurred, and the total number of data bits sent. The program uses the confidence level equation for the error free case if no 
errors have occurred on the first iteration, or if there has not been a change in the number of errors reported from the previous 
iteration. If an error does occur, the error case confidence level equation is used. If the confidence level calculated reaches or 
passes the user defined confidence level, then the program ends and the bit error rate and the confidence level of the data is 
reported. If the confidence level is lower than the confidence the user requested, the BERTwave would run another second and 
Initialize Top Module
Check if Bertwave
Can Close A Link
Move the Motor
 1 Step away
from current
location
Run the Aardvark Program
 to obtain the power values
at the current location
YES
Run The Bertwave Data
acquisition Program
Change direction of motor
Bert cant close link
END Write all power,
error count, error rate and
Conﬁdence  level data to a CSV
Conﬁdence level reached
Run the Aardvark Program
 again obtain the power values
at the current location
to counteract hysteresis
 Motor ran past the initial position twice
END
NO
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then follow the above operation. The code will iterate until the requested confidence level is met. This operation is shown in 
figure 3 below.  
 
Figure 3. BERTwave Data Acquisition Program which is run by the Top-level Code 
 
A Poisson distribution was used to determine the confidence level of the current BERTwave measurement. This was done to 
insure that the reported bit error rate was a statistically sound measurement. Data was taken every second. This runtime between 
measurements was decided upon because data was being sent at a rate of 10 gigabytes per second. This is a sufficient time to 
send enough data to get good reported values and limit latency of the code’s operation. The user is able to set a specific confidence 
level for the BERTwave bit error rate code. The code will run until this confidence level is achieved. The equations used to 
calculate the confidence of the bit error rate measurement are shown below in Equations 1 and 2. 
 The following equation was used in the case where no error had occurred respect to the last reported data3.  
 
                                                                                           𝐶𝑙 = 1 − 𝑒
−𝐵𝑠∗𝐵𝐸𝑅                                                                                      (1) 
 
The equation below was used when errors occurred in the last reported data4. 
 
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                                                  𝐶𝑙 = (1 − (𝑒
−𝐵𝑠∗𝐵𝐸𝑅 ∗
1
𝐵𝑒!
∗ (𝐵𝐸𝑅 ∗ 𝐵𝑠
𝐵𝑒)                                                                    (2) 
 
Where Cl = Confidence Level, Bs = Number of bits sent, BER = Bit Error Rate, and Be = Number of bits in error. 
 
After the confidence level is reached, the motor is moved until a user settable change in power is observed. Then, the code repeats 
and runs the BERTwave until the confidence level is reached again. This approach was chosen over moving a predefined distance 
to ensure enough data points are collected to show a trend of power received vs bit error rate as the relationship between distance 
traveled and power is nonlinear.  
 
III-Results 
The data is stored in a comma separated variable file as shown below. It can be seen that the relationship between the angle of 
transmission and power received is nonlinear. As the angle increased or decreased from boar sight, the power decayed 
exponentially. This can be seen in Figure 4. It can also be seen that the relationship between receive power and bit error rate is 
related. Figure 5 below shows the power vs bit error rate relationship for a test run of the automated misalignment program. 
There is hysteresis in the motor. This is due to inaccuracies in the KDC101 movements. The given commands did not result in 
the motor moving to the exact position requested. Changes in the true positions were seen to be dependent on the direction of 
approach of the motor.  
 
 
  Figure 4, Position vs. Power    Figure 5, Error Rate vs. Power 
 
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IV-Future Work and Conclusion 
Multiple tests were implemented to measure the performance of free space communications links using infrared light. The tests 
were created using Python language and integrated with hardware devices.  
This first test was created for the HiDRA project within NASA’s Space Communication and Navigation  program. Its purpose 
was to determine the relationship between the angle of transmission of a beam, the power received, and the bit error rate of the 
signal.   
The next step in this development will be to add a programmable attenuator to characterize the infrared data beam to baseline 
transceiver performance in a fiber only system for comparison to the free space measurements.  
 
Acknowledgments 
This work is supported by the Advanced Communications and Navigation Division within the NASA Space Communications 
and Navigation (SCaN) Program.   
 
References 
 
1 Sarah A. Tedder, Bryan Schoenholz, Shannon N. Suddath, “Study of Lateral Misalignment Tolerance of a Symmetric Free-
Space Optical Link for Intera International Space Station Communication,” AIAA International Communications Satellite 
Systems Conference; 34th , Oct. 2016. 
 
2 Daniel E. Raible, Robert R. Romanofsky, James M. Budinger, Jennifer Nappier, Alan Hylton, Aaron J. Swank, and Anthony 
L. Nerone. "On the Physical Realizability of Hybrid RF and Optical Communications Platforms for Deep Space Applications", 
32nd AIAA International Communications Satellite Systems Conference, AIAA SPACE Forum, July 14th, 2014,(AIAA 2014-
4482) Available: http://dx.doi.org/10.2514/6.2014-4482 
 
 
3 “How do I measure Bit Error Rate (BER) to a given confidence level on N490xA/B Serial BERTs?,” How do I measure Bit 
Error Rate (BER) to a given confidence level on N490xA/B Serial BERTs? | Keysight (formerly Agilent's Electronic 
Measurement) Available: http://www.keysight.com/main/editorial.jspx%3Fckey%3D1481106%26id%3D...&sa=U&ei=p-
e8VMmWI8n0UsW6gKAE&ved=0CCAQ9QEwBQ&usg=AFQjCNEO_PZyV0U7VnM9OIg1LF8lmICquw?&cc=US&lc=en
g.  
 
 
4 Müller, M., Stephens, R., and McHugh, R., “Total Jitter Measurement at Low Probability Levels, Using Optimized BERT Scan 
Method,” White Paper- Agilent Technologies, 2005, pp. 7–9.  
Published at DesignCon Available: literature.cdn.keysight.com/litweb/pdf/5989-2933EN.pdf 
 
 
 
 
 
 


Infrared Free Space Communication - The Autonomous Testing of Free Space Infrared Communication

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