CLIPPING DISTORTION IN LASER DIODE: MODEL, SIMULATIONS AND STATISTICS

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CLIPPING DISTORTION IN LASER DIODEMODEL,
SIMULATIONS AND STATISTICS

• By Omar Falou

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Introduction

• Telecommunications through optical fiber have
  become a major information-transmission system
• Networks based on optical fiber technology
  provides, fast, efficient and reliable means of
  transmitting information

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Introduction (contd)

• Optical System Transmitter, Fiber and Receiver.
• Transmitter can be either a Light Emitting Diode
  (LED) or a Laser Diode (LD).
• LDs have some advantages over LEDs since they are
  high end, with narrow line width, which make them
  ideal for long transmission

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Optical System an Overview
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Rayleigh Fading Channel

• Simulates a wireless communication channel
• A transmitted signal travels to the receiver
  along several paths that may have different
  lengths therefore, different time delays

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Rayleigh Fading Channel (contd)

• Fading occurs when signals traveling along
  different paths interfere with each other.

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Clipping Distortion

• Clipping distortion has been a subject of
  interest in many recent studies on
  frequency-division subcarrier multiplexing (SCM)
  used in cable television (CATV) applications
• Clipping occurs when the modulating signal
  current, which drives the laser diode,
  occasionally drops below the laser threshold
  current, turning the laser off (figure on the
  next slide).

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Clipping Distortion (contd)

• Region 1 spontaneous emission region
• Region 2 stimulated emission region or the
  linear region.
• Region 3 saturated region.

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Simulink Model
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Simulations

• 2 kinds of fading channel were investigated
• Channel with NO GAIN (0 dB).
• Channel with GAIN (effective gain considering all
  paths is different than 0dB)
• Also, for each type of channel, the clipping
  distortion was studied by varying the number of
  paths (1, 2, 3, 4 and 5)

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Fading Channel Simulation Parameters

• Doppler Frequency 40 Hz
• Sample Time 10-4 s
• Path Delay 0, 10-3, 2x10-3, 3x10-3, 4x10-3 s
• Path Gain 0, -1, -2, -3, -4
• Overall Gain
• 0 dB
• different than 0

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Results and Analysis
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Results Channel with No Gain(1 path)
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Results Channel with No Gain(2 paths)
225/10000 2.25
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Results Channel with No Gain(3 paths)
140/10000 1.40
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Results Channel with No Gain(4 paths)
200/10000 2.00
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Results Channel with No Gain(5 paths)
201/10000 2.01
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Results Channel with Gain(1 path)
201/10000 2.01
159/10000 1.59
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Results Channel with Gain(2 paths)
201/10000 2.01
1160/10000 11.6
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Results Channel with Gain(3 paths)
201/10000 2.01
1842/10000 18.42
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Results Channel with Gain(4 paths)
201/10000 2.01
2495/10000 24.95
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Results Channel with Gain(5 paths)
201/10000 2.01
3003/10000 30.03
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Results Summary
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Conclusion

• One of the main problems with using laser diodes
  is clipping distortion
• Clipping distortion significantly increases with
  the increase of number of paths in channel with
  effective gain
• However, it remains almost constant for channel
  with no gains

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Suggested Solution and Future Work

• Code the original signal at the transmitter side
  before modulating in order to minimize or
  eliminate the effect of clipping
• Decode the signal at the receiver after it has
  been demodulated.
• By doing so, clipped areas of the signal can be
  detected and recovered.

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Acknowledgments

• Dr. Xavier Fernando, Dept. of Electrical and
  Computer Engineering, Ryerson University
• Dr. Michael Kolios, Dept. of Math, Physics and
  Computer Science, Ryerson University