Post on 01-Jan-2016
CLIPPING DISTORTION IN LASER DIODE:
MODEL, SIMULATIONS AND STATISTICS
By: Omar Falou
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
Introduction (cont’d)
• 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
Optical System: an Overview
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
Rayleigh Fading Channel (cont’d)
• Fading occurs when signals traveling along different paths interfere with each other.
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).
Clipping Distortion (cont’d)
• Region 1: spontaneous emission region
• Region 2: stimulated emission region or the linear region.
• Region 3: saturated region.
Simulink Model
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)
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
Results and Analysis
Results: Channel with No Gain(1 path)
-0.5 0 0.5 1 1.5 2 2.5 3 3.5 40
100
200
300
400
500
600
700
800
900
1000
159/10000 = 1.59 %
Results: Channel with No Gain(2 paths)
-0.5 0 0.5 1 1.5 2 2.5 3 3.5 40
100
200
300
400
500
600
700
800
900
225/10000 = 2.25 %
Results: Channel with No Gain(3 paths)
-0.5 0 0.5 1 1.5 2 2.5 3 3.5 40
100
200
300
400
500
600
700
800
900
140/10000 = 1.40 %
Results: Channel with No Gain(4 paths)
-0.5 0 0.5 1 1.5 2 2.5 3 3.5 40
100
200
300
400
500
600
700
800
900
1000
200/10000 = 2.00 %
Results: Channel with No Gain(5 paths)
-0.5 0 0.5 1 1.5 2 2.5 3 3.5 40
100
200
300
400
500
600
700
800
900
1000
201/10000 = 2.01 %
Results: Channel with Gain(1 path)
201/10000 = 2.01 %
-1 0 1 2 3 4 5 6 70
100
200
300
400
500
600
700
800
900
1000
159/10000 = 1.59 %
Results: Channel with Gain(2 paths)
201/10000 = 2.01 %
-1 0 1 2 3 4 5 6 70
100
200
300
400
500
600
700
1160/10000 = 11.6 %
Results: Channel with Gain(3 paths)
201/10000 = 2.01 %
-1 0 1 2 3 4 5 6 70
100
200
300
400
500
600
1842/10000 = 18.42 %
Results: Channel with Gain(4 paths)
201/10000 = 2.01 %
-1 0 1 2 3 4 5 6 70
100
200
300
400
500
600
2495/10000 = 24.95 %
Results: Channel with Gain(5 paths)
201/10000 = 2.01 %
-1 0 1 2 3 4 5 6 70
50
100
150
200
250
300
350
400
450
500
3003/10000 = 30.03 %
Results’ Summary
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
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.
Acknowledgments
• Dr. Xavier Fernando, Dept. of Electrical and Computer Engineering, Ryerson University
• Dr. Michael Kolios, Dept. of Math, Physics and Computer Science, Ryerson University