Transcript Design of an adaptive equalizer for coherent receivers in optical communication systems

TECHNOLOGY
C O R PO RATE
Investigation of Decision Feedback
Equalizer in Optical Systems
Master Thesis
Wissam Haddad
Helsinki University of Technology
E-Mail: [email protected]
Duration: April – September 2004
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Supervisor:
Prof. Dr. Gustav Sven Haggman
Helsinki University of Technology
E-Mail: [email protected]
Supervisor:
Dr. Harald Rohde
Siemens AG - CT IC 2
E-Mail: [email protected]
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Outline
• Introduction
• Signal degradation
• Dispersion
• Receiver Noise
• Nonlinearites
• System setups
• Direct detection receiver
• Coherent receiver
• DFE Structure
• Adaptation Scheme
• Phase adaptation
• Results
• Eye Opening Penalties variations
• Eye Opening penalties (Direct Detection vs. Coherent)
• Eye Opening Diagrams
• Bit Error Rates
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• Conclusions
© Siemens AG, CT IC 2, Wissam Haddad
28.09.2004
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Introduction
• An optical fiber consists of a cylindrical core surrounded by a cladding
• That reflection is usually caused by creating a higher refractive index in the
core of the glass than in the surrounding cladding glass
• Single mode fibers: only one mode is propagated, because the core size
approaches the operational wavelength λ=1.3 μm
• Multimode fibers: The number of modes propagated depends on the core size
and numerical aperture
Coating
Cladding
Core
Reflected light wave
Incident angle
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© Siemens AG, CT IC 2, Wissam Haddad
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Signal degradation
• Dispersion
• Modal Dispersion
• Polarization mode dispersion
• Chromatic dispersion
• Receiver Noise
• Thermal noise
• Shot noise
• Spontaneous emissions
• Nonlinearities
• Self Phase Modulation (SPM)
• Cross Phase Modulation (XPM)
• Stimulated Brillouin Scattering (SBS)
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• Stimulated Raman Scattering (SRS)
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Direct Detection Receiver
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•
•
•
•
•
The number of samples within each signal block used is N = 32768
The total number of bits launched throughout all signal blocks is 128
The number of samples per bit is 256
The center frequency of the signal is adjusted to 193.1 THz
The data rate considered is 10 GHz
© Siemens AG, CT IC 2, Wissam Haddad
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Coherent Receiver
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•
•
•
•
•
The number of samples within each signal block used is N = 32768
The total number of bits launched throughout all signal blocks is 128
The number of samples per bit is 256
The center frequency of the signal is adjusted to 193.1 THz
The data rate considered is 10 GHz
© Siemens AG, CT IC 2, Wissam Haddad
28.09.2004
TECHNOLOGY
DFE Structure
a(k)
FFF
h(k)
+
Data output
Σ
â(k)
C O R PO RATE
_
FBF
f(k)
•Basic problem in linear filtering: desired signal and noise processed
together
•DFE approach: Utilize previous symbol decisions to cancel postcursor ISI
•Both h(k) and f(k) are linear filters
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•Complete filtering system is nonlinear, because nonlinear operation
(slicer) is in the feedback loop
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Adaptation Scheme
ci
Eye Analyzer1
EOP1
ci +β
Eye Analyzer2
EOP2
Comparator
ci+1=ci + β
EOP2 is the minimum
ci - β
Eye Analyzer3
EOP3
• A Pseudo Random Bit Sequence (PRBS) was transmitted for different runs
• Coefficients were adjusted in a way to decrease the eye opening penalty of
the whole system
• This adaptation method is not designed for the purpose of running on real
time system but for the evaluation of the performance of the receivers
considered
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Phase Adaptation
C O R PO RATE
• The phase was changed starting from -90 degrees to 270 degrees
with 10 degrees interval
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• When including nonlinearities of the system, a phase shift is
introduced in the system
• It is important to adapt the system to an optimum phase, which will not
be the same depending on the power that is infused in the system
• The phase of LO is set to be the one who lead to the lowest eye
opening penalty
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Eye Opening Penalties variations
PRBS transmitted 100 runs
Overall Eye Opening penalty is the minimum of EOP1, EOP2, EOP3
Eye Opening Penalty 1
Eye Opening Penaty 2
Eye Opening Penatly 3
System EOP
8
7
6
Eye Opening Penalty, dB
C O R PO RATE
9
5
4
3
2
1
0
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0
10
20
30
40
50
Run, Unit
60
70
80
90
100
© Siemens AG, CT IC 2, Wissam Haddad
28.09.2004
6
6
EOP for back to back
EOP for DD Receiver
EOP for Coherent Receiver
5
EOP for DD receiver
EOP for coherent receiver
EOP for back to back
5
Eye Opening Penatly, dB
4
Eye Opening Penalty, dB
TECHNOLOGY
C O R PO RATE
)
Direct Detection receiver vs. Coherent receiver (1)
3
2
1
4
3
2
1
0
0
-1
0
5
10
15
Run, Unit
Linear Case
20
25
0
5
10
15
20
25
Run, Unit
Nonlinear case (γ = 1.365W/km , P=0dBm)
• The direct detection receiver will have a penalty close to the back to back system
• The coherent detection receiver will stabilize at a higher penalty (≈1.5dB).
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• The adaptation rate of the coefficients when nonlinearities are considered
is slower than that of the linear case
© Siemens AG, CT IC 2, Wissam Haddad
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• Linear Case, 50 -100 Km SSMF
1.5
0.025
Eye opening penalty dB
1.4
Eye opening penalty
C O R PO RATE
1.6
0.03
dB
TECHNOLOGY
Direct Detection receiver vs. Coherent receiver (2)
0.02
0.015
0.01
1.3
1.2
1.1
1
0.9
0.005
0.8
0
50
55
60
65
70
75
80
85
Length Unit Km SSMF
90
95
Direct detection Receiver
100
0.7
50
55
60
65
70
75
80
85
90
Length Unit Km SSMF
95
100
Coherent Receiver
• The eye opening penalty considered for a specific length was taken after
it has stabilized to a certain value (after 50 runs)
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• The eye opening penalties for both systems increase with the length of the fiber
© Siemens AG, CT IC 2, Wissam Haddad
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• Nonlinear Case, 50 -100 Km SSMF, γ = 1.365W/km
0.35
2
0.3
1.8
0.25
1.6
Eye opening penalty dB
Eye opening penalty dB
TECHNOLOGY
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Direct Detection receiver vs. Coherent receiver (3)
0.2
0.15
0.1
0.05
0
50
1.4
1.2
1
0.8
55
60
65
70
75
80
85
Length Unit Km SSMF
Direct detection Receiver
90
95
100
0.6
50
55
60
65
70
75
80
85
90
Length Unit Km SSMF
95
100
Coherent Receiver
• The coefficients of the equalizer were adjusted and adapted to an eye opening
penalty approximately the same for both linear and nonlinear systems
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Interpretation of an eye opening diagram
D
C O R PO RATE
A
B
C
•
•
•
•
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A: the interval over which sampling is error free with a variable margin noise
B: the margin of the noise
C: the spread in data sampling points
D: the sensitivity of noise degradation with timing errors (the slope)
•The vertical eye opening indicates the margin for bit errors due to noise
•The horizontal eye opening indicates the margin for timing errors
due to imperfect clock
© Siemens AG, CT IC 2, Wissam Haddad
28.09.2004
0.07
0.06
0.05
Signal amplitude A
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Eye Opening Diagrams (1)
0.04
0.03
0.02
0.01
0
-0.01
-150
-100
-50
0
time ps
50
100
150
Back to back system
• All eye diagrams are plotted before the slicer of the equalizer
• The optimum detection time is found by fitting a rectangular box with
maximum height in the diagram
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© Siemens AG, CT IC 2, Wissam Haddad
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Eye Opening Diagrams (2)
For a direct detection receiver
-3
3
x 10
0.1
0.09
2.5
0.07
Signal amplitude
2
Signal amplitude
C O R PO RATE
0.08
1.5
0.06
0.05
0.04
1
0.03
0.02
0.5
0.01
0
-150
-100
-50
0
time
50
100
150
Before equalizer, 100 Km SSMF (linear)
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0
-150
-100
-50
0
time
50
100
150
After equalizer, 100 Km SSMF (linear)
© Siemens AG, CT IC 2, Wissam Haddad
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TECHNOLOGY
Eye Opening Diagrams (3)
For a coherent receiver
0.12
0.16
0.14
0.1
Signal amplitude A
Signal amplitude
C O R PO RATE
0.12
0.08
0.06
0.04
0.08
0.06
0.04
0.02
0.02
0
0
-0.02
-150
-100
-50
0
time
50
100
150
Before equalizer, 100 Km SSMF (linear)
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0.1
-0.02
-150
-100
-50
0
time ps
50
100
150
After equalizer, 100 Km SSMF (linear)
© Siemens AG, CT IC 2, Wissam Haddad
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TECHNOLOGY
Eye Opening Diagrams (4)
C O R PO RATE
• The sensitivity of noise degradation with timing errors in coherent receiver
worse than that in the direct detection receiver
The slope of the eye diagram of the direct detection is sharper than that of the
coherent receiver
• The interval over which sampling is error free is similar for the three systems
The width of the eye opening is the same for the three systems
• The spread of data sampling is highest for the coherent receiver
• The noise margin for the direct detection receiver is better that the one
obtained when using the coherent detection receiver
The distance of the eye opening to the center of the diagram is larger for the
direct detection receiver
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• Linear Systems
-1
10
Back to back system
Direct detection
Coherent receiver
-2
10
-3
10
BER
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Bit Error Rates (1)
-4
10
-5
10
-6
10
-7
10
10
12
14
16
18
OSNR (dB)
20
22
24
• The direct detection receiver has a better performance than the coherent receiver
achieving a better bit error rate at different OSNRs.
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• For a BER of 10-5, the difference is approximately of 1.2 dB.
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Bit Error Rates (2)
• Nonlinear Case, 100 Km SSMF, γ = 1.365W/km, P=0 dBm
-1
10
Direct detection
Coherent receiver
-2
-3
10
BER
C O R PO RATE
10
-4
10
-5
10
-6
10
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10
12
14
16
18
OSNR (dB)
20
22
24
• The direct detection receiver outperforms the coherent receiver approximately by
3 dB for a bit error rate of the 10-5.
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Bit Error Rates (3)
• The direct detection receiver leads to lower bit error rates than the coherent receiver.
• The squaring of the field amplitude in the direct detection receiver has a more
important weight on the performance of the system than the effects of the nonlinearities
that arise due to the fact the refractive index depends on the intensity of the applied
field:
~ E   n  n E 2
n
where
n 
3
 3 
8n
and
 3 
the nonlinear susceptibility
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• Coherent Receiver
0
10
Coherent,
Coherent,
Coherent,
Coherent,
-1
10
Linear
Nonlinear, P= 0 dBm
Nonlinear, P= 8 dBm
Nonlinear, P=16dBm
-2
10
BER
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Bit Error Rates (4)
-3
10
-4
10
-5
10
-6
10
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10
12
14
16
18
20
OSNR (dB)
22
24
26
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TECHNOLOGY
Bit Error Rates (5)
C O R PO RATE
• With higher power in the system, the rate of the errors is higher.
• By increasing the power at the input of the fiber, the performance of the system
degrades since the effect of the nonlinearities is increased with the power
injected.
• Since different parts of the pulse undergo different phase shifts which give rise
to pulse chirping, it can be seen that the SPM chirping effect is proportional to
transmitted power and affects the pulse broadening effects of dispersion. I
• The SPM can significantly increase the system penalty due to dispersion
because of increased ISI.
• With increasing power, the phase shifts distort considerably the sensitivity of
the coherent receiver.
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• Direct detection Receiver
0
10
Direct
Direct
Direct
Direct
-1
10
detection,
detection,
detection,
detection,
Linear
Nonlinear, P= 0 dBm
Nonlinear, P= 8 dBm
Nonlinear, P=16dBm
-2
10
BER
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Bit Error Rates (6)
-3
10
-4
10
-5
10
-6
10
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10
12
14
16
OSNR (dB)
18
20
22
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Bit Error Rates (7)
• The performance of the system does not degrade considerably when
increasing the transmitted power, which did not make sense at the first
inspection
• This ensued to check the eye opening penalties after detection,
but before the decision feedback equalizer for different transmitted powers
• The eye opening penalty at the input of the equalizer was checked vs the
transmitted power which was varied in 0.5 dBm steps from 0 to 17 dBm:
4.5
• A difference of 0.076 dB is
observed which is due to
statistical computations
Eye opening penalty dB
4
3.5
3
• That explains the similar
results in the BER curves for
the different powers that
were obtained for the direct
detection receiver
2.5
2
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1.5
0
2
4
6
8
10
12
Injected Power dBm
14
16
18
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Bit Error Rates (8)
• This can be interpreted that the SPM effect with the absence of the dispersion
compensation, add usefully to the system compensating for the pulse
broadening effect of dispersion
• The difference in the performance between the two receivers is more
significant with an increasing input power. This is due to the fact that the effect
of nonlinearities is more important in coherent systems
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Conclusions (1)
• The designed equalizer was used for direct detection receiver and a
coherent receiver and the performances of both receivers were evaluated
and compared
• By computing the bit error rates for different optical signal to noise ratios,
the direct detection receiver leads to lower error rates than the coherent
receiver
• Considering a communication systems with different transmitted powers,
when increasing the power of the signal, the effect of the nonlinearities
(especially SPM) are more eminent in the coherent receiver
• The performance of the direct detection receiver is not affected noticeably
since the SPM effects offset the pulse broadening due to the dispersion
• The results obtained considering a feed forward equalizer were similar to
the one presented above with different indicative results since no ISI
cancellation is done through any feed backward transversal filter
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Conclusions (2)
• Considering a Viterbi receiver based on Maximum Likelihood Sequence
Estimation, the coherent receiver outperforms the direct detection with around
3dB gain for single channel setup and 4-5dB for the dual channel setup
compared to direct detection
• the study can be extended by checking the performance of these equalizers
with different types of fibers, different modulation schemes and an analysis of
the effects of nonlinearities on WDM systems
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Thank you
Questions