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10/25/99 1
IEEE 802.3-99
Duobinary Coding for 10GbE
Greg Copeland, Bertan TezcanEmail: [email protected], [email protected]
Abstract: We propose a new optical codingtechnique for 10GbE, Duobinary coding. Thiscoding has a spectrum bounded within the Nyquistband, which makes it a minimum bandwidthcoding-scheme. This presentation comparesduobinary coding with several other possiblecoding schemes
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Duobinary Coding
• Duobinary coding is a partial response signal:certain amount of ISI is not suppressed by theequalizer but rather left for a simple detector tohandle.
• Permits noise enhancement to remain small evenwhen amplitude distortion is severe.
• Changes input data sequence ak, which has a flatspectrum, into a sequence bk whose spectralcharacteristics match channel.
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• Simple to implement. Output sequence isbk = ak + ak-1
• A precoder is added to eliminate error propagationin receiver, transfer function becomes:
bk=(ak+1)dk-1
dk-1 can not be zero, so estimated ak is:a_estk = {-1 if bk =0
0 else which is just memoryless inverse mapping
Duobinary Coding/Cntd.
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IEEE 802.3-99
D
Gk MIMak
dk-1
dk bk a_estk
Gk
aka_estk
Da_estk-1
a_estk
Figure 1: a: A one pole recursive filter transforms bk into an estimate a_estk b: Toavoid error propogation, the filter is replaced by a precoder and a memoryless
inverse mapping (MIM)
Duobinary System Architecture
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Duobinary Performance
• Duobinary gives very robust solutions for manyproblems in optical channels. It suppressesdispersion and nonlinear effects in optical fiber,waveform distortion and wavelength fluctuation inWDM systems.
• To pack a Terabit/s capacity into the gainbandwidth of WDM systems, spectral efficiencyhas to be more than 0.23 b/s/Hz, or a channelspacing of 0.7nm. Duobinary signals increasedspectral efficiency to 0.6 b/s/Hz.
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• High dispersion tolerance, about twice that of IM(intensity modulated) signals.
• Suppresses Four-wave mixing (FWM) in WDMsystems.
• Typical setup includes push-pull type LiNbO3(LN) Mach-Zehnder modulator in transmitter side.
• Can be demodulated into a binary signal with aconventional direct detection type optical receiver.
Duobinary Performance/Cntd.
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SignalSource
10Gb/s
DuobinaryEncoder
D
ExOR Add
D
LPF
Band LimitingFilter
Modulator PhotoDiode
LPF
BasebandFilter
Output
Transmitter Receiver
Figure 2: Typical experimental setup for duobinary transmission system
Experimental Setup
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Simulation and Experimental Results
• Eye opening degrades due to chromatic dispersionas the transmission distance is extended. Eyeopening penalty gives the performance:
• Penalty = -10log10(B/A * P0/PAVE) (dB)• A: eye-opening of the reference eye diagram• B: eye-opening of the distorted signal due to
chromatic dispersion• P0/PAVE: Averaged optical power if band-limiting
filter inserted
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Eye Opening Penalty
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Spectrum Comparison
• Duobinary is Minimum-Bandwidth signal, hightolerance to chromaticdispersion, powerlimitation due to SBS canbe relaxed.
• Power Spectrum:
P(f) = T2*cos2(πfT)
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BER and Receiver Sensitivity
• Due to more spectral efficiency attained byduobinary coding, there is less receiver sensitivitycompared to other signaling techniques.
• With a powerful FEC scheme added to duobinarysignal, very good BER performance can beattained. This opens the possibility of reaching 2-3times distance in fiber optic cables without anyrepeaters.
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Forward Error Correction
• Reed-Solomon (255, 239) is 8 error correcting FEC scheme. Itis ideal for bursty channels like fiber optic, and wirelesssystems.
• RS(255, 239) is recommended for submarine fiber optic systemsby ITU (ITU G.975), and is being considered for other fiberoptic systems.
• Following is receiver sensitivity versus BER curves for a SMFat 110 km. There is just 2dB degradation in receiver sensitivityfor duobinary signaling. Degradation is 5dB for Mb34 and 8dBfor NRZ signaling. For Duobinary and RS concatenated systems,there is 7dB improvement in receiver sensitivity
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Remarks and Conclusion
• Duobinary coding is more efficient compared to othersignaling techniques.
• Duobinary is MB signal with bandwidth at half the bit rate,and most of the signal power is concentrated in lowerfrequencies. More channel bandwidth reduction than othertechniques.
• 6dB eye opening improvement over NRZ, and 5dB onMB34. Timing recovery constraints are reduced.
• Transmission distance is enhanced, less number ofrepeaters are needed for optical fiber systems
• Very simple algorithm. No separate complex encoder ordecoder needed.
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Remarks and Conclusion/Cntd.
• Less receiver sensitivity,and better BERperformance. If combined with another FECalgorithm, like Reed-Solomon(255,239),significant error correction improvement ispossible. (~12dB)
• Extensive research has been made upon duobinarysignaling for high speed fiber optic systems in last10 years, and very promising results obtained.
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References
[1] Takashi Ono, and Yano Kiyoshi, “ Characteristics of Optical Duobinary signals inTerabit/s Capacity, High-Spectral Efficiency WDM systems” IEEE JLT, vol. 16, pp.788-797 May 1998
[2] S. Aisawa, J. Kani, “A 1580-nm Band WDM Transmission Technology EmployingOptical Duobinary Coding”, IEEE JLT, vol 17, pp. 191-198, Feb 1999
[3] K. Yonenaga, S. Kuwano, “ Dispersion-Tolerant Optical Transmission Systemusing Duobinary Transmitter and Binary Receiver”, IEEE JLT, vol. 15, pp 1530-1537, Aug 1997
[4] A.F. Elrefaie, R.E. Wagner, “Chromatic Dispersion Limitations in CoherentLightWave Transmission systems”, IEEE JLT, vol. 6, pp. 704-709, May, 1988
[5] C.G. Lee, D.I. Lee, and D.Y. Kim, “An Evaluation for high speed optical line code:Minimum Line Code MB34”, proc. Of ICT’98, Porto Carras, Greece, June 22-25,1998
[6] S. B. Wicker, Error Control Systems for Digital Communication and Storage,Prentice Hall, 1995
