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Investigation of Decision Feedback Equalizer in Optical Systems

Investigation of Decision Feedback Equalizer in Optical Systems

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Investigation of Decision Feedback Equalizer in Optical Systems

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  1. Investigation of Decision Feedback Equalizer in Optical Systems Master Thesis Wissam Haddad Helsinki University of Technology E-Mail: wissam.haddad@hut.fi Duration: April – September 2004 Supervisor: Prof. Dr. Gustav Sven Haggman Helsinki University of Technology E-Mail: sgh@hut.fi Supervisor: Dr. Harald Rohde Siemens AG - CT IC 2 E-Mail: harald.rohde@siemens.com

  2. Outline • Introduction • Signaldegradation • 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 • Conclusions

  3. Coating Cladding Core Reflected light wave Incident angle 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

  4. 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) • Stimulated Raman Scattering (SRS)

  5. Direct Detection Receiver • 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

  6. Coherent Receiver • 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

  7. Data output a(k) FFF h(k) + â(k) _ FBF f(k) Σ DFE Structure • 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 • Complete filtering system is nonlinear, because nonlinear operation • (slicer) is in the feedback loop

  8. Eye Analyzer1 ci ci+1=ci + β Eye Analyzer2 Comparator ci+β EOP2 is the minimum Eye Analyzer3 ci - β EOP1 EOP2 EOP3 Adaptation Scheme • 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

  9. Phase Adaptation • 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 was changed starting from -90 degrees to 270 degrees with 10 degrees interval • The phase of LO is set to be the one who lead to the lowest eye opening penalty

  10. Eye Opening Penalties variations PRBS transmitted 100 runs Overall Eye Opening penalty is the minimum of EOP1, EOP2, EOP3

  11. Direct Detection receiver vs. Coherent receiver (1) ) Nonlinear case (γ = 1.365W/km, P=0dBm) Linear Case • 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). • The adaptation rate of the coefficients when nonlinearities are considered • is slower than that of the linear case

  12. Km SSMF Km SSMF Direct Detection receiver vs. Coherent receiver (2) • Linear Case, 50 -100 Km SSMF Coherent Receiver Direct detection Receiver • The eye opening penalty considered for a specific length was taken after • it has stabilized to a certain value (after 50 runs) • The eye opening penalties for both systems increase with the length of the fiber

  13. Km SSMF Km SSMF Direct Detection receiver vs. Coherent receiver (3) • Nonlinear Case, 50 -100 Km SSMF, γ = 1.365W/km Direct detection Receiver 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

  14. D A B C Interpretation of an eye opening diagram • 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

  15. Eye Opening Diagrams (1) 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

  16. Eye Opening Diagrams (2) For a direct detection receiver After equalizer, 100 Km SSMF (linear) Before equalizer, 100 Km SSMF (linear)

  17. Eye Opening Diagrams (3) For a coherent receiver After equalizer, 100 Km SSMF (linear) Before equalizer, 100 Km SSMF (linear)

  18. Eye Opening Diagrams (4) • 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 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 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

  19. Bit Error Rates (1) • Linear Systems • The direct detection receiver has a better performance than the coherent receiver • achieving a better bit error rate at different OSNRs. • For a BER of 10-5, the difference is approximately of 1.2 dB.

  20. Bit Error Rates (2) • Nonlinear Case, 100 Km SSMF, γ = 1.365W/km, P=0 dBm • The direct detection receiver outperforms the coherent receiver approximately by • 3 dB for a bit error rate of the 10-5.

  21. 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: where and the nonlinear susceptibility

  22. Bit Error Rates (4) • Coherent Receiver

  23. Bit Error Rates (5) • 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. • With higher power in the system, the rate of the errors is higher. • 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.

  24. Bit Error Rates (6) • Direct detection Receiver

  25. 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: • A difference of 0.076 dB is • observed which is due to • statistical computations • That explains the similar • results in the BER curves for • the different powers that • were obtained for the direct • detection receiver

  26. 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

  27. 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

  28. 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

  29. Thank you Questions