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Modulation formats for digital fiber transmission

Eric Tell 050329 . Modulation formats for digital fiber transmission. Outline. Fiber performance limitations WDM Optical vs. radio communication Optical modulators Modulation formats Amplitude shift keying Duo-binary signalling Optical single sideband signalling

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Modulation formats for digital fiber transmission

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  1. Eric Tell 050329 Modulation formats for digital fiber transmission

  2. Outline • Fiber performance limitations • WDM • Optical vs. radio communication • Optical modulators • Modulation formats • Amplitude shift keying • Duo-binary signalling • Optical single sideband signalling • Simulation/experimental results • Summary

  3. Fiber performance limitations • Fiber Loss • Chromatic dispersion • different refractive index for different wavelengths • Fiber non-linearities

  4. Chromatic dispersion • Distance limit ~1/(bit rate)² • Example: Single mode fiber @1550nm • chromatic dispersion: 17ps/km-nm • dispersion limited distance: ~100km @10Gbit/s • comparable to loss limit • EDFA => increased loss-limited distance • Chromatic dispersion becomes the limiting factor in single mode long-haul fibers! • We want to decrease the bandwidth for a given datarate!

  5. Wave Division Multiplexing • Decreased channel spacing leads to interchannel interference and makes it difficult to compensate for fiber nonlinearities • Narrower subchannels would be nice...

  6. WDM (cont'd) • In a high capacity link the whole EDFA spectrum is filled with subchannels • The bandwidth of each subchannel is proportional to its bit rate • Total fiber capacity is given by the spectral efficency: (bitrate per channel)/(channel spacing)

  7. WDM (cont'd) • In a practical case using NRZ a spectral efficiency of 40% can be reached Power spectral density of NRZ

  8. WDM (cont'd • More GB/s per channel does not increase total bandwith, however • It results in fewer channels to manage • Increased channel spacing decreases some non-linear distortions • BUT to reach higher spectral efficiency a format with narrower spectrum for a given bandwidth is needed (while at the same time not increasing other impairments)

  9. How can this be achieved? • M-ary Amplitude Shift Keying (ASK) • Duo-binary signaling • Optical Single Sideband (OSSB)

  10. Comparison to radio systems • Much of the same theory can be applied, except • Carrier frequency is different • 1550 nm => 194 Thz • The available components are different • no coherent detection (no PLLs) • The channel is different

  11. Component imperfections • Modulators are nonlinear • difficult to achieve pure AM • PIN photo detectors responds to optical power rather than electrical field amplitude (“square envelope”) • Dispersion introduces a frequency dependent phase shift • “intensity-modulated” approaches are used

  12. Optical Modulators • Direct modulation • directly modulate the drive current of a semiconductor laser • Absorbtion modulation • Modulate the absorption spectrum of reverse-biased diod placed in front of the laser • Faster and more linear than direct modulation (60 GHz) • The Mach-Zender (MZ) modulator • modulation my adding phase shifted signals

  13. Optical modulators (cont'd) • Direct modulators and absorption modulators directly modulates the optical power, but will also generate phase modulation • The MZ modulator is more flexible and can generate different kinds if modulation other than NRZ/RZ/ASK

  14. The MZ modulator contacts V1(t) waveguide LiNbO3 Ein/2 Ein Eout γEin/2 V2(t)

  15. MZ modulator transfer function With γ=1 this can be rewritten as: Amplitude modulation Phasemodulation (chirp) With v1(t)=-v2(t) we remove the phase modulation and get:

  16. MZ modulator biasing “Normal bias”: “Bias at extinction”:

  17. MZ modulators - observations • These modulators are only linear in a small region • A problem for other than RZ/NRZ signaling • There must normally be an unmodulated carrier in order to use non-coherent detection

  18. M-ASK • Less bandwidth • More power needed for a given BER • non-linearities become limiting in long-haul DWDM systems • More complicated (analog and digital) electrical circuits • Possibly useful in multi-mode dispersion limited systems e.g. 10 Gbit/s Ethernet

  19. Duo-binary signaling • Introduce correlation between consecutive symbols • A special case of partial response signaling:

  20. Duo-binary signaling • Add consecutive symbols => three signal levels -1,1,1,-1 MZ modulator -2,0,2,0

  21. AM-PSK Duo-binary • Problem: Normally impractical to handle three levels • Solution: Use 0,E,-E • The detector will detect two levels 0 and E² • By precoding these two levels will correspond to 0 and 1 • a.k.a Amplitude Modulated Phase Shift Keying (AM-PSK) duo-binary signaling

  22. AM-PSK duo-binary system 0,0,1,0,1 1,-1,-1,1,1 map 1 xor 1,1,0,1,0 0,1,1,0,0 0,0,-2,0,2 Precoder MZ modulator biased at extinction 0,0,-E,0,E 0,0,E2,0,E2 |x|2 Photo detector (fiber)

  23. Optical Single Sideband (OSSB) • Observation: The frequency spectrum is symmetrical • Implication: Half of it can be filtered out to save bandwidth => Single Sideband Transmission! • Used e.g. in TV

  24. Subcarrier OSSB • In conventional subcarrier modulation the subcarrier appears on both sides of the optical carrier • Dispersion causes a phase shift between the two signals, which depends on the distance • At certain points the entire signal is canceled out!

  25. Subcarrier OSSB (cont'd) (decided to skip the equations: Optical fiber communications IVB, eq.16.30-16.36)

  26. Creating an SSB signal • Two ways • Use a filter (half the energy is lost) • Use the Hilbert transform • known as a Hartley modulator

  27. Hartley modulator SSB signal: Baseband signal:

  28. Optical SSB modulator “Approximation” of SSB signal: Hilbert transform a(t) â(t) Optical carrier OSSB signal MZ Amplitude modulator Phase modulator

  29. Simulation results: ASK/duo-binary Dispersion induced receiver sensitivity degradation for Gbit/s signalling

  30. More practical issues… • ASK • Nees more power => non-linearities limiting • Duo-binary • Needs extra filtering • Optical dispersion compensation could be an alternative • 225 km @10Gbit/s 1550 nm has been reached

  31. Experimental results: OSSB Experimental receiver sensitivity degradation vs. fiber length @ 10Gbit/s, BER=10-9

  32. DWDM • “Normal” NRZ • 40% spectral efficiency over 150 km • Duo-binary AM-PSK • 100% over 100 km • OSSB • 66% over 300 km

  33. Summary • Distance between repeaters is limited by either of • Fiber loss • Chromatic dispersion • Fiber non-linearities • With the advent of EDFA chromatic dispersion has become the limiting factor in long-haul systems

  34. Summary (cont’d) • We want to limit the bandwidth in order too • Reduce the effects of chromatic dispersion • Reach higher spectral efficiency in DWDM systems • Two potential methods: • Duo-binary signaling • Optical single sideband • Both methods could potentially halve the bandwidth • None of the methods are currently used in commercial systems, but there are some promising experimental results

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