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Special Topics in Optical Engineering II (15/1) M.J. Shin
Paper Review
Special Topics in Optical Engineering II (15/1) M.J. Shin
Contents
• Introduction
• Optical Impairments in Fiber-Wireless Links
• Strategies to Overcome Impairments - Optical Fiber Dispersion
- Optical Spectral Efficiency
- Improving Optical Modulation Depth
- Base-Station(BS) Technologies
- Front-End Nonlinearity
• Conclusion
Special Topics in Optical Engineering II (15/1) M.J. Shin
Introduction
• Hybrid fiber-wireless networks - Advantages
High bandwidth
Spectral congestion X
High-capacity broadband wireless services
- Disadvantages Inherent high propagation loss
Need to deploy picocellular & microcellular architecture
for efficient geographical coverage
• Simplify BS, centralize control center
Special Topics in Optical Engineering II (15/1) M.J. Shin
Centralized Control Architecture
• Typical hybrid fiber-wireless scenario
• Strategy to achieve centralized control architecture
- Move hardware intelligence to Central Office(CO)
- Optically distribute radio signal at mm-wave
Simplify antenna BS
• Susceptible to impairments
Special Topics in Optical Engineering II (15/1) M.J. Shin
Optical Impairments
• Additional impairments - Inefficient spectral usage
- Phase decorrelation(optical carrier, radio signal)
• MM-wave ROF link
Special Topics in Optical Engineering II (15/1) M.J. Shin
Strategies to Overcome Impairment
• Optical Fiber-Dispersion
• Optical Spectral Efficiency
• Improving Optical Modulation Depth
• Base Station Technologies
• Front-End Nonlinearity
Special Topics in Optical Engineering II (15/1) M.J. Shin
Optical Fiber-Dispersion
Modulating Frequencies No
rmalized
RF
Po
wer(
dB
)
• Received RF power varies by phase difference
• RF power: fiber dispersion, transmission distance, mm-wave frequency
• RF power varies periodically, power suppression occur at certain frequency
Special Topics in Optical Engineering II (15/1) M.J. Shin
Mitigation Techniques for Dispersion Effect
• OSSB+C Modulation
• External Filtering
• Optical Carrier Suppression Technique
• Chirped Fiber Gratings
• Fiber Nonlinearities
Special Topics in Optical Engineering II (15/1) M.J. Shin
Dispersion Effect Mitigation Techniques
• Dual-electrode Mach-Zehnder modulator(DEMZM) OSSB+C
• Biased quadrature & 90 degree phase shift between two electrodes
Special Topics in Optical Engineering II (15/1) M.J. Shin
Other Solution for Dispersion
• External Filtering - Reflect unwanted optical sideband(FBG)
- Limited flexibility implemented difficult
• Optical Carrier Suppression Technique - Bias at null point
- Half the desired modulating frequency needed
- Need large RF driver power to obtain desirable modulation depth (nonlinear)
• Fiber Nonlinearities
• Phase conjugation
Special Topics in Optical Engineering II (15/1) M.J. Shin
Optical Spectral Efficiency
• Interleaving multiple mm-wave optical signals - Fiber Bragg gratings(FBGs)
- Arrayed waveguide gratings(AWGs)
Special Topics in Optical Engineering II (15/1) M.J. Shin
AWG Based Wavelength Interleaving Strategy
DEMUX using 2 X N AWG for wavelength-interleaved channels
• DEMUX based on 2XN AWG & high-finesse Fabry-Perot etalon
• Etalon separate optical carriers from sideband signal
• AWG routes optical carriers and corresponding sidebands to same output
Etalon: Monolithic interferometric devices
containing two parallel reflecting surfaces
Special Topics in Optical Engineering II (15/1) M.J. Shin
Improving Optical Modulation Depth
• Low modulation efficiency at mm-wave radio signal
• Huge difference between optical carrier power & modulated sideband power
• Optical filtering by FBGs remove portion of optical carrier
• Improve modulation efficiency
Special Topics in Optical Engineering II (15/1) M.J. Shin
Optical Spectrum of OSSB+C with FBG
• OSSB+C signal carrying 155Mbits/s at 35GHz
• FBG: 95% reflectivity(14dB)
• Bit-error-rate(BER) improved
Special Topics in Optical Engineering II (15/1) M.J. Shin
Need for Base-Station Technologies
• Full-duplex mm-wave fiber-wireless network need optical interface @ BS
• Optical source with narrow linewidth at well-specified wavelength
minimize phase noise degradation
• Ultra-stable, low-cost, narrow-linewidth optical source difficult to realize
Wavelength assignment & source monitoring move to CO
• Source-free base station - CO generate downlink signal & uplink optical carrier with different wavelength
- Wavelength reuse technique
Special Topics in Optical Engineering II (15/1) M.J. Shin
Wavelength Reuse Technique
• OSSB+C modulated signal
• 50% for downstream RF signal
• 50% reuse for uplink optical carrier
Special Topics in Optical Engineering II (15/1) M.J. Shin
Front – End Nonlinearity
• Wireless multicarrier network linearity important(reduce IMD products)
• Feed-forward technique - Suppress IMD3 & reduce laser relative intensity noise(RIN)
• Predistortion technique - Require predistorter at the source to combat IMD
- Same amplitude opposite phase
• Removing dominant IMD technique
Special Topics in Optical Engineering II (15/1) M.J. Shin
Linearization with removing dominant IMD
• Input two tone signal(ω1, ω2)
• Nonlinear DEMZM, photodetector other optical components generate
• ωc─ω1+ω2, ωc+ω1─ω2 dominant IMD3
Special Topics in Optical Engineering II (15/1) M.J. Shin
Linearization with removing dominant IMD
• Remove dominant IMD3 with filter(carrier-to-interference ratio ↑)
• Split optical carrier recombine with clean optical carrier
• Not clear all IMD3 but remove large portion
Special Topics in Optical Engineering II (15/1) M.J. Shin
Conclusion
• Introduction of hybrid fiber-wireless networks - High bandwidth
- Spectral congestion X
- Inherent high propagation loss
• Optical Impairments in Fiber-Wireless Links
• Strategies to Overcome Impairments - Optical Fiber Dispersion
- Optical Spectral Efficiency
- Improving Optical Modulation Depth
- Base-Station(BS) Technologies
- Front-End Nonlinearity
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