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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 Normalized RF Power(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