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