wdm optical interfaces for future fiber radio systems phần 5 pot

Chapter 3: WDM Optical Interface for Simplified Antenna Base Stations signal at DL Drop port and the uplink signal at ADD port, generated by reusing the recovered optical carrier, were also quantified, which are shown in Fig. 3.20. The error-free (at a BER of 10 -9 ) data recovery and the recovered optical spectra verified -50 Wavelength relative to 1552.22 (nm) -30 -10 -0.8 -0.4 00.40.8 Optical Power (dB) DL Drop -50 Wavelength relative to 1552.22 (nm) -30 -10 -0.8 -0.4 00.40.8 Optical Power (dB) IN to Interface (a) (b) -50 Wavelength relative to 1552.22 (nm) -30 -10 -0.8 -0.4 00.40.8 Optical Power (dB) DL Drop -50 Wavelength relative to 1552.22 (nm) -30 -10 -0.8 -0.4 00.40.8 Optical Power (dB) DL Drop -50 Wavelength relative to 1552.22 (nm) -30 -10 -0.8 -0.4 00.40.8 Optical Power (dB) IN to Interface -50 Wavelength relative to 1552.22 (nm) -30 -10 -0.8 -0.4 00.40.8 Optical Power (dB) IN to Interface (a) (b) -50 Wavelength relative to 1552.22 (nm) -30 -20 -0.6 -0.4 00.40.8 Optical Power (dB) ADD to Interface -40 -50 Wavelength relative to 1552.22 (nm) -30 -10 -0.8 -0.4 00.40.8 Optical Power (dB) λ-Re-Use Carrier (c) (d) -50 Wavelength relative to 1552.22 (nm) -30 -20 -0.6 -0.4 00.40.8 Optical Power (dB) ADD to Interface -40 -50 Wavelength relative to 1552.22 (nm) -30 -20 -0.6 -0.4 00.40.8 Optical Power (dB) ADD to Interface -40 -50 Wavelength relative to 1552.22 (nm) -30 -10 -0.8 -0.4 00.40.8 Optical Power (dB) λ-Re-Use Carrier -50 Wavelength relative to 1552.22 (nm) -30 -10 -0.8 -0.4 00.40.8 Optical Power (dB) λ-Re-Use Carrier (c) (d) Fig. 3.19: Optical spectra of the proposed WDM optical interface while modelled by VPI simulator using three WI-DWDM channels: (a): the input signal at port IN, (b): the downlink signal at port DL Drop, (c): the recovered optical carrier at λ-Re-Use port, and (d): the uplink optical mm-wave signal to be added to the interface, generated by reusing the recovered optical carrier. 105 Chapter 3: WDM Optical Interface for Simplified Antenna Base Stations the functionality of the proposed interface, which was later demonstrated in experiment, as described in Section 3.5.3 -5-8 -7 -6 -4 -5 -3 -11 -9 -7 l o g l o g 1 0 1 0 ( ( B E R ) ) Received Optical Power (dBm) Uplink at ADD Port -5 -3 -11 -9 -7 -9 -8 -7 -6 l o g l o g 1 0 1 0 ( ( B E R ) ) Received Optical Power (dBm) Downlink at DL Drop Port (a) (b) -5-8 -7 -6 -4 -5 -3 -11 -9 -7 l o g l o g 1 0 1 0 ( ( B E R ) ) Received Optical Power (dBm) Uplink at ADD Port -5-8 -7 -6 -4 -5 -3 -11 -9 -7 l o g l o g 1 0 1 0 ( ( B E R ) ) Received Optical Power (dBm) Uplink at ADD Port -5 -3 -11 -9 -7 -9 -8 -7 -6 l o g l o g 1 0 1 0 ( ( B E R ) ) Received Optical Power (dBm) Downlink at DL Drop Port -5 -3 -11 -9 -7 -9 -8 -7 -6 l o g l o g 1 0 1 0 ( ( B E R ) ) Received Optical Power (dBm) Downlink at DL Drop Port (a) (b) Fig. 3.20: Simulation BER curves that quantify the degradation of the signals due to traversing the proposed interface: (a): the recovered downlink signal at DL Drop port, and (b): the uplink signal, generated by reusing the recovered optical carrier, at ADD port. 3.7 Effects of the Performance of O/E Devices The overall receiver sensitivity of the experimentally demonstrated system incorporating the proposed interface, irrespective of direction of communication, is less than or equal to -7.7 dBm at a BER of 10 -9 , which is very poor and needs to be improved through further investigation. The performance of the optoelectronic and electrooptic devices such as DE-MZMs and the PD play a very important role in limiting the overall performance of the link. The DE-MZMs used in the experiment exhibit a CSR from 22 to 28 dB. Also, the PD used in the experiment had a responsitivity of less than 0.4. If the performance of O/E devices can be improved either by replacing it with better performing devices or by applying some external 106 Chapter 3: WDM Optical Interface for Simplified Antenna Base Stations performance enhancing techniques (such as CSR reduction by external means), the sensitivity limitation can be resolved quite easily. Fig. 3.21 shows a simulation model developed by using VPI platform, which quantifies the performance enhancement of the system at different values of CSR at the output of the DE-MZMs and the responsitivity of the PD. To make the results comparable, the properties of the modules in the model follow the experimental parameters very closely. To enable variable CSRs in the generated WI-DWDM signals, the sidebands of the OSSB+C signal are separated from the optical carriers using a Fabry Perot filter in conjunction with a 3 port optical circulator, where the intensities of the sidebands were varied by another EDFA (keeping the noise figure unchanged) before combining them back with the separated optical carriers. Fig. 3.22(a) shows the sensitivity at BER = 10 -9 vs. reduction in CSR curve obtained from simulation model, which clearly indicates that, the sensitivity of the system increases almost linearly with reduction in CSR. 1. IN 3. DL Drop 4. λ-Re-Use 5. ADD 7. OUT 10 km SMF WDM Optical Interface 7 3 45 1 Uplink OSSB+C2 Data Recovery 4 x 1 OSSB+C1 OSSB+C2 OSSB+C3 FP 2 x 1 EDFA BPF: band pass filter SMF: single-mode fiber PD: photo detector FP: Febry Perot Filter SB: Sideband PD PD SB EDFA BPF 1. IN 3. DL Drop 4. λ-Re-Use 5. ADD 7. OUT 10 km SMF WDM Optical Interface 7 3 45 1 Uplink OSSB+C2 Data Recovery 4 x 1 OSSB+C1 OSSB+C2 OSSB+C3 FP 2 x 1 EDFA BPF: band pass filter SMF: single-mode fiber PD: photo detector FP: Febry Perot Filter SB: Sideband PD PD SB EDFA BPF Fig. 3.21: Simulation model that quantifies the performance enhancement of the system at different values of the CSR of the DE-MZMs as well as the responsitivity of the photodetector. 107 Chapter 3: WDM Optical Interface for Simplified Antenna Base Stations To verify the impact of the PD on the overall system performance, the responsitivity of the PD module in the simulation model were increased gradually from 20% up to 100% and plotted against the sensitivity of the system at BER = 10 -9 , which is shown in Fig. 3.22(b). This curve also confirms that the sensitivity of the system increases almost linearly with responsitivity of the PD and saturates when the responsitivity > 0.9 A/W. Therefore, both curves (Fig. 3.22a- 3.22b) demonstrate that with proper selection of the O/E devices, the overall performance of the link can be enhanced significantly. -14 -12 -10 -8 -6 -4 Sensitivity (dBm) 0.1 0.3 0.5 0.7 0.9 1.1 PD Responsitivity (A/W) (b)(a) -10 -9 -8 -7 -6 -5 -4 Sensitivity (dBm) -2 0 2 4 6 8 10 12 Reduction in CSR (dB) S C CSR -14 -12 -10 -8 -6 -4 Sensitivity (dBm) 0.1 0.3 0.5 0.7 0.9 1.1 PD Responsitivity (A/W) (b) -14 -12 -10 -8 -6 -4 Sensitivity (dBm) 0.1 0.3 0.5 0.7 0.9 1.1 PD Responsitivity (A/W) (b)(a) -10 -9 -8 -7 -6 -5 -4 Sensitivity (dBm) -2 0 2 4 6 8 10 12 Reduction in CSR (dB) S C CSR (a) -10 -9 -8 -7 -6 -5 -4 Sensitivity (dBm) -2 0 2 4 6 8 10 12 Reduction in CSR (dB) S C CSR S C CSRCSR Fig. 3.22: Simulation graphs that quantify the performance enhancement of the system at different values of the CSR of the DE-MZMs as well as the responsitivity of the photodetector: (a): sensitivity vs. reduction in CSR, and (b): sensitivity vs. PD responsitivity. 108 Chapter 3: WDM Optical Interface for Simplified Antenna Base Stations 3.8 Carrier Reuse over Independent Uplink Light Source As described in the previous sections, the proposed interface enables a carrier extraction technique that provides optical carrier to modulate the uplink mm-wave signals. The downlink optical carrier traverses a series of optical devices, in addition to propagating through a span of optical fibre before being recovered at the interface. This transportation of the optical carrier to the interface may potentially cause broadening of the linewidth of the carrier-pulse due to the Group-Velocity Dispersion (GVD), which can be expressed mathematically [81] as follows: () 1 2 2 2 ωβω ω β ω υω ω ω ∆=∆=∆ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ =∆=∆ L d d L L d d d dT T g where, υ g , is the group velocity, β, is the propagation constant L, is the length of SMF, ∆T, is the amount of pulse broadening, ∆ω, spectral width of the carrier pulse, and 2 2 2 ω β β d d = is the GVD parameter that determines the amount of broadening In terms of range of wavelengths ∆λ, rather than frequency spread ∆ω, the extent of pulse broadening ∆T can be expressed as: () 2 2 21 , 2 β λ π υλ λλ υλ ω ω ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ = ∆=∆ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ =∆=∆ c d d D where DL L d d d dT T g g 109 Chapter 3: WDM Optical Interface for Simplified Antenna Base Stations β 2 , is the dispersion parameter expressed in unit of ps/(km-nm) The above two expressions of pulse broadening demonstrates that there is a definite broadening of downlink carriers before being recovered in the proposed interface to be reused for uplink communication. This dispersion induced pulse broadening contaminates the receiver performance by introducing Intersymbol Interference (ISI) and by reducing the SNR at the decision circuit. To quantify the effects of pulse broadening in a system incorporating the proposed interface, a simulation was carried out using VPITransmissionMaker5.5. The simulation model was very similar to the experiment, where uplink optical mm-wave signal was generated in two different ways: (i) by reusing the recovered downlink carrier, and (ii) by using an independent optical source. In both cases, the BER curves were measured in the CO. The simulation BER curves are presented in Fig. 3.23. It shows that due to pulse broadening, the uplink signal experiences a 0.1 dB -3 -5 -7 -9 -11 -8 -7 -6 -5 -4 carrier reuse independent light-source loglog 1010 (( BER ) ) Received Optical Power (dBm) -3 -5 -7 -9 -11 -3 -5 -7 -9 -11 -8 -7 -6 -5 -4 carrier reuse independent light-source loglog 1010 (( BER ) ) loglog 1010 (( BER ) ) Received Optical Power (dBm) Fig. 3.23: Simulated BER curves as a function of received optical power for uplink transmission while: (i): reused the optical carrier recovered by the proposed interface, and (ii): used an independent optical source as the uplink optical carrier. 110 Chapter 3: WDM Optical Interface for Simplified Antenna Base Stations additional penalty, which is very negligible, and can be ignored. Therefore, the effect of recovered carrier pulse broadening on the overall uplink performance is minimal and hence can be neglected while designing the mm-wave fibre-radio systems incorporating the proposed WDM optical interfaces. 3.9 Conclusion This chapter presented a multifunctional WDM optical interface for future DWDM fibre-radio system that enables dispersion tolerant OSSB+C modulation based wavelength-interleaved networks and capable of providing the optical carrier for the uplink transmission by exploiting a wavelength reuse technique. The functionality of the proposed interface was verified experimentally as well as via simulation for three wavelength-interleaved DWDM channels with a channel spacing of 25 GHz, each carrying 37.5 GHz RF signal with 155 Mb/s BPSK data transported over 10 km of fibre link. The use of the demonstrated interface in the future DWDM fibre-radio networks can improve spectral efficiency and ensure efficient wavelength utilisation, while offers a simplified and consolidated BS architecture by eliminating the need for separate optical source for uplink. In the design process we have taken the benefits of matured and standard component technologies that enhance the possibility of merging the mm-wave fibre-radio based BWA systems with existing optical network infrastructure in the access and metro domains. The effects of the performance of optoelectronic devices (DE-MZM and PD) in the overall performance of the link incorporating the proposed interface were investigated. A simulation model was developed to investigate the impairments contributed by imperfect optical devices such as the DE-MZM and PD. The CSR of the DE-MZM and the responsitivity of the PD were varied and the respective sensitivities were measured. The results indicated that the performance of the links incorporating the proposed interface were largely dependent on the performance of the optoelectronic devices, and by proper selection of these devices, the performance of the link can be significantly enhanced. 111 Chapter 3: WDM Optical Interface for Simplified Antenna Base Stations A comparison was carried out to investigate the effects of pulse-broadening due to dispersion on the optical carriers recovered using wavelength reuse scheme and independent light-sources in the uplink path. The mathematical expressions showed that there was a definite broadening of the optical carrier recovered by the proposed interface to be reused in the uplink path. 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Photonics, MWP, 20 05 [81] G P Agrawal, Fiber- Optic Communication Systems, 3rd ed (Wiley, New York, 2002); Greek Translation, 2000 119 Chapter 3: WDM Optical Interface for Simplified Antenna Base Stations 120 Chapter 4: Characterisation and Enhancement of Links Performance Incorporating WDM Optical Interface 4 Characterisation and Enhancement of Links Performance Incorporating WDM Optical Interface 4.1... vol 31, pp 1848-1849, 19 95 [52 ] R A Griffin, P M Lane, and J J O’Reilly, “Dispersion-tolerant subcarrier data modulation of optical millimeter-wave signals,” Electron Lett., vol 32, no 24, pp 2 258 -2260, 1996 [53 ] G H Smith, D Novak, and Z Ahmed, "Technique for optical SSB generation to overcome dispersion penalties in fiber- radio systems, " Electron Lett., vol 33, pp 74- 75, 1997 [54 ] G H Smith, D Novak,... traversing WDM Optical Interface1 and IN-DROP part of WDM Optical Interface2 demonstrates an improvement of power penalty by approximately 0. 15 dB, although it undergoes several optical components that have the potential to degrade the signal quality significantly This improvement of performance is due to the reduction in Ch1 at A -5 log10 (BER) -4 Ch1 at D Ch1 at E -6 -7 -8 -9 -10 -11 -8 .5 -8 -7 .5 -7 -6.5 . wave WDM fiber- radio, ” Electron. Lett. , vol. 37, pp. 1043 –10 45, 2001. [ 15] H. Toda, T. Yamashita, K. Kitayama, T. Kuri, “A DWDM MM-Wave Fiber Radio system by optical frequency interleaving for. “Dispersion Tolerant Novel Base Station Optical Interface for Future WDM Fiber- Radio Systems, ” Proc. of Conference on Optical Internet/ Australian Conference on Optical Fiber Technology (COIN/ACOFT’03),. systems incorporating the proposed WDM optical interfaces. 3.9 Conclusion This chapter presented a multifunctional WDM optical interface for future DWDM fibre -radio system that enables dispersion