Research On Several Key Techniques Of Broadband Optical Orthogonal Frequency-division Multiplexing Communications

Optical orthogonal frequency division multiplexing(OFDM)has attracted lots of attention due to its high spectral efficiency and robustness to transmission impairments enabled by digital signal processing(DSP).With the emergence of bandwidth-hungry services such as high-definition(HD)television streaming,cloud computing,interactive HD online gaming,the capacity of optical communication system has increased dramatically durding past ten years.Broadband optical OFDM is a competitive candidate for the future communication systems.Several novel key techniques and research results in broadband optical OFDM communication networking have been demonstrated in this dissertation.No overhead scheme to resist subcarrier-subcarrier mixing interference(SSMI)in direct detection optical OFDM(DDO-OFDM),optimized training sequence(TS)for discrete-Fourier-transform Spread(DFT-Spread)direct detection optical OFDM,applying pre?emphasis to resist frequency fading in OFDM radio over fiber(OFDM-RoF)system,improving the robustness to synchronization timing errors with windowing technique for OFDM-RoF system,and recovering few subcarriers OFDM signal with blind equalization have been theoretically and experimentally investigated.High level quadrature amplitude modulation(QAM)optical OFDM transmission in direct detection system,reducing the Peak to Average Power Ratio(PAPR)of OFDM signal in OFDM-RoF system increasing the capacity of fiber wireless OFDM signal transmission system with polarization division multiplexing(PDM),reception of high capacity real-time optical OFDM communication system and 400G 16 ary quadrature amplitude modulation(16QAM)OFDM long haul optical transmission network with one band DFT-spread have been experimentally demonstrated.The main contents of this thesis and innovations are as follows:First,key techniques in the implementation of DDO-OFDM are studied to acheieve good performance.These can be devided into three aspects:1.Half-cycled DDO-OFDM transmission and reception was successfully demonstrated to resist SSMI without spectra efficiency reduction for the first time.The receiver sensitivity was improved by 2 and 1.5 dB in QPSK and 16QAM OFDM with 40-km standard single-mode fiber(SSMF)transmission,respectively.2.High level QAM optical OFDM transmission system utilizing a cost-effective directly modulated laser(DML)has been studied and demostrated.This experiment was enabled by intra-symbol frequency-domain averaging(ISFA)based channel estimation,large fast Fourier transform size to enhance the resistance of intersymbol interference and frequency resoulation in channel estimation,and the DFT-spread technique to reduce PAPR.A 31.7-Gbit/s 2048QAM OFDM signal can be successfully transmitted over 20-km large effective area fiber(LEAF)under a 20%soft-decision forward-error-correction bit error ratio(BER)threshold(2.4×10-2).3.Transmission of 79.86Gb/s discrete-Fourier-transform spread 32QAM OFDM signal over 20-km SSMF utilizing DML has been experimentally demostrated.The experimental results show DFT-spread is effective to reduce PAPR of OFDM signal,and also good at overcoming narrowband interference and high frequencies power attenuation.We compared different types of TS symbol and found the optimized TS for channel estimation is the symbol with digital BPSK/QPSK modulation format due to its best performance against optical link noise during channel estimation.Second,transmission and reception of OFDM signal in optical wireless hybrid systems are studied.Techniques which can be applied to realize broadband OFDM signal goog performance transmission in RoF and Fiber wireless link are studied in the thesis,and it can be devided as follows:1.We have proposed and experimentally demonstrated pre-emphasis technique to resist the frequency fading induced by the dispersion of optical fiber and insufficient bandwidth of components OFDM-RoF system.To overcome the frequency fading,we pre-emphasize the power of the 60GHz millimeter wave(mm-wave)OFDM sub-carriers appropriately in the center station.Experimental result of the proposed system shows the received sensitivity has been improved about 2 dB at the BER of 1×10-4 after 50 km SSMF transmission for 2.5 Gb/s OFDM signal carried on 60 GHz optical mm-wave compared to the original system without pre-emphasis technique.This technique can help to relax the cost of the system as the components with insufficient bandwidth can be applied in the OFDM-RoF system.2.To reduce the nonlinear distortion in electrical devices,optical modulators,and transmission fiber in 60GHz OFDM-RoF system,we adopt companding transform technique to reduce the PAPR of OFDM signal.By adopting this technique,our experimental results show that the receiver sensitivity at BER of 1×10-3 for a 2.98 Gb/s OFDM signal carried on a 60 GHz optical millimeter-wave after 50 km of SSMF transmission is improved by about 0.6,2.2,and 3.8 dB at launch powers of 0,6,and 12 dBm,respectively.3.A reversely modulated optical single sideband scheme(OSSB)in an optical OFDM-RoF system with an in-phase/quadrature(I/Q)modulator is proposed.For the optical mm-wave OFDM transmission,pre-equalization is applied to enhance the power of subcarriers with low signal to noise ratio(SNR)to improve the overall BER performance and the windowing technique is used to improve the robustness to the synchronization timing errors.The experimental results of a 40-GHz 64QAM OFDM-RoF system show that the power penalty of 18.8-Gb/s 64-QAM OFDM signal carried by 38-GHz mm-wave after 40-km SSMF transmission is negligible and the system is immune to the STE with windowing technique.4.A seamlessly integrated fiber-wireless system that delivers 30.67-Gb/s PDM-multiple input multiple output OFDM(PDM-MIMO-OFDM)signal through 40-km fiber and 5-m wireless transmission over free-space at 100 GHz adopting heterodyne coherent detection is experimentally demonstrated.PDM is ulitizled to realize capactity expansion.De-multiplexing is realized by channel estimation based on a pair of time-interleaved TSs.The BER for the 30.67-Gb/s PDM-MIMO-OFDM signal is less than the hard decision for-ward-error-correction(HD-FEC)threshold of 3.8×10-3 when the OSNR is larger than 19.3 dB after both 40-km SSMF transmission and 5-m wireless delivery at 100 GHz.Third,transmission and reception of few subcarriers(Dual-subcarrier and Quad-subcarrier)OFDM with coherent detection is studied.Dual-polarization Dual-/Quad-subcarrier coherent optical orthogonal frequency division multiplexing(CO-OFDM)transmission and reception is successfully demonstrated with blind equalization.A two/Quad-subcarrier quadrature phase shift keyed OFDM(QPSK-OFDM)signal can be equalized as a 9QAM/25QAM signal in the time domain with the cascaded multimodulus algorithm(CMMA)equalization method.The nonlinear effect resistance and transmission distance can be enhanced compared with the traditional CO-OFDM transmission system based on frequency equalization with training sequence.The phase recovery can be realized with simple Viterbi algorithm for QPSK signal after fast ·Fourier transform(FFT)and the frequency offset estimation(FOE)should be done before FFT.Fourth,real-time reception of high throughput optical OFDM signal transmission systems with direct detection or coherent detection are discussed in this thesis.The research can be divided into two parts:1.A fully real-time DDO-OFDM system with 16QAM operating at a new record rate of 100-Gb/s is successfully demonstrated.Two optical subcarriers are applied to transmit the 100Gb/s OFDM signal.Vestigial sideband OFDM(VSB-OFDM)generated by the tunable optical filter with 10-GHz minimum bandwidth is transmitted in the DDO-OFDM system to resist dispersion induced frequcny fading.After transmission of 20-km LEAF,no received optical power penalty is observed for VSB-OFDM and the output BER is lower than 2×10-2,Rather than inserting several TSs,an 11-tap ISFA filter is utilized to reduce ASE noise during the channel estimation with only one TS.2.We have shown a demonstration of field-programmable gate array(FPGA)based single-band real-time dual polarization(DP)16QAM-CO-OFDM receiver at a data rate of 100-Gb/s.The single-band CO-OFDM signal is successfully received and recovered after 200-km SMF-28 transmission under BER of 3.8×10-3.The time synchronization and frequency synchronization are simplified in the DSP of real-time receiver.To the best of our knowledge,it is the first time to transmit 100-Gb/s OFDM signal on one optical carrier.The sampling rate of analog-to-digital converter(ADC)and the bit rate per band are both the highest so far in the demonstration of real-time CO-OFDM.Fifth,transmission of 400Gb/s OFDM signal in the CO-OFDM system with DFT-spread technique to conquer the high frequencies power attenuation and reduce PAPR is studied and demostrated.We have experimentally compared the performance of DFT-spread and pre-equalization in a 244.2-Gb/s PDM-16QAM-OFDM transmission system.400G transmission can be realized with 2-subchannel scheme.The pre-equalization is effective to overcome the high frequency power attenuation in the channel.However,the acquisition of static channel response for pre-equalization is really complicated and the PAPR of the signal after pre-equalization even becomes a little higher.The DFT-spread can be applied to simultaneously resist high frequency power attenuation and reduce the PAPR of OFDM signal.The experimental results also show that one band DFT-Spread demonstrates the best narrow optical filtering tolerance.The transmission distance for 8×244.2-Gb/s wavelength-division-multiplexing(WDM)PDM-16QAM-OFDM at the soft-decision FEC(SD-FEC)threshold of 2.4×10-2 is 2×420km based on pre-equalization while extended to over 3×420km based on one band DFT-spread,which well illustrates one band DFT-spread is more efficient for high bandwidth coherent WDM-OFDM system.