Research On Key Techniques Of Optical Communications Based On Frequency-division Multiplexing

With the explosive growth of data services such as mobile Internet,Internet of Things,big data,and artificial intelligence,as well as the increasing demand for broadband services and bandwidth-hungry applications,the capacity of optical networks should be expanded.Long-reach backbone networks adopt the coherent optical communications to implement the 400G+ high-capacity data transmission.However,due to the limited cost and power consumption,the cost-sensitive and bandwidth-limited optical systems are required in the medium-or short-reach optical networks such as metropolitan area networks,access networks,data center interconnects,and mobile fronthaul.Therefore,how to transmit more data over the limited bandwidth is an attractive and difficult field in the scientific world.Optical communications based on frequency-division multiplexing have the advantages of high spectral efficiency,good chromatic dispersion tolerance,and superior flexibility and expansibility,so that it can utilize the limited bandwidth more effectively and achieve low-cost and high-speed optical transmission.Therefore,the optical communications based on frequency division multiplexing is one of the important technologies for the future large-capacity optical networks.Faced with the demand and challenge of medium-or short-reach optical networks,this dissertation proposes some new frequency-division multiplexing schemes for optical communications.The key techniques such as the novel multiplexing schemes,the peak-to-average power ratio(PAPR)reduction,the equalization algorithms for eliminating the linear and nonlinear distortions,and inter-carrier interference(ICI)cancellation are mainly researched.Based on these techniques,the simulations and experiments are implemented to analyze the performance of the frequency division multiplexing schemes for optical access networks,short-reach optical interconnects,and optical wireless communications.The main innovations of this dissertation are as follows:1.Orthogonal Frequency-Division Multiplexing System Based on Discrete Hartley TransformThe multiplexing/demultiplexing of conventional orthogonal frequency-division multiplexing(OFDM)uses the inverse discrete Fourier transform/discrete Fourier transform(IDFT/DFT).This dissertation studies a novel OFDM scheme based on discrete Hartley transform(DHT).Based on the convolution property of DHT,the channel estimation and equalization of DHT-based OFDM system are proposed.For the low-cost optical communications,intensity-modulation and direct-detection(IM/DD)optical systems are commonly used.The IM/DD optical systems require a real-valued modulated signal.DHT is a real-valued trigonometric transform and has self-inverse property.When the input of DHT is a real-valued signal,the generated OFDM signal is real-valued.However,IDFT is a complex-valued transform.It requires Hermitian symmetry operated on the input signal so as to generate real-valued OFDM signal.Obviously,DHT-based OFDM has a simple structure and low computational complexity,which is more suitable for IM/DD optical system.The main innovations:Based on the convolution property of DHT,this dissertation proposes the theory of channel estimation and equalization for DHT-based OFDM systems,and establishes a theoretical foundation for the application of DHT-based optical OFDM systems.The related results have been published in the IEEE Photonics Technology Letters.2.Asymmetrically Clipped Optical Single-Carrier Frequency-Division Multiplexing SystemThis dissertation first proposes asymmetrically clipped optical single-carrier frequency-division multiplexing system(ACO-SCFDM).Compared with the conventional asymmetrically clipped optical orthogonal frequency-division multiplexing(ACO-OFDM),ACO-SCFDM has a lower PAPR and higher modulation efficiency,and can reduce the influence of high frequency distortion caused by bandwidth limitation.The experimental results show that ACO-SCFDM has better transmission performance.Owing to the above advantages,this dissertation applies ACO-SCFDM to long-haul passive optical network(LR-PON),and first design a simplified multiplexing structure.The computational complexity of transmitter decreases from O(Nlog2N)to O(N).The simulation results show that ACO-SCFDM has a higher signal-to-quantization noise ratio than ACO-OFDM.Therefore,ACO-SCFDM has the potential to reduce the device cost.The experimental results demonstrate that the LR-PON based on ACO-SCFDM has a higher splitting ratio.Due to the nonuse of even subcarrier,ACO-SCFDM has low spectral efficiency.In order to improve the spectral efficiency,this dissertation first proposes the layer/enhanced ACO-SCFDM(L/E-ACO-SCFDM)scheme and applies it to optical wireless communications(OWC).The simplified multiplexing/demultiplexing structure is designed to reduce the computational complexity.The L/E-ACO-SCFDM can simultaneously transmit multiple layers of ACO-SCFDM,which overcomes the disadvantage of low spectral efficiency.In OWC system,the clipping effect of transmitter introduces serious nonlinear clipping noise,and the bandwidth-limited channel introduces serious high-frequency distortion.L/E-ACO-SCFDM can effectively resist nonlinear clipping noise and high-frequency distortion.The main innovations:This dissertation first proposes ACO-SCFDM and design a simplified multiplexing structure,which reduces the computational complexity by an order of magnitude.Meanwhile,the L/E-ACO-SCFDM is first proposed to improve the spectral efficiency.In LR-PON and OWC,ACO-SCFDM has the excellent performance on resisting the nonlinear clipping noise and high-frequency distortion.The relevant results have been published in Optics Letters,Photonics Technology Letters and Photonics Journal,which form a series of papers.3.Optical Communications Based on Interleaved Single-Carrier Frequency-Division MultiplexingThe coherent optical detection can further increase the transmission distance and the splitting ratio of LR-PON.This dissertation proposes a coherent optical interleaved single-carrier frequency-division multiplexing(CO-I-SCFDM)for LR-PON.A simplified multiplexing structure is designed,which reduces the computational complexity of the transmitter from O(Nlog2N)to O(N).When the launch power is set to 0 dBm,after 100-km standard single mode fiber(SSMF)transmission,the splitting ratio of a 10-Gb/s CO-I-SC-FDM scheme can achieve 1:128.For the conventional CO-OFDM scheme,the splitting ratio is only 1:64.Coherent optical detection requires a local oscillator laser and optical hybrid,which has high cost and power consumption.Therefore,the low-cost direct detection is usually employed in the high-speed and short-reach optical interconnects.The direct-detection optical systems can only detect the optical intensity,in which the real-valued signal should be transmitted.In this dissertation,a real-valued I-SCFDM scheme is first proposed for high-speed and short-reach optical interconnects.We experimentally demonstrate 12-Gbit/s QPSK and 24-Gbit/s 16-QAM I-SCFDM systems over 22.5-km SSMF.Compared to OFDM system,I-SCFDM system has a 3-dB improvement of the receiving sensitivity.Meanwhile,128-Gbit/s 16-QAM I-SCFDM system and 40-Gbit/s 256-QAM I-SCFDM system are experimentally demonstrated for high transmission speed and high spectral efficiency,respectively.A joint time-domain and frequency-domain equalization is proposed to effectively eliminate the serious linear and nonlinear distortions.The main innovations:This dissertation proposes the CO-I-SCFDM for increasing the splitting ratio of LR-PON.A simplified multiplexing structure is designed to reduce the computational complexity by an order of magnitude.The real-valued I-SCFDM scheme is first proposed for high-speed short-reach optical interconnects.Meanwhile,a joint time-domain and frequency-domain equalization is proposed to effectively eliminate the serious linear and nonlinear distortions.The relevant results have been published in Optics Express,Photonics Technology Letters and Photonics Journal,which form a series of papers.4.Orthogonal Frequency-Division Multiplexing System Based on Discrete Cosine TransformThis dissertation investigates the discrete cosine transform-based(DCT-based)IM/DD optical OFDM system,in which the subcarrier spacing equals to half of that in the conventional OFDM system.In this dissertation,DCT-based unipolar OFDM(U-OFDM)are first proposed.Meanwhile,a simplified DC-bias optimization scheme is first proposed for DCO-OFDM.The performance of U-OFDM and DCO-OFDM are analyzed and compared in detail.The DCT-based OFDM systems illustrate that when the subcarriers of frequency-division multiplexing system are orthogonal,the required minimum subcarrier spacing equals to a half of the symbol rate per subcarrier.The main innovations:This dissertation first proposes the DCT-based U-OFDM and a simplified DC-bias optimization scheme for DCO-OFDM.Meanwhile,this dissertation verifies the required minimum subcarrier spacing to guarantee the subcarriers of frequency-division multiplexing to be orthogonal.The relevant results have been continuously published in Journal of Lightwave Technology.5.Faster-than-Nyquist Non-Orthogonal Frequency-Division Multiplexing SystemFor further increasing the spectral efficiency,the subcarrier spacing of frequency-division multiplexing can be further compressed.This dissertation first proposes faster-than-Nyquist non-orthogonal frequency-division multiplexing(FTN-NOFDM)system,studies the generation mechanism of FTN-NOFDM signal,and gives the requirement when the transmission rate of NOFDM signal is faster than the Nyquist rate.When the bandwidth compression factor is equal to 0.8,the baseband bandwidth is compressed by 20%and the transmission rate is 25%faster than the Nyquist rate.Shannon theorem gives the maximum capacity limit of the Nyquist signal,but does the Shannon limit apply to the FTN signal?This dissertation first derives the mathematical expression of the capacity limit for FTN-NOFDM signal.This formula reveals that if the ICI is effectively eliminated,the FTN-NOFDM signal has the potential to reach a higher capacity limit than the Nyquist signal,and it has better performance in the bandwidth-limited communication systems.This dissertation apply the FTN-NOFDM to the bandwidth-limited OWC system.Compared to OFDM,FTN-NOFDM has better BER performance.FrCT-NOFDM system has the superior security performance for application in the security communication.The main innovations:This dissertation first proposes FTN-NOFDM and derives the mathematical expression of the capacity limit for FTN-NOFDM signal.The relevant results have been published in Optics Letters,Scientific Reports and IEEE Access,which form a series of papers.In summary,this dissertation systematically studies the optical communications based on frequency-division multiplexing,and proposes a variety of new frequency-division multiplexing schemes.Meanwhile this dissertation investigates the key techniques of the novel multiplexing schemes,PAPR reduction,channel equalization and ICI cancellation.The proposed frequency-division multiplexing schemes are applied to the optical access networks,short-reach optical interconnects and OWC,and the performance of these schemes is detailed studied by simulation and experiment.The research results of this dissertation lay a solid foundation for the application of frequency division multiplexing in optical communications.