Research On Key Technologies Of High Spectral Efficiency Direct Detection Of Medium And Short Distance Optical Transmission

With the significant growth of various Internet applications,including cloud computing,Internet of Things(Io T),live streaming,and Metaverse,the 5G mobile network has been widely commercialized and implemented.Meanwhile,the number of customers and endusers is continuously increasing and data traffic in the network is becoming more distributed.Therefore,to satisfy the data traffic demand for various distributed applications,medium/short-reach optical fiber communications(e.g.,intra-and inter-data center links)tend to play more important roles.However,two key challenges to achieving high-capacity data communications for medium/short-reach applications are(i)limited spectrum resources and(ii)relatively low-cost requirements.As a result,the investigation of high-spectrumefficient modulation/multiplexing techniques and simplified low-cost receivers becomes significantly important for next-generation optical fiber communication systems.In this thesis,I investigate low-cost direct-detection fiber systems through simulation and experiments,explore optimizations of multi-subcarrier multiplexing approaches,and achieve multiple simplified receiver schemes.The main achievements are listed as follows:(1)In view of the spectrum leakage in orthogonal frequency division multiplexing(OFDM)systems,a simplified frame synchronization algorithm was investigated for highspectrum-efficient filter bank multi-carrier(FBMC)systems.In FMBC systems,the prototype filter can release the requirement of orthogonality between different subcarriers.Thus,utilizing FBMC systems offers advantages of reduced cost of cyclic prefix(CP)and higher spectral efficiency,as compared to OFDM systems.However,FMBC systems tend to be more sensitive to frame synchronization errors and conventional synchronization algorithms usually have high computational complexity.To simplify the synchronization for FMBC systems,an algorithm based on sparse fast Fourier transformation(Sparse-FFT)was investigated to reduce the complexity.Simulation results showed that such an algorithm can accurately synchronize the FBMC frames under different sampling rates and signal-to-noise ratio(SNR)conditions.We also verified the algorithm in experiments with different data rates and transmission distances.The proposed Sparse-FFT-based synchronization algorithm can reduce the computational complexity to 10.2% of the FFT-based algorithm.(2)For optical communications,coherent detection can recover both amplitude-and phase-encoded data streams and enable high-spectrum-efficient complex data modulation formats.However,it has a relatively high-complex and high-cost receiver structure.Some simplified coherent receivers reduce the hardware cost through higher computational complexity or by sacrificing spectral efficiency.In this thesis,a simplified receiver scheme for short-reach direct-detection OFDM systems by solving the temporal transport-ofintensity equation(TIE)was investigated.A phase retrieval approach was proposed based on two intensity measurements of the optical signal before and after a chromatic dispersion(CD)medium.A TIE was constructed using the two intensity measurements and retrieved the phase information of the signal by solving the TIE.This approach can reduce the receiver complexity compared to the conventional coherent detection and achieves twice the spectral efficiency of the discrete multitone(DMT)approach.Through numerical simulation,we(i)investigated performance dependence on multiple key systems and channel parameters,including carrier-to-signal power ratio(CSPR),CD value,baud rate,sampling rate,laser linewidth,and frequency offset;(ii)determined the optimal CSPR of the TIE-based approach for a 20-Gbaud 16 QAM OFDM system and found it is about 1～2 d B higher than that of the Kramers-Kronig receiver,and(iii)showed the adaptive modulation scheme in the TIE-based OFDM systems,the effective bitrate is increased by 12.8%.Moreover,power-division multiplexing was also applied in such a TIE-based OFDM system to further increase the spectral efficiency for access networks.The performance of this system was evaluated in both synchronized and unsynchronized channel links through simulation.(3)For conventional direct-detection OFDM systems,fiber CD and limited detector bandwidth would induce spectrum power fading and degrade the system performance.An algorithm was proposed to reconstruct the real-value modulated signal using a single detector with limited bandwidth(e.g.,～half of the signal baud rate).Thus,the spectral efficiency at the receiver is twice that of the intensity modulation direct detection(IM-DD)scheme.Through digitally frequency shifting operation and Hilbert transformation,the data can be reconstructed from a single-sideband signal and,subsequently,the signal-signal beating interference(SSBI)can be mitigated by an iterative SSBI cancellation algorithm.This approach supports the digital CD compensation and potentially enables the utilization of low-bandwidth detectors and DAC/ADC for high-data-rate transmission.The system’s performance was investigated through simulation under different carrier frequency offset,CSPR,optical signal-to-noise ratio(OSNR),and detector bandwidth.We also experimentally demonstrated the scheme using a single laser source for carrier and data signal to reduce the negative effects of laser frequency offset and phase noise.Moreover,the performance of the system was experimentally compared using two different carrier amplification approaches,i.e.(a)using EDFA and(b)using laser injection locking.The results showed that the latter exhibits low-noise,high-selective amplification,enabling a 2 d B increase of receiver sensitivity.