Research On Signal Modulation And Interference Cancellation Techniques For Short-Reach And Bandwidth-Limited Optical Fiber Transmission Systems

With the rapid development of emerging technologies such as Internet of Things,high-definition video,and artificial intelligence,as well as the rapid popularization of traffic-hungry services such as remote office,cloud classroom,and cloud desktop,the data traffic continues to grow with high speed,and the data center architecture is facing with fast updating.In the data center interconnects with high-speed growth of capacity,100-G optical interconnects technology has been widely adopted,400-G age with commercial scale application is coming,and 800-G high-speed optical transceiver has been released.With a large number of connections,the short-reach data center interconnects are very sensitive to cost and power consumption,which prefer to use low-cost,low-power,small-size,and high-integration devices.However,the application of devices with low cost and low power consumption leads to the bandwidth limitation effect for high-speed signal.This dissertation aims at solving the problem of bandwidth limitation in short-reach optical fiber transmission systems,based on multi-carrier or single-carrier modulation,thoroughly studies signal modulation and interference cancellation techniques in bandwidth-limited orthogonal frequency-division multiplexing(OFDM),non-orthogonal frequency-division multiplexing(NOFDM)and single-carrier systems.The main research contents and innovations of this dissertation are summarized as follows:1.Interference Cancellation Scheme Based on Layered Asymmetrically Clipped Optical Orthogonal Frequency-Division MultiplexingFor OFDM systems,the real trigonometric transform represented by discrete Hartley transform(DHT)or discrete cosine transform(DCT)is applied to layered asymmetrically clipped optical orthogonal frequency-division multiplexing(L-ACO-OFDM).In the proposed L-ACO-OFDM based on real trigonometric transform,multiple layers of signals are added to increase the spectral efficiency,which makes L-ACO-OFDM more suitable for short-reach and bandwidth-limited optical fiber transmission systems.In L-ACO-OFDM based on real trigonometric transform,all the signals have real values,and the computational complexity is reduced to half of that of L-ACO-OFDM based on discrete Fourier transform(DFT).To solve the inter-carrier interference(ICI)problem introduced by the clipping noise of the current layer to the subsequent layers,an improved ICI cancellation scheme based on diversity combining technique is proposed in L-ACO-OFDM based on real trigonometric transform.With the improved ICI cancellation scheme,more than 2-dB gain of Eb/N0 can be achieved in the simulation system,and approximately 2-dB receiver sensitivity improvement at the 7%forward error correction(FEC)limit can be achieved in the experimental system.To solve the high peak-to-average power ratio(PAPR)problem of L-ACO-OFDM,the performance of layered asymmetrically clipped optical single-carrier frequency-division multiplexing(L-ACO-SCFDM)with lower PAPR,lower computational complexity and higher resistance to high-frequency distortion properties is first experimentally analyzed in short-reach and bandwidth-limited optical interconnets.With the 3-dB equivalent bandwidth of approximately 2 GHz,18-Gb/s L-ACO-SCFDM and L-ACO-OFDM signals are transmitted.Compared with L-ACO-OFDM,L-ACO-SCFDM has a receiver sensitivity improvement of approximately 4 dB at the 7%FEC limit.At the same time,an optimized method is proposed to remove the fixed interference,which can effectively improve the experimental performance.With this method the Q factor can achieve approximately 2.3-dB improvement.2.Inter-Carrer Interference Cancellation Scheme Based on Faster-Than-Nyquist Non-Orthogonal Frequency-Division MultiplexingFor NOFDM systems,when compressing the subcarrier spacing of OFDM signal to less than half of the symbol rate per subcarrier,faster-than-Nyquist non-orthogonal frequency-division multiplexing(FTN-NOFDM)singal with narrower bandwidth can be generated.Due to the compression of subcarrier spacing,the ICI problem in FTN-NOFDM should be focused on.When the ICI is effectively eliminated by tree-seach based algorithm,the Mazo limit and capacity limit of FTN-NOFDM in additive white Gaussian noise channel are first verified.When the bandwidth compression factor is set to 0.802,QPSK-modulated FTN-NOFDM has almost the same performance as QPSK-modulated OFDM,which agrees well with the Mazo limit.This result also verifies the capacity limit of FTN-NOFDM signal,that if the bandwidth compression factor is between 1 and 0.802,the capacity limit of FTN-NOFDM signal can be higher than that of Nyquist signal.The conventional tree-seach based algorithm has good ICI cancellation performance,while the computational complexity of the conventional tree-seach based algorithm has the potential to be further reduced.In this dissertation,the simplified tree-search based algorithms including radius-aided maximum-likelihood detection with QR decomposition and M-algorithm(RA-QRM-MLD),iterative detection and sphere decoder(ID-SD),iterative detection and maximum-likelihood detection with QR decomposition and M-algorithm(ID-QRM-MLD)are proposed in fractional cosine transform(FrCT)-based FTN-NOFDM for ICI cancellation.In the experimental system,28-Gb/s FTN-NOFDM and OFDM signals are transmitted through the bandwidth-limited system with the 3-dB equivalent bandwidth of approximately 5.5 GHz.Experimental results show that after using the simplified algorithms to effectively eliminate ICI,FTN-NOFDM with bandwidth compression can achive better performance than OFDM in this bandwidth-limited system.When the performance of OFDM is not able to reach the FEC limit,the performance of FTN-NOFDM is able to reach the FEC limit.3.Inter-Symbol Interference Cancellation Scheme Based on Single-Carrier ModulationFor single-carrier systems,based on four-level pulse-amplitude modulation(PAM-4),this dissertation proposes a joint pre-and post-equalization algorithm to eliminate inter-symbol interference(ISI)caused by bandwidth-limited channel.In the joint pre-and post-equalization algorithm,the strength of pre-equalization can be adjusted at the transmitter,so that the problem about reduction of effective signal power caused by high PAPR can be avoided.At the receiver,the post-filter tap coefficient is jointly optimized according to pre-equalization strength.After applying optimal pre-equalization strength and post-filter tap coefficient,in a bandwidth-limited optical fiber transmission system with 10-dB equivalent bandwidth of approximately 14 GHz,2.5-dB gain in receiver sensitivity can be obtained at the 7%FEC limit in a 112-Gb/s PAM-4 transmission over 2-km standard single mode fiber.In conclusion,this dissertation aims at solving the bandwidth limitation problem in short-reach optical fiber transmission systems,systematically and deeply studies three types of signal modulation and interference cancellation schemes.With multi-carrer modulation,for bandwidth-limited OFDM the ICI cancellation scheme and optimized interference cancellation scheme in L-ACO-OFDM and L-ACO-SCFDM are studied,for bandwidth-limited NOFDM the ICI cancellation scheme in FTN-NOFDM is studied.With single-carrier modulation,for single-carrier signal the ISI cancellation scheme is studied.This dissertation applies the proposed signal modulation and interference cancellation techniques in short-reach and bandwidth-limited optical interconnects,and fully verifies the proposed techniques through simulation and experiment.The signal modulation and interference cancellation techniques studied in this dissertation provide feasible solutions for short-reach and bandwidth-limited optical fiber transmission systems.