Research On Polar Code And Constellation Shaping Method Aided By Precoding In Optical Orthogonal Frequency-Division Multiplexing Systems

As new services such as ultra-high-definition video transmission,cloud computing and artificial intelligence are springing up,the bandwidth of information transmission is continuously increasing at an explosive rate.The next-generation optical access network will inevitably require transceivers to maximize throughput and make full use of all available resources,while ensuring coexistence with traditional optical access networks in a cost-effective manner to achieve 5G/6G omni-directional communication.Orthogonal frequency-division multiplexing(OFDM),is favored by people for its advantages such as high er spectrum efficiency,flexible bandwidth allocation and robustness to optical fiber dispersion,and is considered as one of the most potential candidate technologies to realize the ultra-high-speed optical access network.For the practical direct detection optical OFDM system,its frequency response usually exhibits high-frequency power fading characteristics caused by limited equipment ban dwidth(such as digital-to-analog converter,electronic driver,modulator and analog-to-digital converter,etc.)and fiber dispersion during fiber transmission.It will make bit error rate(BER)of high-frequency edge sharply deteriorated,thereby severely reducing the transmission performance of the systems.There are currently two ways to improve communication performance and increase throughput.One is to adopt an effective coding scheme,which will bring a certain coding gain.The second is to properly design and optimize the constellation diagram,which will bring certain shaping gains.Based on direct detection optical OFDM systems,this paper studies the precoding method and assists it to realize polar codes and constellation shaping methods.By using these efficient digital signal processing(DSP)methods to alleviate high-frequency fading effect of the system,resist the nonlinear effect of the system,ensure the transmission quality of the signal,increase the throughput of the system,so as to promote the further development of optical access network.The main research content and innovative results of this thesis are as follows:Firstly,in order to resist high-frequency power fading effect in the direct detection system,an optical OFDM system transmission scheme based on multi-band constant amplitude zero autocorrelation sequence(MB-CAZAC)precoding is proposed and experimentally verified,and 128/64/32/16 quadrature amplitude modulation(QAM)is adopted to improve system performance and enhance network flexibility.All subbands share the same precoding module,whi ch saves space for storing large precoding matrices.Compared to the method that does not use precoding,the proposed MB-CAZAC precoding method can decrease peak-to-average power ratio(PAPR)of the system,equalize the signal-to-noise ratio(SNR)of each sub-band,and improve the bit error rate(BER)of each sub-band,thereby enhancing overall performance of optical transmission systems.Secondly,in order to increase system capacity and ensure transmission quality,a bit interleaved polar coded modulation orthogonal frequency division multiplexing(BIPCM-OFDM)scheme aided by CAZAC precoding is proposed and verified by experiments for cost-effcetive intensity modulation and direct detection(IM-DD)systems.For the high-order 256-QAM modulation format,we compared quadratic polynomial permutation(QPP)interleaver with random interleaver in terms of overcoming influence of fading effects over mapping bit levels.At the same time,to match carefully designed working point of Monte Carlo during polar codes design period,CAZAC precoding is used to balance the uneven SNR between subcarriers.Experimental result illustrates that compared to that BIPCM-OFDM signal without precoding,CAZAC precoding can reduce the PAPR of the system by about 2 d B.Compared to the BIPCM-OFDM method using QPP interleaver,the random interleaver can obtain a Q factor improvement of 1.13 d B.In addition,compared to that no BIPCM,only the CAZAC precoding and only th e random interleaver,the proposed method can obtain Q factor improvements of 4.82 / 2.46 / 1.78 d B,respectively.Thirdly,based on the efficient 256-QAM BIPCM method,twin single-side band OFDM(twin-SSB-OFDM)scheme implemented by one OFDM transmitter is proposed to overcome power fading induced by chromatic dispersion(CD)and extend the transmission distance in direct detection optical systems.Compared to that method which needs two OFDM transmitters,the proposed method has obvious advantages in terms of computational complexity and overhead.The results show that compared to polar coded modulation OFDM without interleaver and OFDM methods,the proposed method can achieve Q factor improvements of 4d B and 5d B,respectively.Furthermore,this method not only improves the sensitivity of the receiver,but also improves the tolerance of the system to the linewidth of the laser.Fourthly,to improve system performance and provide flexible rate adaptation,a probabilistic shaping 64-QAM OFDM(PS-64QAM-OFDM)scheme aided by precoding is proposed,which is verified by experiments in power-constrained and bandwidth-limited direct-detection systems for optical access networks.Since this scheme does not need to consider the channel information obtained by the tran sceiver through round-trip feedback,it can significantly reduce system's overhead and computational complexity compared to adaptive bit,power or entropy loading scheme.The CAZAC precoding is compared with orthogonal cyclic transform(OCT)and discrete Fourier transform(DFT)precoding in reducing PAPR and increasing normalized mutual information(GMI).In order to obtain better fine granularity,the system performance of PS-64QAM-OFDM is compared with that of uniformly distributed OFDM at the same information rate.Compared with conventional64QAM-OFDM signal,when the information rate is 4.0/3.6/3.0 bits/QAM symbol,the PS-64QAM-OFDM signal can provide shaping gains of 1.04/ 0.94/0.64 d B and1.0/1.0/1.0 d B receiver sensitivity gain s,respectively.At the same time,only by adjusting probability distribution,a net r ate from 32.66 to 43.55 Gb/s can be obtained.In addition,in order to further improve the communication capacity and receiver sensitivity in the optical access network s,a high-order encoded PS-OFDM transmission scheme based on CAZAC precoding is proposed and experimentally verified.Since 256 QAM is used in PS OFDM,the spectral efficiency and shaping gain of the system can be remarkably improved.Experimental results exhibit,it can achieve a capacity increase of 2.45-Gbit/s/? in terms of capacity.When the information rate is 6.0 bits/QAM symbol,the proposed scheme can achieve a performance improvement of 2.0 d B in terms of receiver sensitivity and achievable information rate,respectively.Finally,in order to effectively overcome the nonlinear effect,a geometrically shaped OFDM(GS-OFDM)scheme based on precoding is proposed,and it is experimentally verified in a cost-effective IM-DD system.At the same time,in improving the performance of PAPR,six precoding methods including CAZAC,DFT,OCT,etc.are explored.In addition,in order to comprehensively explore the system performance under different single-mode fiber transmission distances,including 20 /50 / 80 kilometers,the geometric shaping scheme is compared with the uniformly distributed QAM modulation scheme in terms of mutual information(MI).When the information rate is 4.5 bits/QAM symbol,after 20 kilometers of optical fiber transmission,the two proposed GS methods can achieve a receiver sensitivity gain of0.24/0.61 d B respectively compared with uniformly distributed 64 QAM.After 50 kilometers,the two proposed GS methods can achieve a receiver sensitivity gain of0.32/0.74 d B respectively compared with uniformly distributed 64 QAM.After 80 kilometers,the two proposed GS methods can achieve 0.36/0.87 d B receiver sensitivity gains respectively compared with uniformly distributed 64 QAM.With the increase of the fiber distance,the nonlinear effect of the fiber is gradually obvi ous,and the shaping gain gradually becomes larger.Experimental results prove that the proposed precoding-assisted GS-OFDM scheme has strong robustness in resisting nonlinear effects.