Research On Digital Signal Processing Algorithms For High Spectral Efficiency Optical Orthogonal Frequency-Division Multiplexing Communication Systems

With the rapid development of information technology,people have higher requirements on the data rate.However,limited spectrum resources are becoming increasingly scarce.In order to increase the data rate and provide higher spectral efficiency?SE?under limited frequency band resources,Optical Orthogonal Frequency Division Multiplexing?O-OFDM?has become an essential technology for modern optical fiber communication and visible light communication,and thus received widespread attention from the industry.OFDM modulation technology can not only achieve efficient spectrum utilization,but also can effectively counter the inter-code interference generated by optical fiber and indoor multi-path effects,thereby ensuring the stability of optical fiber communication and visible light communication systems.Compared with coherent light detection?CO-OFDM?,direct detection of OFDM systems?DDO-OFDM?has the advantage of low cost.How to improve spectrum efficiency,save cost and reduce complexity in OFDM?DDO-OFDM?communication system is the research focus of this article.In this paper,a short-distance intensity modulation direct detection?IM-DD?communication system based on optical fiber and visible light is studied,and digital signal processing algorithms in optical OFDM communication systems with high spectral efficiency are analyzed.The primary research contents and related achievements of this paper are as follows:Firstly,the FFT-size efficient combined with 4096 QAM modulation is proposed in an intensity-modulated and direct-detection?IMDD?optical communication system based on directly modulating laser to reduce hardware implementation complexity and system overhead,and it is demonstrated by numerical simulation and experimental verification.Compared to the traditional method of using the Hermitian symmetry of the frequency symbols to generate real OFDM signals,the proposed technique has saved 55%in the number of the FFT/IFFT operations,which results in a great reduction of the power consumption and the occupied chip area.Also,an intra-symbol frequency-domain average channel estimation algorithm is used to obtain an accurate channel response estimate,and a direct modulation laser?DML?is used to reduce the cost of the optical fiber transmission system.The experimental results show that the OFDM-signal which combined effective FFT-size with 4096 QAM can be successfully transmitted over 10km standard single-mode fiber?SSMF?,and the bit error rate?BER?is lower than the threshold of 20%soft-decision forward error correction code?FEC?.The spectrum efficiency reaches 9.5bit/s/Hz,and it has similar BER performance with traditional OFDM signals.Secondly,a method combining DFT-spread technique and FFT-size algorithm is proposed and experimentally demonstrated in the hybrid Fiber-VLLC system.Also,a high spectral-efficiency adaptive modulation scheme is introduced and experimentally demonstrated in the VLLC system.The proposed method can efficiently reduce the peak-to-average power ratio,improve the robustness of high-frequency power attenuation and narrow-band interference in the system,thus can improve the transmission performance of the system,and the communication speed while ensuring the reliability of communication.The experimental results show that after 20 km of standard single-mode fiber and 5 meters of free space transmission,the BER of 4.75Gbps 64QAM DFT-spread OFDM signal is less than 10^-2.And the BER of 3.17 Gbps16QAM DFT-spread OFDM signal is below at 10^-5.Also,the performance of the adaptive DFT-spread scheme and the conventional OFDM scheme has been compared.The experimental results show that the adaptive DFT-spread scheme can achieve an excellent overall performance and higher-rate communications than the conventional OFDM scheme.Thirdly,a joint preprocessing scheme based on Orthogonal Circulant Matrix Transform?OCT?precoding and pre-emphasis has been proposed and experimental demonstrated in the visible light system based on light emitting diodes?LED?to overcome the attenuation effect of OFDM signal.The experimental results show that compared with a single pre-enhancement or pre-equalization method,the joint pre-processing scheme can efficiently improve the BER performance of OFDM systems based on visible light communication.Finally,a decision-based sampling clock frequency offset algorithm has been proposed and experimentally demonstrated in the VLC-LED system and direct detection of optical fiber communication system to compensate for sampling clock frequency deviation and reduce the length of the cyclic prefix.Experimental results show that the method is not affected by the estimation error.The algorithm can reduce the overhead of the generation of the cyclic prefix introduced by the sampling frequency offset,and is suitable for the environment of automatic sampling frequency offset caused by the time domain change.At the same time,the algorithm can implement OFDM symbol recovery based on symbols,and do not require buffers and high-power electronic devices.By using this algorithm,the standard sampling frequency offset can be directly compensated,without any pilot assistance.And it can compensate for the residual sampling frequency offset caused by other estimation and the sampling frequency offset of the OFDM system.