| Faced with the rapid development of the communications industry,people make more demand on communications-related services.In order to achieve high-speed access to the Internet in the manner of"anytime,anywhere,and anyway",researchers seek for new opportunities given overcrowded and limited frequency band resources.Thanks to the huge progress made in electronic device technology,communications technology is expected to make another leap forward.Recently,researches on Ultra-Wideband?UWB?technology becomes a hotspot,which is widely used in the radio over fiber networks.The multiband orthogonal frequency division multiplexing ultra-wideband signals in the optical fiber?MB-OFDM UWBoF?system shows advantages of low cost,long reach and large transmission capacity.However,the MB-OFDM UWBoF system suffers from a number of transmission damage,such as packet optical fiber chromatic dispersion,photoelectric device nonlinearity and non-ideal frequency response.They result in transmission performance degradation.This paper discusses the DSP processing algorithms that aim to solve the related problems in the MB OFDM UWBoF system.The main research works and achievements of the specific papers are as follows:Firstly,a modified channel estimation scheme based on Golay TS,which is orthogonal in time domain,is proposed to resist the subcarrier cross beat interference of the MB-OFDM UWBoF system.The simulation results show that the receiver sensitivity of the modified Golay TS channel estimation scheme is 1dB better than the conventional Golay TS channel estimation scheme,in the case of the 70km single mode fiber transmission at the BER of 3.8×10-3.At the same time,the synchronization and channel estimation performance adopting Golay TS and frequency domain orthogonal TSs are compared.The receiver sensitivity using TSs is improved by about2.7dB compared with the modified Golay TS channel estimation scheme as simulation results show.Besides,the performance of the LS and MMSE channel estimation algorithms based on pilot is also evaluated.The results show that the LS algorithm is superior in the 64QAM MB-OFDM UWBoF system because of its simplicity and feasibility.Secondly,a scheme based on pre-compensation is proposed to overcome the effect of fiber dispersion in the MB-OFDM UWBoF system.The intra-symbol frequency domain averaging?ISFA?scheme is adopted to improve the accuracy of channel estimation.Based on the pre-compensation combined with ISFA,the performance of pilot estimation and TSs estimation is studied.Compared with the scheme of only pilot aided channel estimation and the scheme of pilot aided and pre-compensation combined with ISFA enhanced channel estimation,the experimental results show that the receiver sensitivity of the proposed scheme is improved by about0.37dB and 1.25dB respectively in the case of the 70km standard single mode fiber?SSMF?transmitted with BER at 3.8×10-3.Therefore,the TSs channel estimation scheme is more suitable in 64QAM MB-OFDM UWBoF system based on ISFA pre-compensation scheme.Thirdly,the effects of frequency selective fading on the three sub-bands of MB-OFDM UWB signals is studied.An optimization scheme for selecting the best ISFA window size for each MB-OFDM UWB sub-band is proposed,and verified by the experiment of 128-QAM MB-OFDM UWB transmission system in optical fiber.The experimental results show that after the 60km standard single mode fiber?SSMF?transmission at the error rate of 3.8×10-3,the receiver sensitivity of channel estimation scheme based on the modified ISFA optimization with training sequences?TSs?increases by 1.9dB in contrast to the traditional channel estimation scheme.Fourthly,a scheme based on DFT-spread combined with pre-compensation is proposed to suppress high PAPR and frequency selective fading effects.It is studied by experiment in the MB-OFDM UWBoF system.We theoretically analyze the PAPR of baseband OFDM signal and MB-OFDM UWB signal.Moreover,the DFT spread scheme and CF scheme are compared for their PAPR performance given MB-OFDM UWB signal.The simulation results show that after 70km SSMF transmission at the bit error rate?BER?of 1×10-3,the receiver sensitivity of the system is improved by3.3dB compared with the system without DFT and pre-compensation.Therefore,the proposed pre-compensation scheme combined with DFT-spread is simple and effective.Finally,a scheme based on OCT precoding and LDPC coding is proposed to improve the transmission performance of MB-OFDM UWBoF system.Moreover,the performance of MB-OFDM UWBoF system based on OCT precoding is studied,and the influence of OCT pre-coding on each sub band is analyzed.The simulation results show that the OCT pre-coding scheme can average the SNR of each subcarrier to reduce the difference between the sub channels.The OCT pre-coding combined with LDPC scheme can not only average the SNR across the subcarriers to ease the diversity of sub-channels in practical application,but also improve the transmission performance of the system. |
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