Digital Signal Processing Algorithms For Impairments Compensation In Optical Fiber OFDM Systems

Orthogonal frequency division multiplexing(OFDM)technology is widely used in traditional wired and wireless communication systems such as ADSL,Wi Fi,4G LTE,due to its high spectral efficiency,strong resistance to multipath and the ability to compensate for system impairments with powerful digital signal processing algorithms(DSP).In order to improve people's increasing demand for broadband in access networks,the OFDM-based PON system can effectively solve the problems related to low spectral efficiency and weak dispersion resistance in PON system with the traditional on-off keying(OOK)modulation.However,there are many impairments in the OFDM-PON system,such as the imperfect frequency response of the optical or/and electrical devices and the offset mismatch of sub-ADCs in the high-speed time-interleaved analog-to-digital converter(TI-ADC).In view of the above problems,this paper uses low-cost digital signal processing algorithms to compensate for the corresponding impairments.The research contents and results of this paper are concluded as follows.Firstly,a layered precoding technique is proposed based on the layered asymmetrically clipped orthogonal frequency division multiplexing(L-ACO-OFDM)system.The proposed layer-precoding technique can effectively reduce the signal-to-noise ratio(SNR)difference among sub-carriers induced by the imperfect channel response in the L-ACO-OFDM system.Also it can reduce inter-layer error transmission and thus improves the receiver performance.Numerical simulation shows that he layered discrete Fourier transform(DFT)precoded L-ACO-OFDM has the similar receiver sensitivity as that of the layered orthogonal circulant matrix transform(OCT)precoding.However,the receiver sensitivity is improved by 3.4 dB when the BER is 1e-3 compared with the L-ACO-OFDM system without any precoding techniques.Secondly,a low-complexity online digital offset mismatch compensation(OMC)scheme based on time-domain averaging is proposed for high-speed TI-ADCs.One or more data subcarriers in the receiving OFDM signal may be interfered due to the offset mismatch of each sub-ADC chip,which will worsen the overall performance of the receiver.The proposed OMC has low implementation complexity and can accurately estimate the DC offset of each sub-ADC,effectively improving the BER of the receiver.Through real-time experiment and numerical simulation,the feasibility of the proposed OMC algorithm is verified effectively.The experimental results show that in the real-time OFDM receiver based on 5 GSa/s TI-ADC,the proposed OMC algorithm can improve the BER performance by an order of magnitude.When the BER is 1e-3,the receiver sensitivity can be improved by more than 5 d B.Simulation results show that DC offset mismatch will lead to the “floor effect” of BER performance,and the proposed OMC algorithm can eliminate this effect of BER performance and significantly improve BER performance.Compared with that of offset mismatch-free case,the SNR penalty is negligible.