Research On Fast-OFDM-based Digital Signal Processing Techniques In IM/DD Optical Comunication Systems

The everincreasing demand for high bandwidth applications are contributing to the exponentially growth of access network traffic.The access network is responsible for connecting useres,thus its traffic rate restricts the available bandwidth resources of end users.Intensity modulation and direct detection(IM/DD)optical communication systems,with simple structure and low cost,are very appropriate for cost sensitive access network.In addition,advanced modulation formats and relevant digital signal processing(DSP)techniques play important role in increasing the the rate of optical communication systems.Hence,it is of great significance for optical access network upgrading to investigate the advanced modulation formats appropriate for the IM/DD optical communication systems and use relevant DSP techniques to overcome the transmission impairment,improve the communication quality and increase the system capacity.The fast orthogonal frequency division multiplexing(FOFDM),which is based on discrete cosine transformation(DCT)and simple one-dimensional modulation,is very appropriate for IM/DD optical communication systems,and it is regarded as one of the most promising technologies for future optical access network.In the thesis,FOFDM is deeply studied in IM/DD optical communication system.In addition,a series of DSP techniques are proposed based on FOFDM.The main work of the thesis can be concluded as follows.Firstly,the research of static adaptive modulation and overlapping frequency domain equalization(OFDE)based on FOFDM.In order to improve the transmission rate as much as possible under the limited bandwidth of photoelectric devices,an adaptively modulated FOFDM scheme is proposed when the channel state information is obtained.In addition,in order to reduce the overhead in FOFDM system furtherly,an OFDE technique for FOFDM is proposed.Compared with the uniformly modulated FOFDM scheme,the adaptively modulated FOFDM scheme has a great improvement in transmission quality.Through transmission of 20km standard single mode fiber(SSMF),the adaptively modulated optical FOFDM system with 16/8/4-pulse amplitude modulation(PAM)can improve the receiver sensitivity by 2.3d B at the bit error rate(BER)of 2.4×10^-2 when compared to uniformly modulated optical FOFDM system with 16-PAM.Moreover,the FOFDM scheme based on OFDE technique can achieve FDE without cyclic prefix(CP)and obtain similar performance as CP-based FDE.Thus,it can improve the transmission efficiency by 12.37%Secondly,the research of the sampling frequency offset(SFO)estimation and compensation technique based on FOFDM.In order to reduce the influence of SFO caused by unsynchronized sampling clock frequency in optical transceiver,an SFO estimation and compensation method based on FOFDM is proposed.By inserting training symbols at the frame head and frame tail,the SFO can be estimated and compensated among the single-tap equalization of FOFDM.As a result,the SFO estimation error can be controlled within±0.1 ppm under 200ppm SFO,so as to compensate the SFO-indcued subcarrier phase offset(SPO)effectively.It is found in the experiment that,through transmission of 20km SSMF,the average error vector amplitude(EVM)on each subcarrier of FOFDM is 3.3db higher than that on each subcarrier of OFDM under 200ppm SFO at the received optical power(ROP)of-1d Bm.It indicates that the FOFDM is more robust than OFDM for SFO-indcued inter-carrier interference(ICI).Thirdly,the research of real-value precoding technique based on FOFDM.This part of work can be divided as the following two sections.1)The applied research of real-value precoding technique based on FOFDM in Radio over Fiber(Ro F)system.In order to reduce the influence of nonlinear effects in FOFDM-Ro F system,a DCT-precoding technique is proposed to reduce the peak to average power ratio(PAPR)of FOFDM signals.Experimental results show that the DCT-precoded FOFDM signal can obtain a significant PAPR as well as relatively flat signal to noise ratio(SNR)for different subcarriers.Consequently,the DCT-precoding technique can improve the system performance greatly.2)The applied research of real-value precoding technique based on FOFDM in Visible light communication(VLC)system.In order to improve the system capacity without channel prior information,a real-value precoding method for FOFDM is proposed.In addition,it is verified by simulation and experiment in optical wireless channel.Both simulation and experimental results show that FOFDM can effectively overcome the frequency-selective fading of optical wireless channels by using the DCT or walsh-hadamard Transformation(WHT)precoding technique.Furthermore,WHT precoding can further resist the SFO-indcued ICI in FOFDM system.