Researches On The Key Technologies Of Upstream Transmission In Direct Detection OFDM PON

With the rapid development of high-bandwidth services,especially video services,the current commercial EPON and GPON networks have been unable to meet the high capacity and wide coverage of access networks demand.However,OFDM PON based on Orthogonal Frequency Division Multiplexing(OFDM)has become an important option for next-generation PON(NGPON)because of its high spectrum efficiency,good anti-dispersion performance,and its flexible and free subcarrier allocation.Compared with coherent detection,OFDM PON with intensity modulation(IM)and direct detection(DD)has reduced the complexity of the device and the algorithm,and meets the requirement of the access network for low cost,becoming one of the current research hot spots.However,a large number of researches on direct detection of OFDM PON have been focused on the downlink in recent years.In fact,with the development of services such as live webcasting and video surveillance,users' requirements for uplink transmission performance of access networks continue to increase.It is necessary to study the large-capacity OFDM PON uplink transmission technology.In this paper,the key issues such as direct detection of uplink system capacity in OFDM PON uplink transmission,compatibility with existing TDMA PON and PAPR suppression are studied.The main research work is as follows.1.In order to solve the problem of wasting system capacity caused by transmitting signals of the same modulation format on ONU channels with different signal-to-noise ratios in uplink multiple access,this paper designs an uplink scheme to achieve system capacity increasing based on hierarchical modulation.The principle of layered modulation is expounded.The modeling of transmission and reception for layered signals are analyzed in the uplink IMDD OFDM PON scenario.The effects of the layered power ratio and the phase noise of the laser lines on the performance of the layered modulated signal are analyzed,and the scheme carries on the damage suppression at the receiving unit using iterative detection based on pilot-aided and decision feedback.The simulation results of hierarchical 8QAM modulated 15Gbps IMDD OFDM PON uplink system show that when the information of two ONUs whose SNR differ 6dB is transmitted on the same subcarriers and the hierarchical power ratio is 2,the capacity of whole system is increasing about 15.3%than the traditional OFDMA PON.2.In order to solve the problems of low efficiency,high demand for linearity of DAC/ADC and non-linearity of optical fiber caused by high PAPR in OFDM systems,a scheme of reducing the PAPR in the uplink IMDD OFDM PON system based on hierarchical modulation and suppressing the out-of-band spectrum dispersion caused by limiting noise through frequency domain filtering has been used in this paper.After several times of limiting,the system's PAPR can be effectively reduced and the transmission performance of the system can be improved.The simulation results of uplink IMDD OFDM PON system based on hierarchical modulation show that the system PAPR can be reduced by about 2.8dB with 2 asymmetric clipping methods when the layered power ratio is 2 and the number of subcarriers is 128.3.For the power fading caused by dispersion in intensity modulation direct detection OFDM PON system,a scheme of large capacity,high branch ratio IMDD OFDM PON based on Volterra filter to suppress nonlinear impairments and using low cost DML is designed in this paper.The principle of dispersion-caused power fading and the principle of increasing the SPM effect of the originating optical power to stimulate the fiber to move the valley of power fading to the high frequency are analyzed by modeling.The time-domain Volterra filter is used to suppress the square-law detection and the modulator chirp Chirp caused by beat frequency interference and fiber non-linear damage.The experimental results of 20Gbps,16QAM and 60Km off-line transmission show that the power fading trend of 5G Baud OFDM signal at 5GHz is reduced by 3dB after EDFA is used to increase the transmit power.And by using the time-domain Volterra filter to equal a variety of nonlinear damage of whole system,the system receiver sensitivity can be up to-7.2dBm.