Research On Modulation Coding And Probabilistic Shaping In The Next Generation High-speed PON

With the development of 4K/8K video,5G mobile transmission,virtual reality,Internet of Things,cloud services and other technologies,network traffic presents an explosive growth trend,driving the evolution of the next generation passive optical network(PON)towards a higher rate.At present,the single wavelength transmission technology of 25 Gb/s and 50 Gb/s is relatively mature,and the single wavelength transmission of 100 Gb/s needs to be further studied.In the next generation of PON,how to balance the demands of high rate transmission,low bandwidth devices and low complexity equalizer is an urgent problem to be solved.At the same time,high-speed signals are inevitably more prone to error codewords in bandwidth-limited transmission.In order to ensure transmission performance,so it is necessary to introduce error correction code to ensure transmission performance.In addition,probabilistic shaping has become a research hotspot in recent years because of its advantages in power-constrained systems,such as increasing transmission capacity and improving error performance.Research on probabilistic shaping technology applicable to PON has important reference significance for the next generation of PON.In this paper,researches and experiments are carried out on key issues such as an economical transmission scheme based on a new modulation format in single-wavelength 100 Gb/s transmission,and probabilistic shaping technology combined with low density parity check code(LDPC).The main research content and results are as follows.1.In order to transmit single-wavelength 100 Gb/s signal on the basis of existing low-bandwidth optical devices as much as possible,an economical transmission scheme based on low-bandwidth receiver and DB-PAM-4 modulation format is proposed and verified by experiments.Since one optical line terminal(OLT)in PON can correspond to multiple optical network units(ONU),the cost of ONU terminal will be multiplied by using the original low-band devices.The proposed scheme uses 12-GHz PIN as the receiver to convert PAM-4 to DB-PAM-4 signal,uses 25-GHz EML as the transmitter to reduce the equalization pressure,and uses 35-tap forward feedback equalizer(FFE)with 500 training sequence length as the equalizer.Soft decision LDPC(SD-LDPC)is used as error correction code to provide a high-speed and economically feasible solution for the next generation PON.This paper also conducted a comparative experiment of equalizing the signal to DB-PAM-4 and PAM-4.The results show that equalizing to DB-PAM-4 when the bandwidth is limited can reduce the bit error rate(BER)to the LDPC threshold(1×10^-2),but the BER equalized to PAM-4 is still higher than 10^-1.On this basis,the calculation method of logarithmic likelihood ratio in SD-LDPC decoding for Gray coded DB-PAM-4 and PAM-4 signals is simplified.After SD-LDPC error correction,the BER of 10⁷ data can be reduced to 0.2.In order to apply probabilistic shaping to PON transmission,a low-complexity probabilistic shaping scheme is proposed in this paper.The core of this scheme is to use an improved probabilistic amplitude shaping(PAS)architecture and combine prefixes free distribution matching(PCDM)algorithm with LDPC to change the distribution probability of signals.In this paper,PCDM codebook with particle size of 0.01 was generated.In 100 Gb/s PAM-4 system,probabilistic shaping signals and uniformly distributed signals at different LDPC and PCDM code rates were compared,in which the optical transceiver devices were 25-GHz EML and 33-GHz PIN to better recover the four levels and observe the distribution of each level.The experimental results show that the probabilistic shape signal based on PCDM can provide the receiver sensitivity gain compared with the uniformly distributed signal at the same total information rate,and the gain increases with the decrease of LDPC or PCDM bit rate.In the 25km optical fiber transmission scenario,when the total code rate is 3/5 and the LDPC code rate is8/9,5/6 and 2/3,the corresponding probabilistic shaping gain is 0.9 d B,1.2 d B and 1.9d B,respectively.