Study On Direct Detection Of PAM4 Signals With FTN For Optical Transmission System

With the emerging applications such as cloud computing, big data,Internet of things, etc, Internet era is coming. Living in this new information and digital age, the amount of Internet users is increasing, leading to the exponential growth of data transmission rate. Thus, how to improve the spectrum efficiency and reduce the cost is an important issue to address in the short-reach optical transmission system, which is close to the users. In this paper, we studied the points in short-reach optical transmission systems,which is detailed as follows:Firstly, we study two methods about how to improve the spectrum efficiency in short-reach direct modulating direct detecting system. The first technology improves the spectrum efficiency through compressing spectrum such as Faster Than Nyquist (FTN). The second technology is through employing high modulation format including Pulse Amplitude Modulation(PAM), Carrierless Amplitude/Phase modulation (CAP), Discrete Multi-Tone(DMT). We then focuse on PAM-4 based short-reach optical transmission system, including the transmitter, transmission system, and receiver combined with digital signal processing (DSP).Secondly, in the research of PAM-4 short-reach optical transmission system, the DSP process at the receiver consists of the equalization,post-filtering, and threshold decision. The equalization algorithm employs DD-LMS algorithm and we improve the stypticity of the algorithm. In the study of post-filter, we compared the impact of 2-tap filter and the improved 3-tap filter on the system. Simulation results show that at the scenario of SNR ranging from 9 dB to14 dB and at high, the bit error ratio (BER) of 3-tap filter is obviously lower than that of 2-tap filter. For example, at SNR = 14 dB,bandwidth = 16 GHz, the BER decreases by almost one level. We verifie the theory in which the optimal taps are calculated through conducting the simulations. The contour line of the results show that the practical optimal taps are as same as that calculated according to the autocorrelation matrix of the noise, meantime the Q-value is optimal. In our study, the decision algorithm employs MLSE algorithm and BCJR algorithm and we further improve all these algorithms. The results of the simulation for MLSE and BCJR show that the system with BCJR achieves better performance than MLSE. Specially, Q-value of BCJR improves 1 unit compared with that of MLSE at the case where the number of states equals to 8 and SNR is 14 dB at bandwidth =12 GHz. Finally, we conducte the simulation according to the accuracy of DAC module in the BCJR based system. Simulation results show that as long as the number of quantization bit exceeds 4, the performance of the system is close to the theory. In conclusion, BCJR is a promising solution in the future where the FTN based short-reach optical transmission system is combined with technologies such as recurrent code.