The Research On Symbol Synchronization Technology In Real-time Intensity-modulation And Direct-detection OFDM-PON

The bandwidth requirements of the access network will increase dramatically with the rapid development of multimedia services such as high definition television and online games.Optical Orthogonal Frequency Division Multiplexing(OFDM)has received wide attention due to its excellent dispersion-tolerant energy,high spectral efficiency,and flexible dynamic bandwidth scheduling capability.Appling t OFDM in Passive Optical Network(PON)has become a research hotspot for the next generation.At the same time,the Intensity Modulation Direct Detection(IMDD)optical OFDM system has the characteristics of simple system structure and low cost,and is especially suitable for the cost-sensitive optical access network field.Therefore,IMDD OFDM-PON is the object of this paper.The requirement of the accuracy of symbol timing synchronization in OFDM-PON systems is extremely high,and poor synchronization has a serious impact on system performance.Symbol timing errors can cause severe inter symbol interference(ISI).And with the increasing attention of real-time OFDM-PON systems,real-time symbol synchronization algorithms have become the focus of research.This paper mainly studies the symbol synchronization technology in real-time IMDD OFDM-PON system.The main contributions include the following:1)In the traditional methods,the method of judging symbol synchronization performance can only be indirect,such as the BER performance.It's the lack of a way to directly evaluate the accuracy of symbol synchronization.Therefore,we propose a time domain symbol synchronization accuracy evaluation criteria according to a time domain non-return to zero code.By detecting the NRZ code of the time domain in groups,the symbol timing deviation can be accurately estimated.Due to our simple design and high scalability,we can design NRZ codes with different samples according to different application scenarios.Therefore,the proposed synchronization accuracy evaluation criteria can be applied to other algorithms.2)Combining our proposed accuracy criteria,we propose a low-complexity and high-precision symbol synchronization algorithm using NRZ code.By adding a smoothing filter,the robustness of the algorithm to the phase deviation of the sampling clock is improved.Furthermore,in order to apply the algorithm to the real-time OFDM-PON system,multiplication operation will occupy a large amount of resources.Therefore,we simplify the algorithm implementation and replace the multiplier by using the sign bit XOR operation,which greatly reduces the algorithm complexity.At the same time,the accuracy of the algorithm is guaranteed.The simulation results show that 98% synchronization accuracy can still be achieved when the received optical power is as low as-21 dBm.Meanwhile,due to the unique frame structure for synchronization,compared with the traditional synchronization algorithm using training sequences,the synchronization performance is not affected by the size of FFT,so the algorithm is more robust.3)In order to further reduce the complexity of real-time algorithm and improve the robustness of the algorithm,a novel dual-threshold symbol synchronization algorithm based on amplitude decision is proposed,and it is verified in both offline OFDM-PON system and FPGA-based real-time OFDM-PON system.Experimental verification is performed on which the coarse synchronization threshold is used to find the approximate starting position of the frame and the fine synchronization is used to perform accurate symbol synchronization.Unlike traditional synchronization algorithms that use correlation calculations or use sign bit addition to obtain timing metric functions,they require extensive experimentation to obtain an optimized detection threshold.In the proposed algorithm,a very simple amplitude threshold based on theoretical analysis of the optical channel stable channel characteristics is obtained by amplitude comparison.And a fixed threshold can be used at different signal to noise ratios,which greatly simplifies the implementation of the algorithm.When the received optical power of the FPGA-based real-time IMDD-OFDM PON platform is as low as-21 dBm,the proposed algorithm can achieve synchronization accuracy of up to 95%.In addition,our algorithm can cover the transmission distance requirement of the optical access network within 100 km.Secondly,due to our unique algorithm design,our algorithm performance is also independent of the size of FFT,and it is proved that there is a good sampling frequency offset tolerance.For sampling frequency offsets up to 500 ppm,there is no impact on our synchronization performance.