Research On Discrete Multi-tone Modulation Technology Of High Speed Based On Direct Detection Technology In Optical Metro Network

The modern society has now entered the information era.The communication has become one kind of basic necessity of life,as important as clothing,food,housing,transportation,etc.Communication is an essential mean of exchanging information which plays a crucial role in promoting the development of the society and human culture.In the recent decades,the increasing need for communication has facilitated the development of optical fiber communication technology(FOCT).At present,the OFCT has become one of the most important parts of modern communication technologies,and it also plays a vital role in the modern communication network.With the newly emerging business such as the cloud computing and the data center,the communication needs witness an explosive growth.To solve the contradiction between the heavy demand and the under capacity,it is necessary to promote the transmission rate up to 100-Gb/s or even T bit level.The optical network transmission needs a lot of optical radiating and receiving devices.On consideration of this,the optical network requires being low-power consumption and cost-effective.On the other hand,the direct detection technology is widely used due to the simplicity of the system structure and less requisite devices.At present,the so-called'hundred G and hundred kilometers' standard single mode fiber(SSMF)transmission technology based on direct detection has been realized.Discrete multi-tone(DMT)is one kind of multi-carrier transmission modulation method based on discrete Fourier transform of which the theoretical capacity is close to the Shannon capacity.Recent research focuses on the modulation methods which aim to increase the transmission rate of metropolitan area network.DMT modulation is one of the hotspots of which the sub-carrier can be modulated using multiple quadrature amplitude modulations.This method has the advantage of the efficient frequency spectrum.Besides,it can dynamically adjust the bit distribution and modulation format according to the ability of sub-channels which results in higher channel capacity and allows high-speed transmission using low rate devices.This paper investigates the application of direct detection discrete multi-frequency modulation system in an optical metropolitan network.The research focuses on the introduction of constellation shaping,wavelet transform and discrete cosine transform technologies,the improvement of discrete multi-frequency modulation system(the design of the system and the analysis on the transmission properties based on simulations).The main contributions and innovations include the following:(1)A direct detection self-adaption discrete multi-frequency modulation system scheme with high frequency spectrum and low complexity by analyzing discrete multi-frequency modulation technology was proposed.This scheme includes the construction of discrete multi sub-carrier modulation system in the optical area and the realization of the self-adaption algorithm.The self-adaption algorithm can complete the self-adaption process of the multi-carrier modulation by adjusting baseband modulation format using the self-adaption bit distribution algorithm in the multi-frequency modulation system according to the quality of the sub channels.Also,the simulation established on the optical channel was conducted considering the influence of the dispersion of the optical fiber.Results illustrate the proposed method can realize simple but efficient signal transmission.(2)Proposed a new probabilistic shaping(PS)scheme.Compared with the traditional constellation shaping scheme,the communication system,which uses probabilistic shaping technology,has certain advantages in transmission capacity and performance.Combined with probability shaping technology and Discrete Wavelet Transform(DWT),a signal modulation and demodulation scheme is proposed.The research results show that the use of probabilistic shaping technology instead of the traditional constellation forming technology can obtain better forming gain,and reduce the average power of the signal and save transmission power.By using wavelet analysis,inter-symbol interference and inter-carrier interference can be well suppressed,the interference between adjacent signals can be reduced,and the spectrum utilization of the system can be improved.(3)PAM-DCMT modulation system was proposed by investigating the discrete cosine transform technology.This scheme replaces the IFFT/FFT with IDCT/DCT in DMT modulation system which can obtain higher frequency spectrum compression property thus to reduce the disturbance between signals.Results prove that the DCT is more simple and efficient and can reduce the out-of-balance in phase because it operates in real field compared with the complex discrete Fourier transform.Besides,it can combine with self-adaption,the sub-carriers with high signal-noise ratio can transmit more bits compared with low ratio sub-carriers,thus to increase the transmission rate.