Application Of Volterra Filter In Intensity Modulation And Direct Detection System

Driven by the rapid development of broadband services,the increasing bandwidth demand of short-reach optical transmissions grows even faster than that in backbone networks.Combined with advanced modulation formats and digital signal processing,coherent detection has been widely used in backbone transmission systems.However,the cost and energy consumption are much more sensitive in short reach optical transmissions.Intensity modulation and direct detection(IM/DD)system is more suitable for short-reach applications.However,there are some transmission impairments which seriously degrade the transmission performance of the DML-based IM/DD transmission system,such as laser chirp,chromatic dispersion(CD),limited bandwidth of the optoelectronic devices and fiber nonlinearity.Based on the above problems,the compensation of nonlinear has become a hot topic in optical short-reach systems.On the basis of the key technologies in adaptive equalizer,we use the Volterra filter to compensate for nonlinear distortions in Orthogonal Frequency Division Multiplexing(OFDM)and Pulse Amplitude Modulation(PAM)systems in the experiments.The main contents of this paper are as follows:(1)This paper describes the principle of adaptive Volterra algorithm based on Least mean square(LMS)and Recursive Least Square(RLS)algorithms.The two adaptive algorithms are compared by experimental setup.Then,two kinds of high order modulation formats,i.e.OFDM and PAM-4,and their advantages and disadvantages are introduced.(2)The IM/DD OFDM system has been built through experimental platform.Firstly,the Volterra equalizer has been used to compensate for the nonlinear distortions.Then,on the basis of the bitloding algorithm,Volterra algorithm is used to enhance the transmission capacity of the system.In the OFDM IM/DD system,we finally realize the 36 Gbps OFDM signal transmission over 50 km standard single mode fiber(SSMF)experiment.The system transmission capacity has been increased by 22%.(3)The IM/DD PAM-4 system has been built through experimental platform.Firstly,the traditional Feed-Forward Equalizer(FFE)and Volterra algorithm in PAM-4 system are compared,and the Volterra algorithm is modified according to the characteristics of the PAM-4 system.Then,the influence of different memory lengths on the performance of Volterra filter has been discussed,and the main interference is derived from linear and the second order terms.At last,in order to solve the problem of high complexity of Volterra equalizer in PAM-4 system,a truncated algorithm is introduced,which greatly reduces the complexity of Volterra algorithm and of guarantee the transmission performance of the PAM-4 system.