Research On Equalization Technology Of Mode Division Multiplexing

In recent years, to meet the rapid growth of the information and data requirement, a variety of new optical fiber communication technology emerged at the right moment. Based on existing infrastructure of optical communication system, how to reduce the cost and maximize the transmission capacity of optical fiber communication system is a key topic in the field of optical communications. Mode-division multiplexing (MDM) technology based on coherent detection can be a good solution to this problem and is expected to make optical fiber communication into a new stage of development.Few-mode fiber can transmit fewer modes than multimode fiber, which makes better use of mutually orthogonal mode channel for transmission. Using Few-mode fiber can effectively reduce the mode crosstalk. Therefore, mode division multiplexing system using few-mode fiber for the transmission can effectively enhance the optical transmission capacity and suppress nonlinear effects interference. However, each mode signal transmitting in few-mode fiber link will be influenced by coupling among different modes and different symbols crosstalk of one mode. To ensure reliable transmission of MDM system, the receiving signal should be processed by MIMO equalizer. The main work of this paper include:(1) The article analyzes the impact of mode dispersion and mode coupling on transmission signal and gives theoretical model of few mode fiber transmission link based on mode dispersion, mode coupling and optical fiber attenuation.(2) A single channel adaptive Constant Modulus Algorithm (CMA) was deduced in theory. CMA was simulated with MATLAB to analyze the influence of iterative step and the filter length on the algorithm convergence and estimation error. However, CMA cannot combine both the convergence and estimation error. To solve this problem we adopt variable step size CMA and compare the effects of two equalization algorithms based on MATLAB simulation. Then, the single channel variable step size CMA is extended to MIMO variable step size CMA:(3) A 2 x 2 coherent detection MDM simulation system is built on OPTISYSTEM platform based on few-mode fiber link model. The simulation concludes, MIMO equalization algorithm can eliminate crosstalk between mode signals and effectively improve the quality of the transmission signal. With the increase of coupling strength, equalization effect will be decreased. MIMO equalizer adopting variable step size CMA can obtain a lower system bit error rate than adopting CMA.