Research On Signal Equalization Technology At The Receiver Of Wireless Optical Communication System

Wireless optical communication?WOC?refers to the technology of transmitting information in the atmospheric channel using optical signals.Because of its high communication rate and good confidentiality,it has become the leading edge of communication research.The optical signal is affected by various adverse factors when it transmitted in the atmospheric channel.Those will result in the attenuation of the optical signal energy and the inter-symbol interferences?ISI?appearance,which leads to an increase of the bit error rate?BER?.In order to improve the performance of the communication system,this paper study the equalization technology at the receiver.Aiming at decreasing the ISI and reducing the BER of the WOC system.Firstly,this paper analyzes the influence of atmospheric channel by establishing the WOC system model.The atmospheric attenuation effects and the atmospheric turbulence effects is studied in the thesis,Then the beam expansion in time domain when it transmitted in atmospheric channel is analyzed,and its mathematical model is established.This paper studies the signal adaptive equalization technique and the blind equalization technique.Two basic algorithms of adaptive equalization technique are introduced:least mean square?LMS?algorithm and recursive least squares?RLS?algorithm.Then,the blind equalization techniques based on Bussgang are studied,the constant modulus algorithm?CMA?,MCMA and SAG-CMA algorithms are analyzed.A cascaded equalization algorithm is proposed based on CMA.The adaptive equalization algorithm and the blind equalization algorithm are simulated.The effects of different parameters on the convergence performance of the equalization algorithm are analyzed.The BER with equalization are compared with without equalization under different SNRs.The simulation results show that,the convergence of the algorithm in this paper proposed has been improved,and the BER is also reduced.In order to verify the effectiveness of the algorithm this paper proposed,two parts of the experimental under indoor environment and in the real atmosphere channel are conducted.The measured data by experimental are processed offline,the results show that,the BER with equalization is reduced to about 10^-6 under different experimental conditions,which is reduced by about 2 orders of magnitude.The eye diagrams of the signal are compared,the opening degree of eye diagram become larger than without equalization.The effectiveness of the equalization algorithm is verified,and it improves the communication performance of the WOC system.It provides a reference for the application of the equalization technology in the real WOC system.