Experimental Study On The Control Method Of The Peak To Average Ratio In FSO-OFDM System

The orthogonal frequency division multiplexing is introduced into free-space laser communications because of its high spectral efficiency and the anti-inter-symbol interference property.By using this technology,the information transfer rate can be increased under the same bandwidth conditions.Compared with the traditional single-carrier technology,the multi-carrier modulation technology still has some drawbacks.High PAPR is one of the main weaknesses of the multi-carrier system.The uncertainty in information transfer will cause peak overlap among a plurality of sub-carriers,which leads to overlarge peak and laser saturation,and thus increases the bit error rate of the system.Therefore,we must control the PAPR of OFDM signals.This paper focuses on simulations and experiments of the peak to average ratio control technique in the atmospheric laser OFDM modulation system.The primary work and achievements of this paper are as follows:(1)The theory of atmospheric attenuation and atmospheric turbulence effects on FSO system is described.Meanwhile,the optical signal transmission attenuation characteristic is studied,and the principle of wireless laser communication system is analyzed.This paper introduces basic principles of OFDM systems,emphasizes the role of IDFT/DFT technology to OFDM systems,and introduces how the guard interval and cyclic prefix can solve the inter-carrier interference.Besides,this paper introduces the principles of DC biased optical OFDM systems.(2)This paper describes the reasons for PAPR in FSO-OFDM system,and then introduces three kinds of methods to reduce PAPR,as well as advantages and disadvantages of each method.This paper focuses on introducing selected mapping(SLM)algorithm and partial transmitted sequences(PTS)algorithm,and also do some simulations of these two algorithms on the transmitter side to verify the validity to reduce PAPR in FSO-OFDM system.(3)Based on the principle of optical wireless OFDM system,the system model is built.this paper analysis the influence of SLM and PTS algorithms on FSO-OFDM system through the constellation.Simulation results show that due to the blocking of the PTS algorithm,influence of the noise on the signal is increased,and the bit error rate is not improved.By simulating channel transmission of the SLM algorithm,we analyzed the constellation,the eye diagram and bit error rate curve.The results verified that this algorithm can reduce the error rate of the system and improve the reliability of signal transmission.Finally experimental results also verified the effectiveness of the application of SLM algorithm in FSO-OFDM system.The results showed that:the transmission of optical OFDM signals in the atmosphere can be affected by atmospheric turbulence and atmospheric molecular scattering.Applying of SLM algorithm on the transmitter side can effectively reduce the peak to average ratio of optical OFDM signals.By normalizing the amplitude of SLM-OFDM and FSO-OFDM into the same interval,the average power ratio of the SLM-OFDM signal and the signal to noise ratio are improved,thus the influence of the noise on optical OFDM system is reduced.Outfield short distance laboratory tests showed that the transmission of SLM-OFDM system is better than that of the FSO-OFDM system.