Research On Peak-to-average Power Ratio Reduction Of PS-OFDM System In Vehicular Millimeter Wave Channel

With the advent of The Fifth Generation Communication System（5G）era,the number of cars is constantly increasing,and the massive wireless service data is developing in the direction of diversification and intelligence.In addition,high frequency millimeter wave（mm Wave）has the advantages of narrow beam,high bandwidth,good confidentiality,strong penetration ability and etc..And mm Wave is considered as the key technology of the future vehicular communication system.Internet of vehicles communication system based on probabilistic shaping（PS）and orthogonal frequency division multiplexing（OFDM）system,it has many advantages such as high spectral efficiency,anti-multipath fading and close to Shannon limit.However,for the traditional OFDM system,high Peak to Average Power Ratio（PAPR）is one of its main disadvantage.Furthermore,the association of PS and OFDM technology further aggravates the problem of PAPR enlargement,which seriously affects the system performance.In addition,Orbital Angular Momentum（OAM）System based on Uniform Circular Array（UCA）,as the key technology of The Sixth Generation Communication System（6G）in the future.However,OAM also has the problem of high PAPR similar to OFDM System.The signal with high PAPR enter into the nonlinear region of the transmitter power amplifier,resulting in signal distortion,which seriously affects the transmission reliability of the system and increases the difficulty of the transmitter hardware implementation.This paper focuses on and solves the problem of high PAPR in PS-OFDM system and UCA-OAM system,mainly by studying the PAPR reduction technology in OFDM system,and focusing on the PAPR reduction technology based on clipping noise compression and precoding,from the aspects of improving the PAPR reduction and bit error rate（BER）performance.And two improved PAPR mitigation methods are proposed to reduce the high PAPR in PS-OFDM system and UCA-OAM system.The main research work and innovation are as follows:（1）The basic principle of OFDM system and the problem of PAPR are introduced,and three classical PAPR mitigation algorithms are summarized,which are signal predistortion,probability and precoding technologies.What’s more,the principles,advantages and disadvantages of these techniques are studied and the experimental simulation analysis is carried out.In order to obtain a lower complexity,this paper focuses on clipping noise mitigation technology and the precoding technology.（2）The avantages and disadvantages of PS technology and PS-OFDM combined system are studied.For the problem that PS technology will aggravate the high PAPR of OFDM system,an improved clipping noise compression method is proposed,and the feasibility and validity of the proposed method are proved by experimental simulation in the additive white gaussian noise channel and vehicular-mm Wave channel scenario.Firstly,the Airy compression function is improved,and an improved clipping noise compression method based on improved Airy function（CNC-IAF）is proposed.In this method,the PS-OFDM signal peak is clipped,and the improved Airy compression function is used to compress the clipping noise.Finally,the transmitted signal is modified by the clipped signal and the compressed noise,so as to reduce the BER and PAPR.The simulation results show that the proposed CNC-IAF method is significantly superior to the CNC method in PAPR suppression performance and the NCNC method 0.49 dB and 0.63 dB when the value of the complementary cumulative distribution function（CCDF）is 10^-3 andBit error ratio（BER）is 10^-3.In AWGN channel,the proposed CNC-IAF method is about 0.4～1.0 dB better than the CNC and NCNC methods in improving the system BER performance.And in the Vehicle-millimeter wave channel,the proposed CNC-IAF method is about 0.45 dB better than the NCNC method.（3）The principle of UCA-OAM system and the same high PAPR problem as OFDM system are studied.Aiming at the serious harm caused by high PAPR to UCA-OAM system,A conjugate symmetric discrete orthogonal transform（CS-DOT）precoding method is proposed to reduce the PAPR of UCA-OAM system.The experimental simulation is carried out in the scenarios of aligned UCA and misaligned UCA.The simulation results show that the PAPR reduction performance of the proposed CS-DOT precoding method is about 0.13～2.31 dB better than that of the reference precoding method when CCDF is 10^-3.In the UCA-aligned channel scenario,the proposed CS-DOT precoding method is about 0.5～6.75 dB better than the reference precoding method in improving the system BER performance.And in the UCA unaligned channel scenario,the proposed CS-DOT precoding method has similar error performance as the reference method,but is about 5 dB better than the conventional UCA-OAM system.