Research On The PAPR Reduction Based On Precoding Techniques In VLC OFDM Systems

The visible light communication(VLC)technology is a communication technology based on light emitting diode(LED)to transmit the signal,and is regarded as a wireless communication technology that can be applied to the next generation due to its rich spectrum resources,wide application scenarios and better security performance,and has become a Research hotspot.The performance of the VLC system is restricted by the limited modulation bandwidth of LED.Due to its better spectrum utilization,anti-multipath fading and easy combination of other technologies,orthogonal frequency division multiplexing(OFDM)technology is regarded as a modulation technology that can be applied by VLC systems to realizes high-speed transmission.A signal input to the LED is required to be a positive real number.The VLC OFDM technology constructs Hermitian symmetry on the input signal,so that the real number signal is generated after IFFT operation,and then the conversion of negative to positive is adopted to obtain a signal with unipolar real number,which meets the requirement.Therefore,it is suitable for VLC system.However,VLC OFDM has the problem of the high peak-to-average power ratio(PAPR),which affects the performance of VLC system.Thus,it is of great significance to study the PAPR reduction problem.The PAPR reduction technique in VLC OFDM is the main research direction.The main research contents of this thesis are as follows:1.The visible light OFDM systems are firstly studied.The theories of the DC bias optical OFDM(DCO-OFDM)system and the asymmetrically clipped optical OFDM(ACO-OFDM)system are expounded and derived in detail.Then,starting with the definition of PAPR,the PAPR formulas of the DCO-OFDM systems and the ACO-OFDM systems are derived theoretically,and are verified by simulation.A nonlinear model of LED is built,which illustrates the importance of studying the PAPR reduction technique in VLC OFDM system.Three types of PAPR reduction techniques are discussed,and the precoding technique is selected as the main research object.2.The PAPR reduction performance of the classic precoding technique is fixed and limited.The precoding matrix(PM)method can flexibly reduce the PAPR,but its computational complexity is relatively large.Aiming at this,an improved discrete sine transform(DST)precoding method based on the DST precoding technique and generalized precoding(GP)method is proposed.The complexity analysis shows that compared with the PM method,the improved method reduces the computational complexity by about 19% to 33% when the extension length is the same,and only a small amount of computational complexity is increased compared to the original DST precoding technique.Simulation result shows that when the redundancy is 12.5%,25%,37.5% and 50%,compared with the DST precoding technique,the proposed method improves the PAPR reduction performance about 0.30 d B,0.57 d B,0.85 d B and 1.07 d B respectively in the ACO-OFDM system,and 0.22 d B,0.38 d B,0.55 d B,0.64 d B respectively in the DCO-OFDM system.In addition,under the ideal conditions,as redundancy increases,the system's bit error ratio(BER)performance is improved.Therefore,the proposed improved method can reduce the PAPR while taking the computational complexity and BER performance into consideration.3.In order to further reduce the PAPR,combining the proposed improved DST precoding method and the low complexity selective mapping(LC-SLM)technique,two joint improved DST-SLM(I-DST-SLM)PAPR reduction schemes based on the conventional DST-SLM schemes are proposed.The complexity analysis shows that when the same number of candidate signals are generated,compared with the conventional DST-SLM scheme 1,the improved scheme 1 reduce the computational by about 41% to 65% in the ACO-OFDM system and 42% to 57% in DCO-OFDM system.Besides,compared with the conventional DST-SLM scheme 2,the improved scheme 2reduce the computational by about 51% to 81% in ACO-OFDM system and 69% to85% in DCO-OFDM system.Simulation result shows that when 4 candidate signals are generated,compared with the improved DST precoding method and DST-SLM scheme1,in the ACO-OFDM system,the improved scheme 1 improves the PAPR reduction performance by about 0.26 d B and 0.62 d B,respectively.In the DCO-OFDM system,the improvement is about 0.13 d B and 0.53 d B,respectively.Compared with the improved DST precoding method and the conventional DST-SLM scheme 2,in the ACO-OFMD system,the improved scheme 2 improves the PAPR reduction performance by about 1.19 d B and 0.55 d B,respectively.In the DCO-OFDM system,the improvement is about 0.67 d B and 0.42 d B,respectively.In addition,both improved schemes have better BER performance than the conventional DST-SLM schemes.Therefore,the two improved schemes can achieve a balance between PAPR reduction performance,computational complexity and BER performance.