Research On PAPR Reduction Based On Companding Transforms In OFDM Systems

Orthogonal frequency division multiplexing(OFDM)has been extensively applied in many wireless communication systems due to its high spectral efficiency,great immunity to frequency-selective fading and lower implementation complexity.Whereas,OFDM systems suffer from high out-of-band(OOB)radiation,sensitivity to frequency and time offset,and most crucially,high peak-to-average power ratio(PAPR).In this case,large amplitude signals can easily overstep the range of linear work area with nonlinear high-power amplifier(HPA),resulting in unexpected signal distortion.Compared to other current PAPR reduction techniques,companding transform techniques have shown its unique superiority,which are great PAPR reduction capability and low algorithmic complexity.Yet companding transform techniques still have some problems,such as poor bit error ratio(BER),poor OOB radiation,high algorithmic complexity and other aspects,which demand further improvement.Therefore,the research based on companding transforms for PAPR reduction in OFDM systems is of great significance and necessity.The main contents and innovations of the thesis are as follows:Firstly,aiming at the problem that the complexity of nonlinear companding schemes are generally high,this paper proposes a general "Three-stage" design model according to the design criteria of companding algorithm mentioned in references.And on this basis,a high-efficiency,adjustable and low complexity nonlinear companding algorithm based on quadratic function is proposed.In this scheme,small amplitude signals remains unchanged,large amplitude signals are clipped,and the middle part of the signals is amplified by the designed quadratic function,so as to reduce the distortion and OOB as much as possible.Under the joint constraints of power conservation and piecewise point continuity,the solution method of the parameters of the companding function are analyzed and obtained,and then four groups of alternative parameters which can achieve better comprehensive performance are given.In addlition?the paper also proposes a method to analyze the complexity of the algorithm.This method directly counts and compares the number of various arithmetic operations and the number of companded samples of the companding function and the simplicity of the proposed scheme are also proved.Simulation results in terms of PAPR reduction,BER performance and OOB radiation under different circumstances are compared and analyzed,which show that regardless of modulation order,range of signal-to-noise ratio(SNR)and nonlinear HPA,the proposd scheme can outperform other compared companding schemes,thereby satisfying variable system requirements.Secondly,fully considering the problem of floor effect in hard limited nonlinear companding schemes with high modulation order,an effective piecewise nonlinear companding algorithm based on a sigmod function is proposed.In this scheme,small amplitude signals remains unchanged,large amplitude signals are expanded and clipped by a designed quadratic function.This scheme makes use of the characteristics of sigmod function,completing the two operations of soft limiting and power compensation at the same time,and avoiding the "multi-to-one" mapping of large amplitude signals before and after companding,thus leading to better resistance to the floor effect produced by BER curve with high modulation order.In this paper,the constraint equations of power conservation and piecewise point continuity are given,and the basis of parameter selection to minimize the companding interference is obtained through theoretical analysis,which is helpful to adjust the parameter selection according to the communication environment and demand in practical application.Through the comparison and analysis of simulation results,it is verified that the proposed scheme has a high PAPR adjustment range,and has better BER performance and power spectral density(PSD)performance in the case of high modulation order.