Peak-to-average Power Ratio Reduction Techniques For Rateless Coded OFDM Systems

Orthogonal Frequency Division Multiplexing(OFDM)is an attractive technique for wireless communications due to its higher spectral efficiency,lower computational complexity,and greater immunity to multipath fading channels.It has been used in the Fourth Generation(4G)of mobile telecommunications systems and been adopted in many international standards,such as the Long Term Evolution(LTE).At the same time,OFDM will still act a significant role in the “Internet +” strategy and the Fifth Generation(5G)communications systems.However,a major drawback of OFDM signals is the high peak-to-average power ratio(PAPR),which occurs when output signals are the coherent superposition of multiple subcarriers.High PAPR signals will not only impose higher requirements for high power amplifier(HPA),digital to analog converter(DAC),and analog to digital converter(ADC)but also decrease the communication coverage and communication range.In order to deal with the high PAPR problem,a number of methods have been proposed.This thesis investigates the fountain coded OFDM systems and applies Luby Transform(LT)codes into PAPR reduction.The main contributions are as follows.1.Two mapping schemes between LT packets and OFDM signals are firstly studied,and their transformation method is presented.Through the transformation method,no matter which mapping scheme is adopted,it is easy to make more than one LT packets map into an OFDM symbol.In such case,we exchange the locations of LT packets in an OFDM symbol to generate different permutations,which fully explores the spatial freedom.Then,permutations multiplied a phase rotation vector is to generate candidates for OFDM PAPR reduction;therefore,we propose a PAPR reduction algorithm based on scrambling LT packets.Simulation results demonstrate that the proposed algorithm can reduce the PAPR effectively.2.With the increase of the number of LT packets mapping into an OFDM symbol,the number of the candidates shows an approximately exponential increase,which will consume huge calculation resources at transmitters.In order to reduce the complexity brought by searching the candidate with the smallest PAPR,we propose an improved PAPR reduction algorithm.In the improved algorithm,a predetermined threshold is introduced during the candidate generation process to prevent redundant Inverse Fast Fourier Transformation(IFFT)operations.Moreover,we use the number of IFFT operations as an indicator to model and formulate the theoretical algorithm complexity.Simulation results show that the improved algorithm can effectively reduce PAPR with a huge complexity reduction.Compared with the existing scheme,a maximum complexity reduction of 81% can be obtained concerning the total number of IFFT operations.Regarding the IFFT operations per degree,experimental curves are also consistent with the mathematical analysis.Additionally,the aggregate fitness value is defined to evaluate the impact of PAPR reduction and complexity reduction on communication systems.Finally,the improved algorithm is implemented in Lab VIEW.