Research On Key Techniques Of Orthogonal Time Frequency Space Waveform Optimization

The sixth-generation of mobile communications(6G)will boast superior performance and novel capabilities across multiple dimensions.Given the ubiquitous nature of 6G services and the diverse range of industries it will serve,6G will encompass seven typical application scenarios,including high moving speed broadband(HMSBB),ultra high rate broadband(UHRBB),ultra low latency and high reliability(ULLHR)and so on.The HMSBB scenario is designed to meet the high data rate coverage demands in high-speed mobility.However,overcoming the Doppler frequency shift induced by high mobility is essential to ensure communication quality in this scenario.Orthogonal time frequency space(OTFS)is a new type of multi-carrier technology proposed in recent years.It can transform the wireless time-varying channel into an approximately stationary two-dimensional channel in the delay-Doppler domain through corresponding transformations,demonstrating robustness to Doppler frequency shifts and having promising applications in 6G.However,as a multi-carrier technology,OTFS also suffers from the problem of high peak-to-average power ratio(PAPR).Generally,the dynamic range of power amplifiers in communication systems is limited,and when the instantaneous power of high PAPR signals exceeds the linear range of the amplifier,it will lead to nonlinear distortion of the signal and degrade system performance.On the other hand,higher out-of-band(OOB)power radiation is also a problem for OTFS because it can cause adjacent channel interference,ultimately affecting communication quality.In this thesis,we optimize the OTFS waveform in terms of both PAPR and OOB power radiation,and the main contribution is as follows:Chapter 1 provides an overview of the background and future prospects of OTFS,as well as the current research status of OTFS.Chapter 2 presents a detailed description of the basic principles and signal model of the OTFS system,where an upper bound on the PAPR of the OTFS signal is derived.Furthermore,an analytical expression for the complementary cumulative distribution function(CCDF)that characterizes the PAPR of the OTFS signal is also provided.Chapter 3 first employs some classic methods to suppress PAPR of OTFS and analyzes their performance based on simulation results.To be specific,based on the traditional companding algorithm,a piecewise companding function scheme is proposed,which maximally reduces the distortion of the signal during the companding process by modifying the amplitude distribution of the signal.Simulation results show that the piecewise companding function scheme outperforms the traditional μ law companding in both PAPR and bit error rate(BER).For the classic selective mapping(SLM)algorithm,the imperialist competition algorithm(Im CA)is used to improve it,and the Im CA-SLM scheme is proposed.Simulation results show that the proposed Im CA-SLM is capable of productive attractive performance compared to conventional SLM.Chapter 4 extends the research conducted in chapter 3.Firstly,in multiple-input multi-output systems,the feasibility of the above two schemes,namely piecewise companding function and Im CA-SLM,is once again demonstrated through experiments.Subsequently,based on Im CA-SLM,the signal is temporally windowed using CP-OTFS to reduce PAPR and suppress OOB power radiation,with signal reconstruction performed at the receiving end to compensate for signal distortion.Simulation results indicate that this scheme effectively suppresses PAPR and OOB power radiation without causing signal distortion.Furthermore,the performance of the piecewise companding function scheme under nonlinear conditions is studied based on the Saleh model.Simulation results show that,under two types of nonlinear conditions considered,the BER performance of the piecewise companding function scheme is superior to that of the traditional μ law companding scheme.In the final chapter,a concise summary of the entire thesis is presented,highlighting the insufficiencies encountered during the course of the research and outlining potential avenues for future investigation in the relevant field.