Research On Massive Machine-type Communications And Non-orthogonal Multiple Access Technology

The Internet of Things(IoT)is one of the key technologies facing the future.It aims to make every physical object around us a source of information,thereby forming a huge network to share and process information more intelligently.To this end,the 5th Generation Mobile Communication System(5G)has proposed a Massive Machine-type Communications(mMTC)scenario.In the typical scenario of mMTC,the access users are mainly sensors and smart devices with low cost,low power consumption and low complexity.The massive access nodes bring huge challenges to current wireless communication systems.The Non-Orthogonal Multiple Access(NOMA)scheme achieves a multiplexing of time and frequency resources and is one of the key technologies to solve the massive access problem in mMTC.The grant-free(GF)based NOMA schemes have proven to reduce signaling overhead.However,most of the stateof-art NOMA schemes still require pilots for channel estimation.The mMTC scenario is characterized by short-packet transmission,and the time-frequency resources occupied by the pilot symbols are even close to the data symbols themselves,which will result in low utilization of time-frequency resources.In this regard,this thesis believes that the non-coherent transmission which needs no pilots for channel estimation during the communication is a promising framework in mMTC.This thesis will take the non-coherent NOMA scheme in the mMTC scenario as the research background,and introduce the cube-split codebook as the non-coherent codebook to study the non-coherent multiple access scheme in the mMTC scenario.First of all,this thesis elaborates the construction process of cube-split codebook on real domain and complex domain.For the cube-split codebook in the complex domain,a single-user non-coherent access scheme is designed in this thesis.Simulation results show that the single-user non-coherent access has a good performance,coming from introducing no channel estimation error.In addition,the simulation results also explore the impact of the minimum distance between codewords and the different codebook parameters on transmission performance.Secondly,for the multi-user non-coherent multiple access problem with one single antenna receiver,this thesis introduces the orthogonal short spreading sequence as the signature of multiple access.A linear expansion matrix is designed in the transmitter in order to tolerate some collisions of the spreading sequence among users,where the extended codeword is in the subspace spanned by the original codeword.At the same time a spreading pattern table is designed in this paper.The spreading pattern with a fixed symbol length is used to balance the randomness of selection and accuracy of detection.Simulation results show that the proposed scheme obtains performance gain under five typical transmission packet sizes.In addition,the simulation also explores the effects of different spreading sequences,different size of spreading pattern tables,and the approximation algorithms of log-likelihood ratio on transmission performance.Finally,a multi-access non-coherent multiple-access problem with a multipleantennas receiver is introduced.A multiple-access scheme based on linear precoding matrix is introduced.Each user needs to construct a different linear precoding matrix,and uses the linear precoding matrix to generate a signature.Simulation results show that the transmission scheme based on signal reconstruction can achieve higher performance gain than the baseline transmission scheme under five typical transmission packet sizes.At the same time,the simulation also verified the impact of the number of access users and the number of receiver antennas.