Research On Key Technology Based On Scma System

The Fifth Generation Mobile Communication Technology(5G)demands higher spectral efficiency,massive connected devices and lower power consumption device.However,the traditional orthogonal multiple access technologies can not meet the requirements of the future system.Non-orthogonal multiple access technologies have drawn much attention due to the capacity advantages.Sparse Code Multiple Access(SCMA)is a typical code domain non-orthogonal multiple access scheme.In SCMA,the proceduce of modulation and spreading are combined together as a codebook selector,which is responsible for designing dedicated codebook for each user,and incoming bits are directly mapped to a multi-dimensional codeword of an SCMA codebook set.In SCMA,multiple codewords from different users can be multiplexed for transmission to support system overload.The receiver exploits Message Passing Algorithm(MPA)to recover multi-user signals.However,the high complexity of MPA may limits the application of SCMA.In 5G,the Massive data connection,low power consumption and low delay scene,which urges receiver has lower complexity,no matter in base station or the terminal side.Based on this,the article studies the low complexity decoding algorithm in SCMA system.Firstly,the SCMA system is briefly introduced,including the technical principle,system model,link model and the codebook design scheme in transimitter,which lay the foundation for the research and simulation of the decoding algorithm.Then,the principle of MPA and related low-complexity decoding algorithm is analyzed.In view of the menthod of partial marginalized message delivery algorithm,this paper takes the reliability of nodes into decoding and proposes a dynamic message passing algorithm based on reliability of nodes(RN-DMPA).RN-DMPA takes the residual values of posteriori probability as symbol reliability for nodes,which makes the decoding structure can be dynamically adjusted and the accuracy of the message is improved to approach the decoding performance of MPA.What's more,RN-MPA can also determine whether the algorithm converges according to the reliable node sets,so that the algorithm is not limited by the fixed iterations and has greater flexibility.The simulation results prove that RN-DMPA can save 28.7%of the computation load compared with MPA,only the signal-to-noise ratio with a loss of 0.2dB.Inspired by RN-DMPA,this paper explores the decoding structure and proposes a message passing algorithm based on variable nodes(VMPA).During each iteration,VMPA expands the flow path of the message on the eage of factor graph and updates the messages according to the order of the variable nodes,so that extrinsic information can be fully utilized.With this strategy,the convergence speed of the algorithm is improved and the overall calculational complexity is reduced.Furthermore,this paper creatively introduces a grouping algorithm based on factor matrix and proposes a Group Message Passing Algorithm Based on Variable Nodes(G-VMPA).By means of orthogonalization of variable node sets,unrelated variable nodes are permited to update message at the same time.The simulation results show that VMPA and G-VMPA can save 59.8%and 59.3%of the computational complexity only signal-to-noise ratio with a loss of 0.06dB.