Research On The Multiuser Detection Schemes In Sparse Code Multiple Access

With the rapid development of the mobile internet and Internet of Things(IoT),multiple access technology has gradually become a hot research issue in wireless mobile communication systems.For the sake of achieving the diverse communication scenarios in fifth generation(5G),conventional orthogonal multiple access(OMA)technology can not meet the requirements,so some non-orthogonal multiple access(NOMA)technologies are proposed correspondingly.Sparse code multiple access(SCMA)is a competitive NOMA scheme,which can enhance spectrum efficiency and support massive connections.As the optimal multiuser detection scheme in SCMA system,the maximum a posterior(MAP)algorithm is hard to implement due to its high computational complexity.However,the message passing algorithm(MPA),as a suboptimal multi-user detection scheme,can reduce the computational complexity in a degree and approach the performance of the MAP algorithm by exploiting the sparsity of the codewords.In order to promote the commercialization of SCMA technology,efficient multi-user detection schemes and codebook assignment optimization schemes are studied in this paper,and the main work is organized as follows:(1)In uplink SCMA system,the computational complexity of conventional MPA experiences exponential growth with the increasing of users and codebooks size,so this paper studies from the constellations and the factor graph to reduce the decoding complexity of the computational multiuser detection schemes.On the one hand,a low complexity MPA scheme based on dynamic constellation point is propoed in this paper,which reduces the computational complexity by cutting down the number of superposed constellation points(SCPs)participated during the message update process.On the other hand,a low complexity MPA scheme based on dynamic factor graph is propoed in this paper,which depresses the decoding complexity via decreasing the number of edges during the message update process.Compared with conventional MPA scheme,the proposed multiuser detection schemes in this paper can make a better tradeoff between computational complexity and bit error rate(BER)by setting reasonable parameters.(2)Due to the random feature of channels between base station and users,the performance of SCMA system will be promoted by adaptively adjusting transmitted codebook of users according to the condition of channel gain.Considering the problem that the conventional random codebook assignment scheme does not make full use of the channel state information(CSI)between base station and users,Codebook assignment based on fixed resource nodes order and codebook assignment based on dynamic resource nodes order are proposed in this paper.Compared with conventional random codebook assignment,the proposed schemes can obviously improve bit error ratio(BER)performance and maintain the same convergence rate.(3)Considering the problem that conventional MPA is unable to realize multiuser detection under the condition that the activity states of users are unknown,this paper firstly proposes a codebook design(CD)based MPA scheme(called CD-MPA)to jointly detect the activity state information and the transmitted data.On this foundation,a sphere decoding(SD)based MPA scheme(called SCD-MPA)is proposed to further reduce the decoding complexity of multiuser blind detection algorithm.Simulation results show that the SCD-MPA scheme is capable of reducing the computational complexity significantly without changing symbol error ratio(SER),misdetection probability and convergence rate compared with CD-MPA scheme.