Research On Sparse Code Multiple Access Technology

Sparse Code Multiple Access(SCMA)is a Low Density Signature(LDS)based non-orthogonal multiple access technology,which spreads users' signals in the complex space into sparse code words via index vector and overlay them before transmission.Since much more index vectors can be used in non-orthogonal format than that in orthogonal format,SCMA can support much more access users and thus has higher spectrum efficiency than the orthogonal multiple access technology.However,there is neither scheme to design index vectors nor scheme to support multiuser multirate transmission.In this thesis,a flexible SCMA scheme termed Grouped Codeword Sparse Code Multiple Access(CG-SCMA)is proposed.Firstly,non-orthogonal multiple access technologies and the LDS-based SCMA techniques are introduced and then followed by the analysis of the pros and cons of SCMA.The SCMA predefind codebook can't adapt to varying subscribers and can hardly support multi-user multi-rate transmission either.In view of these shortcomings,a new scheme is designed which combines the grouped index matrix design and grouped constellation rotation design.Simulation results show that the proposed CG-SCMA outperforms both SCMA and LDS.Secondly,a scheme to flexibly support multi-user multi-rate transmission is addressed for CG-SCMA.Multiple modulation level schemes with optimal constellation rotation is employed to support multi-rate transmission such that it can adapt itself to the varying radio environment.In this thesis,the CG-SCMA scheme to support QPSK and BPSK simultaneously is designed.Simulation results show that the detection performance of the proposed method falls in between the scheme with fixed BPSK and the one with fixed QPSK.Finally,Passing Algorithm Message(MPA)-based detection algorithm is developed for SCMA and LDS system.By exploiting LDS,an optimal decision formula of receiver is derived under principle of a posteriori maximum(MAP).Since the computational complexity depends on the user number and iteration,a fast detection algorithm is proposed to terminate the iteration on condition that the extrinsic information of users'converge.The simulation results verify the reduced complexity of the proposed method with little detection performance loss.