Research On Sparse Code Multiple Access And Its Key Technologies

To keep up with the explosive growth of terminal numbers due to the developing of mobile internet and internet of things(IoT),5th generation mobile communication network(5G)need to have a character of massive connection.Sparse code multiple access(SCMA)proposed by China's Huawei in 2013,is one of the candidate multiple access schemes for its high overload capacity.The SCMA directly maps the input binary bit stream into multi-dimensional complex sparse codewords from the prearranged codebook in the transmitter,instead of the traditional modulation and spreading spectrum processing.On the receiver side,the message passing algorithm(MPA)with iterative properties is used for decoding based on the sparse characteristics of codewords.This thesis proposes detailed research results of SCMA in terms of key performance analysis and key technology optimization design,further the main contribution of this thesis can be concluded as follows:1.This thesis firstly analyzes the SCMA system structure,and then gives the basic evaluation results from the perspective of overload rate and decoding complexity.Furthermore the SCMA is compared with single carrier frequency division multiple access(SC-FDMA)used in LTE uplink because of multiuser access features.Studies show that SCMA has a high overload capacity which is more suitable for mass connection of 5G.However,SCMA has increased bit error rate(BER)at the case of high overload rate condition,and the receiver will be more complex.2.Aimed at the high BER problem under the condition of high overload of SCMA,the influence of phase rotation on the SCMA code is studied firstly.Two schemes of optimizing the phase rotation angle are then proposed based on the controlled-distance among the constellation points of constellation.The first scheme is called the rotation based on maximizing minimum euclidean distance(M-rotation)and the second is the rotation based on exponential average(E-rotation).E-rotation achieves better BER performance than M-rotation,but M-rotation is outstanding at the case of high signal to noise ratio(SNR).With respect to the decoding complexity,both schemes have no distinction but E-rotation gets better performance than M-rotation when the partial extrinsic information transmission of MPA(PEIT-MPA)proposed in chapter 5 is adopted.3.Aimed at the high decoding computation of SCMA under the condition of high overload,the originating source and influence of extrinsic information in the procedure of MPA iteration decoding is researched.A named PEIT-MPA algorithm based on threshold decision of probability density function is proposed to reduce the FN load according to the relationship between the extrinsic information value and the received signal probability density function value.Results show that PEIT-MPA reduces the implementation complexity especially in high SNR without effects on BER and iterative convergence rate of original MPA.