Design And Analysis Of Multi-user Codebook For Sparse Code Non-orthogonal Multiple Access Systems

After nearly 40 years of development,the mobile communication network has put forward more stringent requirements for system capacity,latency and the number of loadable connections.In the next generation mobile communication system,a new air interface technology is urgently needed to meet the above-mentioned system performance requirements while ensuring the low cost of the equipment.As a new type of non-orthogonal multiple access,sparse code multiple access can fully meet the stringent requirements of system performance in 5G system.Therefore,it becomes one of 5G candidate air interface technologies and has received extensive attention and research in the industry and academic.Multiple access is one of the core technologies in wireless communication,which enables the wireless base station to distinguish and serve multiple users simultaneously.Most of the existing systems use orthogonal multiple access,which means that multiple users are connected to the system by resources that are partitioned orthogonally across multiple dimensions,such as frequency division,time division,code division,etc.In the 4G system,the OFDMA multiple access technology is also an orthogonal multiple access mode,in which orthogonal access is achieved among two-dimensional time-frequency resources on the grid.Orthogonal access has the advantage of simple implementation,but its disadvantage is that the number of users it serviced is strictly proportional to the number of orthogonal resources.So when the number of users changes dramatically or more users connected to the system,orthogonal access technology is not sufficient to meet these demand,so the non-orthogonal multiple access technology has become a hotspot in the current 5G research.Sparse Code Multiple Access(SCMA)is a non-orthogonal multiple access(NONMA)technique to meet the requirements of large capacity,massive user connections,and low latency.At the transmitter,it maps the coded bits directly into the multidimensional codewords of the complex domain.The codewords of different users are non-orthogonal superimposed on the same time-frequency resources with sparse spreading.The receiver uses this sparse characteristic for low complexity multi-user joint detection,and then combined with channel decoding to complete the multi-user bit string recovery.So its biggest feature is: the number of nonorthogonal code word can be superimposed over the number of resources.Compared to 4G OFDMA technology,it can serve more users at the same time under the condition of equal resource number,which can effectively enhance the system capacity and spectral efficiency.The codebook design plays an important role in the SCMA system,and SCMA multiuser codebook design has been a bottleneck restricting system performance.The existing codebook design methods don't consider the system capacity while generating multi-user codebook.Therefore,the main innovation and contribution of this paper is not only to optimize and improve the existing codebook design method based on minimum Euclidean distance,but also to propose a SCMA high-dimensional codebook design method based on capacity maximization.And the complexity of its realization compared with the existing codebook design methods have dropped significantly.In this paper,a novel SCMA codebook design scheme is proposed to maximize the sum rate,where we transfer the design of multi-dimensional constellation sets to the optimizing of series of 1-dimensional complex codewords.Specifically,a basic M-order pulse-amplitude modulation(PAM)is optimized and then the angles of rotation between the input 1-dimensional constellation and the basic M-PAM constellation can be obtained within a feasible calculation complexity to improve the sum-rate.Finally,the series of 1-dimensional complex codewords are combined to construct multi-dimensional codebooks based on Latin square criterion.Numerical results illustrate that the proposed codebook outperforms the existing codebook by 1.3 dB over AWGN channel and 1.1 dB over Rayleigh channel in terms of bit error rate(BER)performance.In terms of capacity,the proposed codebook achieves a 0.1dB gain at low SNR(SNR=8dB)and0.3dB at high signal-to-noise ratios(SNR=12dB).In the appendix,we give the optimal codebook of 6 users & 4 resource blocks and 15 users& 6 resource blocks obtained by using the codebook design method of capacity maximization described in this paper.