Research On SCMA Technology In Non-orthogonal Multiple Access

Since the birth in the 1980 s,mobile communication systems have undergone four generations of evolutions.Multiple access technologies have also been evolving in this process.In the 5G era of interconnected things,non-orthogonal multiple access(NMA)technologies become much more popular than traditional orthogonal multiple access technologies,because of their potentials to significantly increase the spectral efficiency and to accommodate an increased number of user connections under limited resources.Sparse code multiple access(SCMA)is a representative technology in NMA,which not only significantly improves the capacity and spectrum efficiency of communication systems,but also enable the systems to introduce contention-based access mechanisms without scheduling.After briefly introducing the research background,performance advantages and technical principles of SCMA,this thesis focuses on the analysis of the specific impact of the overloading factor on the uplink SCMA system and proposes two new types of uplink SCMA system receivers.The main contributions of this thesis are as follows:1.In order to solve the overload problems caused by accessing of large number of users simultaneously in 5G,the influence of the overloading factor on the detection performance and transmission rate of uplink SCMA system is analyzed.SCMA is more suitable for the uplink of mobile communication systems due to its unique characteristics.In the 5G era of interconnected things,ubiquitous access requires the SCMA be capable of handing traffic overload situations.Therefore,the overload performance of an uplink SCMA system is studied in this thesis from two aspects.On the one hand,the influence of the overloading factor on system detections,in terms of bit error rate(BER)and computation complexity,is analyzed,and the quantitative relationship between the overloading factor and the calculation amount of system detection is derived.On the other hand,by establishing the relationship model between the overloading factor and the system average sum rate,the influence of the overloading factor on the system transmission rate is analyzed.Moreover,these analytical results are validated via simulations.2.In order to solve the problems of the priori error and the high complexity of existing receivers,two novel receivers for uplink SCMA systems are proposed.There is mutual interference among users due to the non-orthogonal characteristics of uplink SCMA systems,thus the performance of the receiver is very important for uplink SCMA systems.At present,the message passing algorithm(MPA)based receiver is mostly applied in the uplink SCMA system.In this thesis,the principle of the MPA based receiver is explained in detail,and its shortcomings in multi-user detection are pointed out,such as the priori error and the high computational complexity.Then,a new detection algorithm,called L-S-MPA,is obtained by optimizing both the update strategy and the computation method of the MPA.Moreover,another new detection algorithm(G-MMSE)is obtained through improving the soft MMSE by exploiting the characteristics of the SCMA codebook.Accordingly,two novel receivers for uplink SCMA systems are proposed based on joint detection,named as R-MPA receiver and G-M-MPA receiver respectively.The error performance and computational complexities of the two proposed receivers are compared with existing MPA receiver analytically and via simulation,showing their much improved performances.