Research And Improvement Of Non Orthogonal Code Multiple Access Techniques

The fundamental requirements for the fifth generation(5G)wireless networks are capable of supporting massive capacity,massive connectivity and achieving the goal of low latency.Non orthogonal code multiple access techniques distinguish users in code domain.Due to its advantages of enhancing spectral efficiency and serving more users simultaneously without occupying more resources,it can satisfy the challenging requirements in 5GThis paper investigates multiple non orthogonal code multiple access techniques and further studies a representative technique of them,namely Sparse Code Multiple Access(SCMA).During the research,we find that due to the fixed transmission pattern in traditional SCMA scheme,each time the multiuser detection is performed based on factor graph with small girth,which limits the further enhancement of detection performance.So this paper introduces interleaving technique into traditional SCMA scheme and proposes an improved SCMA scheme,namely Interleave Division SCMA(ID-SCMA)which distinguishes users by different codewords and interleavers.ID-SCMA attempts to enhance detection performance by breaking the independency among different factor graphs during the traditional SCMA scheme's detection process,making them connect with each other and constructing a factor graph with bigger girth.The simulation results show that in AWGN channel,the ID-SCMA scheme can achieve a significant performance gain over traditional SCMA scheme.Besides,for the interleaver design problem in ID-SCMA scheme,this paper firstly designs a kind of interleavers,named by PEG(Progressive Edge Growth)interleavers which guarantee the corresponding factor graph has a large girth and improve the detection performance of ID-SCMA scheme.Then this paper designs a group of QPP(Quadratic Permutation Polynomial)interleavers which can simplify the structure of interleavers and make ID-SCMA scheme obtain a detection performance close to the performance of random interleaving situation.In ID-SCMA scheme,users' transmitted symbols can be carried by OFDM(Orthogonal Frequency Division Multiplexing)subcarriers.This paper analyses the frequency domain correlation between subcarriers in several kinds of typical multipath channels.Based on the analysis,we state the reasonability of Rayleigh flat fading channel and independent fading channel used for assessing the performances of traditional SCMA scheme and ID-SCMA scheme.We simulate the performances of traditional SCMA and ID-SCMA in these two channels and obtain different performances.Based on the analysis of performance differences,we demonstrate that if the system in fading channel adopts ID-SCMA scheme,due to the use of the scheme's interleaving technique,we can make the subcarriers which carry the non-zero symbols of each user's codeword experience mutually independent channel fading and introduce"diversity" effect to the system.ID-SCMA scheme guarantees each user can achieve good detection performance under different channel conditions.