Research On Multi-user Shared Access And Its Key Technologies

With the fast development of the mobile Internet and the growing demands for Internet of things,data traffic will show explosive growth,which makes high requirements on the fifth generation mobile communication,such as high spectrum efficiency,massive connections and low latency.Most of the traditional multiple access schemes adopt orthogonal access technology which leads to a low spectrum efficiency.Multi-User Shared Access(MUSA)is a non-orthogonal multiple access,which can support several times connections and has a higher spectrum efficiency compared with the access technology in LTE.Moreover,it also simplifies the resource scheduling process and reduces the signaling overhead.However,due to the adoption of the spread spectrum technology based on triple-level complex spreading sequence and the multi-user detection technology based on MMSE-SIC,the receiver of MUSA has a high complexity and a large processing delay.In addition,the spreading sequences have an influence on system performance.Aiming at these problems,this thesis studies on the MUSA system,and the main works are shown as follows.1.The complex spreading sequence is studied in the thesis.In order to achieve a better performance,the sequences should be optimized and improved as the balance and correlation of the sequences have a great effect on the performance of MUSA system.Firstly,aiming at the problem that only the influence of cross-correlation peak is taken into account in the process of cross-correlation optimization in the existing optimization algorithm,an improved spreading sequence optimization algorithm for MUSA system is proposed.Both the influences of cross-correlation peak and the cross-correlation mean square are taken into consideration,thus the optimized sequences have a better performance.The simulation results show that the proposed algorithm has a better performance than the existing algorithm in BER and user overloading ratio.Besides,the random selection of the spreading sequence will lead to the collision of the sequences.In order to reduce the collision probability,a set of improved complex spreading sequences based on constellation figure of merit is proposed.The proposed complex sequence has a higher constellation figure of merit than the triple-level complex spreading sequence.Additionally,more excellent spreading sequences can be obtained after optimization,which can further promote the performance of MUSA system.2.Multi user detection algorithm is studied in the thesis.Aiming at the high complexity and the large processing delay of MMSE-SIC detection,two improved detection schemes are proposed.The first scheme of the improved MMSE-SIC receiver is based on iterative method to overcome the high complexity of MMSE-SIC detection algorithm caused by multiple matrix inversion operation.This scheme computes MMSE weight matrix by multiple iterations,which avoids the direct matrix inversion operation.The simulation results show that the proposed algorithm reduces the complexity by an order of magnitude without performance degradation when compared with traditional MMSE-SIC algorithm,which satisfies the application of large-scale connections in 5G mMTC scenario.The second scheme of the improved detection algorithm is based on PIC structure to tackle the large processing delay of MMSE-SIC receiver caused by the adoption of successive processing mechanism.The proposed detection algorithm can efficiently reduce the processing delay by adopting parallel elimination strategy.The simulation results show that the proposed algorithm can reduce the processing delay by 40% while ensuring the detection performance within 150% user overloading ratio,which satisfies low latency demands in 5G uRLLC scenario.