Optimization and Improvement of the Spreading Sequence in Multi-user Shared Access System

Multi-User Shared Access (MUSA) is a non-orthogonal multiple access technology, and its performance will be affected by the spreading sequence due to the usage of complex domain spread spectrum technology, so the spreading sequences should be optimized and improved. Considering 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 is proposed. Both the influences of cross-correlation peak and the cross-correlation mean square are taken into consideration. The simulation results show that the proposed algorithm can promote 0.9 dB SNR performance in the same BER than the existing algorithm. In addition, the random selection of the spreading sequences will lead to the collision of the sequence. 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, and when compared with the optimized triple-level complex spreading sequence, the SNR performance can promote about 1 dB in the same BER and the user overloading ratio can improve 15% when BER is 10^-3, which can further promote the performance of MUSA system.