关键词: 码分多址, 联合稀疏图, 多用户检测, 信道译码

Abstract: The conventional code division multiple access ( CDMA) is of poor performance when user numbers are overloaded.In order to solve this problem,a united low-density graph is constructed.In the graph,variable nodes are taken as the bridge,and chip nodes and parity check nodes are linked through low density edges,thus combining the low-density spreading matrix of CDMA and the low-density parity check matrix of LDPC codes.Then,by using the message passing algorithm,the multiuser detection and the channel decoding are performed in the graph,thus accomplishing the united detection and decoding scheme.Simulation results show that the united low-density graph can effectively resist multiple access interference and channel noise,and the receiver on the basis of the united low-density graph can achieve a better performance than a Turbo receiver,and that,under overloaded conditions,the united low density graph CDMA outperforms the single low-density graph CDMA in terms of bit error rate，average iteration number,message convergence and near-far effect，and even under severely overloaded conditions,the united low density graph CDMA still can achieve a satisfactory performance.

Key words: code division multiple access, united low-density graph, multiuser detection, channel decoding