基于联合稀疏图的CDMA系统建模与分析

Modeling and Analysis of CDMA System Based on United Low-Density Graph

针对传统码分多址(CDMA)在接入用户数量过载传输时性能低下的问题,构造了联合稀疏图模型,该联合稀疏图以变量节点为桥梁,通过低密度的边同时连接码片节点和校验节点而将CDMA的低密度扩频矩阵和LDPC码的低密度校验矩阵相结合;然后在联合稀疏图上采用消息传递方式进行多用户检测和信道译码,完成联合检测译码方案.仿真测试表明:联合稀疏图具有抗多址干扰和信道干扰等特点,基于联合稀疏图的接收机能够获得比Turbo接收机更优的传输性能;在过载情况下,基于联合稀疏图的CDMA系统的误比特性、平均迭代次数、消息收敛特性和远近效应优于基于单一稀疏图的CDMA系统,即使严重过载,文中系统也能获得理想的通信性能.

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 combi- ning 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 unit- ed low-density graph can achieve a better performance than a Turbo receiver, and that, under overloaded condi- tions, 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 con- ditions, the united low density graph CDMA still can achieve a satisfactory performance.