Performance comparison of LDPC-coded IDMA system with equal spreading and unequal spreading

A scheme of applying unequal spreading to LDPCCoded IDMA system was proposed. The issue of LDPC-Coded DDMA system with equal spreading and unequal spreading was studied. Gaussian approximation was applied to the messages probability density distribution function between nodes on factor graph of LDPC-Coded spreading system. We compare two cases, the first is optimization of LDPC with fixed spreading factor, and the second is joint optimization of LDPC and spreading degree. By means of iteration and applying the method of differential evolution we obtained degree profiles in the case of 8 users with equal spreading and unequal spreading on condition that the system spectral efficiency is the same. We constructed the check matrix randomly based on the optimized degree profiles and made simulations. It has shown that the performance of LDPC-Coded spreading IDMA system cannot be improved by joint optimization of LDPC code and spreading degree for AWGN Multiple Access Channel (MAC) from the perspective of approaching MAC channel capacity. The result gives us guidance in designing LDPC-Coded IDMA spreading system.