Performance Analysis and Optimization for the LDPC-Coded Multi-Carrier LDS System

With the common property of the sparse structure, multi-carrier low density signature (MC-LDS) and low density parity-check (LDPC) codes can be combined into a joint sparse factor graph (JSFG) while the joint detection and decoding (JDD) algorithm could bring significant gains in terms of the bit error rate (BER). In this paper, considering the coupling character of the soft information interaction in the elaborate JSFG, we carve the distribution of the merged decoding soft information from both sides and theoretically analyze the error performance of the uplink LDPC-coded MC-LDS system. Correspondingly, not only the average BER but also the practical decoding threshold could be effectively derived for any given number of iterations. Furthermore, based on the theoretical analysis, we optimize the degree distribution of the JSFG by leveraging the differential evolution method. Finally, simulation results show that the practical threshold of the JDD receiver with the optimized code approaches the signal to noise ratio of the theoretical capacity.

I. Introduction

Non-orthogonal multiple access (NOMA) [1]-[3], as a state-of-art multiple access technique which achieves high spectral efficiencies and enhances the connectivity, can support massive access devices communications for the cellular based Internet of Things [4]-[6]. Specifically, benefiting from the superposition coding at the transmitters and the Belief Propagation (BP) based nonlinear detection at the receiver [7], multi-carrier low density signature (MC-LDS), a code domain multiplexing NOMA, has attracted much attention to confronting the overloaded requirement [4]. However, the introduced multiuser interference [8] is a serious issue, which inevitably impairs the transmit reliability.