A novel code index modulation for an efficient sparse code multiple access differential chaos shift keying system

Abstract

In this paper, we propose a novel design of sparse code differential chaos shift keying with code index modulation as an efficient and reliable multiple access chaos communication system, termed CIM-SCMA-DCSK. Orthogonal subspaces with quasi-orthonormal chaotic bases are presented to serve as indices and to spread the information symbols. The chaotic bases from multiple subspaces are utilized to enlarge the shape gain of the designed codebook, which leads to enhancing the reliability of the system. The information bits are first mapped to a sparse pattern using a codebook. The sparse symbols are then modulated by a chaotic basis, which is selected according to additional bits that represent an index for the basis. Thus, we attain a high data rate by conveying both indexed and modulated bits without demanding extra bandwidth or energy. The new codebook ensures the use of an optimal, low-complexity decoder that relies on a simple comparator detector. Furthermore, the mathematical expressions for bit error rate (BER), energy efficiency, spectrum efficiency, throughput, and system complexity are derived and analyzed under different channel conditions. To demonstrate the efficiency of the proposed scheme, we compare the CIM-SCMA-DCSK performance with that of relevant benchmark schemes using different objective metrics.