In this study, a new chaos modulation scheme which combines M ‐ary differential chaos shift keying (DCSK) with code‐index modulation (CIM‐MDCSK) is proposed. In the CIM‐MDCSK scheme, the two Walsh codes that are used to generate two orthogonal chaos components are selected from the candidate codes, whose code indices are determined by information symbols. Thus, the information bits are conveyed not only by M ‐ary DCSK signal but also by the indices of the Walsh codes. Theoretical symbol error rate (SER) and bit error rate (BER) of the proposed scheme are derived and confirmed by simulations. Furthermore, by applying the noise‐reduction to reference signals of CIM‐MDCSK, the SER/BER expressions are also derived. Simulation results show that the proposed CIM‐MDCSK outperforms DCSK and M ‐ary DCSK based on Hilbert transform. Especially, the CIM‐MDCSK with noise‐reduction shows significant advantages. Moreover, the performance of CIM‐MDCSK scheme is compared with direct sequences spread‐spectrum system with code‐index modulation over flat and frequency selective fading channel. The proposed CIM‐MDCSK can offer good performance over frequency selective fading channel with a low complexity non‐coherent receiver which does not require any channel state information and channel equalisation.

Energy usage and data rate of differential chaos shift keying (DCSK) are both inefficient since half of the symbol duration is used to transmit reference. To improve the energy efficiency and data rate, we propose a new chaos modulation scheme which combines DCSK with code index modulation (CIM) referred to as CIM-DCSK. In the proposed scheme, a short reference signal is transmitted first. Then P repeated reference signals, which serve as information-bearing signals, are simultaneously modulated by an information bit and a Walsh code with a specific index which is determined by an extra information symbol. The analytical bit error rate (BER) performance of the proposed scheme is derived and verified by simulations. Further, we improve the performance of the proposed system by applying noise-reduction and optimizing power coefficients on the reference and the information-bearing signals of CIM-DCSK. Simulation and analytical results show that these optimization methods significantly boost the BER performances of the proposed schemes. Finally, the BER performances of the proposed systems are compared with that of the conventional DCSK system and the short reference DCSK (SR-DCSK) system. Numerical results show that the proposed schemes outperform both DCSK and SR-DCSK.

In this paper, by applying M-ary constellation modulation to code shifted differential chaos shift keying (CS-DCSK), a new M-ary DCSK which is called CS-MDCSK, is proposed. In the proposed scheme, the reference chaos signal is the same with the CS-DCSK, and the two orthogonal components of information bearing signal are obtained by two Walsh codes. Furthermore, the proposed CS-MDCSK is extended to general scheme (GCS-MDCSK), which transmits more than one M-ary constellation information bearing signals with one reference. Since high order constellations are used, the proposed scheme enhances the spectrum efficiency compared to CS-DCSK and GCS-DCSK. By computer simulations, the bit error rate (BER) performance of two proposed systems are compared to that of CS-DCSK and GCS-DCSK, respectively. Simulation results show that the proposed systems outperform the CS-DCSK and GCS-DCSK when 4-ary constellation is applied.