###### Abstract
In order to enhance the transmission throughput, the IM technique can be employed in cooperative communication [31]. A two-way multi-user half-duplex cooperative communication system based on the joint grouping subcarrier-permutation index modulation-DCSK (GSPIM-DCSK) system is proposed in [32]. In [33], the SR-DCSK cooperative communication system with a code index modulation (CIM-SR-DCSK-CC) system is proposed in order to increase the throughput without negatively impacting the BER performance. In order to maintain the advantages of the CIM-SR-DCSK-CC system and improve the data rate, we propose an SR-DCSK cooperative communication system based on modified code index modulation (MCIM), referred to as the MCIM-SR-DCSK-CC system in this paper. The main contributions of this paper are as follows:  
In the proposed MCIM-SR-DCSK-CC system, the SR-DCSK scheme is employed to achieve the communication between the source and the destination, and the modified code index modulation scheme based on M-ary DCSK (MCIM-MDCSK) is adopted to realize the communication between the relay and the destination. On the other hand, in the proposed system, each transmission period is divided into two phases: the source transmits the polarity-modulated bits to the relay and the destination in the first phase; the relay transmits its own information bits and the polarity-modulated bits to the destination based on a given decode-and-forward (DF) cooperative protocol in the second phase. The DF protocol is a widely used relaying strategy in wireless communication systems, particularly in cooperative communication scenarios. It is designed to improve the reliability and performance of communication links by leveraging intermediate relay nodes [33]. The DF protocol operates in two main phases: the transmission phase and the relay phase. In the transmission phase, the information signal is transmitted from the source to the relay. The source node S transmits the data signal to the relay node R. The relay node R receives the signal, which may be corrupted by noise and channel impairments. The relay node attempts to decode the received signal. If the signal is successfully decoded, the relay node prepares to forward the data to the destination node D. In the relay phase, the information signal is transmitted from relay to destination. After successfully decoding the signal, the relay node R re-encodes the data and retransmits them to the destination node D. The destination node D receives the signal from both the source node S and the relay node R. The destination node combines the received signals (e.g., using maximum ratio combining or selection combining) to improve the reliability of the received data. The destination node decodes the combined signal to retrieve the original data. The theoretical BER formula of the proposed MCIM-SR-DCSK-CC system is obtained over the AWGN and multipath Rayleigh fading channels. The simulation results closely match the theoretical analysis results. The comparative analysis between the proposed system and the CIM-SR-DCSK-CC system is given to confirm the excellence of the proposed system. The EE, data rate, and throughput of the proposed system are analyzed and compared with other DCSK cooperative communication systems. The proposed system has better EE, a higher date rate, and higher throughput than its competitor. The computational complexity and the memory requirement are also analyzed. The rest of this paper is as follows: The system model of the proposed system is given in Section 2. The theoretical analysis of the BER performance, the EE, the data rate, the throughput, the computational complexity, and the memory requirement are shown in Section 3. Section 4 shows the simulation results and discussions. Section 5 gives the conclusion.