In this paper, a quadruple code index modulation (QCIM) technique is proposed for low-complexity and spectral-efficient wireless transmission. The proposed QCIM scheme utilizes quadruple code indices in addition to M-ary phase shift keying symbols to transmit more information bits compared to conventional code index modulation (CIM) schemes. To recover the transmitted information bits, we propose maximum likelihood detection (MLD) and greedy detection (GD) algorithms for the proposed QCIM, with the GD algorithm offering almost the same bit error rate (BER) performance to the MLD algorithm but with significantly lower system complexity. Additionally, we extend QCIM into a reconfigurable intelligent surface (RIS)-assisted QCIM (RIS-QCIM) system, which significantly enhances BER performance and eliminates the need for channel state information at the receiver. Numerical results demonstrate that the proposed QCIM and RIS-QCIM achieve higher spectral efficiency and superior BER performance in contrast to benchmark schemes.
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