Design of Efficient OCDM Transmission System Based on Dual-Mode Index Modulation

Orthogonal chirp division multiplexing (OCDM) is a promising technique for multi-carrier communication systems operating on chirp spread spectrum (CSS), which is more robust to multipath interference. In this paper, we propose a novel OCDM multi-carrier system based on dual-mode index modulation (IM), named DM-OCDM. In this scheme, particular subcarrier indices are designed to convey extra bits implicitly, with two M-QAM signal constellations adopted to modulate information bits in OCDM subcarriers ordinarily. Then, two frequency domain equalizers are proposed to compensate for the channel response, and a subsequent maximum likelihood (ML) detector is utilized to demodulate symbols. To address the computational burden caused by ML detection, we further propose a novel reduced complexity log-likelihood ratio (RCLLR) detector. Numerical simulation results demonstrate that the proposed DM-OCDM scheme can improve spectral efficiency significantly while guaranteeing the bit error rate performance compared to the traditional OCDM scheme.