This paper discusses two innovative schemes for orthogonal frequency division multiplexing (OFDM): indexed modulated OFDM (OFDM-IM) and dual-mode dual-indexed OFDM-IM (DMDI-OFDM-IM). The aim is to improve spectral efficiency and error performance under frequency-selective and fast time-varying fading channels. The OFDM-IM scheme de- livers information through the indexing of activated subcarriers beyond the multiple signal constellations in conventional OFDM. This paper briefs outline a low-complexity transceiver archi- tecture for OFDM-IM based on maximum likelihood detection and log-likelihood ratio computation. A brief analysis of the error performance of conventional OFDM and DM-OFDM-IM is presented. The DMDI-OFDM-IM scheme provides additional information transmission through dual indexing by relocating symbols and selecting relocation vectors in active subcarriers, as well as utilizing inactive subcarriers, resulting in improved frequency performance. Thus, higher spectral efficiency can be achieved. And there is evidence of significant improvement in the error performance of both schemes compared to conventional OFDM under different channel conditions, confirming their potential application in efficient wireless communication systems. Index Terms—DM OFDM-IM, Error Performance, Fad- ing Channels, Index Modulation, Low-Complexity Transceiver DesignOFDM-IM, Orthogonal Frequency Division Multiplexing (OFDM), Spectral Efficiency