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This paper proposes a novel uplink multiple access scheme to enhance the overall spectral efficiency of wireless systems. The core idea is to exploit index modulation (IM) concept in orthogonal frequency division multiple access (OFDMA) systems. In the proposed scheme, entitled IM-OFDMA, part of the transmitted bits of each user is conventionally modulated, while the rest of the bits determine the index of the active subcarrier that will be used to communicate the modulated symbol. Unlike OFDMA, IM-OFDMA does not require any scheduling algorithm, which results in data collision between the user's received messages. Yet, it is revealed that IM-OFDMA outperforms OFDMA and state of the art uplink non-orthogonal multiple access (NOMA) over a wide range of system and channel parameters. Besides, it is reported that IM-OFDMA can serve up to 3 times the number of served users in OFDMA system while …