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Orthogonal time frequency space (OTFS) is a recent two-dimensional modulation technique utilizing delay-Doppler domain and nominated as a promising candidate waveform for beyond 5G communication systems. By leveraging both time and frequency selectivity of doubly-dispersive radio channels, it has shown a better performance compared to traditional schemes. OTFS offers different options for accommodating multiple users on a given resource block. This article introduces a novel OTFS based uplink multiple access scheme by means of index modulation (IM) in delay-Doppler domain. IM based OTFS multiple access (OTFS-IMMA) proposes the coexistence of multiple users over the same delay-Doppler resources. Unlike conventional OTFS multiple access schemes, OTFS-IMMA allows users to employ IM to activate a subset of the available delay-Doppler resources without a coordination by the base station. Although data collision may occur between the users, our analysis shows that the achievable performance is still better than other schemes. Moreover, the proposed OTFS-IMMA scheme paves the way for flexible resource utilization in OTFS-MA to satisfy various users' demands in beyond 5G systems. It has been shown via both mathematical analysis and simulations that the proposed OTFS-IMMA scheme offers significant enhancement not only on bit error rate performance but also on the overall system efficiency. In addition, the achieved performance gain through OTFS-IMMA further increases as the number of channel taps and/or the number of delay-Doppler bins increase.

This article introduces the secrecy analysis of the index modulation-based orthogonal frequency division multiple access (IM-OFDMA) schemes by considering the in-phase and quadrature imbalance (IQI) presence at the users' RF front ends. In particular, by assuming the existence of an eavesdropper, the secrecy performance of the considered uplink multiple-access system is investigated through the strictly positive secrecy capacity and secrecy outage probability, and closed-form expressions are derived for both quantities. Moreover, the secrecy performance is conducted regarding IQI compensation based on a preamble-based estimation method. Further, analysis of the impact of the different parameters of IM-OFDMA, IQI, and estimation method is included via analytical and simulation-based results. The results reveal that IM-OFDMA is more reliable in secrecy performance than OFDMA, regardless of whether IQI is present. Also, IQI compensation can lead to a better secrecy performance compared to the case of no IQI.