A new subcarrier-index modulation schemes for downlink NOMA systems

In this paper, we propose two downlink multiple access architectures for networks where human-type communication users (HTCUs) and machine-type communication devices (MTCDs) coexist. The proposed schemes combine non-orthogonal multiple access (NOMA), orthogonal frequency division multiplexing (OFDM), and index modulation (OFDM-IM) concepts. In the first scheme, the base station (BS) transmits bits of HTCUs using modulated symbols and bits of MTCDs by the subcarrier activation pattern (SAP). This approach called IM-NOMA with null subcarriers (IM-NOMA-NS) ensures that inactive subcarriers are always null, which improves the system bit error rate (BER) performance. To improve the spectral efficiency (SE), we propose a second approach, termed IM-NOMA with dual-mode modulation (IM-NOMA-DM), in which the HTCUs' bits are transmitted using two-dimensional modulation and the MTCDs' bits are transmitted using one-dimensional modulation and the SAP. For each proposed system, a near-optimal low-complexity detector, based on the energy-detection (ED) and the log-likelihood ratio (LLR) criterion, is provided to mitigate the detection burden of the optimal maximum-likelihood (ML) detector. The BER performances and SE of the proposed schemes are investigated. The average BERs of IM-NOMA-NS and IM-NOMA-DM are derived in closed-form expressions corroborated by the simulation results. We have proved numerically that the proposed schemes achieve a good trade-off between BER performance, SE, and the number of supported users, making them more suitable for Internet of Things (IoT) applications.