A downlink index-modulation based nonorthogonal multiple access scheme

Massive connectivity and spectral efficiency are among the main defining terms of the future communications era. Therefore, revolutionary technologies are required to keep up with the connectivity and spectrum demands. Non-orthogonal multiple access (NOMA) and Index modulation (IM) techniques have emerged as promising candidates to satisfy the spectral efficiency and capacity demands. In this paper, a novel downlink multiple access scheme, called IM-NOMA, is proposed reaping the advantages of both NOMA and IM. In IM-NOMA, the bit block to be sent to each user is divided into two sub-blocks. While the first sub-block is modulated, the second sub-block determines a subset from the available set of channels to carry the modulated symbol. As in power-domain NOMA, the base station (BS) superimposes the modulated symbols carried by the same frequency carrier after a proper power allocation. In such, spectral efficiency is boosted by the extra bits carried by the indexes of the used carriers and the error performance is improved by spreading the users' signals in power domain as well as diversifying the symbols of each user over multiple channels. An upper bound on the average BER is derived in a closed form expression under maximum likelihood (ML) detection method and Rayleigh fading channel. Simulation results show a significant improvement in the BER of the proposed IM-NOMA as compared to the conventional NOMA scheme.