Abstract
In this letter, an enhanced index modulation (IM)-aided grant-free non-orthogonal multiple access (GF-NOMA) scheme is presented in the multiple-antenna massive access system. In the IM-aided GF-NOMA, each user is allocated <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${N}$ </tex-math></inline-formula> spread sequences, from which only <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$N_{a}, N_{a} < N$ </tex-math></inline-formula> , sequences are activated to transmit information. In the conventional scheme, only the indexes of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$N_{a}$ </tex-math></inline-formula> active sequences are utilized to convey information. In the proposed enhanced IM (EIM)-aided scheme, besides these indexes, constellation symbols are also utilized to convey information. Concretely, the first active spread sequence carries pilot while the others carry constellation symbols. At receiver, two multiuser detection algorithms, hybrid approximate message passing and covariance-based maximum-likelihood detection are presented. Finally, simulation results are given to demonstrate the performance of the proposed scheme.
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