  Orthogonal frequency division multiplexing (OFDM) with index modulation
(OFDM-IM) appears as a promising multi-carrier waveform candidate for beyond 5G
due to its attractive advantages such as operational flexibility and ease of
implementation. However, OFDM-IM may not be a proper choice for 5G services
such as enhanced mobile broadband (eMBB) since achieving high data rates is
challenging because of its null subcarriers. One solution to enhance the
spectral efficiency of OFDM-IM is the employment of multiple distinguishable
constellations (modes) by also exploiting its null subcarriers for data
transmission. This paper proposes a novel IM technique called super-mode
OFDM-IM (SuM-OFDM-IM), where mode activation patterns (MAPs) and subcarrier
activation patterns (SAPs) are jointly selected and conventional data symbols
are repetition coded over multiple subcarriers to achieve a diversity gain. For
the proposed scheme, a low-complexity detector is designed, theoretical
analyses are performed and a bit error rate (BER) upper bound is derived. The
performance of the proposed system is also investigated through real-time
experiments using a software-defined radio (SDR) based prototype. We show that
SuM-OFDM-IM exhibits promising results in terms of spectral efficiency and
error performance; thus, appears as a potential candidate for 5G and beyond
communication systems.
