  Spectral efficiency improvement is a key focus in most wireless communication
systems and achieved by various means such as using large antenna arrays and/or
advanced modulation schemes and signal formats. This work proposes to further
improve spectral efficiency through combining non-orthogonal spectrally
efficient frequency division multiplexing (SEFDM) systems with index modulation
(IM), which can efficiently make use of the indices of activated subcarriers as
communication information. Recent research has verified that IM may be used
with SEFDM to alleviate inter-carrier interference (ICI) and improve error
performance. This work proposes new SEFDM signal formats based on novel
activation pattern designs, which limit the locations of activated subcarriers
and enable a variable number of activated subcarriers in each SEFDM subblock.
SEFDM-IM system designs are developed by jointly considering activation
patterns, modulation schemes and signal waveform formats, with a set of
solutions evaluated under different spectral efficiency scenarios. Detailed
modelling of coded systems and simulation studies reveal that the proposed
designs not only lead to better bit error rate (BER) but also lower
peak-to-average power ratio (PAPR) and reduced computational complexity
relative to other reported index-modulated systems.
