  In this paper, we present a new multi-antenna modulation scheme, termed as
{\em space-time index modulation (STIM)}. In STIM, information bits are
conveyed through antenna indexing in the spatial domain, slot indexing in the
time domain, and $M$-ary modulation symbols. A time slot in a given frame can
be used or unused, and the choice of the slots used for transmission conveys
slot index bits. In addition, antenna index bits are conveyed in every used
time slot by activating one among the available antennas. $M$-ary symbols are
sent on the active antenna in a used time slot. We study STIM in a
cyclic-prefixed single-carrier (CPSC) system in frequency-selective fading
channels. It is shown that, for the same spectral efficiency, STIM can achieve
better performance compared to conventional orthogonal frequency division
multiplexing (OFDM). Low-complexity iterative algorithms for the detection of
large-dimensional STIM signals are also presented.
