  In this paper, we present three new signal designs for Enhanced Spatial
Modulation (ESM), which was recently introduced by the present authors. The
basic idea of ESM is to convey information bits not only by the index(es) of
the active transmit antenna(s) as in conventional Spatial Modulation (SM), but
also by the types of the signal constellations used. The original ESM schemes
were designed with reference to single-stream SM and involved one or more
secondary modulations in addition to the primary modulation. Compared to
single-stream SM, they provided either higher throughput or improved
signal-to-noise ratio (SNR). In the present paper, we focus on multi-stream SM
(MSM) and present three new ESM designs leading to increasing SNR gains when
they are operated at the same spectral efficiency. The secondary signal
constellations used in the first two designs are based on a single geometric
interpolation step in the signal constellation plane, while the third design
also makes use of additional constellations derived through a second
interpolation step. The new ESM signal designs are described for MIMO systems
with four transmit antennas two of which are active, but we also briefly
present extensions to higher numbers of antennas. Theoretical analysis and
simulation results indicate that the proposed designs provide a significant SNR
gain over MSM.
