New signal design for enhanced spatial modulation with multiple constellations

In this paper, we introduce a new signal design for the Enhanced Spatial Modulation (ESM) concept, which was recently proposed by the present authors. The basic idea of ESM is to convey information bits not only by the indexes of the active transmit antennas, but also by the types of the multiple constellations used. Design of the original ESM schemes involved one or more secondary modulations, which were derived using a single step of geometric interpolation in the signal constellation plane. In the new design, we go one step further in the interpolation process and derive additional modulations leading to a significant increase of the number of active antenna and modulation combinations used. A design example is given for 10 bits per channel use (bpcu) transmission using four transmit antennas two of which are active at a time. The new design is compared to conventional multi-stream spatial modulation (MSM) and to our previously proposed ESM scheme. The analysis and the simulation results confirm a signal-to-noise ratio (SNR) gain of 2.2 dB over MSM and 0.4 dB over ESM when the three schemes are operated at the same spectral efficiency.