Spatial modulation with signature constellations for increased robustness to antenna and channel correlations

Spatial modulation (SM) has a strong sensitivity to transmit antenna and channel correlations, because some of the information bits are assigned to active antenna selection, and the correlation limits the detection reliability of these bits. Recent approaches to solve this problem rely on either unequal error protection (UEP) of antenna and symbol bits with the addition of a channel encoder/decoder pair to the transceiver or precoding in the form of antenna-dependent rotation (or joint rotation and amplitude scaling) of the signal constellation. The UEP approaches have been shown to offer only limited efficiency in compensating for the adverse channel effects while increasing the latency and complexity due to the addition of the encoder/decoder. The precoding based approaches achieve good results for small number of transmit antennas and for small constellations such as BPSK and QPSK, but the performance quickly degrades for higher-level signal constellations. Moreover, the complexity of the precoder optimization problem increases with the number of transmit antennas and the modulation order, making this approach not very practical to use for high spectral efficiencies. This paper introduces a novel approach to this problem, which is particularly suited for high-level signal constellations. This is achieved by the use of active-antenna dependent signature constellations which are derived from each other through successive geometric interpolations. This approach preserves the minimum inter-constellation Euclidean distance given the primary constellation. The theoretical analysis and the simulation results show that compared to previous methods it gives significant performance improvements in terms of robustness to transmit antenna correlation, particularly for Rician fading channels.