Quaternion-Neural-Networks-Based Decoder for RIS-Aided Polarization-Space Modulation

Abstract

Contemporary communications systems use large arrays in order to exploit the spatial domain requiring multiple radio-frequency (RF) chains leading to prohibitive cost and power. Spatial degrees of freedom are also achieved by utilizing the polarization states of an electromagnetic wave. To this end, we propose a polarization-state modulation system based on carrier wave reflections from a reconfigurable intelligent surface (RIS), where a sequence of data bits is mapped to polarization states. We consider a system with an upper millimeter-wave or low-THz RF source so that the channel model is line-of-sight-dominant. The data symbol constellation consists of sixteen quaternion-valued symbols, of which two correspond to linear and the remaining twelve represent elliptical polarizations. Our numerical experiments indicate that the RIS is capable of directing signal power toward the location of the receiver. The bit error rate performance of our rudimentary demodulator, although worse than OFDM-BPSK, is improved using a quaternion neural network.