Irregularly Activated Spatial Modulation Schemes with RIS as a Modulator

Keeping in mind the device size and power constraints of future wireless devices, transmitters free of radio frequency chains need to be designed using reconfigurable intelligent surfaces (RIS) as modulators. In this context, spatial modulation (SM) can be applied to selectively activate the desired RIS elements for fulfilling spectral- and energy-efficient communications with low error rates. In this paper, we deploy a RIS as a modulator and propose SM and generalized SM (GSM) based on irregular activation of the RIS elements, where the elements are activated with different probabilities. This can be further combined with element selection, where channel conditions are considered for element activation. It is shown that element selection based on channel conditions in conjunction with irregularly activated SM schemes perform better than conventional SM and GSM schemes in terms of error rates and energy efficiency while maintaining an acceptable signalling overhead and computational complexity.