Reconfigurable Intelligent Surface Enabled Simultaneous Wireless Information and Energy Transfer Under Transmit Antenna Selection

We consider a novel and comprehensive model in which the source has fewer radio frequency (RF) chains than the number of transmit antennas and is aided by a reconfigurable intelligent surface (RIS) in the simultaneous transfer of wireless information and energy to two heterogeneous sets of users. These users are the information users (IUs) that have data decoding capability and the energy users (EUs) that have energy harvesting capability. Given this model, we propose near-optimal heuristic algorithms to determine the antenna subset that should be selected at the source and the corresponding phase shift pattern at RIS to (i) maximize the sum energy harvested (EH) at EUs subject to signal to interference plus noise ratio constraints at IUs and, (ii) maximize sum spectral efficiency (SE) at IUs subject to EH constraints at EUs. We observe that the RF chains which are active and increase the hardware complexity and power consumption at the source can be traded with the RIS elements that are passive and low-cost, leading to improvement in the sum SE at IUs and sum EH at EUs under the respective constraints. Additionally, we analyze the system when no dedicated energy beams are used. We also show that it suffices to have a 4 bit programmable RIS to obtain sum EH identical to that with a continuous phase RIS.