| Abstract |
Internet of things (IoT)[1] applications are wide spreading, recently empowered by the dissemination of technologies like smart cities, autonomous vehicles, smart grids, industry 4.0, home automation, wearables, etc. On the other hand, the global warming and climate crisis demand urgent, bold actions toward energy-efficient (EE) technologies for telecommunication systems, which are responsible for a significant part of global energy consumption. To this end, reconfigurable intelligent surfaces (RISs) appear as an important technology to improve the propagation channel gain at the expense of very little power expenditures, recognized as a key element in achieving green telecommunication systems. In this chapter, we focus on the EE uplink (UL) transmission of massive multiple-input multiple-output (M-MIMO) IoT systems aided by an RIS. We propose and evaluate different schemes to minimize the total UL transmit power by optimizing the transmit power of IoT devices, the RIS phase-shift element, and the combining matrix at the base station (BS). Particularly, we give special attention to manifold optimization techniques, which are well suited to the RIS phase-shifts optimization problem. Herein, we treat jointly via iterative alternating optimization (i-AO) approach the three optimization variables: RIS phase-shift vector; BS combining matrix, and unit terminal (UT) power allocation vector. Extensive numerical results are provided and discussed, revealing that the proposed conjugate gradient (CG) method based on Riemannian manifold (RM) with the zero-forcing (ZF) combining achieves the highest power savings, being able to reduce the UL transmit power by up to 89% under typical operation conditions scenarios in comparison with conventional systems without RIS. © 2025 by The Institute of Electrical and Electronics Engineers, Inc. All rights reserved. |
