Spectrum and Energy-Efficiency Maximization in RIS-Aided IoT Networks

New device capabilities supporting spectrum and energy-efficient internet of things (IoT) deployments are expected to drive the evolution of fifth-generation (5G) and beyond networks. Due to operating on the same spectrum of resources, every IoT pair interferes with each other. This paper utilizes reconfigurable intelligent surfaces (RISs) to mitigate inter-node interference in an IoT network through directional beamforming by adjusting the phase shifts of the passive elements of the RIS. We consider an RIS-assisted IoT network consisting of multiple pairs of IoT devices that utilize both direct paths and one-hop reflected paths. Two resource allocation problems, namely spectrum-efficiency maximization (SEM) and energy-efficiency maximization (EEM), are studied. Since the formulated optimization problems are non-convex in nature, we divide them into two sub-problems and solve them alternately. To obtain the optimal phase shifts of the elements of the RIS for both SEM and EEM, we extend the conjugate gradient technique to Riemannian manifolds. On the other hand, to obtain the optimal transmit power, we solve the transmit power allocation sub-problem as a difference of concave functions for the SEM and use a pricing-based technique for the EEM. Numerical results validate the effectiveness of deploying RISs to assist IoT networks, albeit concerning the RIS’s location, its number of elements, and the number of IoT node pairs. In particular, considerable spectrum and energy efficiency gains are achieved in comparison to baseline state-of-the-art networks.