A New Framework of RIS-Aided User-Centric Cell-Free Massive MIMO System for IoT Networks

With its merit of intercell interference elimination and enhanced throughput, cell-free (CF) massive multiple-input multiple-output (MIMO) system, has attracted considerable interests in evolution of 6G Internet of Things (IoT) networks. Meanwhile, reconfigurable intelligent surface (RIS) has shown its potential benefit in enhancing both capacity and energy efficiency (EE). In this article, a new framework of RIS-aided CF massive MIMO system is proposed for IoT networks, in which extra active access points (APs) are deployed closely to the passive RISs, in that way, the RIS-user channels can be approximately obtained by parameters estimated at those extra APs with acceptable loss. To avoid extra power consumption, a user-centric AP selection strategy in the CF system is suggested, on the basis of which, an optimization problem related to power control, precoding, and RIS phase shift is formulated to maximize the sum rate (SR). To deal with this tough three-variable optimization, a Lagrangian dual transformation and fractional programming-based algorithm is proposed. In particular, the proposed algorithm can extend to optimize EE and well adapt to other RIS-aided CF systems. Simulation results reveal that the proposed algorithm can achieve superior performance in terms of both SR and EE.