Interference-Free RIS-Aided Cell-Free Massive MIMO with Physical Layer Security

In this paper, a reconfigurable intelligent surface (RIS) assisted cell free massive MIMO (CFmMIMO) framework is designed to enhance physical layer security (PLS) and mitigate multi user (MU) interference in next generation wireless networks. A channel state information (CSI) based precoder is designed at the access point (AP) to suppress MU interference, enabling interference free reception for the legitimate users. To further enhance secrecy performance, we formulate a joint optimization problem that maximizes the secrecy sum rate using an alternating optimization (AO) framework, which iteratively updates the active beamforming at the AP, user power allocation, and the RIS phase shift matrix. The highly nonconvex problem is addressed under the Riemannian manifold optimization (RMO) framework and solved using a Riemannian Conjugate Gradient (RCG) algorithm for RIS phase shift design. Simulation results verify that the proposed framework effectively enhances the secrecy sum rate and eliminates interference, demonstrating its potential for secure and scalable CFmMIMO networks in dense wireless environments.

💡 Research Summary

The paper proposes a novel framework that integrates reconfigurable intelligent surfaces (RIS) with cell‑free massive MIMO (CF‑mMIMO) to simultaneously address multi‑user (MU) interference and physical‑layer security (PLS) in dense 6G‑era networks. The system consists of a single access point (AP) equipped with Nₜ antennas, K single‑antenna users, a full‑duplex eavesdropper (Eve) with N_E > 1 antennas, and a RIS with M programmable phase‑shift elements. Direct AP‑to‑user links are assumed blocked, so all downlink communication is routed through the RIS. The AP’s antennas are partitioned into K blocks, each dedicated to a specific user, which already reduces inter‑user coupling.

Interference‑free precoder design
Given perfect CSI of the cascaded AP‑RIS‑user channels, the received signal at user k can be expressed as a sum of the desired term and interference from other users. By constructing a K × K interference matrix Q that captures the contributions of all users (including power allocation, beamforming vectors w_k, RIS phase matrix Θ, and symbol amplitudes), and a desired‑signal vector q, the authors formulate a linear system Q ϖ = q. Solving for the precoder vector ϖ_opt = Q⁻¹q orthogonalizes the transmitted streams, guaranteeing that the interference term vanishes for every user. A scaling factor β̄ is introduced to keep the average transmit power constant; it can be pre‑computed offline using statistical channel realizations. With ϖ_opt applied, each user observes an interference‑free signal that can be detected by a simple maximum‑likelihood (ML) detector.

Joint secrecy‑rate maximization
Beyond interference suppression, the paper seeks to maximize the secrecy sum‑rate R_s = ∑_k