RIS-Assisted Survivable Fronthaul Design in Cell-Free Massive MIMO System

TL;DR

This work tackles fronthaul survivability in cell-free massive MIMO by leveraging reconfigurable intelligent surfaces (RIS) to maintain reliable wireless backup links and by enabling a resource-sharing scheme that offloads traffic to a nearby master AP when primary cables fail. The authors formulate a redundant capacity minimization problem and solve it via an alternating optimization framework that combines a modified weighted MMSE-based precoding with RIS phase-shift optimization using Riemannian gradient descent, along with dynamic Lagrange multiplier updates. Results show that RIS can reduce the required redundant capacity by up to 65.6% to achieve 99% survivability, with the gains most pronounced when the direct master AP–CPU link is outage-disabled, demonstrating substantial reliability improvements at lower infrastructure cost. Overall, the paper presents a practical, RIS-enabled approach to enhancing fronthaul reliability in next-generation wireless networks through joint optimization of wireless backhaul paths and load-sharing strategies.

Abstract

This paper investigates the application of reconfigurable intelligent surfaces (RISs) to improve fronthaul link survivability in cell-free massive MIMO (CF mMIMO) systems. To enhance the fronthaul survivability, two complementary mechanisms are considered. Firstly, RIS is set to provide reliable line-of-sight (LOS) connectivity and enhance the mmWave backup link. Secondly, a resource-sharing scheme that leverages redundant cable capacity through neighboring master access points (APs) to guarantee availability is considered. We formulate the redundant capacity minimization problem as a RIS-assisted multi-user MIMO rate control optimization problem, developing a novel solution that combines a modified weighted minimum mean square error (WMMSE) algorithm for precoding design with Riemannian gradient descent for RIS phase shift optimization. Our numerical evaluations show that RIS reduces the required redundant capacity by 65.6% compared to the no RIS case to reach a 99% survivability. The results show that the most substantial gains of RIS occur during complete outages of the direct disconnected master AP-CPU channel. These results demonstrate RIS's potential to significantly enhance fronthaul reliability while minimizing infrastructure costs in next-generation wireless networks.

Figures (4)

• Figure 1: Illustration of the RIS-assisted CF mMIMO system model.
• Figure 2: Illustration of the simulation setup. The height of different entities is neglected for simplicity.
• Figure 3: Convergence condition of the algorithm. In the evaluated case, $N_\text{used}=400$, $C_0=1.6$ Gbps, $d_\text{AP}=50$ m, $d_\text{CPU}=200$ m, and $d_\text{RIS-CPU}=5$ m.
• Figure 4: Relation between the redundant capacity and the resulting survivability. (a) $d_\text{CPU}=175$ m, $N_\text{used}=400$, $C_0=1.6$ Gbps; (b) $d_\text{CPU}=200$ m, $N_\text{used}=400$, $C_0=1.6$ Gbps; (c) $d_\text{CPU}=175$ m, $N_\text{used}=1200$, $C_0=4.9$ Gbps; (d) $d_\text{CPU}=200$ m, $N_\text{used}=1200$, $C_0=4.9$ Gbps.