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

TL;DR

This paper proposes a RIS-assisted approach to improve fronthaul link survivability in cell-free massive MIMO systems by combining LOS enhancement, backup links, and resource sharing, resulting in significant capacity savings.

Contribution

It introduces a novel RIS-assisted optimization framework for fronthaul survivability, combining WMMSE and Riemannian gradient methods for the first time in this context.

Findings

RIS reduces redundant capacity by 65.6% for 99% survivability.

RIS provides substantial gains during complete outages of direct links.

The approach enhances reliability while minimizing infrastructure costs.

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.