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Unlocking the Potential of Local CSI in Cell-Free Networks with Channel Aging and Fronthaul Delays

Lorenzo Miretti, Sławomir Stańczak

TL;DR

This work tackles the performance gap in downlink cell-free networks caused by fronthaul delays and channel aging by introducing a distributed Team MMSE precoding framework that merges timely local CSI with delayed global CSI. The main contribution is a rigorous optimal solution yielding a two-stage precoding structure $m{T}_l=m{F}_lm{C}_l$, where the local MMSE stage $m{F}_l$ depends on $H_l$ and the coupling matrix $m{C}_l$ depends on $Z=H[t-d]$, computed via a linear system with conditional expectations. The authors also analyze C-RAN functional splits, propose practical suboptimal schemes (local, centralized delay-tolerant, naive distributed, and structure-aware distributed), and demonstrate through simulations that carefully designed distributed precoding can surpass both fully centralized and fully distributed approaches even with realistic delays and mobility. The results highlight the value of delegating partial precoding to APs and provide guidance for functional split decisions in future C-RAN deployments, offering robust performance gains under realistic fronthaul constraints. Overall, the paper advances practical interference management in cell-free networks by formalizing and solving a robust distributed precoding problem under delayed CSI, with clear implications for scalable, high-performance wireless architectures.

Abstract

It is generally believed that downlink cell-free networks perform best under centralized implementations where the local channel state information (CSI) acquired by the access-points (AP) is forwarded to one or more central processing units (CPU) for the computation of the joint precoders based on global CSI. However, mostly due to limited fronthaul capabilities, this procedure incurs some delay that may lead to partially outdated precoding decisions and hence performance degradation. In some scenarios, this may even lead to worse performance than distributed implementations where the precoders are locally computed by the APs based on partial yet timely local CSI. To address this issue, this study considers the problem of robust precoding design merging the benefits of timely local CSI and delayed global CSI. As main result, we provide a novel distributed precoding design based on the recently proposed team minimum mean-square error method. As a byproduct, we also obtain novel insights related to the AP-CPU functional split problem. Our main conclusion, corroborated by simulations, is that the opportunity of performing some local precoding computations at the APs should not be neglected, even in centralized implementations.

Unlocking the Potential of Local CSI in Cell-Free Networks with Channel Aging and Fronthaul Delays

TL;DR

This work tackles the performance gap in downlink cell-free networks caused by fronthaul delays and channel aging by introducing a distributed Team MMSE precoding framework that merges timely local CSI with delayed global CSI. The main contribution is a rigorous optimal solution yielding a two-stage precoding structure , where the local MMSE stage depends on and the coupling matrix depends on , computed via a linear system with conditional expectations. The authors also analyze C-RAN functional splits, propose practical suboptimal schemes (local, centralized delay-tolerant, naive distributed, and structure-aware distributed), and demonstrate through simulations that carefully designed distributed precoding can surpass both fully centralized and fully distributed approaches even with realistic delays and mobility. The results highlight the value of delegating partial precoding to APs and provide guidance for functional split decisions in future C-RAN deployments, offering robust performance gains under realistic fronthaul constraints. Overall, the paper advances practical interference management in cell-free networks by formalizing and solving a robust distributed precoding problem under delayed CSI, with clear implications for scalable, high-performance wireless architectures.

Abstract

It is generally believed that downlink cell-free networks perform best under centralized implementations where the local channel state information (CSI) acquired by the access-points (AP) is forwarded to one or more central processing units (CPU) for the computation of the joint precoders based on global CSI. However, mostly due to limited fronthaul capabilities, this procedure incurs some delay that may lead to partially outdated precoding decisions and hence performance degradation. In some scenarios, this may even lead to worse performance than distributed implementations where the precoders are locally computed by the APs based on partial yet timely local CSI. To address this issue, this study considers the problem of robust precoding design merging the benefits of timely local CSI and delayed global CSI. As main result, we provide a novel distributed precoding design based on the recently proposed team minimum mean-square error method. As a byproduct, we also obtain novel insights related to the AP-CPU functional split problem. Our main conclusion, corroborated by simulations, is that the opportunity of performing some local precoding computations at the APs should not be neglected, even in centralized implementations.
Paper Structure (17 sections, 2 theorems, 16 equations, 2 figures)

This paper contains 17 sections, 2 theorems, 16 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Problem statement
  3. Basic definitions and assumptions
  4. Delayed CSI sharing
  5. Team MMSE precoding
  6. Problem solution
  7. Optimal solution
  8. Interpretation and computation of the optimal solution
  9. C-RAN functional split aspects
  10. Suboptimal solutions
  11. Local precoding
  12. Centralized (delay-tolerant) precoding
  13. Naïve distributed precoding
  14. Structure-aware distributed precoding
  15. Numerical simulations and conclusions
  16. ...and 2 more sections

Key Result

Proposition 1

For given $\bm{p}\in\mathbbmss{R}_{++}^K$ and $\bm{\sigma}\in \mathbbmss{R}_{++}^L$, Problem prob:TMMSE admits a unique solution, which is also the unique $\mathbbm{T} \in \mathcal{T}$ satisfying where $\mathbbm{F}_l:=\left(\mathbbm{H}_{l}\bm{P}\mathbbm{H}_{l}^\mathsf{H} + \sigma_l\bm{I}_N\right)^{-1}\mathbbm{H}_{l}\bm{P}^{\frac{1}{2}} \in \mathcal{H}^{N\times K}$.

Figures (2)

  • Figure 1: Pictorial representation of possible implementations of the proposed team MMSE solution \ref{['eq:DTMMSE']} in C-RAN architectures: (a) distributed precoding with CSI sharing; (b) locally refined centralized precoding (compress-before-precoding); (c) locally refined centralized precoding (compress-after-precoding).
  • Figure 2: Cumulative density function of downlink ergodic rates achieved by the optimal team MMSE solution \ref{['eq:DTMMSE']} and the suboptimal designs in Sect. \ref{['ssec:approximate']}, assuming pedestrian mobility and a CSI sharing delay of (a) $10$ ms ($r = 0.9$) and (b) $1$ ms ($r = 0.99$).

Theorems & Definitions (7)

  • Remark 1
  • Remark 2
  • Remark 3
  • Proposition 1
  • proof
  • Proposition 2
  • proof