# Achieving Optimal Performance-Cost Trade-Off in Hierarchical Cell-Free Massive MIMO

## TL;DR

This paper introduces a hierarchical cell-free massive MIMO system that balances performance and cost by replacing some access points with a central base station, supported by a new analytical framework and hierarchical combining scheme.

## Contribution

It provides the first comprehensive analysis of uplink configuration in hierarchical cell-free massive MIMO and proposes a novel hierarchical combining method for optimal performance-cost trade-off.

## Key findings

- HCF with centralized zero-forcing achieves optimal performance-cost balance.
- HCF reduces fronthaul requirements compared to traditional CF systems.
- The analytical framework supports arbitrary combining schemes.

## Abstract

Cell-free (CF) massive MIMO offers uniform service via distributed access points (APs), which impose high deployment costs. A novel design called hierarchical cell-free (HCF) addresses this problem by replacing some APs with a central base station, thereby lowering the costs of fronthaul network (wireless sites and fiber cables) while preserving performance. To identify the optimal uplink configuration in HCF massive MIMO, this paper provides the first comprehensive analysis, benchmarking it against cellular and CF systems. We develop a unified analytical framework for spectral efficiency that supports arbitrary combining schemes and introduce a novel hierarchical combining approach tailored to HCF two-tier architecture. Through analysis and evaluation of user fairness, system capacity, fronthaul requirements, and computational complexity, this paper identifies that HCF using centralized zero-forcing combining achieves the optimal balance between performance and cost-efficiency.