# Cell-Free Millimeter-Wave Massive MIMO Systems with Limited Fronthaul Capacity

## TL;DR

This paper introduces a novel cell-free mmWave massive MIMO framework that incorporates low-complexity hybrid precoding, limited fronthaul capacity, and optimized pilot allocation to enhance network performance and fairness.

## Contribution

It proposes a new framework for cell-free mmWave massive MIMO with hybrid precoding and fronthaul constraints, including a clustering-based pilot allocation and power optimization algorithms.

## Key findings

- The pilot allocation strategy outperforms classical random approaches.
- Trade-offs identified among user rate, fronthaul capacity, and hardware complexity.
- Simulation confirms improved performance and fairness.

## Abstract

Network densification, massive multiple-input multiple-output (MIMO) and millimeter-wave (mmWave) bands have recently emerged as some of the physical layer enablers for the future generations of wireless communication networks (5G and beyond). Grounded on prior work on sub-6~GHz cell-free massive MIMO architectures, a novel framework for cell-free mmWave massive MIMO systems is introduced that considers the use of low-complexity hybrid precoders/decoders while factors in the impact of using capacity-constrained fronthaul links. A suboptimal pilot allocation strategy is proposed that is grounded on the idea of clustering by dissimilarity. Furthermore, based on mathematically tractable expressions for the per-user achievable rates and the fronthaul capacity consumption, max-min power allocation and fronthaul quantization optimization algorithms are proposed that, combining the use of block coordinate descent methods with sequential linear optimization programs, ensure a uniformly good quality of service over the whole coverage area of the network. Simulation results show that the proposed pilot allocation strategy eludes the computational burden of the optimal small-scale CSI-based scheme while clearly outperforming the classical random pilot allocation approaches. Moreover, they also reveal the various existing trade-offs among the achievable max-min per-user rate, the fronthaul requirements and the optimal hardware complexity (i.e., number of antennas, number of RF chains).