Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Hierarchical Cell-Free Massive MIMO for High Capacity with Simple Implementation

Wei Jiang, Hans D. Schotten

TL;DR

This work addresses the high fronthaul cost and uniform service limitations of cell-free massive MIMO by introducing a hierarchical architecture that places a central base station with $N_b$ antennas at the center and distributes APs around the edge. By classifying users into near and far groups and performing partial data delivery to APs, the system achieves higher sum throughput while maintaining comparable worst-case per-user spectral efficiency; the CBS can act as both central processor and CPU, reducing signaling overhead. The paper derives uplink and downlink SE expressions under MMSE CSI and conjugate beamforming, and demonstrates via simulations (e.g., $M=256$, $K=16$) that the proposed HCF setup yields significant sum-rate gains with about 50% lower fronthaul cost compared to conventional CF/UC. The results support the practical viability of HCF for 6G-like private networks, offering scalable performance improvements with simpler fronthaul requirements.

Abstract

Cell-free massive multi-input multi-output (MIMO) has recently gained much attention for its potential in shaping the landscape of sixth-generation (6G) wireless systems. This paper proposes a hierarchical network architecture tailored for cell-free massive MIMO, seamlessly integrating co-located and distributed antennas. A central base station (CBS), equipped with an antenna array, positions itself near the center of the coverage area, complemented by distributed access points spanning the periphery. The proposed architecture remarkably outperforms conventional cell-free networks, demonstrating superior sum throughput while maintaining a comparable worst-case per-user spectral efficiency. Meanwhile, the implementation cost associated with the fronthaul network is substantially diminished.

Hierarchical Cell-Free Massive MIMO for High Capacity with Simple Implementation

TL;DR

This work addresses the high fronthaul cost and uniform service limitations of cell-free massive MIMO by introducing a hierarchical architecture that places a central base station with antennas at the center and distributes APs around the edge. By classifying users into near and far groups and performing partial data delivery to APs, the system achieves higher sum throughput while maintaining comparable worst-case per-user spectral efficiency; the CBS can act as both central processor and CPU, reducing signaling overhead. The paper derives uplink and downlink SE expressions under MMSE CSI and conjugate beamforming, and demonstrates via simulations (e.g., , ) that the proposed HCF setup yields significant sum-rate gains with about 50% lower fronthaul cost compared to conventional CF/UC. The results support the practical viability of HCF for 6G-like private networks, offering scalable performance improvements with simpler fronthaul requirements.

Abstract

Cell-free massive multi-input multi-output (MIMO) has recently gained much attention for its potential in shaping the landscape of sixth-generation (6G) wireless systems. This paper proposes a hierarchical network architecture tailored for cell-free massive MIMO, seamlessly integrating co-located and distributed antennas. A central base station (CBS), equipped with an antenna array, positions itself near the center of the coverage area, complemented by distributed access points spanning the periphery. The proposed architecture remarkably outperforms conventional cell-free networks, demonstrating superior sum throughput while maintaining a comparable worst-case per-user spectral efficiency. Meanwhile, the implementation cost associated with the fronthaul network is substantially diminished.
Paper Structure (11 sections, 21 equations, 3 figures)

This paper contains 11 sections, 21 equations, 3 figures.

Table of Contents

  1. Introduction
  2. System Model
  3. The Communication Process
  4. Uplink Training
  5. Uplink Data Transmission
  6. Downlink Data Transmission
  7. Performance Analysis
  8. Uplink Spectral Efficiency
  9. Downlink Spectral Efficiency
  10. Numerical Results
  11. Conclusion

Figures (3)

  • Figure 1: An illustrative comparison between the proposed hierarchical cell-free massive MIMO (right) and the conventional cell-free architecture (left).
  • Figure 2: The simulation scenarios of CF in left and HCF in right. It shows the snapshot for a simulation epoch, where the locations of APs and UEs randomly vary in different epochs for ergodic evaluation.
  • Figure 3: Performance comparison of CF/UC and HCF massive MIMO systems: (a) the CDF curves in terms of per-user SE; (b) the CDF curves of sum SE; and (c) the required number of complex-valued symbols delivered over the fronthaul network at each symbol period.