Virtual Sectorization to Enable Hybrid Beamforming in mm-Wave mMIMO

Abstract: Hybrid beamforming (HBF) is a key technology to enable mm-wave Massive multiple-input multiple-output (mMIMO) receivers for future-generation wireless communications. It combines beamforming in both analog (via phase shifters) and digital domains, resulting in low power consumption and high spectral efficiency. In practice, the problem of joint beamforming in multi-user scenarios is still open because an analog beam can't cover all users simultaneously. In this paper, we propose a hierarchical approach to divide users into clusters. Each cluster consists of users inside a virtual sector produced by the analog beamforming of an HBF-based mMIMO receiver. Thus, inside each sector, a lower-cost digital beamforming serves a limited number of users within the same cluster. Simulations with realistic non-line-of-sight scenarios generated by the QuaDRiGa 2.0 demonstrate that our methods outperform standard FFT-based alternatives and almost achieve SVD-based beamspace performance bound.

• The paper proposes a hierarchical clustering algorithm that uses virtual sectorization to enhance multi-user hybrid beamforming.
• It demonstrates near SVD-based beamspace performance with a performance loss of only 1.4 dB in NLOS simulations.
• The approach reduces hardware complexity by utilizing a partially-connected hybrid beamforming architecture tailored for mmWave mMIMO systems.

Virtual Sectorization to Enable Hybrid Beamforming in mm-Wave mMIMO (2404.02161)

The paper presents a hierarchical approach to enhance the hybrid beamforming (HBF) technology in millimeter-wave (mmWave) massive multi-input multiple-output (mMIMO) systems. By employing a technique called virtual sectorization, this research addresses the challenge of joint beamforming in multi-user scenarios, where a single analog beam is insufficient to cover all users effectively.

Introduction

Enhanced mobile broadband (eMBB) typically relies on wide frequency bands, high signal-to-noise ratios, and spectrum reuse to increase data transmission rates. Massive MIMO technology leverages spatial domain separation using a large array of antennas. As communication systems transition to 6G, there is potential to exploit the upper millimeter-wave spectrum for higher data rates. While this transition allows for more antennas on a conventional form factor, it presents challenges related to cost and energy consumption due to greater integration complexity compared to earlier networks.

Implementing hybrid beamforming (HBF) technology offers a solution to these challenges. HBF combines analog and digital beamforming, allowing for reduced power consumption while maximizing the sum rate performance of mMIMO systems. It calls for a decrease in the number of ADC and DAC converters at base stations, aiding higher data rate achievement in future wireless networks.

Proposed Hierarchical Clustering

To address the ongoing issue of developing a beamforming algorithm for multi-user scenarios, a hierarchical clustering methodology is proposed. Users are grouped into clusters, each represented by a virtual sector or an analog beam sector, with a focus on minimizing performance degradation. Within each sector, digital beamforming efficiently serves the limited number of users.

The proposed approach involves a positional clustering method using a partially-connected (PC) HBF architecture. Users are clustered per subarray based on their spatial correlation, calculated using the dot product of their AoA steering vectors. This allows for better maximization of the beamforming gain for users within each cluster.

System Model and Algorithms

The proposed hierarchical clustering algorithm operates under a partially-connected HBF architecture utilizing OFDM and uplink operations. This involves clustering users into groups that can be effectively covered with analog beams configured by phase shifters and using FFT for digital beamspace selection. The algorithm defines user clusters (g_i) and uses SVD-based beamspace approximation for enhancement in a spatially correlated user distribution.

Hybrid Beamforming Architectures

The proposed solution utilizes a subarray-based or sub-connected HBF architecture due to its reduced complexity compared to fully-connected architectures. Each digital channel connects to a subarray of antenna elements through dedicated phase shifters facilitating efficient analog beamforming.

Performance Analysis

Simulation results suggest that the proposed method achieves significant performance improvements over standard FFT-based alternatives, nearing the SVD-based beamspace performance bound. Implemented within QuaDRiGa 2.0 non-line-of-sight (NLOS) scenarios typical of future 6G settings, the new algorithm demonstrated a performance loss of only 1.4 dB compared to full digital beamforming, highlighting substantial benefits in terms of hardware requirements and computational complexity.

Conclusion

The study introduces a novel clustering algorithm for managing the sectorization challenge within HBF systems, important for future cellular communication systems aiming to operate in the upper millimeter wave bands with UM-MIMO infrastructure. The hierarchical approach proposes efficient clusterization of spatially correlated users to optimize analog beam allocation with limited degradation in performance. Given its promising results in maintaining proximity to SVD-based beamspace performance metrics, while reducing hardware complexity, the proposed HBF method presents a practical advantage for next-generation massive MIMO networks. Future research should explore the real-world deployment scenarios further and extend the methodology to more dynamic environments.