Hybrid Digital and Analog Beamforming Design for Large-Scale Antenna Arrays (1601.06814v1)

Abstract: The potential of using of millimeter wave (mmWave) frequency for future wireless cellular communication systems has motivated the study of large-scale antenna arrays for achieving highly directional beamforming. However, the conventional fully digital beamforming methods which require one radio frequency (RF) chain per antenna element is not viable for large-scale antenna arrays due to the high cost and high power consumption of RF chain components in high frequencies. To address the challenge of this hardware limitation, this paper considers a hybrid beamforming architecture in which the overall beamformer consists of a low-dimensional digital beamformer followed by an RF beamformer implemented using analog phase shifters. Our aim is to show that such an architecture can approach the performance of a fully digital scheme with much fewer number of RF chains. Specifically, this paper establishes that if the number of RF chains is twice the total number of data streams, the hybrid beamforming structure can realize any fully digital beamformer exactly, regardless of the number of antenna elements. For cases with fewer number of RF chains, this paper further considers the hybrid beamforming design problem for both the transmission scenario of a point-to-point multipleinput multiple-output (MIMO) system and a downlink multiuser multiple-input single-output (MU-MISO) system. For each scenario, we propose a heuristic hybrid beamforming design that achieves a performance close to the performance of the fully digital beamforming baseline. Finally, the proposed algorithms are modified for the more practical setting in which only finite resolution phase shifters are available. Numerical simulations show that the proposed schemes are effective even when phase shifters with very low resolution are used.

• The paper establishes that a hybrid beamforming architecture achieves near-digital performance when the number of RF chains is at least twice the number of data streams.
• It introduces iterative algorithms for point-to-point MIMO and multi-user MISO designs, demonstrating high spectral efficiency and effective interference mitigation.
• The study rigorously analyzes finite resolution phase shifter effects, outlining practical trade-offs for cost-effective and scalable mmWave system implementations.

Hybrid Digital and Analog Beamforming Design for Large-Scale Antenna Arrays: Technical Overview

In the paper titled "Hybrid Digital and Analog Beamforming Design for Large-Scale Antenna Arrays," Sohrabi and Yu investigate a hybrid beamforming architecture for millimeter wave (mmWave) communications, specifically targeting large-scale antenna arrays. Traditional fully digital beamforming methods necessitate an individual radio frequency (RF) chain for each antenna element, which, at mmWave frequencies, incurs high costs and substantial power consumption. The authors propose a hybrid scheme that combines a low-dimensional digital beamformer and an RF beamformer using analog phase shifters to address these issues.

Key Contributions

1. Hybrid Beamforming Feasibility: The paper establishes that a hybrid architecture can emulate a fully digital beamformer if the number of RF chains is at least twice the number of data streams, regardless of the antenna count. This assertion sets a fundamental requirement for hybrid structures aiming to match digital beamforming performance.
2. Design for Point-to-Point MIMO Systems:
   • Algorithm Development: When the number of RF chains equals the number of data streams, the authors propose an iterative algorithm for designing the hybrid precoder and combiner. The algorithm leverages the properties of the RF and digital stages to achieve near-optimal spectral efficiency.
   • Numerical Performance: Simulations confirm that the proposed method achieves spectral efficiency close to the theoretical capacity, demonstrating the practical effectiveness of the hybrid approach.
3. Design for Multi-User MISO Systems:
   • Iterative Refinement: For multi-user downlink systems, an iterative method is introduced that alternates between refining the RF and digital beamforming stages while managing inter-user interference. The paper shows that increasing the number of RF chains beyond the number of users significantly improves the system performance.
   • Zero-Forcing Precoding: The digital beamformer part employs Zero-Forcing (ZF) precoding to mitigate inter-user interference, a crucial aspect for maintaining high spectral efficiency in multi-user environments.
4. Finite Resolution Phase Shifters:
   • Quantization Effects: The impact of finite resolution phase shifters, a practical consideration given the expense and complexity of precise components, is rigorously analyzed. The proposed algorithms are adapted to account for phase quantization, demonstrating that hybrid beamforming remains effective even with low-resolution (e.g., 1-bit) phase shifters.
   • Performance Trade-Offs: The paper introduces methods to mitigate performance loss due to quantization, such as slightly increasing the number of RF chains, which balances cost, complexity, and performance.

Implications and Future Directions

The findings of this research have significant implications for both theoretical studies and practical implementations of mmWave communications and massive MIMO systems. Key implications include:

• Cost-Effective Massive MIMO: By reducing the necessary number of RF chains, hybrid beamforming enables cost-effective and power-efficient deployments of massive MIMO systems, crucial for 5G and beyond.
• Flexibility in Hardware Design: The ability to operate effectively with low-resolution phase shifters introduces flexibility in hardware design, allowing for scalable and economically viable mmWave systems.

Conclusion

Sohrabi and Yu's work extensively contributes to the understanding and implementation of hybrid beamforming in large-scale antenna arrays, providing robust frameworks for both point-to-point MIMO and multi-user MISO applications. Future research could explore the robustness of hybrid designs under imperfect channel state information (CSI) conditions or extend these strategies to frequency-selective channels in more dynamic environments, further amplifying the relevance and application range of hybrid beamforming architectures in next-generation wireless networks.