Millimeter Wave Beamforming for Wireless Backhaul and Access in Small Cell Networks (1306.6659v1)

Abstract: Recently, there has been considerable interest in new tiered network cellular architectures, which would likely use many more cell sites than found today. Two major challenges will be i) providing backhaul to all of these cells and ii) finding efficient techniques to leverage higher frequency bands for mobile access and backhaul. This paper proposes the use of outdoor millimeter wave communications for backhaul networking between cells and mobile access within a cell. To overcome the outdoor impairments found in millimeter wave propagation, this paper studies beamforming using large arrays. However, such systems will require narrow beams, increasing sensitivity to movement caused by pole sway and other environmental concerns. To overcome this, we propose an efficient beam alignment technique using adaptive subspace sampling and hierarchical beam codebooks. A wind sway analysis is presented to establish a notion of beam coherence time. This highlights a previously unexplored tradeoff between array size and wind-induced movement. Generally, it is not possible to use larger arrays without risking a corresponding performance loss from wind-induced beam misalignment. The performance of the proposed alignment technique is analyzed and compared with other search and alignment methods. The results show significant performance improvement with reduced search time.

• The paper introduces an adaptive beam alignment method using hierarchical codebooks and iterative subspace sampling to overcome high path loss in mmWave systems.
• The paper's method achieves up to a 13 dB improvement in beamforming gain over non-adaptive techniques while significantly reducing search time at high SNR.
• The paper quantifies wind-induced misalignment on large antenna arrays, highlighting the need for frequent realignment to maintain link reliability.

Millimeter Wave Beamforming for Wireless Backhaul and Access in Small Cell Networks

The paper "Millimeter Wave Beamforming for Wireless Backhaul and Access in Small Cell Networks" by Sooyoung Hur et al. explores the utilization of outdoor millimeter wave (mmWave) communications for addressing the challenges in tiered network cellular architectures, specifically in providing reliable and scalable backhaul and access solutions in small cell networks.

Background and Motivations

The exponential growth in demand for mobile multimedia services necessitates innovations to increase spectrum efficiency. The concept of multi-tier cell deployment, such as picocells and femtocells, aims to enhance geographic spectrum reusability. However, the dense deployment of picocells requires cost-effective and scalable backhaul solutions that are not feasible with conventional wired infrastructures. Additionally, the limited sub-3 GHz spectrum further motivates the exploration of higher frequency bands, including mmWave bands (60 GHz to 80 GHz), which offer substantial underutilized spectrum but pose significant propagation challenges.

Methodology

The authors propose leveraging large array antennas for beamforming at mmWave frequencies to counteract high path loss and environmental impairments such as rain and oxygen absorption. Beamforming with large arrays, while providing necessary directional gain, introduces sensitivity to movement and misalignments caused by environmental factors like wind. This paper centers on efficient beam alignment techniques and their practical deployment implications in small cell networks.

Beam Alignment Technique

A novel beam alignment technique is proposed, employing adaptive subspace sampling and hierarchical beam codebooks to optimize the alignment process. The algorithm utilizes a multi-round "ping-pong" adaptive sampling strategy where the transmitter and receiver iteratively refine their beam directions based on previously observed samples. Subcodebooks of increasing resolution are designed to cover the channel subspace efficiently, minimizing search time and computational complexity.

Wind Sway Analysis

The paper also explores the impact of wind-induced vibrations on beam alignment performance. Wind sway can substantially disrupt beam alignment, especially for large antenna arrays with narrow beams. The authors introduce the notion of beam coherence time—the expected duration a beam remains aligned before wind-induced misalignment causes performance degradation. The analysis highlights a tradeoff between array size and vulnerability to environmental disturbances, underscoring the need for frequent beam realignment in windy conditions.

Numerical Results and Performance Evaluation

The proposed beam alignment technique is evaluated against non-adaptive joint alignment and single-sided alignment methods through simulations. The results demonstrate significant performance improvements in terms of beamforming gain and reduced search time. For instance, the proposed adaptive alignment method achieves a gain improvement of up to 13 dB over non-adaptive techniques at high SNR.

In addition, the paper quantifies wind-induced performance penalties, showing that larger arrays, while offering higher potential gains, are more susceptible to misalignments. For example, a 32-element array experiences a substantial gain reduction in high wind conditions, reinforcing the importance of adaptive beam alignment for maintaining link reliability.

Implications and Future Directions

The research presented in this paper has profound implications for the deployment of small cell networks. By demonstrating effective beamforming and alignment techniques at mmWave frequencies, this paper paves the way for practical implementations of high-capacity wireless backhaul and access solutions. The findings on wind-induced misalignment stress the necessity of robust beam management algorithms in real-world deployments, particularly in urban environments where infrastructure is prone to environmental disturbances.

Future developments in this area could explore more sophisticated adaptive algorithms, multi-dimensional arrays to mitigate wind effects, and integration with network-level strategies for dynamic resource allocation. Moreover, extending this research to include mobility scenarios, such as vehicular networks, would further enhance the applicability of mmWave communications in next-generation wireless networks.

Conclusion

In conclusion, Hur et al.'s work on mmWave beamforming and alignment provides a comprehensive framework for addressing the challenges of small cell backhaul and access in urban deployments. The combination of adaptive subspace sampling, hierarchical codebooks, and wind sway analysis offers a pathway to scalable, high-performance mmWave networks, laying the groundwork for future advancements in this critical area of wireless communications.