[Paper Review] Dual Sub-6 GHz -- Millimeter Wave Beamforming and Communications to Achieve Low Latency and High Energy Efficiency in 5G Systems

Morteza Hashemi, C. Emre Koksal+1 more|arXiv (Cornell University)|Jan 23, 2017

Millimeter-Wave Propagation and Modeling22 references4 citations

TL;DR

This paper proposes a dual-band RF/mmWave architecture that leverages sub-6 GHz beamforming to enable low-latency, high-energy-efficiency mmWave communications in 5G systems. By exploiting spatio-temporal correlation between RF and mmWave channels, the system reduces mmWave beam training overhead and uses the RF link as a fallback to maintain throughput and minimize delay, achieving optimal performance through a threshold-based scheduling policy with proven monotonic structure.

ABSTRACT

We propose a hybrid architecture that integrates RF (i.e., sub-6 GHz) and millimeter wave (mmWave) technologies for 5G cellular systems. In particular, communications in the mmWave band faces significant challenges due to variable channels, intermittent connectivity, and high energy usage. On the other hand, speeds for electronic processing of data is of the same order as typical rates for mmWave interfaces which makes the use of complex algorithms for tracking channel variations and adjusting resources accordingly impractical. Our proposed architecture integrates the RF and mmWave interfaces for beamforming and data transfer, and exploits the spatio-temporal correlations between the interfaces. Based on extensive experimentation in indoor and outdoor settings, we demonstrate that an integrated RF/mmWave signaling and channel estimation scheme can remedy the problem of high energy usage and delay associated with mmWave beamforming. In addition, cooperation between two interfaces at the higher layers effectively addresses the high delays caused by highly intermittent mmWave connectivity. We design a scheduler that fully exploits the mmWave bandwidth, while the RF link acts as a fallback mechanism to prevent high delay. To this end, we formulate an optimal scheduling problem over the RF and mmWave interfaces where the goal is to maximize the delay-constrained throughput of the mmWave interface. We prove using subadditivity analysis that the optimal scheduling policy is based on a single threshold that can be easily adopted despite high link variations.

Motivation & Objective

• Address the high energy consumption and beamforming delay in mmWave communications due to complex signal training and hardware constraints.
• Overcome intermittent mmWave connectivity caused by blockage and rapid channel variations in mobile environments.
• Enable efficient utilization of mmWave's high bandwidth while maintaining low delay through RF-assisted beamforming and data transfer.
• Formulate an optimal scheduling policy that maximizes delay-constrained throughput over RF and mmWave interfaces.
• Design a practical, low-complexity threshold-based admission control mechanism that balances throughput and delay.

Proposed method

• Use RF band for coarse angle-of-arrival (AoA) estimation to reduce the angular search range for mmWave beamforming from 180° to ≤20°.
• Leverage strong spatial correlation between RF and mmWave signals—94% of measurements show ±10° AoA accuracy under LOS conditions.
• Implement fully analog beamforming on mmWave with initial beam alignment guided by RF estimates, reducing ADC power consumption.
• Integrate RF and mmWave interfaces at higher layers to enable cooperative data transfer, using RF as a fallback during mmWave outages.
• Formulate a constrained throughput maximization problem as a reward optimization, proving the optimal policy has a single threshold structure.
• Apply subadditivity analysis to derive a monotonic, easily implementable threshold policy that adapts to dynamic channel conditions.

Experimental results

Research questions

• RQ1 Can RF-assisted beamforming significantly reduce the beam training overhead and energy consumption in mmWave systems?
• RQ2 How does the correlation between RF and mmWave channel characteristics affect beam alignment accuracy and efficiency?
• RQ3 What is the optimal scheduling policy for balancing mmWave throughput and delay when the mmWave link is intermittent?
• RQ4 Can the RF interface effectively serve as a reliable fallback to maintain low-latency communication during mmWave outages?
• RQ5 What is the structure of the optimal admission control policy for mmWave queues under delay and energy constraints?

Key findings

• 94% of RF and mmWave AoA measurements were within ±10° under line-of-sight conditions, validating strong spatial correlation.
• The angular search range for mmWave beamforming was reduced from 180° to an average of ≤20° using RF-based coarse beam alignment.
• The optimal scheduling policy is a single-threshold policy, proven via subadditivity analysis, enabling low-complexity implementation.
• Increasing the reneging cost reduces the optimal threshold, decreasing mmWave queue delay at the expense of lower throughput.
• Increasing the reward value raises the threshold, increasing mmWave throughput at the cost of higher waiting time, demonstrating effective trade-off control.
• The integrated RF/mmWave architecture achieves high energy efficiency and low latency by minimizing mmWave beam training and leveraging RF for reliable data transfer during outages.