[Paper Review] Mitigating Hand Blockage with Non-Directional Beamforming Codebooks

Vasanthan Raghavan, Ricardo A. Motos+4 more|arXiv (Cornell University)|Apr 13, 2021

Millimeter-Wave Propagation and Modeling22 references4 citations

TL;DR

This paper proposes a non-directional beamforming codebook that jointly optimizes amplitudes and quantized phases to mitigate hand blockage in millimeter wave systems. By estimating and compensating for phase distortions induced by the hand, the codebook coherently combines signals across antenna elements, reducing blockage-induced SNR degradation by up to 10 dB in measured scenarios with a 4×1 array at 28 GHz.

ABSTRACT

Hand blockage leads to significant performance impairments at millimeter wave carrier frequencies. A number of prior works have characterized the loss in signal strength with the hand using studies with horn antennas and form-factor user equipments (UEs). However, the impact of the hand on the effective phase response seen by the antenna elements has not been studied so far. Towards this goal, we consider a measurement framework that uses a hand phantom holding the UE in relaxed positions reflective of talk mode, watching videos, and playing games. We first study the impact of blockage on a directional beam steering codebook. The tight phase relationship across antenna elements needed to steer beams leads to a significant performance degradation as the hand surface can distort the observed amplitudes and phases across the antenna elements, which cannot be matched by this codebook. To overcome this loss, we propose a non-directional beamforming codebook made of amplitudes and/or quantized phases with both these quantities estimated as necessary. Theoretical as well as numerical studies show that the proposed codebook can de-randomize the phase distortions induced by the hand and coherently combine the energy across antenna elements and thus help in mitigating hand blockage losses.

Motivation & Objective

• Address the lack of understanding in how hand blockage distorts phase responses across mmWave antenna elements, beyond just amplitude loss.
• Overcome the limitations of conventional directional beamforming codebooks, which fail under hand blockage due to rigid phase relationships.
• Develop a practical beamforming strategy that adapts to hand-induced phase distortions without requiring full CSI feedback.
• Demonstrate that coherently combining energy across elements using estimated amplitudes and phases can significantly reduce blockage-related SNR degradation.

Proposed method

• Design a non-directional beamforming codebook that independently optimizes amplitudes and quantized phases based on measured channel state information.
• Use a measurement framework with a hand phantom in relaxed user positions (talk, video, gaming) to capture realistic phase and amplitude distortions.
• Formulate the received SNR as a function of element-wise amplitudes and phases, incorporating both in-phase and quadrature components to model coherent combining.
• Derive theoretical bounds on SNR improvement by analyzing the impact of phase quantization (B bits) and amplitude compensation on constructive interference.
• Introduce a phase compensation term that de-randomizes hand-induced phase distortions, enabling coherent combining even under blockage.
• Validate the approach using both theoretical analysis and numerical simulations based on measured data from a 4×1 dual-polarized patch array at 28 GHz.

Experimental results

Research questions

• RQ1 How does hand blockage affect the phase response across mmWave antenna elements, and why do conventional directional codebooks fail under such conditions?
• RQ2 Can a non-directional beamforming codebook that jointly optimizes amplitudes and quantized phases outperform directional codebooks under hand blockage?
• RQ3 What is the theoretical SNR improvement achievable by compensating for hand-induced phase distortions through adaptive beamforming?
• RQ4 How does the number of phase quantization bits (B) affect the performance gain in mitigating blockage losses?
• RQ5 To what extent can coherent combining of signals across elements reduce blockage-induced SNR degradation in realistic user scenarios?

Key findings

• The proposed non-directional beamforming codebook achieves up to 10 dB of SNR improvement over conventional directional codebooks under hand blockage in measured 28 GHz scenarios.
• Phase distortion induced by the hand significantly degrades performance in directional codebooks due to mismatched phase relationships across elements.
• Joint optimization of amplitudes and quantized phases enables coherent combining of signals, effectively de-randomizing phase distortions caused by the hand.
• Theoretical analysis shows that SNR gain scales with the number of phase quantization bits (B), with gains approaching theoretical limits as B increases.
• Numerical results confirm that the proposed method maintains high performance even with limited feedback, making it suitable for practical mmWave systems.
• The method is robust across different user postures (talk, video, gaming), demonstrating consistent performance improvement under relaxed hand positions.