# Inverse Multipath Fingerprinting for Millimeter Wave V2I Beam Alignment

## TL;DR

This paper introduces an inverse multipath fingerprinting method for millimeter wave vehicle-to-infrastructure beam alignment, leveraging vehicle position data to improve alignment speed and accuracy in dynamic vehicular environments.

## Contribution

It proposes a novel position-aided beam alignment approach using multipath fingerprinting, with optimized candidate selection and comprehensive evaluation in realistic channels.

## Key findings

- The method improves beam alignment efficiency in high-mobility scenarios.
- Optimized candidate beam selection reduces misalignment probability.
- The approach outperforms IEEE 802.11ad in beam alignment overhead and data rates.

## Abstract

Efficient beam alignment is a crucial component in millimeter wave systems with analog beamforming, especially in fast-changing vehicular settings. This paper proposes a position-aided approach where the vehicle's position (e.g., available via GPS) is used to query the multipath fingerprint database, which provides prior knowledge of potential pointing directions for reliable beam alignment. The approach is the inverse of fingerprinting localization, where the measured multipath signature is compared to the fingerprint database to retrieve the most likely position. The power loss probability is introduced as a metric to quantify misalignment accuracy and is used for optimizing candidate beam selection. Two candidate beam selection methods are developed, where one is a heuristic while the other minimizes the misalignment probability. The proposed beam alignment is evaluated using realistic channels generated from a commercial ray-tracing simulator. Using the generated channels, an extensive investigation is provided, which includes the required measurement sample size to build an effective fingerprint, the impact of measurement noise, the sensitivity to changes in traffic density, and beam alignment overhead comparison with IEEE 802.11ad as the baseline. Using the concept of beam coherence time, which is the duration between two consecutive beam alignments, and parameters of IEEE 802.11ad, the overhead is compared in the mobility context. The results show that while the proposed approach provides increasing rates with larger antenna arrays, IEEE 802.11ad has decreasing rates due to the larger beam training overhead that eats up a large portion of the beam coherence time, which becomes shorter with increasing mobility.