[Paper Review] Multipath-assisted Radio Sensing and Occupancy Detection for Smart In-house Parking in ITS

Jonas Ninnemann, Paul Schwarzbach+1 more|arXiv (Cornell University)|Jan 16, 2022

Smart Parking Systems Research4 citations

TL;DR

This paper proposes a passive occupancy detection system for in-house parking using multipath information from Ultra-wideband (UWB) Channel Impulse Responses (CIRs). By applying exponentially weighted moving average filtering and background subtraction to detect time-variant CIR changes, the method identifies parked vehicles without active transponders, achieving reliable occupancy detection in controlled scenarios with a 25 m path length recognition range.

ABSTRACT

Joint, radio-based communication, localization and sensing is a rapidly emerging research field with various application potentials. Greatly benefiting from these capabilities, smart city, mobility, and logistic concepts are key components for maximizing the efficiency of modern transportation systems. In urban environments, both the search for parking space and freight transport are time- and space-consuming and present the bottlenecks for these transportation chains. Providing location information for these heterogeneous requirement profiles (both active and passive localization of objects), can be realized by using retrofittable wireless sensor networks, which are typically only deployed for active localization. An additional passive detection of objects can be achieved by assessing signal reflections and multipath properties of the transmission channel stored within the Channel Impulse Response (CIR). In this work, a proof-of-concept realization and preliminary experimental results of a CIR-based occupancy detection for parking lots are presented. As the time resolution is dependent on available bandwidth, the CIR of Ultra-wideband transceivers are used. For this, the CIR is smoothed and time-variant changes within it are detected by performing a background subtraction. Finally, the reflecting objects are mapped to individual parking lots. The developed method is tested in an in-house parking garage. The work provided is a foundation for passive occupancy detection, whose capabilities can prospectively be enhanced by exploiting additional physical layers, such as 5G or even 6G.

Motivation & Objective

• To enable passive occupancy detection in in-house parking garages without requiring active transponders on vehicles or freight trailers.
• To leverage existing UWB infrastructure for joint communication, localization, and sensing to reduce deployment costs and increase system efficiency.
• To develop a CIR-based method that detects environmental changes due to parked objects using multipath propagation characteristics.
• To validate the feasibility of passive occupancy detection in real-world parking scenarios using UWB signal analysis.
• To explore future integration with 5G/6G and Li-Fi physical layers for enhanced performance and broader application.

Proposed method

• Utilizes UWB transceivers to capture Channel Impulse Responses (CIRs) with a time resolution of 0.3 m, enabling detection of multipath reflections.
• Applies an exponentially weighted moving average (EWMA) filter to smooth the CIR and extract a stable background reference.
• Performs background subtraction between the current CIR and the smoothed reference to detect time-variant changes indicating object presence.
• Maps detected reflections to spatial positions using an elliptical model and constructs an interpolated heatmap for visualization and occupancy mapping.
• Employs a multi-sensor setup along walls to enable coverage of multiple parking lots and improve detection robustness.
• Uses path length estimation from CIR peaks to infer object positions and validate detection accuracy.

Experimental results

Research questions

• RQ1 Can multipath components in UWB CIRs be reliably used to detect passive objects such as parked vehicles or freight trailers in indoor parking environments?
• RQ2 How effective is background subtraction of smoothed CIRs in identifying occupancy changes without active transponders?
• RQ3 What are the limitations of current UWB bandwidth in resolving closely spaced multipath reflections from nearby objects?
• RQ4 To what extent can the proposed method detect occupancy across multiple parking spaces using a single UWB network?
• RQ5 How can the method be extended to support multi-object detection and integration with future physical layers like 5G or Li-Fi?

Key findings

• The method successfully detected parked vehicles by identifying subtle CIR changes due to multipath reflections, even with a 0.15 m path difference between direct and reflected paths.
• A recognition range of approximately 25 m was achieved, demonstrating the feasibility of detecting reflections from objects at moderate distances in indoor environments.
• The system identified that bandwidth limitations prevent resolution of closely spaced multipath components, such as when the path difference is below 0.3 m.
• Shadowing effects were observed, where reflections blocked by vehicles could not be detected in the presence of larger objects, similar to occlusion in camera-based systems.
• Field tests confirmed the proof-of-concept for passive occupancy detection, though detection reliability was limited to specific geometric configurations and signal conditions.
• The approach enables multi-use deployment of UWB sensors for communication, active localization, and passive sensing, improving cost-efficiency compared to dedicated occupancy systems.