Leveraging the Properties of mmWave Signals for 3D Finger Motion Tracking for Interactive IoT Applications
Authors: Liu Yilin , Zhang Shijia , Gowda Mahanth , Nelakuditi Srihari 
Published: June 2023

Wireless signals, which are mainly used for communication networks, also have the potential to extend our senses, enabling us to see behind closed doors and track moving objects through walls [3, 10]. Accordingly, there is a growing interest in the community recently to develop novel IoT applications for sensing by exploiting radio frequency signals [7]. Given the compact size of modern wireless devices, this enables ubiquitous applications in the areas of smart healthcare, sports analytics, AR/VR etc. Specifically, as these signals travel in the medium, they traverse occlusions and bounce off different objects before arriving at a receiver; hence, the reflected signals carry information about the environment. By exploiting this property, this paper shows the feasibility of tracking precise 3D finger motion using mmWave signals that are popularly used in 5G networks.
Motivation and Application: This paper presents mm4Arm, a system that quantifies the performance of finger motion tracking for interactive applications using mmWave signals through a carefully designed simulation and measurement study. We considered using mmWave signal because FMCW-based radars are being used for ubiquitous applications in the areas of smart healthcare [4], sports analytics, AR/VR [17], autonomous driving [5], etc. Similar to the popular Google Soli platform [15], our main motivation is to enable wearable, mobile computing, and AR/VR applications where conventional touch interaction may be hard. Finger motion-based interfaces over the air are known to be a popular form of human-computer interaction [9, 14]. In contrast to Soli, which can only detect 11 predefined gestures, mm4Arm can perform arbitrary 3D motion tracking, thus allowing highly precise control. Decades of prior research have shown that such a finer control can enable rapid and fluid manipulation for highly intuitive interaction [16]. Therefore, regardless of the application, we focus on enabling the core motion tracking framework by solving the underlying challenges.
Tracking Fingers by Observing the ForeArm: In this paper, we not only focus on tracking the 3D finger motion using mmWave reflections, but based on observations via simulations and measurements, we also identify the underlying conditions that enable precise tracking. A critical observation is that the small size of fingers does not provide stable reflections to the level required for tracking. However, the data-driven analysis reveals that it is possible to indirectly track fingers by measuring reflections from the forearm. Finger motion activation involves neuro-muscular interactions, which induce minute muscular motions in the forearm. Such muscular motion produces vibrations in the forearm. Thanks to the short wavelength of mmWave signals, the phase measurements are extremely sensitive to small vibrations (up to 0.63 μm), thus opening up opportunities for precise motion tracking. Moreover, the forearm offers a rich texture and curvature and a much bigger surface for reflections, in contrast to the small size of fingers, which facilitates robust tracking. mm4Arm analyzes such forearm vibrations for 3D finger motion tracking.
We reiterate two critical observations made in this paper: (i) When 3D finger motion tracking is of interest in contrast to predefined gesture classification, the reflections obtained directly from fingers do not provide sufficient information. Very few reflections come back to the radar due to the small size of fingers and dominant specular reflections. A similar observation on specularity has been made earlier in the context of autonomous cars [6, 13]. (ii) Vibrations in the forearm during finger motion can capture rich information. Because of the large surface of the forearm and its curvature, the reflections are more stable and robust to natural variation in arm position, height, and orientation. This can be leveraged for 3D finger motion tracking.