Co-opting Mobile Device Cellular RF Front Ends for HCI Sensing

Modern mobile devices, most notably smartphones (but also some smartwatches, tablets, and laptops), contain sophisticated radio frequency front-ends (RFFE) for cellular communication. These enable reliable and power-efficient wireless connectivity, automatically compensating for factors such as environmental loading, user interaction, and device orientation. Unlike early mobile radios that relied on static antenna-matching architectures, contemporary modems continuously monitor transmitted and reflected RF power and actively reconfigure antenna behavior using integrated impedance and aperture tuners.

This dynamic tuning capability implicitly enables sensing of antenna behavior, including changes induced by proximate external electromagnetic loading. By operating across multiple cellular frequency bands, the modem can further probe antenna response as a function of frequency. Conceptually, this process mirrors the operation of a vector network analyzer (VNA), which characterizes RF components by sweeping frequencies and measuring reflected and transmitted signals, particularly antenna reflection (i.e., S11). While smartphone RFFE subsystems are designed for communication rather than precision RF instrumentation, their ability to programmatically observe frequency-dependent antenna performance provides a VNA-like sensing capability already embedded within commodity consumer devices.

Simultaneously, prior research in Human-Computer Interaction (HCI) has demonstrated that RF measurements obtained from VNAs can serve as a powerful sensing modality. These works established that antenna impedance measurements are sensitive to human hand pose, face pose, environmental interaction and nearby widgets. However, these approaches have relied on specialized laboratory instrumentation and custom RF hardware, limiting their scalability and deployment.

In this paper, we present the first demonstration that RF sensing for HCI applications can be realized using the existing RFFE hardware already embedded in mobile devices. Unlocking RF sensing on widely deployed consumer devices significantly expands the accessibility and scalability of RF-driven HCI interaction techniques. This insight not only offers a pathway for prior VNA-based interaction paradigms to move beyond laboratory prototypes, but also introduces new opportunities for continuous, real-world, and context-aware sensing scenarios that leverage the pervasive presence of mobile devices. By reframing communication subsystems as sensing infrastructure, our work demonstrates how latent RF reconfigurability in commodity hardware can serve as a new avenue for ubiquitous interactive sensing.

To demonstrate the feasibility and generality of our idea, we re-implement five use cases previously motivated and demonstrated in the HCI literature, using an off-the-shelf smartphone. We then use these functional implementations to evaluate sensing performance, which we can directly compare with prior work implementing similar functionality. Our experiments show that commonplace mobile hardware can achieve sensing accuracy comparable to prior work, while operating from a standard device rather than hardware tailored to individual use cases.

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Kim, D., and Harrison, C. 2026. Co-opting Mobile Device Cellular Antennas for HCI Sensing. In Proceedings of the 39th Annual ACM Symposium on User Interface Software and Technology (November 2 – 5, 2026). UIST '26. ACM, New York, NY.

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Co-opting Mobile Device Cellular RF Front Ends for HCI Sensing Modern mobile devices, most notably smartphones (but also some smartwatches, tablets, and laptops), contain sophisticated radio frequency front-ends (RFFE) for cellular communication. These enable reliable and power-efficient wireless connectivity, automatically compensating for factors such as environmental loading, user interaction, and device orientation. Unlike early mobile radios that relied on static antenna-matching architectures, contemporary modems continuously monitor transmitted and reflected RF power and actively reconfigure antenna behavior using integrated impedance and aperture tuners. This dynamic tuning capability implicitly enables sensing of antenna behavior, including changes induced by proximate external electromagnetic loading. By operating across multiple cellular frequency bands, the modem can further probe antenna response as a function of frequency. Conceptually, this process mirrors the operation of a vector network analyzer (VNA), which characterizes RF components by sweeping frequencies and measuring reflected and transmitted signals, particularly antenna reflection (i.e., S11). While smartphone RFFE subsystems are designed for communication rather than precision RF instrumentation, their ability to programmatically observe frequency-dependent antenna performance provides a VNA-like sensing capability already embedded within commodity consumer devices. Simultaneously, prior research in Human-Computer Interaction (HCI) has demonstrated that RF measurements obtained from VNAs can serve as a powerful sensing modality. These works established that antenna impedance measurements are sensitive to human hand pose, face pose, environmental interaction and nearby widgets. However, these approaches have relied on specialized laboratory instrumentation and custom RF hardware, limiting their scalability and deployment. In this paper, we present the first demonstration that RF sensing for HCI applications can be realized using the existing RFFE hardware already embedded in mobile devices. Unlocking RF sensing on widely deployed consumer devices significantly expands the accessibility and scalability of RF-driven HCI interaction techniques. This insight not only offers a pathway for prior VNA-based interaction paradigms to move beyond laboratory prototypes, but also introduces new opportunities for continuous, real-world, and context-aware sensing scenarios that leverage the pervasive presence of mobile devices. By reframing communication subsystems as sensing infrastructure, our work demonstrates how latent RF reconfigurability in commodity hardware can serve as a new avenue for ubiquitous interactive sensing. To demonstrate the feasibility and generality of our idea, we re-implement five use cases previously motivated and demonstrated in the HCI literature, using an off-the-shelf smartphone. We then use these functional implementations to evaluate sensing performance, which we can directly compare with prior work implementing similar functionality. Our experiments show that commonplace mobile hardware can achieve sensing accuracy comparable to prior work, while operating from a standard device rather than hardware tailored to individual use cases. Download Paper PDF Reference Kim, D., and Harrison, C. 2026. Co-opting Mobile Device Cellular Antennas for HCI Sensing. In Proceedings of the 39th Annual ACM Symposium on User Interface Software and Technology (November 2 – 5, 2026). UIST '26. ACM, New York, NY.
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