Head-Mounted Mode Usage

Description

Head-mounted mode usage is a 3GPP term describing an operating and test mode for wearable devices that are worn on the head. The definition states that this mode corresponds to a wearable device for head-mounted mode, and that the device — for example XR glasses or a head-mounted display (HMD) — is tested via specific head phantoms for head-mounted devices. It is therefore both a usage classification and a test methodology: it identifies a class of devices by how they are worn, and it prescribes the phantom-based setup used to evaluate them.

Architecturally, this concept sits in the domain of UE RF performance, radiated testing and electromagnetic-field (EMF) assessment rather than in core network or protocol architecture. Wearable devices present a challenge for traditional testing because their RF behaviour is strongly influenced by the presence and proximity of the human body, and in particular the head. A device worn on the head has its antennas close to the head, and the head both absorbs and scatters radio energy, altering radiation patterns, efficiency and specific absorption rate (SAR). Testing such a device in free space would not represent real usage; testing it with a generic body phantom would also be inaccurate because the geometry and dielectric properties of the head differ from those of the torso.

How it works: 3GPP defines specific head phantoms for head-mounted devices. These phantoms model the size, shape and electromagnetic properties of a human head (and, depending on the setup, may include relevant tissue-equivalent materials) so that measurements of radiated performance — such as total radiated power (TRP), total isotropic sensitivity (TIS), or EMF exposure metrics — reflect realistic on-head conditions. The device under test is positioned on or near the phantom in a defined orientation, and the measurement is performed in an anechoic or reverberation chamber using over-the-air (OTA) methods. Because the head phantom is standardised, results are repeatable and comparable across vendors and test labs.

The term is closely tied to the broader trend of OTA testing for UEs with integrated antennas, and to the specific challenges of extended reality (XR) devices. XR glasses and HMDs are typically small, have multiple antennas for connectivity and sensing, and are worn in direct contact with or very close to the head. Their RF performance and exposure characteristics therefore depend heavily on head proximity. By defining a head-mounted mode and associated phantoms, 3GPP enables consistent evaluation of these devices for both performance (coverage, throughput, link budget) and compliance (SAR/EMF limits).

In Rel-19, specifications 38.161 and 38.870 address UE radio transmission and reception for such devices and the associated study items. The definition of head-mounted mode usage provides the vocabulary that links device categories to test setups. For engineers, the practical implications are: when testing a head-worn wearable, use the specified head phantom and positioning; report results in the context of that mode; and recognise that free-space or body-phantom results are not substitutes. The concept also informs device design, because antenna placement, power control and thermal management must account for head loading and exposure constraints. As XR and wearable use cases grow, this mode is expected to remain a foundational element of 3GPP's radiated testing framework.

Purpose & Motivation

Head-mounted mode usage exists because wearable devices worn on the head cannot be accurately evaluated using conventional UE test methods. Traditional handset testing assumes a device held in the hand or placed near the head in a specific talk position; body-worn devices assume torso proximity. Head-mounted devices such as XR glasses and HMDs have a different physical relationship with the user: the antennas are integrated into a frame or visor that sits on the face and head, often with multiple radios and sensors. The head's presence materially changes radiation patterns, efficiency and exposure, so a dedicated mode and phantom are needed to make measurements meaningful and comparable.

The motivation is twofold. First, performance: operators and vendors need to know how well a head-worn device will actually perform in the field, including how much power reaches the network and how sensitive the receiver is in the presence of head loading. Second, compliance: regulators require assessment of RF exposure (SAR and, increasingly, power density) under realistic conditions. A standardised head phantom and test mode allow both performance and exposure to be assessed consistently, supporting type approval and market access.

Historically, 3GPP's radiated testing work evolved from simple free-space and hand/head talk-position measurements towards more diverse device form factors, including wearables. The emergence of XR as a major use case in Rel-18/Rel-19 accelerated the need for head-mounted-specific methods. By defining head-mounted mode usage and the associated phantoms, 3GPP addresses a gap in previous approaches, which either ignored head-worn devices or approximated them poorly. The result is a framework that supports innovation in wearables while maintaining rigorous, repeatable assessment of RF performance and exposure.