Movable-Element STAR-RIS for 6G Non-Terrestrial Networks: Architectures, Enabling Technologies, and Open Challenges

Abstract

Non-terrestrial networks (NTNs) are emerging as a key component of 6G systems by extending ubiquitous and resilient connectivity across satellite, aerial, and terrestrial layers. However, severe propagation loss, rapidly varying geometry, Doppler, blockage, and stringent payload and energy constraints make efficient beam and propagation control particularly challenging. Movable-element STAR-RIS (ME-STAR-RIS) offers a new approach by combining full-space transmission/reflection control with reconfigurable surface geometry, thereby introducing an additional spatial degree of freedom for geometry-aware NTN operation. In this article, we first describe the operating principle, movement constraints, and potential hardware realizations of ME-STAR-RIS, and then present representative satellite-borne, aerial-relay, and cross-tier NTN architectures. We discuss the key enabling mechanisms, including joint geometry--electromagnetic optimization, mobility-aware channel acquisition, multi-timescale control, and predictive configuration. A LEO-centered numerical case study compares ME-STAR-RIS with an otherwise identical fixed STAR-RIS and demonstrates the benefit of element mobility across different transmit powers, movement ranges, and surface sizes, while revealing diminishing returns from increasingly large displacements. Finally, we discuss the main hardware, CSI, scalability, near-field, sensing, security, reliability, and standardization challenges, and highlight promising directions toward practical ME-STAR-RIS-enabled NTNs