This paper investigates the design of integrated sensing and communication (ISAC) systems assisted by simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs), which act as multi-functional programmable metasurfaces capable of supporting concurrent communication and sensing within a unified architecture. We propose a two-stage ISAC protocol, in which the preparation phase performs direction estimation for outdoor users located in the reflection space, while maintaining communication with both outdoor and indoor users in the transmission space. The subsequent communication phase exploits the estimated directions to enhance information transfer. The directions of outdoor users are modeled as Gaussian random variables to capture estimation uncertainty, and the corresponding average communication performance is incorporated into the design. Building on this framework, we formulate a performance-balanced optimization problem that maximizes the communication sum-rate while guaranteeing the required sensing accuracy, jointly determining the beamforming vectors at the base station (BS), the STAR-RIS transmission and reflection coefficients, and the metasurface partition between energy-splitting and transmit-only modes. The physical constraints of STAR-RIS elements and the required sensing performance are explicitly enforced. To address the non-convex nature of the problem, we combine fractional programming, Lagrangian dual reformulation, and successive convex approximation. The binary metasurface partition is ultimately recovered via continuous relaxation followed by projection-based binarization. Numerical results demonstrate that the proposed design achieves an effective trade-off between sensing accuracy and communication throughput, by significantly outperforming conventional STAR-RIS-aided ISAC schemes.
M ETASURFACES are artificial electromagnetic (EM) structures composed of subwavelength elements ("meta-atoms") possibly integrated with tunable microelectronic components, such as diodes and varactors, which enable programmable control over the amplitude, phase, and polarization of incident waves [1]. When empowered with reconfigurability, these structures give rise to reconfigurable intelligent surfaces (RISs), which have emerged as a key enabler for next-generation wireless networks due to their ability to reshape the propagation environment in a software-defined manner [2]- [4]. By dynamically manipulating the reflected or transmitted wavefronts, RISs allow the establishment of favorable communication links, mitigation of blockages, and enhancement of coverage in complex propagation scenarios [5]- [7]. Owing to their low power consumption, compact form factor, and deployment flexibility, RISs are particularly attractive for integration into urban infrastructures, vehicles, and buildings, thus supporting the vision of environment-aware and user-centric wireless networks [8]- [11].
Despite their advantages, conventional RIS architectures typically rely on reflective elements, which inherently limit their angular coverage to a single half-space. This constraint restricts their applicability in scenarios where users are distributed on both sides of the surface. To overcome this limitation, simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) have been recently proposed [12]- [14]. By leveraging advanced meta-atom designs and interlayer structures, STAR-RISs are capable of concurrently reflecting and transmitting incident signals, thereby enabling full-space EM control [15], [16]. This unique capability makes STAR-RISs a promising solution for supporting heterogeneous users located in distinct spatial regions, such as indoor and outdoor environments separated by a building facade.
STAR-RISs can operate under different working modes, including energy splitting (ES), mode switching (MS), time division (TD), polarization division (PD), and frequency division (FD) [17], [18]. Among these, the ES mode has received the most attention in the literature due to its ability to simultaneously support transmission and reflection while respecting energy conservation principles [19]- [25]. In this mode, the transmitted and reflected signals are intrinsically coupled through both amplitude and phase constraints. Other modes, such as MS and TD [26], can be interpreted as special cases or extensions of the ES mode with additional constraints, while PD and FD impose stringent requirements on signal polarization or frequency selectivity [27], [28], and have therefore received limited attention in the literature.
In parallel with the development of programmable metasurfaces, integrated sensing and communication (ISAC) has emerged as a fundamental paradigm for sixth-generation (6G) wireless networks [29], [30]. ISAC aims to unify sensing and communication functionalities within a single system, enabling spectrum-and hardware-efficient operation. STAR-RISs are particularly well suited for ISAC applications, as their full-space control capability allows simultaneous information delivery and environmental sensing in different spatial regions. Recent works have investigated STAR-RIS-assisted ISAC systems under various assumptions [31]- [34]. However, most existing designs rely on idealized knowledge of user locations or directions and do not explicitly account for the intrinsic uncertainty arising from sensing and estimation processes. Indeed, in practical ISAC deployments, especially in dynamic outdoor environments, user positions or directions are not perfectly known and must be inferred through sensing mechanisms.
Prior studies [35]- [38] have shown that direction-of-arrival (DOA) estimation accuracy in metasurface-assisted systems is fundamentally limited by noise, propagation conditions, and hardware constraints. Moreover, most existing works focus on optimizing beamforming and STAR-RIS coefficients within a single transmission stage. In contrast, the protocol-level design (i.e., the joint optimization of system parameters across multiple phases within a slot) accounting for the distinct roles of sensing-and communication-oriented signaling has received comparatively limited attention.
This paper investigates a STAR-RIS-aided ISAC system deployed on the fac ¸ade of a smart building (see Fig. 1), where a base station (BS) serves both indoor and outdoor users while sensing the directions of outdoor users through dedicated sensors mounted on the STAR-RIS. Beyond conventional broadband services, the proposed STAR-RIS-assisted ISAC framework is particularly well suited to support Internet-of-Things (IoT) ecosystems in smart cities [39]. Its low-power, compact metasurface deployment on building facades enables wide-area, environment-aware connectivity and accurate device localization -suppo
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