Simultaneously Transmitting And Reflecting (STAR) RIS Aided Wireless Communications (2104.01421v2)

Abstract: The novel concept of simultaneously transmitting and reflecting (STAR) reconfigurable intelligent surfaces (RISs) is investigated, where the incident wireless signal is divided into transmitted and reflected signals passing into both sides of the space surrounding the surface, thus facilitating a full-space manipulation of signal propagation. Based on the introduced basic signal model of `STAR', three practical operating protocols for STAR-RISs are proposed, namely energy splitting (ES), mode switching (MS), and time switching (TS). Moreover, a STAR-RIS aided downlink communication system is considered for both unicast and multicast transmission, where a multi-antenna base station (BS) sends information to two users, i.e., one on each side of the STAR-RIS. A power consumption minimization problem for the joint optimization of the active beamforming at the BS and the passive transmission and reflection beamforming at the STAR-RIS is formulated for each of the proposed operating protocols, subject to communication rate constraints of the users. For ES, the resulting highly-coupled non-convex optimization problem is solved by an iterative algorithm, which exploits the penalty method and successive convex approximation. Then, the proposed penalty-based iterative algorithm is extended to solve the mixed-integer non-convex optimization problem for MS. For TS, the optimization problem is decomposed into two subproblems, which can be consecutively solved using state-of-the-art algorithms and convex optimization techniques. Finally, our numerical results reveal that: 1) the TS and ES operating protocols are generally preferable for unicast and multicast transmission, respectively; and 2) the required power consumption for both scenarios is significantly reduced by employing the proposed STAR-RIS instead of conventional reflecting/transmiting-only RISs.

• The paper introduces three STAR-RIS operating protocols—Energy Splitting, Mode Switching, and Time Switching—for full-space signal control and enhanced network coverage.
• It employs penalty-based iterative algorithms with successive convex approximation to jointly optimize active and passive beamforming while minimizing power usage.
• Numerical results demonstrate that STAR-RIS outperforms conventional RIS setups in unicast and multicast transmissions, with scalable benefits as RIS elements increase.

Analyzing STAR-RIS Aided Wireless Communications

The paper under discussion explores the innovative concept of Simultaneously Transmitting and Reflecting (STAR) Reconfigurable Intelligent Surfaces (RISs) and explores their integration into wireless communication systems. This work primarily focuses on how STAR-RIS can manipulate signal propagation in a full-space manner, which contrasts sharply with conventional RIS setups that operate solely on reflections.

Overview of STAR-RIS

STAR-RIS is characterized by its ability to split the incident wireless signal into transmitted and reflected components, enabling communication coverage on both sides of the surface, achieving a full-space communication environment. The paper introduces three operating protocols tailored for STAR-RIS: Energy Splitting (ES), Mode Switching (MS), and Time Switching (TS). Each protocol is crafted to optimize the interaction between the STAR-RIS and the communication network, capitalizing on STAR-RIS’s unique capabilities.

System Architecture and Protocol Design

The authors design a STAR-RIS aided downlink communication system and consider both unicast and multicast transmission cases. The paper involves a multi-antenna base station that communicates with two distinct users. The base station and the users employ active beamforming and passive transmission/reflection beamforming strategies, respectively.

For ES, each RIS element can independently adjust transmission and reflection coefficients, offering a high degree of operational flexibility. This flexibility, however, comes at the computational cost of handling a large number of design variables. MS simplifies the control by switching the RIS elements between pure transmission and reflection states, reducing complexity and potential overhead. TS leverages the time domain, alternately serving users in different time slots, which simplifies control at the expense of time resource efficiency.

Optimization and Algorithms

The core contribution of the paper is the development of optimization frameworks for active and passive beamforming that minimize power consumption while satisfying rate constraints. Due to the non-convex nature of these problems, especially given the coupling between transmission and reflection beamforming, solutions are formulated using penalty-based iterative algorithms with successive convex approximation—a potent technique that locates energy-efficient beamforming configurations.

For ES and MS, the strategies depend on iteratively approaching the objective through penalty transformations that relax certain constraints, ensuring computational feasibility. For TS, the problem decomposes into subproblems that align with state-of-the-art optimization methods.

Numerical Insights and Comparisons

Through extensive numerical simulations, the paper reveals significant reductions in power consumption when utilizing STAR-RIS compared to traditional RIS configurations. Particularly, the results indicate that TS and ES outperform MS in unicast and multicast scenarios, respectively. The paper also highlights that the STAR-RIS solution scales favorable with the number of RIS elements, enhancing the strategic advantages over conventional RIS deployments.

Theoretical and Practical Implications

The research establishes a foundation for enhanced RIS deployment. In practice, the full space coverage of STAR-RIS can be revolutionary for urban connectivity, where obstacles commonly obstruct typical line-of-sight paths. Theoretically, this work extends the paradigm of RIS by introducing a more versatile surface that aligns with the future generation of wireless networks' demands.

Speculation on Future Developments

Moving forward, the introduction of STAR-RIS could shift design preferences towards more flexible network deployments. Furthermore, the necessity for efficient CSI acquisition in RIS-aided systems, especially for simultaneous transmission and reflection scenarios, invites further research. Additionally, the deployment of STAR-RIS in tandem with other technologies like Massive MIMO or mmWave promises novel hybrid systems that leverage the spatial agnosticism of STAR RIS designs.

In conclusion, this paper contributes meaningfully to the evolutionary trajectory of RIS technology, offering a paradigm shift that enhances wireless networks' operational efficiency and spatial reach. It prompts the academic and industrial sectors to consider the broader ramifications and opportunities presented by STAR-RIS in the continuously evolving communication landscapes.