STAR: Simultaneous Transmission And Reflection for 360° Coverage by Intelligent Surfaces (2103.09104v2)

Abstract: A novel simultaneously transmitting and reflecting (STAR) system design relying on reconfigurable intelligent surfaces (RISs) is conceived. First, an existing prototype is reviewed and the potential benefits of STAR-RISs are discussed. Then, the key differences between conventional reflecting-only RISs and STAR-RISs are identified from the perspectives of hardware design, physics principles, and communication system design. Furthermore, the basic signal model of STAR-RISs is introduced, and three practical protocols are proposed for their operation, namely energy splitting, mode switching, and time switching. Based on the proposed protocols, a range of promising application scenarios are put forward for integrating STAR-RISs into next-generation wireless networks. By considering the downlink of a typical RIS-aided multiple-input single-output (MISO) system, numerical case studies are provided for revealing the superiority of STAR-RISs over other baselines, when employing the proposed protocols. Finally, several open research problems are discussed.

• The paper introduces STAR-RIS, which achieves full 360° wireless coverage by enabling both simultaneous transmission and reflection.
• It proposes three protocols—energy splitting, mode switching, and time switching—to optimize signal processing and network performance.
• Numerical comparisons show that STAR-RIS reduces AP transmit power and outperforms traditional RIS in diverse communication scenarios.

Overview of STAR: Simultaneous Transmission And Reflection for 360° Coverage by Intelligent Surfaces

The paper introduces an innovative approach to enhancing wireless communication systems through the deployment of simultaneous transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs). The authors begin by differentiating between STAR-RISs and conventional reflecting-only RISs, examining them through the lenses of hardware design, underlying physics, and communication system architecture. Thereafter, they present a fundamental signal model for STAR-RISs and propose three practical protocols: energy splitting (ES), mode switching (MS), and time switching (TS). Furthermore, they articulate several promising scenarios in which STAR-RISs can be implemented within future wireless networks and conclude with numerical comparisons highlighting the superior performance of STAR-RISs.

The STAR-RIS technology distinguishes itself by facilitating both the transmission and reflection of wireless signals across a complete 360° coverage area. This marks a significant improvement over conventional RISs, which are limited to reflecting signals within a single half-space. The introduction of STAR-RISs permits full-space coverage, allowing for increased flexibility in network deployment and presenting new opportunities for improving wireless communication efficiency, as demonstrated through various scenarios.

Key Contributions and Practical Implications

1. Enhanced Coverage and Flexibility: STAR-RISs provide full-space coverage which allows for seamless wireless communication in both the transmission and reflection spaces. This capability significantly extends the coverage area and provides additional degrees of freedom (DoFs) in signal propagation manipulation.
2. Three Practical Operating Protocols:

The authors propose three practical protocols (ES, MS, and TS) for managing the distinct capabilities of STAR-RISs, each with unique advantages and considerations: - Energy Splitting (ES) offers high flexibility with independent transmission and reflection from each element. - Mode Switching (MS) simplifies hardware implementation with less complexity but sacrifices some performance gain. - Time Switching (TS) allows independent design for transmission and reflection but is complex due to synchronization requirements.

1. Applications in Next-Generation Networks: Several applications of STAR-RISs are discussed for diverse environments, including outdoor-to-indoor communications, non-orthogonal multiple access (NOMA), coordinated multi-point (CoMP) communication, physical layer security, and localization and sensing.
2. Performance Comparison and Numerical Results: Through case studies, STAR-RISs are shown to outperform traditional RIS configurations and omni-surfaces in both unicast and multicast scenarios in terms of required AP transmit power. This is attributed to their additional DoFs and full-space coverage capabilities.

Future Research Directions

The paper identifies several open research areas crucial for the further development of STAR-RISs:

• Spatial Analysis Using Stochastic Geometry: Investigating new models for capturing spatial randomness and orientation dependencies in STAR-RIS-aided networks can advance the performance analysis of large-scale deployments.
• Channel Estimation Challenges: Developing efficient channel estimation techniques remains crucial given the added complexity of simultaneous transmission and reflection paths.
• Deployment Strategies: Optimal placement of STAR-RISs is essential to balance user distribution in transmission and reflection zones, which presents unique challenges in practical deployments.

The research on STAR-RISs signals a paradigm shift in wireless communication systems, providing a basis for designing next-generation networks with enhanced coverage and flexibility. Continued exploration in this field promises to further refine these models and protocols, cementing STAR-RISs as a vital technology in the future landscape of wireless communications.