IEEE ComSoc RCC Special Interest Group (SIG) on
Beyond Diagonal
Reconfigurable Intelligent Surface (BD-RIS)

About the SIG

Reconfigurable intelligent surface (RIS) is expected to be a key technology for 6G to efficiently and effectively enhance wireless communication performance. RIS consists of a large number of reconfigurable scattering elements, each with the capability to manipulate the phase of an incident electromagnetic wave, so that the phase shifts of these elements can be coordinated to direct the scattered electromagnetic signals toward the intended receivers.

The literature has so far developed and focused on diagonal RIS (D-RIS) where each element is independently controlled by a tunable impedance connected to ground and is not connected to other elements. This simple architecture, denoted as single-connected, results in a diagonal scattering matrix, which is also commonly known as the phase shift matrix. However, this simple D-RIS architecture limits its flexibility in manipulating waves and thus limits the achievable performance. D-RIS is the conventional RIS architecture used throughout the entire literature so far.

To fully exploit the benefits of RIS and improve its performance, this SIG focuses on beyond diagonal RIS (BD-RIS). BD-RIS has been introduced as a generalization of D-RIS in which the scattering matrix is not restricted to being diagonal. Consequently, BD-RIS explores a broader range of architectures compared to conventional D-RIS. The key novelty in BD-RIS is the presence of interconnections among the RIS elements, which enhances the flexibility and performance of RIS at the expense of additional circuit complexity. For example, in the so-called fully-connected architecture, all the RIS elements are connected to each other through tunable impedance components.

BD-RIS-aided wireless systems have been modeled through scattering parameter network analysis, confirming the significant performance gain achieved over D-RIS in various scenarios. Specifically, the key benefits of BD-RIS can be summarized as follows.

First, compared with D-RIS which can only manipulate the phase of the incident wave, BD-RIS has higher flexibility in manipulating both the magnitude and phase, which further boosts the performance.
Second, compared with D-RIS which can only support reflective mode, BD-RIS can work on transmissive, hybrid, and multi-sector modes to enable full-space coverage while generating highly directional beams.
Third, BD-RIS with hybrid and multi-sector modes highly facilitates practical deployments benefiting from flexible antenna array arrangements and full-space coverage.
Fourth, when considering RIS with discrete values, BD-RIS is shown to achieve better performance than D-RIS with fewer resolution bits due to the high flexibility of the reconfigurable impedance network.
Fifth, the required number of RIS elements in BD-RIS can be effectively reduced compared to D-RIS, given the same performance requirement.

Thanks to its benefits in flexibly manipulating wave, enlarging coverage, facilitating the deployment, and simplifying the hardware implementation, BD-RIS has the potential to tackle the challenges of modern communication systems and is a gold mine of research problems for academia and industry, spanning fundamental limits, modeling, architecture design, optimization, and implementation.

Scope of the SIG

In this SIG on BD-RIS, we provide a platform to bring together Ph.D. students, researchers, and engineers in academia and industry interested in the lower layers of wireless (communication, sensing, and power) systems and in particular the design of 6G physical layer to share their ideas and discuss the major technical challenges, recent breakthroughs, new applications, open problems and challenges related to BD-RIS. Topics of interest of this SIG include, but are not limited to:

Fundamental limits of BD-RIS-aided wireless networks.
BD-RIS characterization and modeling.
Development of novel BD-RIS architectures with a favorable performance-complexity trade-off.
BD-RIS for multi-user/multi-cell multi-antenna networks and integration with different multiple access schemes.
Cross-layer design, optimization, and performance analysis of BD-RIS.
BD-RIS in B5G services such as enhanced MBB, URLLC, massive MTC, massive IoT, V2X, cellular, wireless power transfer, integrated communications and sensing, and mobile edge computing.
Channel estimation for BD-RIS-aided wireless networks.
Artificial Intelligence-empowered design for BD-RIS-aided wireless networks.
Modeling of radio-frequency (RF) impairments in BD-RIS.
Hardware implementation of BD-RIS.
BD-RIS for RF, millimeter waves, terahertz, and optical.
BD-RIS prototyping and experimentation.
Applications of BD-RIS in various verticals.

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