27 research outputs found
A Dynamic Grouping Strategy for Beyond Diagonal Reconfigurable Intelligent Surfaces with Hybrid Transmitting and Reflecting Mode
Beyond diagonal reconfigurable intelligent surface (BD-RIS) is a novel branch
of RIS which breaks through the limitation of traditional RIS with diagonal
scattering matrices. However, the existing research focuses on BD-RIS with
fixed architectures regardless of channel state information (CSI), which limit
the achievable performance of BD-RIS. To solve this issue, in this paper, we
propose a novel dynamically group-connected BD-RIS based on a dynamic grouping
strategy. Specifically, RIS antennas are dynamically divided into several
subsets adapting to the CSI, yielding a permuted block-diagonal scattering
matrix. To verify the effectiveness of the proposed dynamically group-connected
BD-RIS, we propose an efficient algorithm to optimize the BD-RIS with dynamic
grouping for a BD-RIS-assisted multi-user multiple-input single-output system.
Simulation results show that the proposed dynamically group-connected
architecture outperforms fixed group-connected architectures.Comment: 6 pages, 6 figures, accepted by IEEE Trans. Veh. Techno
Discrete-Value Group and Fully Connected Architectures for Beyond Diagonal Reconfigurable Intelligent Surfaces
Reconfigurable intelligent surfaces (RISs) allow controlling the propagation
environment in wireless networks through reconfigurable elements. Recently,
beyond diagonal RISs (BD-RISs) have been proposed as novel RIS architectures
whose scattering matrix is not limited to being diagonal. However, BDRISs have
been studied assuming continuous-value scattering matrices, which are hard to
implement in practice. In this paper, we address this problem by proposing two
solutions to realize discrete-value group and fully connected RISs. First, we
propose scalar-discrete RISs, in which each entry of the RIS impedance matrix
is independently discretized. Second, we propose vector-discrete RISs, where
the entries in each group of the RIS impedance matrix are jointly discretized.
In both solutions, the codebook is designed offline such as to minimize the
distortion caused in the RIS impedance matrix by the discretization operation.
Numerical results show that vector-discrete RISs achieve higher performance
than scalar discrete RISs at the cost of increased optimization complexity.
Furthermore, fewer resolution bits per impedance are necessary to achieve the
performance upper bound as the group size of the group connected architecture
increases. In particular, only a single resolution bit is sufficient in fully
connected RISs to approximately achieve the performance upper bound.Comment: Accepted by IEEE for publicatio
Beyond Diagonal Reconfigurable Intelligent Surfaces: A Multi-Sector Mode Enabling Highly Directional Full-Space Wireless Coverage
Reconfigurable intelligent surface (RIS) has gained much traction due to its
potential to manipulate the propagation environment via nearly-passive
reconfigurable elements. In our previous work, we have analyzed and proposed a
beyond diagonal RIS (BD-RIS) model, which is not limited to traditional
diagonal phase shift matrices, to unify different RIS modes/architectures. In
this paper, we create a new branch of BD-RIS supporting a multi-sector mode. A
multi-sector BD-RIS is modeled as multiple antennas connected to a multi-port
group-connected reconfigurable impedance network. More specifically, antennas
are divided into () sectors and arranged as a polygon prism with
each sector covering space. Different from the recently introduced
concept of intelligent omni-surface (or simultaneously transmitting and
reflecting RIS), the multi-sector BD-RIS not only achieves a full-space
coverage, but also has significant performance gains thanks to the highly
directional beam of each sector.We derive the constraint of the multi-sector
BD-RIS and the corresponding channel model taking into account the relationship
between antenna beamwidth and gain. With the proposed model, we first derive
the scaling law of the received signal power for a multi-sector BD-RIS-assisted
single-user system. We then propose efficient beamforming design algorithms to
maximize the sum-rate of the multi-sector BD-RIS-assisted multiuser system.
Simulation results verify the effectiveness of the proposed design and
demonstrate the performance enhancement of the proposed multi-sector BD-RIS.Comment: 14 pages, 10 figures, submitted to IEEE journa
Reconfigurable Intelligent Surfaces 2.0: Beyond Diagonal Phase Shift Matrices
Reconfigurable intelligent surface (RIS) has been envisioned as a promising
technique to enable and enhance future wireless communications due to its
potential to engineer the wireless channels in a cost-effective manner.
Extensive research attention has been drawn to the use of conventional RIS 1.0
with diagonal phase shift matrices, where each RIS element is connected to its
own load to ground but not connected to other elements. However, the simple
architecture of RIS 1.0 limits its flexibility of manipulating passive
beamforming. To fully exploit the benefits of RIS, in this paper, we introduce
RIS 2.0 beyond diagonal phase shift matrices, namely beyond diagonal RIS
(BD-RIS). We first explain the modeling of BD-RIS based on the scattering
parameter network analysis and classify BD-RIS by the mathematical
characteristics of the scattering matrix, supported modes, and architectures.
Then, we provide simulations to evaluate the sum-rate performance with
different modes/architectures of BD-RIS. We summarize the benefits of BD-RIS in
providing high flexibility in wave manipulation, enlarging coverage,
facilitating the deployment, and requiring low complexity in resolution bit and
element numbers. Inspired by the benefits of BD-RIS, we also discuss potential
applications of BD-RIS in various wireless systems. Finally, we list key
challenges in modeling, designing, and implementing BD-RIS in practice and
point to possible future research directions for BD-RIS.Comment: 7 pages, 5 figures, submitted to IEEE journal for possible
publicatio
A Universal Framework for Multiport Network Analysis of Reconfigurable Intelligent Surfaces
Reconfigurable intelligent surface (RIS) is an emerging paradigm able to
control the propagation environment in wireless systems. Most of the research
on RIS has been dedicated to system-level optimization and, with the advent of
beyond diagonal RIS (BD-RIS), to RIS architecture design. However, developing
general and unified electromagnetic (EM)-compliant models for RIS-aided systems
remains an open problem. In this study, we propose a universal framework for
the multiport network analysis of RIS-aided systems. With our framework, we
model RIS-aided systems and RIS architectures through impedance, admittance,
and scattering parameter analysis. Based on these analyses, three equivalent
models are derived accounting for the effects of impedance mismatching and
mutual coupling. The three models are then simplified by assuming large
transmission distances, perfect matching, and no mutual coupling to understand
the role of the RIS in the communication model. The derived simplified models
are consistent with the model used in related literature, although we show that
an additional approximation is commonly considered in the literature. We
discuss the benefits of each analysis in characterizing and optimizing the RIS
and how to select the most suitable parameters according to the needs.
Numerical results provide additional evidence of the equivalence of the three
analyses.Comment: Submitted to IEEE for publicatio