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D-STAR: Dual Simultaneously Transmitting and Reflecting Reconfigurable Intelligent Surfaces for Joint Uplink/Downlink Transmission

Li-Hsiang Shen, Po-Chen Wu, Chia-Jou Ku, Yu-Ting Li, Kai-Ten Feng, Yuanwei Liu, Lajos Hanzo

TL;DR

The paper tackles the limited 180-degree coverage of conventional RIS by introducing a dual STAR-RIS (D-STAR) architecture that provides 360-degree coverage for joint uplink/downlink transmission. It develops the DBAP optimization framework, combining active BS beamforming with passive D-STAR amplitude and phase design via a suite of techniques including Lagrangian dual transform, Dinkelbach's method, ADMM with SCA, and PCCP in an alternating optimization loop. The approach yields substantial DL rate gains under UL constraints and outperforms conventional RIS, STAR-RIS, and HDx baselines across various deployment, quantization, and network settings, while offering practical guidance on inter-D-STAR distances and partitioning. Overall, the work enables robust full-plane service for UL/DL with improved spectral efficiency and interference management, suitable for next-generation wireless systems.

Abstract

The joint uplink/downlink (JUD) design of simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RIS) is conceived in support of both uplink (UL) and downlink (DL) users. Furthermore, the dual STAR-RISs (D-STAR) concept is conceived as a promising architecture for 360-degree full-plane service coverage, including UL/DL users located between the base station (BS) and the D-STAR as well as beyond. The corresponding regions are termed as primary (P) and secondary (S) regions. Both BS/users exist in the P-region, but only users are located in the S-region. The primary STAR-RIS (STAR-P) plays an important role in terms of tackling the P-region inter-user interference, the self-interference (SI) from the BS and from the reflective as well as refractive UL users imposed on the DL receiver. By contrast, the secondary STAR-RIS (STAR-S) aims for mitigating the S-region interferences. The non-linear and non-convex rate-maximization problem formulated is solved by alternating optimization amongst the decomposed convex sub-problems of the BS beamformer, and the D-STAR amplitude as well as phase shift configurations. We also propose a D-STAR based active beamforming and passive STAR-RIS amplitude/phase (DBAP) optimization scheme to solve the respective sub-problems by Lagrange dual with Dinkelbach's transformation, alternating direction method of multipliers (ADMM) with successive convex approximation (SCA), and penalty convex-concave procedure (PCCP). Our simulation results reveal that the proposed D-STAR architecture outperforms the conventional single RIS, single STAR-RIS, and half-duplex networks. The proposed DBAP of D-STAR outperforms the state-of-the-art solutions found in the open literature for different numbers of quantization levels, geographic deployment, transmit power and for diverse numbers of transmit antennas, patch partitions as well as D-STAR elements.

D-STAR: Dual Simultaneously Transmitting and Reflecting Reconfigurable Intelligent Surfaces for Joint Uplink/Downlink Transmission

TL;DR

The paper tackles the limited 180-degree coverage of conventional RIS by introducing a dual STAR-RIS (D-STAR) architecture that provides 360-degree coverage for joint uplink/downlink transmission. It develops the DBAP optimization framework, combining active BS beamforming with passive D-STAR amplitude and phase design via a suite of techniques including Lagrangian dual transform, Dinkelbach's method, ADMM with SCA, and PCCP in an alternating optimization loop. The approach yields substantial DL rate gains under UL constraints and outperforms conventional RIS, STAR-RIS, and HDx baselines across various deployment, quantization, and network settings, while offering practical guidance on inter-D-STAR distances and partitioning. Overall, the work enables robust full-plane service for UL/DL with improved spectral efficiency and interference management, suitable for next-generation wireless systems.

Abstract

The joint uplink/downlink (JUD) design of simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RIS) is conceived in support of both uplink (UL) and downlink (DL) users. Furthermore, the dual STAR-RISs (D-STAR) concept is conceived as a promising architecture for 360-degree full-plane service coverage, including UL/DL users located between the base station (BS) and the D-STAR as well as beyond. The corresponding regions are termed as primary (P) and secondary (S) regions. Both BS/users exist in the P-region, but only users are located in the S-region. The primary STAR-RIS (STAR-P) plays an important role in terms of tackling the P-region inter-user interference, the self-interference (SI) from the BS and from the reflective as well as refractive UL users imposed on the DL receiver. By contrast, the secondary STAR-RIS (STAR-S) aims for mitigating the S-region interferences. The non-linear and non-convex rate-maximization problem formulated is solved by alternating optimization amongst the decomposed convex sub-problems of the BS beamformer, and the D-STAR amplitude as well as phase shift configurations. We also propose a D-STAR based active beamforming and passive STAR-RIS amplitude/phase (DBAP) optimization scheme to solve the respective sub-problems by Lagrange dual with Dinkelbach's transformation, alternating direction method of multipliers (ADMM) with successive convex approximation (SCA), and penalty convex-concave procedure (PCCP). Our simulation results reveal that the proposed D-STAR architecture outperforms the conventional single RIS, single STAR-RIS, and half-duplex networks. The proposed DBAP of D-STAR outperforms the state-of-the-art solutions found in the open literature for different numbers of quantization levels, geographic deployment, transmit power and for diverse numbers of transmit antennas, patch partitions as well as D-STAR elements.
Paper Structure (26 sections, 6 theorems, 43 equations, 12 figures, 3 tables, 1 algorithm)

This paper contains 26 sections, 6 theorems, 43 equations, 12 figures, 3 tables, 1 algorithm.

Table of Contents

  1. Introduction
  2. System Model and Problem Formulation
  3. D-STAR Architecture
  4. SINR Model
  5. Problem Formulation
  6. Proposed DBAP Scheme in D-STAR Architecture
  7. Optimization of Active Transmit Beamformer
  8. Optimization of D-STAR
  9. Amplitude of D-STAR
  10. Phase shifts of D-STAR
  11. Convergence Analysis
  12. Numerical Results
  13. Quantization Effects
  14. Deployment of D-STAR
  15. Partitioning D-STAR
  16. ...and 11 more sections

Key Result

Lemma 1

(Modified Lagrangian Dual Transform): The original problem is equivalent to the transformed one associated with the auxiliary variable $\gamma_{u,k}$ for each ratio term in the SINR as where $A_k(\boldsymbol{\Xi})$ and $B_k(\boldsymbol{\Xi})$ represent the nominator and denominator terms of the SINR $\gamma_{u,k}$, respectively. Note that $\boldsymbol{\Xi}=\{{\mathbf{w}}_{\text{PD}},{\mathbf{w}}_

Figures (12)

  • Figure 1: Architectures of STAR-RIS for (a) conventional STAR-RIS and (b) proposed D-STAR.
  • Figure 2: The overall architecture of proposed D-STAR system.
  • Figure 3: Separate architectures of received signal paths for (a) primary DL, (b) secondary DL, (c) primary UL, and (d) secondary UL users.
  • Figure 4: The relative distances of the deployed D-STAR architecture.
  • Figure 5: Convergence of the proposed DBAP scheme both with and without coupled phase shifts.
  • ...and 7 more figures

Theorems & Definitions (11)

  • Lemma 1
  • proof
  • Lemma 2
  • proof
  • Lemma 3
  • Lemma 4
  • proof
  • Lemma 5
  • proof
  • Corollary 1
  • ...and 1 more