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Enhancing User Fairness in Wireless Powered Communication Networks with STAR-RIS

Guangyu Zhu, Xidong Mu, Li Guo, Ao Huang, Shibiao Xu

TL;DR

To fully eliminate the doubly-near-far-effect in WPCNs, two STAR-RIS operating protocol-driven transmission strategies, namely energy splitting nonorthogonal multiple access (ES-NOMA) and time switching time division multiple access (TS-TDMA) are proposed.

Abstract

A simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted wireless powered communication network (WPCN) is proposed, where two energy-limited devices first harvest energy from a hybrid access point (HAP) and then use that energy to transmit information back. To fully eliminate the doubly-near-far effect in WPCNs, two STAR-RIS operating protocol-driven transmission strategies, namely energy splitting non-orthogonal multiple access (ES- NOMA) and time switching time division multiple access (TS- TDMA) are proposed. For each strategy, the corresponding optimization problem is formulated to maximize the minimum throughput by jointly optimizing time allocation, user transmit power, active HAP beamforming, and passive STAR-RIS beamforming. For ES-NOMA, the resulting intractable problem is solved via a two-layer algorithm, which exploits the one-dimensional search and block coordinate descent methods in an iterative manner. For TS-TDMA, the optimal active beamforming and passive beamforming are first determined according to the maximum-ratio transmission beamformer. Then, the optimal solution of the time allocation variables is obtained by solving a standard convex problem. Numerical results show that: 1) the STAR-RIS can achieve considerable performance improvements for both strategies compared to the conventional RIS; 2) TS- TDMA is preferred for single-antenna scenarios, whereas ES- NOMA is better suited for multi-antenna scenarios; and 3) the superiority of ES-NOMA over TS-TDMA is enhanced as the number of STAR-RIS elements increases.

Enhancing User Fairness in Wireless Powered Communication Networks with STAR-RIS

TL;DR

To fully eliminate the doubly-near-far-effect in WPCNs, two STAR-RIS operating protocol-driven transmission strategies, namely energy splitting nonorthogonal multiple access (ES-NOMA) and time switching time division multiple access (TS-TDMA) are proposed.

Abstract

A simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted wireless powered communication network (WPCN) is proposed, where two energy-limited devices first harvest energy from a hybrid access point (HAP) and then use that energy to transmit information back. To fully eliminate the doubly-near-far effect in WPCNs, two STAR-RIS operating protocol-driven transmission strategies, namely energy splitting non-orthogonal multiple access (ES- NOMA) and time switching time division multiple access (TS- TDMA) are proposed. For each strategy, the corresponding optimization problem is formulated to maximize the minimum throughput by jointly optimizing time allocation, user transmit power, active HAP beamforming, and passive STAR-RIS beamforming. For ES-NOMA, the resulting intractable problem is solved via a two-layer algorithm, which exploits the one-dimensional search and block coordinate descent methods in an iterative manner. For TS-TDMA, the optimal active beamforming and passive beamforming are first determined according to the maximum-ratio transmission beamformer. Then, the optimal solution of the time allocation variables is obtained by solving a standard convex problem. Numerical results show that: 1) the STAR-RIS can achieve considerable performance improvements for both strategies compared to the conventional RIS; 2) TS- TDMA is preferred for single-antenna scenarios, whereas ES- NOMA is better suited for multi-antenna scenarios; and 3) the superiority of ES-NOMA over TS-TDMA is enhanced as the number of STAR-RIS elements increases.
Paper Structure (22 sections, 1 theorem, 32 equations, 10 figures, 1 table, 2 algorithms)

This paper contains 22 sections, 1 theorem, 32 equations, 10 figures, 1 table, 2 algorithms.

Table of Contents

  1. Introduction
  2. Prior Works
  3. Motivations and Contributions
  4. Organization and Notations
  5. System Model and Problem Formulation
  6. STAR-RIS Protocols and Models
  7. Signal Transmission Models
  8. Problem Formulation
  9. Proposed Solutions to the MTM Problems
  10. Proposed Solution for ES-NOMA
  11. Proposed Solution for TS-TDMA
  12. Computational Complexity Analysis
  13. Numerical Results
  14. Simulation Setup
  15. Convergence of Proposed Algorithm 1
  16. ...and 7 more sections

Key Result

Theorem 1

For any given $P_A>0$, the optimal solutions for the simplified problem ES_Transmit always satisfy $\mathrm{Rank}(\mathbf{V^{\textup{ES}*}})=1$.

Figures (10)

  • Figure 1: Illustration of a SATR-RIS assisted WPCN.
  • Figure 2: Illustration of transmission block structure.
  • Figure 3: Simulation setup.
  • Figure 4: Convergence of Algorithm 1.
  • Figure 5: User throughput in a single-antenna HAP case.
  • ...and 5 more figures

Theorems & Definitions (5)

  • Remark 1
  • Theorem 1
  • proof
  • Remark 2
  • Remark 3