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Hybrid Active-Passive RIS Transmitter Enabled Energy-Efficient Multi-User Communications

Ao Huang, Xidong Mu, Li Guo, Guangyu Zhu

TL;DR

The paper investigates an energy-efficient downlink multi-user system that leverages a hybrid active-passive RIS transmitter, where each RIS element can switch between active amplification and passive phase shifting. It formulates a joint EE optimization problem over RIS element mode scheduling, RIS beamforming, and feed-antenna power allocation, and solves it via a Dinkelbach-transformed, alternating-optimization framework; a high-complexity exhaustive-search variant and a low-complexity Big-M based scheme are developed. The results show that the hybrid RIS approach outperforms fully active/passive RIS and conventional RF chains, with EE gains attributed to extra degrees of freedom from mode switching, while achieving near-optimal performance with only a minority of active elements. The study also reveals that for a fixed RIS size, maximum EE is obtained by activating only a subset of elements, highlighting a practical trade-off between performance and energy consumption and offering guidance for hardware design in future wireless networks.

Abstract

A novel hybrid active-passive reconfigurable intelligent surface (RIS) transmitter enabled downlink multi-user communication system is investigated. Specifically, RISs are exploited to serve as transmitter antennas, where each element can flexibly switch between active and passive modes to deliver information to multiple users. The system energy efficiency (EE) maximization problem is formulated by jointly optimizing the RIS element scheduling and beamforming coefficients, as well as the power allocation coefficients, subject to the user's individual rate requirement and the maximum RIS amplification power constraint. Using the Dinkelbach relaxation, the original mixed-integer nonlinear programming problem is transformed into a nonfractional optimization problem with a two-layer structure, which is solved by the alternating optimization approach. In particular, an exhaustive search method is proposed to determine the optimal operating mode for each RIS element. Then, the RIS beamforming and power allocation coefficients are properly designed in an alternating manner. To overcome the potentially high complexity caused by exhaustive searching, we further develop a joint RIS element mode and beamforming optimization scheme by exploiting the Big-M formulation technique. Numerical results validate that: 1) The proposed hybrid RIS scheme yields higher EE than the baseline multi-antenna schemes employing fully active/passive RIS or conventional radio frequency chains; 2) Both proposed algorithms are effective in improving the system performance, especially the latter can achieve precise design of RIS elements with low complexity; and 3) For a fixed-size hybrid RIS, maximum EE can be reaped by setting only a minority of elements to operate in the active mode.

Hybrid Active-Passive RIS Transmitter Enabled Energy-Efficient Multi-User Communications

TL;DR

The paper investigates an energy-efficient downlink multi-user system that leverages a hybrid active-passive RIS transmitter, where each RIS element can switch between active amplification and passive phase shifting. It formulates a joint EE optimization problem over RIS element mode scheduling, RIS beamforming, and feed-antenna power allocation, and solves it via a Dinkelbach-transformed, alternating-optimization framework; a high-complexity exhaustive-search variant and a low-complexity Big-M based scheme are developed. The results show that the hybrid RIS approach outperforms fully active/passive RIS and conventional RF chains, with EE gains attributed to extra degrees of freedom from mode switching, while achieving near-optimal performance with only a minority of active elements. The study also reveals that for a fixed RIS size, maximum EE is obtained by activating only a subset of elements, highlighting a practical trade-off between performance and energy consumption and offering guidance for hardware design in future wireless networks.

Abstract

A novel hybrid active-passive reconfigurable intelligent surface (RIS) transmitter enabled downlink multi-user communication system is investigated. Specifically, RISs are exploited to serve as transmitter antennas, where each element can flexibly switch between active and passive modes to deliver information to multiple users. The system energy efficiency (EE) maximization problem is formulated by jointly optimizing the RIS element scheduling and beamforming coefficients, as well as the power allocation coefficients, subject to the user's individual rate requirement and the maximum RIS amplification power constraint. Using the Dinkelbach relaxation, the original mixed-integer nonlinear programming problem is transformed into a nonfractional optimization problem with a two-layer structure, which is solved by the alternating optimization approach. In particular, an exhaustive search method is proposed to determine the optimal operating mode for each RIS element. Then, the RIS beamforming and power allocation coefficients are properly designed in an alternating manner. To overcome the potentially high complexity caused by exhaustive searching, we further develop a joint RIS element mode and beamforming optimization scheme by exploiting the Big-M formulation technique. Numerical results validate that: 1) The proposed hybrid RIS scheme yields higher EE than the baseline multi-antenna schemes employing fully active/passive RIS or conventional radio frequency chains; 2) Both proposed algorithms are effective in improving the system performance, especially the latter can achieve precise design of RIS elements with low complexity; and 3) For a fixed-size hybrid RIS, maximum EE can be reaped by setting only a minority of elements to operate in the active mode.
Paper Structure (25 sections, 50 equations, 8 figures, 2 tables, 2 algorithms)

This paper contains 25 sections, 50 equations, 8 figures, 2 tables, 2 algorithms.

Table of Contents

  1. Introduction
  2. Prior Works
  3. Motivations and Contributions
  4. Organization and Notation
  5. System Model and Problem Formulation
  6. System Model
  7. Hybrid Active-Passive RIS Transmitter Architecture
  8. Signal Model and Problem Formulation
  9. Solution Analysis
  10. Alternating Optimization Based Algorithm
  11. Element Mode Scheduling Optimization for the Hybrid RIS
  12. Transmission Beamforming Optimization at the Hybrid RIS
  13. Power Allocation Optimization at the Feed Antenna
  14. Proposed Algorithm and Computational Complexity
  15. Low-Complexity Algorithm
  16. ...and 10 more sections

Figures (8)

  • Figure 1: Illustration of the hybrid active-passive RIS transmitter enabled multi-user communication network.
  • Figure 2: Algorithm flowchart.
  • Figure 3: The simulated setup.
  • Figure 4: Convergence of the proposed two algorithms.
  • Figure 5: System EE versus the number of transmitter antennas $M$.
  • ...and 3 more figures

Theorems & Definitions (1)

  • Remark 1