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Secrecy Sum-Rate Maximization for Active IRS-Assisted MIMO-OFDM SWIPT System

Xingxiang Peng, Peiran Wu, Junhui Zhao, Minghua Xia

Abstract

The propagation loss of RF signals is a significant issue in simultaneous wireless information and power transfer (SWIPT) systems. Additionally, ensuring information security is crucial due to the broadcasting nature of wireless channels. To address these challenges, we exploit the potential of active intelligent reflecting surface (IRS) in a multiple-input and multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) SWIPT system. The active IRS provides better beamforming gain than the passive IRS, reducing the "double-fading" effect. Moreover, the noise introduced at the active IRS can be used as artificial noise (AN) to jam eavesdroppers. This paper formulates a secrecy sum-rate maximization problem related to precoding matrices, power splitting (PS) ratios, and the IRS matrix. Since the problem is highly non-convex, we propose a block coordinate descent (BCD)-based algorithm to find a sub-optimal solution. Moreover, we develop a heuristic algorithm based on the zero-forcing precoding scheme to reduce computational complexity. Simulation results show that the active IRS achieves a higher secrecy sum rate than the passive and non-IRS systems, especially when the transmit power is low or the direct link is blocked. Moreover, increasing the power budget at the active IRS can significantly improve the secrecy sum rate.

Secrecy Sum-Rate Maximization for Active IRS-Assisted MIMO-OFDM SWIPT System

Abstract

The propagation loss of RF signals is a significant issue in simultaneous wireless information and power transfer (SWIPT) systems. Additionally, ensuring information security is crucial due to the broadcasting nature of wireless channels. To address these challenges, we exploit the potential of active intelligent reflecting surface (IRS) in a multiple-input and multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) SWIPT system. The active IRS provides better beamforming gain than the passive IRS, reducing the "double-fading" effect. Moreover, the noise introduced at the active IRS can be used as artificial noise (AN) to jam eavesdroppers. This paper formulates a secrecy sum-rate maximization problem related to precoding matrices, power splitting (PS) ratios, and the IRS matrix. Since the problem is highly non-convex, we propose a block coordinate descent (BCD)-based algorithm to find a sub-optimal solution. Moreover, we develop a heuristic algorithm based on the zero-forcing precoding scheme to reduce computational complexity. Simulation results show that the active IRS achieves a higher secrecy sum rate than the passive and non-IRS systems, especially when the transmit power is low or the direct link is blocked. Moreover, increasing the power budget at the active IRS can significantly improve the secrecy sum rate.

Paper Structure

This paper contains 19 sections, 1 theorem, 50 equations, 6 figures, 4 tables, 4 algorithms.

Table of Contents

  1. Introduction
  2. System Model And Problem Formulation
  3. System Model
  4. Precoding at Alice
  5. Power Splitting at Bob
  6. Information Decoding at Bob
  7. Energy Harvesting at Bob
  8. Information Eavesdropping at Eve
  9. Problem Formulation
  10. BCD-Based Algorithm
  11. Reformulation of the Original Problem
  12. Optimization of the Auxiliary Variables $\mathbf{\Xi}$
  13. Optimization of the Precoding Matrices $\{\mathbf{B}_k,\mathbf{A}_k\}_{k=1}^N$
  14. Optimization of the PS Matrix $\mathbf{D}$
  15. Optimization of the IRS Matrix $\boldsymbol{\Phi}$
  16. ...and 4 more sections

Key Result

Lemma 1

Given $\mathbf{E} \succ 0 \in \mathbb{C}^{N \times N}$, then we have where $f(\mathbf{S}) \triangleq -\operatorname{Tr}(\mathbf{S E})+\log_2\det(\mathbf{S}) + N$.

Figures (6)

  • Figure 1: Schematic diagram of a MIMO-OFDM SWIPT system, where Alice sends information to Bob in the presence of an active IRS and an eavesdropper Eve.
  • Figure 2: The geometric setting of users and IRS in the simulation experiments.
  • Figure 3: Convergence behavior of the BCD-based algorithms with random initial blocks.
  • Figure 4: Secrecy sum-rate vs. (left) the Tx power and (right) the reflecting/compensated power (For the passive- and non-IRS schemes, $P_{\rm I}$ is compensated to the transmitter).
  • Figure 5: Secrecy sum-rate vs. the Alice-Bob link's path-loss exponent.
  • ...and 1 more figures

Theorems & Definitions (5)

  • Remark 1
  • Remark 2
  • Remark 3: Problem formulation for the passive-IRS case
  • Lemma 1
  • Remark 4