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STAR-RIS-Assisted Communication Radar Coexistence: Analysis and Optimization

Anastasios Papazafeiropoulos, Pandelis Kourtessis, Symeon Chatzinotas

TL;DR

This work tackles mutual interference in radar-communication coexistence by introducing a STAR-RIS that provides 360-degree coverage in a correlated Rayleigh fading environment. It relies on a two-timescale statistical CSI framework and uses alternating optimization to jointly design STAR-RIS amplitudes/phases and radar beamformers, yielding SE expressions that depend on large-scale statistics, e.g., $SE=\frac{1}{\tau_c}\sum_{k=1}^K \log_2(1+\gamma_k)$. The gradient-based PGAM updates derive complex gradients with respect to $\boldsymbol{\theta}$ and $\boldsymbol{\beta}$ for ES/MS protocols, while UaTF provides tractable SINR bounds for both radar and UEs. Numerical results show that the STAR-RIS architecture outperforms conventional RIS and random-phase baselines, and that the proposed statistical CSI design achieves substantial gains with significantly reduced signaling overhead compared to instantaneous CSI approaches.

Abstract

Integrated sensing and communication (ISAC) is expected to play a prominent role among emerging technologies in future wireless communications. In particular, a communication radar coexistence system is degraded significantly by mutual interference. In this work, given the advantages of promising reconfigurable intelligent surface (RIS), we propose a simultaneously transmitting and reflecting RIS (STAR-RIS)-assisted radar coexistence system where a STAR-RIS is introduced to improve the communication performance while suppressing the mutual interference and providing full space coverage. Based on the realistic conditions of correlated fading, and the presence of multiple user equipments (UEs) at both sides of the RIS, we derive the achievable rates at the radar and the communication receiver side in closed forms in terms of statistical channel state information (CSI). Next, we perform alternating optimization (AO) for optimizing the STAR-RIS and the radar beamforming. Regarding the former, we optimize the amplitudes and phase shifts of the STAR-RIS through a projected gradient ascent algorithm (PGAM) simultaneously with respect to the amplitudes and phase shifts of the surface for both energy splitting (ES) and mode switching (MS) operation protocols. The proposed optimization saves enough overhead since it can be performed every several coherence intervals. This property is particularly beneficial compared to reflecting-only RIS because a STAR-RIS includes the double number of variables, which require increased overhead. Finally, simulation results illustrate how the proposed architecture outperforms the conventional RIS counterpart, and show how the various parameters affect the performance. Moreover, a benchmark full instantaneous CSI (I-CSI) based design is provided and shown to result in higher sum-rate but also in large overhead associated with complexity.

STAR-RIS-Assisted Communication Radar Coexistence: Analysis and Optimization

TL;DR

This work tackles mutual interference in radar-communication coexistence by introducing a STAR-RIS that provides 360-degree coverage in a correlated Rayleigh fading environment. It relies on a two-timescale statistical CSI framework and uses alternating optimization to jointly design STAR-RIS amplitudes/phases and radar beamformers, yielding SE expressions that depend on large-scale statistics, e.g., . The gradient-based PGAM updates derive complex gradients with respect to and for ES/MS protocols, while UaTF provides tractable SINR bounds for both radar and UEs. Numerical results show that the STAR-RIS architecture outperforms conventional RIS and random-phase baselines, and that the proposed statistical CSI design achieves substantial gains with significantly reduced signaling overhead compared to instantaneous CSI approaches.

Abstract

Integrated sensing and communication (ISAC) is expected to play a prominent role among emerging technologies in future wireless communications. In particular, a communication radar coexistence system is degraded significantly by mutual interference. In this work, given the advantages of promising reconfigurable intelligent surface (RIS), we propose a simultaneously transmitting and reflecting RIS (STAR-RIS)-assisted radar coexistence system where a STAR-RIS is introduced to improve the communication performance while suppressing the mutual interference and providing full space coverage. Based on the realistic conditions of correlated fading, and the presence of multiple user equipments (UEs) at both sides of the RIS, we derive the achievable rates at the radar and the communication receiver side in closed forms in terms of statistical channel state information (CSI). Next, we perform alternating optimization (AO) for optimizing the STAR-RIS and the radar beamforming. Regarding the former, we optimize the amplitudes and phase shifts of the STAR-RIS through a projected gradient ascent algorithm (PGAM) simultaneously with respect to the amplitudes and phase shifts of the surface for both energy splitting (ES) and mode switching (MS) operation protocols. The proposed optimization saves enough overhead since it can be performed every several coherence intervals. This property is particularly beneficial compared to reflecting-only RIS because a STAR-RIS includes the double number of variables, which require increased overhead. Finally, simulation results illustrate how the proposed architecture outperforms the conventional RIS counterpart, and show how the various parameters affect the performance. Moreover, a benchmark full instantaneous CSI (I-CSI) based design is provided and shown to result in higher sum-rate but also in large overhead associated with complexity.
Paper Structure (19 sections, 5 theorems, 64 equations, 7 figures, 2 algorithms)

This paper contains 19 sections, 5 theorems, 64 equations, 7 figures, 2 algorithms.

Table of Contents

  1. Introduction
  2. System Model
  3. STAR-RIS Model
  4. Operation protocols
  5. Radar Model
  6. Channel Model
  7. Data Transmission
  8. Sum SE Maximization Design
  9. Problem Formulation for STAR-RIS
  10. Problem Formulation for Radar
  11. Numerical Results
  12. Simulation Setup
  13. Benchmark Schemes
  14. Conclusion
  15. Proof of Theorem \ref{['Theorem1']}
  16. ...and 4 more sections

Key Result

Theorem 1

Given the PB ${\boldsymbol{\Phi}}_{w_{k}}$, the achievable SINR of the received signal at the radar in a RIS-assisted communication radar coexistence, accounting for correlated fading, is given by SINR10, where

Figures (7)

  • Figure 1: A STAR-RIS-assisted communication radar coexistence system.
  • Figure 2: Downlink achievable communication sum SE versus the number of RIS elements antennas $N$ for varying conditions (Analytical results and MC simulations).
  • Figure 3: Downlink achievable communication sum SE versus the number of BS antennas $M$ for varying conditions (Analytical results and MC simulations).
  • Figure 4: Downlink achievable communication sum SE versus the number of radar antennas $Q$ for different precoders/decoders and element spacing (MC simulations).
  • Figure 5: Downlink achievable communication sum SE versus the SNR for varying conditions.
  • ...and 2 more figures

Theorems & Definitions (8)

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