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On the performance of Active STAR-RIS-Assisted Cell-Free Massive MIMO Systems with Phase Errors and Channel Aging

Jun Qian, Ross Murch, Khaled B. Letaief

TL;DR

This paper analyzes the performance of active STAR-RIS-assisted cell-free massive MIMO systems under practical impairments, specifically phase errors and channel aging. By adopting a spatially correlated Rayleigh fading model and MMSE channel estimation, it derives closed-form downlink SE expressions that explicitly account for phase errors and aging. The results show that increasing the number of APs, STAR-RIS elements, and amplification can mitigate degradation, and a resource-block-length design is proposed to counter aging effects. The findings underscore the potential of active STAR-RISs to enhance robustness and SE in 6G networks, while also indicating that amplification and element count must be jointly optimized to balance gains against amplified interference and noise.

Abstract

Active reconfigurable intelligent surfaces (RISs) employ amplification to overcome attenuation caused by the RIS cascaded link. In this paper, we analyze the effects of phase errors and channel aging in active simultaneously transmitting and reflecting (STAR) RIS-assisted cell-free massive multiple-input multiple-output (MIMO) systems. By leveraging a spatially correlated Rayleigh fading model, this paper derives minimum mean square error estimate-based channel estimates and formulates closed-form expressions for downlink spectral efficiency. This analytical framework enables a comprehensive evaluation of the effects of channel aging and uniformly distributed phase errors on system performance. The results demonstrate that active STAR-RISs can effectively compensate for the adverse effects of phase errors and channel aging. To counteract the impact of channel aging, we propose practical guidelines for resource-block-length design. Also, an increase in APs and STAR-RIS elements, along with a larger amplification factor, can alleviate performance degradation.

On the performance of Active STAR-RIS-Assisted Cell-Free Massive MIMO Systems with Phase Errors and Channel Aging

TL;DR

This paper analyzes the performance of active STAR-RIS-assisted cell-free massive MIMO systems under practical impairments, specifically phase errors and channel aging. By adopting a spatially correlated Rayleigh fading model and MMSE channel estimation, it derives closed-form downlink SE expressions that explicitly account for phase errors and aging. The results show that increasing the number of APs, STAR-RIS elements, and amplification can mitigate degradation, and a resource-block-length design is proposed to counter aging effects. The findings underscore the potential of active STAR-RISs to enhance robustness and SE in 6G networks, while also indicating that amplification and element count must be jointly optimized to balance gains against amplified interference and noise.

Abstract

Active reconfigurable intelligent surfaces (RISs) employ amplification to overcome attenuation caused by the RIS cascaded link. In this paper, we analyze the effects of phase errors and channel aging in active simultaneously transmitting and reflecting (STAR) RIS-assisted cell-free massive multiple-input multiple-output (MIMO) systems. By leveraging a spatially correlated Rayleigh fading model, this paper derives minimum mean square error estimate-based channel estimates and formulates closed-form expressions for downlink spectral efficiency. This analytical framework enables a comprehensive evaluation of the effects of channel aging and uniformly distributed phase errors on system performance. The results demonstrate that active STAR-RISs can effectively compensate for the adverse effects of phase errors and channel aging. To counteract the impact of channel aging, we propose practical guidelines for resource-block-length design. Also, an increase in APs and STAR-RIS elements, along with a larger amplification factor, can alleviate performance degradation.
Paper Structure (11 sections, 18 equations, 2 figures, 1 table)

This paper contains 11 sections, 18 equations, 2 figures, 1 table.

Table of Contents

  1. Introduction
  2. System Model
  3. STAR-RIS-assisted Channel Model with Phase Errors
  4. Channel Aging
  5. Uplink Channel Estimation and Downlink SE Analysis
  6. Uplink Channel Estimation
  7. Downlink SE Analysis
  8. Numerical Results and Discussion
  9. Parameter Setup
  10. Results and Discussion
  11. Conclusion

Figures (2)

  • Figure 1: Downlink SE versus time instant index with $M=10$, $N=4$, $K=10$, $K_t=K_r=5$, $L=64$ (Analytical Results).
  • Figure 3: Downlink SE versus number of STAR-RIS elements with $M=10$, $N=4$, $K=10$, $K_t=K_r=5$, $v=60$ km/h (Analytical Results).