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Location-Aided Distributed Beamforming for Near-Field Communications with Element-Wise RIS

Xiao Zheng, Wenchi Cheng, Jingqing Wang, Zhuohui Yao, Jiangzhou Wang

TL;DR

A new element-wise RIS architecture is designed and a distributed location-aided transmission scheme with low complexity is proposed to enhance the reflected gain for channel state information (CSI)-limited RIS-assisted near-field communications.

Abstract

Active reconfigurable intelligent surface (RIS) emerges as an effective technique to resist the double-fading attenuation of passive RIS. By embedding with power harvesting function, it further evolves to zero-power active RIS, which can effectively enhance the flexibility of RIS deployment without external power demand. Nevertheless, existing works neglected the inherent difficulty of channel estimation (CE) for RIS-assisted systems, and the discrete phase shift constraint in practical deployment. In this paper we design a new element-wise RIS architecture and propose a distributed location-aided transmission scheme with low complexity to enhance the reflected gain for channel state information (CSI)-limited RIS-assisted near-field communications. Specifically, the new element-wise RIS provides dynamic element selection capability with low hardware resources. Based on Fresnel diffraction theory, we construct the mapping from locations in space-domain to phase distributions of waves in phase-domain and reveal the priority of elements for harvesting and reflecting. {Then, the distributed beamforming design with the phase of determine-then-align is proposed, where the estimation overhead reduction stems from exempted requirements of RIS-associated CE at base station (BS).} The asymptotic analysis indicates that the proposed scheme can achieve the optimal gain with a fixed proportion of reflective elements when RIS is large, followed by simulations to verify its superiority to other protocols.

Location-Aided Distributed Beamforming for Near-Field Communications with Element-Wise RIS

TL;DR

A new element-wise RIS architecture is designed and a distributed location-aided transmission scheme with low complexity is proposed to enhance the reflected gain for channel state information (CSI)-limited RIS-assisted near-field communications.

Abstract

Active reconfigurable intelligent surface (RIS) emerges as an effective technique to resist the double-fading attenuation of passive RIS. By embedding with power harvesting function, it further evolves to zero-power active RIS, which can effectively enhance the flexibility of RIS deployment without external power demand. Nevertheless, existing works neglected the inherent difficulty of channel estimation (CE) for RIS-assisted systems, and the discrete phase shift constraint in practical deployment. In this paper we design a new element-wise RIS architecture and propose a distributed location-aided transmission scheme with low complexity to enhance the reflected gain for channel state information (CSI)-limited RIS-assisted near-field communications. Specifically, the new element-wise RIS provides dynamic element selection capability with low hardware resources. Based on Fresnel diffraction theory, we construct the mapping from locations in space-domain to phase distributions of waves in phase-domain and reveal the priority of elements for harvesting and reflecting. {Then, the distributed beamforming design with the phase of determine-then-align is proposed, where the estimation overhead reduction stems from exempted requirements of RIS-associated CE at base station (BS).} The asymptotic analysis indicates that the proposed scheme can achieve the optimal gain with a fixed proportion of reflective elements when RIS is large, followed by simulations to verify its superiority to other protocols.
Paper Structure (16 sections, 56 equations, 14 figures, 1 algorithm)

This paper contains 16 sections, 56 equations, 14 figures, 1 algorithm.

Table of Contents

  1. Introduction
  2. System Model
  3. Element-Wise RIS Model
  4. Channel Model
  5. Problem Formulation
  6. Fresnel Zone Model
  7. Parametric Fresnel Ellipsoid
  8. Extension to Fractional Fresnel Zone
  9. Interaction between RIS and Fresnel Ellipsoid
  10. Parametric Curve
  11. Distributed Transmission Design
  12. Initialization stage
  13. Distributed Beamforming Design
  14. Asymptotic Results
  15. Numerical Results
  16. ...and 1 more sections

Figures (14)

  • Figure 1: The system model for self-sustainable active RIS assisted near-field communications.
  • Figure 2: The diagram of the first and second Fresnel Zones.
  • Figure 3: The diagrams of configuration criterion between reflective elements and absorptive elements: (a) the phase resolution of reflective element $D=1$; (b) the phase resolution of reflective element $D=2$.
  • Figure 4: The proposed frame structure.
  • Figure 5: One example of joint reflective and absorptive coefficients design with 1-bit phase resolution.
  • ...and 9 more figures

Theorems & Definitions (3)

  • Remark 1
  • Remark 2
  • Remark 3