Location-Driven Beamforming for RIS-Assisted Near-Field Communications

TL;DR

The paper tackles CSI acquisition challenges in RIS-assisted near-field communications for 6G by proposing a location-driven beamforming paradigm grounded in the Fresnel zone model. It develops a near-field beamforming framework that uses computed RIS phase distributions from UE location, introduces the two-step position-aided On/Off state judgment (TPOSJ) with an adjustable threshold $\xi$, and defines a time-division frame structure that leverages localization in uplink/downlink phases. Key contributions include formalizing the Fresnel-zone–RIS interaction, detailing a practical location-driven beamforming scheme without explicit CSI, and demonstrating reduced computational complexity with robustness to localization errors through absorption-mode elements. The results highlight the potential for high-precision localization, mobility support, and ISAC-compatible operation to reduce pilot overhead and enable scalable RIS deployments in future 6G networks.

Abstract

Future wireless communications are promising to support ubiquitous connections and high data rates with cost-effective devices. Benefiting from the energy-efficient elements with low cost, reconfigurable intelligent surface (RIS) emerges as a potential solution to fulfill such demands, which has the capability to flexibly manipulate the wireless signals with a tunable phase. Recently, as the operational frequency ascends to the sub-terahertz (THz) bands or higher bands for wireless communications in six-generation (6G), it becomes imperative to consider the near-field propagation in RIS-assisted communications. The challenging acquisition of channel parameters is an inherent issue for near-field RIS-assisted communications, where the complex design is essential to acquire the informative near-field channel embedded with both the angle and distance information. Hence, in this paper we systematically investigate the potential of exploiting location information for near-field RIS-assisted communications. Firstly, we present the progresses in the near-field RIS-assisted communications, which are compatible with existing wireless communications and show the potential to achieve the fine-grained localization accuracy to support location-driven scheme. Then, the Fresnel zone based model is introduced, with which the location-driven beamforming scheme and corresponding frame structure are developed. Also, we elaborate on four unique advantages for leveraging location information in RIS-assisted communications, followed by numerical simulations. Finally, several key challenges and corresponding potential solutions are pointed out.

Figures (6)

• Figure 1: Three RIS-assisted localization schemes without CE.
• Figure 2: The schematic diagrams of Fresnel zones and phase distributions on the RIS in the near-field and the far-field.
• Figure 3: The geometric relations formed by Fresnel zone based model.
• Figure 4: The time-division frame structure in location-driven beamforming.
• Figure 5: Complexity comparison between location-driven beamforming and CSI-driven beamforming.