Large-Scale Intelligent Surfaces Enabled Unified Near-Field and Far-Field Communications: Codebook Design and Beam Training

Benefiting from the large-scale antenna arrays, the forthcoming 6G networks are capable of providing high spectral efficiency by utilizing spatial diversity. The near-field region also expands with the large aperture, and thus, users may randomly distribute in both the near and far fields, leading to near-far (NF) field communications. In this article, by deploying large-scale reconfigurable intelligent surfaces (RISs) that serve as an energy-efficient solution, a general codebook-based beam training scheme for the RIS-aided NF-field communications is proposed, which does not rely on any channel state information. Starting from the basics of NF-field communications, we present the design principles and key technologies of the NF-field codebook-based beam training, targeting at serving users located either in the near field or the far field without prior knowledge of their locations. Utilizing the general scheme, a case study is presented for a large-scale RIS-enabled multiuser communication system. The proposed scheme is further extended to the wideband transmission, the omni-directional surface case, and the communication-assisting sensing. Future research directions for NF-field communications enabled by intelligent surfaces are also discussed.

Introduction

Future 6G mobile networks are anticipated to meet the increasing demands on high data rates for supporting various applications [1]. To fulfill the stringent requirements, massive MIMO has played a significant role in improving the data rate through spatial multiplexing. Owing to the recent development of metamaterials, reconfigurable intelligent surfaces (RISs) have emerged as a promising implementation of extremely large-scale arrays (ELSAs) to achieve massive MIMO [2]. Specifically, RISs comprise multiple low-cost metamaterial elements, which have the capability to manipulate the radio propagation environment by controlling the phase shifts of the impinging waves. It can be easily extended to ELSAs in a cost-efficient way, breaking the scale limit of traditional phased arrays caused by expensive phase shifters and complicated circuits.