The far-field channel model has historically been used in wireless communications due to the simplicity of mathematical modeling and convenience for algorithm design. With the need for high data rates, low latency, and ubiquitous connectivity in the sixth generation (6G) of communication systems, new technology enablers such as extremely large antenna arrays (ELAAs), reconfigurable intelligent surfaces (RISs), and distributed multiple-input-multiple-output (D-MIMO) systems will be adopted. These enablers not only aim to improve communication services but also have an impact on localization and sensing (L&S), which are expected to be fundamentally built-in functionalities in future wireless systems. Despite appearing in different scenarios and supporting different frequency bands, such enablers share the so-called near-field (NF) features, which will provide extra geometric information conducive to L&S. In this work, we describe the NF features, namely, the spherical wave model, spatial non-stationarity, and beam squint effect. After discussing how L&S see NF differently from communication, the opportunities and open research challenges are provided.

翻译：远场信道模型由于数学建模简单且便于算法设计，在无线通信中历史上一直得到广泛应用。随着第六代（6G）通信系统对高数据速率、低时延及泛在连接的需求，超大规模天线阵列（ELAA）、可重构智能超表面（RIS）以及分布式多输入多输出（D-MIMO）系统等新型技术使能器将被采用。这些使能器不仅旨在提升通信服务质量，还将对定位与感知（L&S）功能产生影响——该功能预计将成为未来无线系统的基础内置能力。尽管这些使能器在不同场景中出现且支持不同频段，但它们共同具备所谓的近场（NF）特征，这将为L&S提供额外的几何信息。本文描述了这些近场特征，即球面波模型、空间非平稳性和波束斜视效应。在讨论L&S如何从不同于通信的角度看待近场后，本文进一步阐述了相关机遇与开放性研究挑战。