This paper considers the problem of downlink localization of user equipment devices (UEs) that are either in the near-field (NF) or in the far-field (FF) of the array of the serving base station (BS). We propose a dual signaling scheme, which can be implemented at the BS, for localizing such UEs. The first scheme assumes FF, while the other assumes NF conditions. Both schemes comprise a beam-sweeping technique, employed by the BS, and a localization algorithm, employed by the UEs. The FF-based scheme enables beam-steering with a low signaling overhead, which is utilized for the proposed localization algorithm, while the NF-based scheme operates with a higher complexity. Specifically, our proposed localization scheme takes advantage of the relaxed structure of the FF, which yields low computational complexity, but is not suitable for operating in the NF. Since the compatibility and the performance of the FF- based scheme depends on the BS-to-UE distance, we study the limitations of FF-based procedure, explore the trade-off in terms of performance and resource requirements for the two schemes, and propose a triggering condition for operating the component schemes of the dual scheme. Also, we study the performance of an iterative localization algorithm that takes into account the accuracy-complexity trade-off and adapts to the actual position of the UE. We find that the conventional Fraunhofer distance is not sufficient for adapting localization algorithms in the mixed NF and FF environment.

翻译：本文研究了位于服务基站（BS）阵列近场或远场区域的用户设备（UE）下行链路定位问题。我们提出一种可在基站端实现的双信令方案以实现对上述UE的定位。第一方案基于远场假设，第二方案则基于近场条件。两种方案均包含由基站执行的波束扫描技术，以及由用户设备执行的定位算法。基于远场的方案能够以低信令开销实现波束导向，所提定位算法充分利用该特性，而基于近场的方案则需更高复杂度。具体而言，所提定位方案利用了远场结构的松弛特性以降低计算复杂度，但该方案不适用于近场环境。由于基于远场方案的兼容性与性能受限于基站到用户设备的距离，我们深入探究了远场处理方法的局限性，揭示了两种方案在性能与资源需求间的权衡关系，并提出了双方案中各子方案运行的触发条件。此外，我们研究了兼顾精度-复杂度权衡并适应用户设备实际位置的迭代定位算法性能。研究表明，传统夫琅禾费距离不足以指导混合近场与远场环境下的定位算法自适应调整。