Massive MIMO, among other ground-breaking technologies, is being developed for the next-generation wireless systems to support requirements in terms of data rates, reliability, latency, intelligence, security and energy efficiency. Accurate channel estimation remains a key challenge in fully exploiting massive MIMO. While recent research has explored aspects such as near-field effects, spatial non-stationarity, and channel sparsity, many practical estimation and modeling techniques still provide limited CSI, often dominated by aggregate channel gain and delay, without full spatial characteristics. Although wideband models and phased-array techniques can capture delay and angular information, many practical estimation methods still lack comprehensive spatial resolution, including polarization, which limits their effectiveness for advanced massive MIMO techniques. This article introduces the concept of radio radiance field (RRF), which captures the spatial distribution and directionality of radio propagation. From RRF, a comprehensive spatial representation of the wireless channel, referred to as Spatial-CSI, can be derived. Owing to the comprehensive geometric and radio information, RRF can be implemented directly for beamforming, delay-alignment modulation, and many other techniques in massive MIMO and reflective intelligent surface implementations. An RRF can also serve as a digital radio twin, which is a virtual representation of the radio environment that includes both geometric structure and radio propagation characteristics, enabling real-time simulation and optimization of wireless systems. It paves the way for various applications from communications to sensing in the next-generation wireless communication systems.

翻译：大规模MIMO等突破性技术正在为下一代无线系统开发，以满足数据速率、可靠性、时延、智能性、安全性和能效等方面的需求。准确的信道估计仍然是充分发挥大规模MIMO潜能的关键挑战。尽管近期研究已探索了诸如近场效应、空间非平稳性和信道稀疏性等方面，许多实用的估计与建模技术仍仅提供有限的信道状态信息，通常以聚合信道增益和时延为主导，缺乏完整的空间特性。虽然宽带模型和相控阵技术能够捕获时延和角度信息，但许多实用估计方法仍缺乏包括极化在内的全面空间分辨率，这限制了其在先进大规模MIMO技术中的有效性。本文引入无线电辐射场概念，该概念能够捕获无线电传播的空间分布与方向性特性。基于RRF可推导出无线信道的完整空间表征，称为空间CSI。凭借其全面的几何与无线电信息，RRF可直接应用于大规模MIMO和反射智能表面实现中的波束成形、时延对齐调制及众多其他技术。RRF还可作为数字无线电孪生体——即包含几何结构与无线电传播特性的无线电环境虚拟表征，实现无线系统的实时仿真与优化。这为下一代无线通信系统中从通信到感知的多种应用铺平了道路。