Modern MIMO communication systems are almost exclusively designed under the assumption of locally plane wavefronts over antenna arrays. This is known as the far-field approximation and is soundly justified at sub-6-GHz frequencies at most relevant transmission ranges. However, when higher frequencies and shorter transmission ranges are used, the wave curvature over the array is no longer negligible, and arrays operate in the so-called radiative near-field region. This letter aims to show that the classical far-field approximation may significantly underestimate the achievable spectral efficiency of multi-user MIMO communications operating in the 30-GHz bands and above, even at ranges beyond the Fraunhofer distance. For planar arrays with typical sizes, we show that computing combining schemes based on the far-field model significantly reduces the channel gain and spatial multiplexing capability. When the radiative near-field model is used, interference rejection schemes, such as the optimal minimum mean-square-error combiner, appear to be very promising, when combined with electrically large arrays, to meet the stringent requirements of next-generation networks.
Continuous Wavefront Design via Virtual Point Sources: A Holographic Paradigm for Near-Field XL-MIMO
Tongji University（同济大学）；Shanghai Institute of Intelligent Science and Technology, Tongji University（同济大学上海智能科学与技术研究院）；Shanghai Jiao Tong University（上海交通大学）；University of Macau（澳门大学）；Fudan University（复旦大学）；Shanghai Qi Zhi Institute（上海人工智能实验室）
Near-Field Channel Estimation for XL-MIMO: A Deep Generative Model Guided by Side Information
National Mobile Communications Research Laboratory, Southeast University, Nanjing 210096, China（国家移动通信研究中心，东南大学，南京）；Purple Mountain Laboratories, Nanjing 211100, China（紫金山实验室，南京）；School of Electrical Engineering and Telecommunications, University of New South Wales, Sydney, 2052, Australia（电气工程与电信学院，新南威尔士大学，悉尼）；Department of Electrical and Computer Engineering, University of Delaware, Newark, DE 19716, USA（电气与计算机工程系，德雷克塞尔大学，纽卡斯尔）
