Next-generation mobile networks promise to support high throughput, massive connectivity, and improved energy efficiency. To achieve these ambitious goals, extremely large-scale antenna arrays (ELAAs) and terahertz communications constitute a pair of promising technologies. This will result in future wireless communications occurring in the near-field regions. To accurately portray the channel characteristics of near-field wireless propagation, spherical wavefront-based models are required and present both opportunities as well as challenges. Following the basics of near-field communications (NFC), we contrast it to conventional far-field communications. Moreover, we cover the key challenges of NFC, including its channel modeling and estimation, near-field beamfocusing, as well as hardware design. Our numerical results demonstrate the potential of NFC in improving the spatial multiplexing gain and positioning accuracy. Finally, a suite of open issues are identified for motivating future research.

翻译：下一代移动网络有望支持高吞吐量、大规模连接和提升的能源效率。为实现这些宏伟目标，超大规模天线阵列与太赫兹通信构成了一对极具前景的技术。这将使未来无线通信发生在近场区域。为精确刻画近场无线传播的信道特性，需要基于球面波前的模型，这既带来了机遇也带来了挑战。基于近场通信的基础知识，我们将其与传统远场通信进行对比。此外，我们涵盖了近场通信的关键挑战，包括其信道建模与估计、近场波束聚焦以及硬件设计。我们的数值结果展示了近场通信在提升空间复用增益与定位精度方面的潜力。最后，指出了一系列开放性问题以推动未来研究。