Multiple-antenna technologies are advancing towards large-scale aperture sizes and extremely high frequencies, leading to the emergence of near-field communications (NFC) in future wireless systems. To this context, we investigate the degree of freedom (DoF) in near-field multiple-input multiple-output (MIMO) systems. We consider both spatially discrete (SPD) antennas and continuous aperture (CAP) antennas. Additionally, we explore three important DoF-related performance metrics and examine their relationships with the classic DoF. Numerical results demonstrate the benefits of NFC over far-field communications (FFC) in terms of providing increased spatial DoFs. We also identify promising research directions for NFC from a DoF perspective.

翻译：多天线技术正朝着大孔径尺寸和极高频率的方向发展，这促使未来无线系统中近场通信（NFC）的出现。在此背景下，我们研究了近场多输入多输出（MIMO）系统中的自由度（DoF）。我们考虑了空间离散（SPD）天线和连续孔径（CAP）天线。此外，我们探索了三种与DoF相关的重要性能指标，并考察了它们与经典DoF的关系。数值结果表明，近场通信在提供更多空间自由度方面优于远场通信（FFC）。我们还从自由度的角度指出了NFC的有前景的研究方向。