Performance analysis is carried out in a near-field multiple-input multiple-output (MIMO) system for both discrete and continuous aperture antennas. The effective degrees of freedom (EDoF) is first derived. It is shown that near-field MIMO systems have a higher EDoF than free-space far-field ones. Additionally, the near-field EDoF further depends on the communication distance. Based on the derived EDoF, closed-form expressions of channel capacity with a fixed distance are obtained. As a further advance, with randomly deployed receivers, ergodic capacity is derived. Simulation results reveal that near-field MIMO has an enhanced multiplexing gain even under line-of-sight transmissions. In addition, the performance of discrete MIMO converges to that of continuous-aperture MIMO.

翻译：针对近场多输入多输出（MIMO）系统中离散与连续孔径天线进行了性能分析。首先推导了有效自由度（EDoF），结果表明近场MIMO系统相比于自由空间远场系统具有更高的EDoF，且其EDoF进一步依赖于通信距离。基于所推导的EDoF，得到了固定距离下的信道容量闭合表达式。进一步地，针对随机部署的接收端，推导了遍历容量。仿真结果表明，即使在视距传输条件下，近场MIMO仍具有增强的复用增益。此外，离散MIMO的性能趋于收敛至连续孔径MIMO的性能。