Hybrid Near-Field and Far-Field XL-MIMO: How Many Users Can Be Supported?

Extremely large-scale multiple-input multiple-output (XL-MIMO) has been regarded as a potential technique to achieve high spectral efficiency in the upcoming sixth-generation cellular network. As the scale increases, the near-field effect becomes non-negligible, which makes the traditional far-field-only assumption no longer applicable. Recent works have proved that by exploiting the near-field effect, the degrees of freedom can be increased significantly. However, some data streams are unable to convey information at a high data rate. Therefore, how much gain can be harvested regarding the supported user number remains an open question. In this paper, we derive the mathematical relationship between the supported user number and the antenna number in a downlink XL-MIMO system, showing that the supported user number is related to the degree of correlation (DoC) of the channels. The relationship between the near-field range and the DoC is then analyzed, implying that users in the same direction can be better supported in the near field due to the decline of the DoC of the channels.