The channel capacity of near-field (NF) communications is characterized by considering three types of multiuser channels: i) multiple access channel (MAC), ii) broadcast channel (BC), and iii) multicast channel (MC). For NF MAC and BC, closed-form expressions are derived for the sum-rate capacity as well as the capacity region under a two-user scenario. These results are further extended to scenarios with an arbitrary number of users. For NF MC, closed-form expressions are derived for the two-user channel capacity and the capacity upper bound with more users. Further insights are gleaned by exploring special cases, including scenarios with infinitely large array apertures, co-directional users, and linear arrays. Theoretical and numerical results are presented and compared with far-field communications to demonstrate that: i) the NF capacity of these three channels converges to finite values rather than growing unboundedly as the number of array elements increases; ii) the capacity of the MAC and BC with co-directional users can be improved by using the additional range dimensions in NF channels to reduce inter-user interference (IUI); and iii) the MC capacity benefits less from the NF effect compared to the MAC and BC, as multicasting is less sensitive to IUI.

翻译：近场（NF）通信的信道容量通过考虑三种多用户信道进行表征：i）多址信道（MAC），ii）广播信道（BC），以及iii）组播信道（MC）。针对近场MAC和BC，本文推导了在双用户场景下和容量以及容量区域的闭式表达式，并将这些结果进一步扩展至任意用户数量的场景。对于近场MC，推导了双用户信道容量及更多用户时的容量上界的闭式表达式。通过探索特殊情形（包括无限大阵列孔径、共向用户及线性阵列）获得了更深层次的见解。理论分析与数值结果被给出并与远场通信进行对比，以证明：i）这三种信道的近场容量趋于有限值，而非随阵列单元数量增加而无界增长；ii）通过利用近场信道中的额外距离维度减少共向用户的用户间干扰（IUI），可提升MAC和BC的容量；iii）与MAC和BC相比，MC容量受近场效应的增益较小，原因在于组播对IUI的敏感度较低。