The channel capacity of near-field (NF) communications is characterized by considering three types of line-of-sight 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. For comparison, the MAC and BC sum-rates achieved by typical linear combiners and precoders are also analyzed. 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：本文通过考虑三种视距多用户信道类型来刻画近场通信的信道容量：i) 多址信道，ii) 广播信道，以及 iii) 组播信道。针对近场多址信道与广播信道，推导了双用户场景下和速率容量以及容量区域的闭式表达式，并将这些结果进一步推广至任意用户数的场景。对于近场组播信道，推导了双用户信道容量以及多用户场景下的容量上界的闭式表达式。通过探究特殊场景获得了进一步的见解，包括无限大阵列孔径、同向用户以及线性阵列等情况。作为对比，还分析了典型线性合并器与预编码器在多址信道与广播信道中实现的和速率。理论与数值结果均被给出，并与远场通信进行了比较，结果表明：i) 这三种信道的近场容量随阵列单元数增加收敛于有限值，而非无限增长；ii) 通过利用近场信道中额外的距离维度来降低用户间干扰，可以提升同向用户多址信道与广播信道的容量；iii) 与多址信道和广播信道相比，组播信道从近场效应中获益较少，因为组播对用户间干扰较不敏感。