Capacity Limits of Near-Field Multiple Access Channel

Capacity limits of near-field (NF) multiple access channel (MAC) is characterized. Closed-form expressions are derived for the sum-rate capacity and the capacity region under a two-user scenario. These results are further extended to scenarios with an arbitrary number of users. Further insights are gleaned by exploring special cases, including scenarios with infinitely large array apertures and co-directional users. Theoretical and numerical results are presented and compared with far-field communications to demonstrate that: i) the NF capacity of the MAC converges to finite values rather than growing unboundedly as the number of array elements increases; ii) the MAC capacity with co-directional users can be improved by using the additional range dimensions in NF channels to reduce inter-user interference.