  This letter investigates the coexistence between near-field (NF) and
far-field (FF) communications, where multiple FF users are clustered to be
served on the beams of legacy NF users, via non-orthogonal multiple access
(NOMA). Three different successive interference cancellation (SIC) decoding
strategies are proposed and a sum rate maximization problem is formulated to
optimize the beam assignment and decoding order. The beam assignment problem is
further reformulated as an overlapping coalitional game, which facilitates the
design of the proposed clustering algorithm. The optimal decoding order in each
cluster is also derived, which can be integrated into the proposed clustering.
Simulation results demonstrate that the proposed clustering algorithm is able
to significantly improve the sum rate of the considered system, and the
developed strategies achieve different trade-offs between sum rate and
fairness.
