RIS Assisted Near-Field NOMA Communications: A Security-Fairness Trade-Off

A reconfigurable intelligent surface (RIS) assisted near-field secure non-orthogonal multiple access (NOMA) communication system is investigated. In particular, a challenging secure NOMA communication scenario is considered, where the user closing to the RIS is untrusted. Exploiting the near-field beamfocusing capability, a far-to-near successive interference cancellation (SIC) operation is employed to facilitate the secure NOMA communications. Based on this, the trade-off between security and fairness is characterized by maximizing the weighted sum of security capacity and minimum capacity. An alternating optimization based algorithm is developed to solve this highly coupled problem, where RIS beamforming and power allocation are optimized using semidefinite relaxation and successive convex approximation methods, respectively. Numerical results demonstrate: (1) The secure communication for the far user can be achieved in the near field but is impossible in the far field; (2) As the distance between two users increases, the security capacity initially increases and then decreases, while the minimum capacity continuously declines.