Sum-Rate Maximization for NOMA-Assisted Pinching-Antenna Systems

In this letter, we investigate a non-orthogonal multiple access (NOMA) assisted downlink pinching-antenna system. Leveraging the ability of pinching antennas to flexibly adjust users' wireless channel conditions, we formulate an optimization problem to maximize the sum rate by optimizing both the users' power allocation coefficients and the positions of pinching antennas. The optimal power allocation coefficients are obtained in closed-form by using the Karush-Kuhn-Tucker (KKT) conditions. The optimization problem of pinching antenna placements is more challenging than the power allocation problem, and is solved by a bisection-based search algorithm. In particular, the algorithm first optimizes the antenna placements to create favorable channel disparities between users, followed by fine-tuning the antenna positions to ensure the phase alignment for users, thus maximizing the sum rate. Simulation results demonstrate that, compared to conventional-antenna systems, pinching antennas can significantly enhance the sum rate in NOMA scenarios, and the proposed bisection-based search algorithm can achieve a sum rate nearly equivalent to that of an exhaustive search.