Multicast with Multi-Waveguide PASS via Position and Beam Co-Design

Pinching-antenna systems (PASS) route energy through low-loss dielectric waveguides and radiate via reconfigurable pinching antennas (PAs), enabling large, shapeable apertures with a minimal number of radio chains. We study a near-field multicast downlink network obtained by extending PASS from a single waveguide to a coordinated multi-waveguide array and by jointly optimizing PA positions and beams. First, we build a cascaded channel that couples in-waveguide and free-space propagation, posing a worst-case multicast objective under spacing, coupling span, and power constraints. A two-stage co-design then follows. Stage I performs layout planning as a constrained bi-objective placement that maximizes worst-user channel state information while minimizing a wrapped-phase residual; when solved with the non-dominated sorting genetic algorithm (NSGA) II yields feasible Pareto layouts. Stage II fixes a knee layout obtained from stage I and refines the multicast beam via a convex semi-definite relaxation (SDR)-successive convex approximation (SCA) formulation with a feasibility warm start that recovers rank-one beams. Numerical results reveal that across a range of transmit power, coupling span, PA per waveguide, number of waveguides, user count, user range, and base-station height, the proposed design outperforms a single-waveguide PASS and x or y-aligned uniform linear arrays, delivering higher worst-user rates as well as sharply lowering the rate dispersion. The study also identifies broad coupling-length ranges where gains saturate and shows that a moderate pinch counts and added guides help improve spatial coverage until a geometry-limited plateau hits. These effects arise from in-waveguide proximity and lateral phase control, positioning multi-waveguide PASS as a practical, flexible antenna option for next-generation communication.