Multi-Mode Pinching-Antenna Systems: Mode Selection or Mode Combining?

TL;DR

This paper explores multi-mode pinching antenna systems for multi-user communications, proposing mode selection and mode combining protocols, and introduces an optimization algorithm that enhances spectral efficiency and reduces hardware complexity.

Contribution

It introduces two novel protocols for multi-mode PASS, formulates a sum rate maximization problem, and develops a PSO-based KKT-guided algorithm for efficient beamforming optimization.

Findings

Multi-mode PASS outperforms single-mode and hybrid systems.

Mode combining achieves high spectral efficiency.

Mode selection offers lower hardware complexity.

Abstract

This letter investigates multi-mode pinching antenna systems (PASS), where signals of multiple orthogonal modes can be transmitted within a dielectric waveguide and radiated by pinching antennas (PAs). This enables mode-domain multiplexing for efficient multi-user communications using a single waveguide. In particular, two operating protocols are proposed, namely mode selection and mode combining. Mode selection enforces each PA to predominantly radiate signal power of one single mode, while mode combining allows each PA to flexibly radiate power of multiple modes. Based on the two protocols, a sum rate maximization problem is formulated for multi-mode PASS-enabled multi-user downlink communications, where the transmit beamforming, PA positions, and PA propagation constants are jointly optimized. To address this rapidly oscillating and highly nonconvex problem, a particle swarm optimization (PSO) based Karush-Kuhn-Tucker (KKT)-parameterized beamforming (PSO- KPBF) algorithm is proposed. KKT-conditioned solutions are exploited to guide the swarm search, thus reducing the search space and achieving fast convergence. Numerical results demonstrate that: 1) Even using a simple uniform mode-combining design, the multi-mode PASS significantly outperform conventional single-mode PASS and hybrid beamforming systems; and 2) Mode combining achieves high spectral efficiency, while mode selection approximates its performance with a lower hardware complexity. Code is released at https://github.com/xiaoxiaxusummer/multi_mode_pinching_antenna