Sum Capacity Characterization of Pinching Antennas-assisted Multiple Access Channels

TL;DR

This paper analyzes the maximum achievable sum rate in a pinching antenna-assisted multiple access channel, revealing that optimal capacity can be achieved with a number of beamforming vectors equal to the number of users, without needing NOMA.

Contribution

It introduces a capacity analysis for PASS-assisted channels, showing that optimal sum capacity is attainable with a number of beamforming vectors equal to users, and derives related bounds.

Findings

Optimal sum rate achieved with beamforming vectors equal to number of users

NOMA is unnecessary for sum capacity in this setup

Derived closed-form sum rate and bounds for finite beamforming vectors

Abstract

Pinching antenna system (PASS) has recently shown its promising ability to flexibly reconfigure wireless channels via dynamically adjusting the positions of pinching antennas over a dielectric waveguide, termed as pinching beamforming. This paper studies the fundamental limit of the sum rate for a PASS-assisted multiple access channel, where multiple users transmit individual messages to a base station under the average power constraint. To this end, a dynamic pinching beamforming setup is conceived, where multiple pinching beamforming vectors are employed in a transmission period and the capacity-achieving non-orthogonal multiple access (NOMA) based scheme is considered. For the ideal case with an asymptotically large number of pinching beamforming vectors, the optimal transmission scheme is unveiled to carry out alternating transmission among each user whose channel power gain is maximized with the tailored pinching beamforming. This implies that NOMA is not needed for achieving the sum capacity and the required optimal number of pinching beamforming vectors is equal to the number of users. With this insight, the corresponding sum rate is derived in closed-form expression, which serves as the upper bound of the sum rate. Inspired by this result, a lower bound of the sum rate under an arbitrarily finite number of pinching beamforming vectors is obtained. Numerical results validate our theoretical findings and also illustrate the practical significance of using dynamic pinching beamforming to improve the sum rate.