STAR-RIS Assisted SWIPT Systems: Active or Passive?
Guangyu Zhu, Xidong Mu, Li Guo, Ao Huang, Shibiao Xu
TL;DR
This work compares active and passive STAR-RIS implementations in a multi-user SWIPT system with blocked direct channels. It formulates a power-minimization problem and develops two optimization frameworks: a penalty-based alternating optimization for the passive STAR-RIS to handle coupled phase shifts, and an SDR/SCA-based joint beamforming plus amplification design for the active STAR-RIS. Numerical results show that active STAR-RIS provides gains at small aperture sizes, while passive STAR-RIS offers robustness under a fixed power budget, with the gap between them narrowing as aperture grows. The findings inform STAR-RIS hardware decisions for SWIPT in future networks by clarifying the trade-offs between amplification power, hardware cost, and near-field effects.
Abstract
A simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted simultaneous wireless information and power transfer (SWIPT) system is investigated. Both active and passive STAR-RISs are considered. Passive STAR-RISs can be cost-efficiently fabricated to large aperture sizes with significant near-field regions, but the design flexibility is limited by the coupled phase-shifts. Active STAR-RISs can further amplify signals and have independent phase-shifts, but their aperture sizes are relatively small due to the high cost. To characterize and compare their performance, a power consumption minimization problem is formulated by jointly designing the beamforming at the access point (AP) and the STAR-RIS, subject to both the power and information quality-of-service requirements. To solve the resulting highly-coupled non-convex problem, the original problem is first decomposed into simpler subproblems and then an alternating optimization framework is proposed. For the passive STAR-RIS, the coupled phase-shift constraint is tackled by employing a vector-driven weight penalty method. While for the active STAR-RIS, the independent phase-shift is optimized with AP beamforming via matrix-driven semidefinite programming, and the amplitude matrix is updated using convex optimization techniques in each iteration. Numerical results show that: 1) given the same aperture sizes, the active STAR-RIS exhibits superior performance over the passive one when the aperture size is small, but the performance gap decreases with the increase in aperture size; and 2) given identical power budgets, the passive STAR-RIS is generally preferred, whereas the active STAR-RIS typically suffers performance loss for balancing between the hardware power and the amplification power.