# Power-Efficient Resource Allocation for Active STAR-RIS-Aided SWIPT Communication Systems
## 1 Introduction
One study [16] investigated an algorithm for combined emission beamforming and energy-containing RIS reflection matrices to maximize energy efficiency using quadratic transform-based fractional order planning techniques. A different study [17] examined the complex and non-convex robust secrecy rate optimization problem when AP and RIS share identical power source. The research findings indicated that, in the context of active RIS, the proposed algorithm delivers superior secrecy rate performance in contrast to passive RIS. Another study [18] addressed the problem of limited performance gain of conventional passive RIS by proposing an active RIS-assisted multi-user multiple-input multiple-output SWIPT system, which achieved the system power minimization while satisfying the minimum communication rate demand and energy collection constraints of both the IRs and ERs. However, both passive and active RIS only consider the case of reflection and not transmission. The tunable components are integrated into the substrate of the RIS, and the wireless signal cannot penetrate the RIS, allowing no energy leakage into the space behind the RIS; therefore, the AP and users must be placed on identical side of RIS, resulting in utilization of only a half-space intelligent radio environment with very limited spectrum of space utilization, severely limiting the flexibility and effectiveness of the RIS. To overcome the limitations of RIS and thus make fuller use of spectrum resources, researchers have proposed a new concept--STAR-RIS, which is not only reflective but also transmissive compared to RIS, thus achieving 360\({}^{\circ}\) wireless coverage [19]. One study [20] investigated a multi-input single-output passive STAR-RIS-assisted NOMA downlink network, achieving maximum energy efficiency while balancing total rate and power consumption. A different study [21] proposed a network of passive STAR-RIS-assisted SWIPT systems applying the time switching (TS) protocol to maximize the total rate of all the information receivers by co-optimizing energy allocation of IRs and the beamforming vectors of passive STAR-RIS. [PERSON] investigated the max-min fairness optimization problem for SWIPT systems with assistance of passive STAR-RIS in imperfect channel states under the constraints of the signal-to-interference-plus-noise Ratio (SINR) of IRs, aiming to maximize the minimum power collected by the ERs [22]. Although the above work has achieved good results in passive RIS, active RIS, and passive STAR-RIS-assisted SWIPT systems, respectively, passive RIS-assisted SWIPT systems still have high power consumption due to signal fading issues, and passive RIS is only reflective but not transmissive, resulting in limited spectrum space utilization. Although active RIS can reduce the attenuation by amplifying the signal, it still does not have the function of transmission, and although passive STAR-RIS has the function of transmission and reflection, it cannot amplify the signal; therefore, an active STAR-RIS that combines signal amplification, transmission, and reflection to reduce the AP transmitting power to a greater extent has become a hotspot in current research. However, few existing studies involve the combination of active STAR-RIS and SWIPT techniques. To solve this issue, the present paper explores the problem of energy minimization in an active STAR-RIS assisted SWIPT system. The contributions of this paper are as below:  
* This paper constructs a SWIPT system model assisted by active STAR-RIS and consisting of AP, active STAR-RIS, multiple IRs, and multiple ERs.