# Power-Efficient Resource Allocation for Active STAR-RIS-Aided SWIPT Communication Systems
## 1 Introduction  
With the development of Internet of Things (IoT) technology, the need for ubiquitous communication between devices will become stronger [1]. Low-power devices will be employed on a large scale; however, this will lead to very high energy consumption as well as significant carbon emissions [2]. To address energy and environmental issues, SWIPT technology is expected to be an integral part of 6G [3, 4]. SWIPT technology relies on wireless energy transmission technology, which allows simultaneous transmission of energy and information using the same frequency bands and waveforms in order to realize the synergy between ERs and IRs. The access point (AP) balances the energy reception and information transmission performance by optimizing the energy beamforming and information beamforming. However, the transmission efficiency and range of the SWIPT system are not ideal for long-distance transmission due to the fact that the energy andinformation emission are concentrated at the AP, and the path loss is high when the receiver is in the shadow area of the obstacle. With the aim of decreasing the SWIPT system's energy use, researchers have conducted research on full-duplex technology [5] and reconfigurable intelligent surface (RIS). A lot of reflective unit elements are distributed on the RIS plane, and the amplitude and direction of incoming signal can be adjusted by changing phase or amplitude parameters of units through coding control of circuits such as programmable gate arrays, thus expanding the coverage of the signal [6]. Therefore, the use of RIS can alleviate the problem of transmission loss caused by long-distance communication in SWIPT systems. A great deal of research has been conducted by scholars on passive RIS-assisted SWIPT techniques. One study [7] explored the simultaneous optimization of message rate and received power in passive RIS-assisted multi-input single-output downlink multi-user wireless networks and developed a practical algorithm. Another study [8] addressed the challenge of low far-field power transmission efficiency with a passive RIS-assisted SWIPT system. Utilizing sorting and iterative optimization algorithms, it sought to maximize the minimum rate for information receivers, both in ideal and non-ideal channels. In one study [9], passive RIS was employed to enhance the performance of cooperative non-orthogonal multiple access (C-NOMA). The objective was to maximize the data rate for cell-edge users while considering the energy constraints and minimum data rate requirements of cell-center users. Another study [10] presented the problem of maximum average secrecy rate, using RIS to enhance physical layer security in Rayleigh fading channels. One study [11] combined RIS with UAVs to simultaneously transmit information and collect energy by segmenting passive reflective arrays in geometric space, ensuring quality of service in dynamic wireless environments. A further study [12] investigated the deep integration of RIS and NOMA, which dramatically increased the data transfer rate in the Internet of Medical Things (IOMT) while significantly improving the safety of private users in RIS-assisted NOMA networks. These findings suggest that the RIS-assisted SWIPT system is able to further boost wireless system capability. However, to further decrease the power consumption and improve the performance of the system, active RIS has been widely used [13]. One study [14] proposed an active RIS-assisted SWIPT system to maximize the rate of IRs. The research results indicated that an active RIS-assisted SWIPT system can obtain superior performance gains compared to other systems. Another study [15] explored the issue of active RIS-assisted multi-user downlink transmission and rate maximization based on actual hardware impairments. The simulation results showed that active RIS achieves a higher total transmission rate compared to passive RIS.