# Performance Tradeoff Analysis of Hybrid Signaling SWIPT Systems with Nonlinear Power Amplifiers
## 1 Introduction  
Along with the rapid commercialization of the 5 th generation (5G) mobile communication technology, research and development of the 6 th generation (6G) network is gaining momentum [1]. Enabled by 5G and 6G technologies, Internet of things (IoT) is quickly becoming ubiquitous in people's daily lives. A key challenge hindering the proliferation of IoT is to provide power-deficient sensor devices with a reliable and sustainable energy supply [2]. Conventional energy supply technologies, such as wired charging [3], near-field wireless charging [4], and renewable energy harvesting (EH) [5], are limited by either the short distance or the availability of energy sources; hence, their applicability is constrained to specific scenarios. Exploiting the fact that radio signals are widely used for modern wireless communication systems, radio frequency energy transfer (RFET) technology was proposed as a ubiquitous energy supply technology that can continuously harvest the weak energy of far-field radio signals from the environment [6]. Simultaneous wireless information and power transfer (SWIPT) systems [7] take a step further to propose that concurrent information transmission and energy transfer can be integrated into a single system and jointly optimized at both the signaling and device levels. In a typical SWIPT scenario, information and energy transmitters share the same transmitting device, whereas receivers for energy harvesting (EH) and information decoding (ID) are not necessarily co-located. Based on whether the EH and ID receivers are co-located, SWIPT systems have two operational models: standalone and co-located models. The former uses separated antennas on different devices for EH and ID, respectively, as shown in Figure 1a. The latter has one device for signal reception and needs toseparate the analog signal received by the antenna (or antenna array) into two signals by means of time division [8; 9; 10], power division [11; 12; 13; 14], space division [15; 16; 17], or frequency-splitting [18]. The separated signals are then fed into the ID receiver and EH receiver, respectively, as shown in Figure 1b. The potential benefits of SWIPT systems include improved spectrum utilization, reduced overall energy consumption, and reduced cost with shared infrastructure. Nevertheless, significant challenges remain in SWIPT systems to balance the conflicting goals of information transmission and energy transfer and ensure the coexistence of the two systems [19; 20; 21]. As a potentially disruptive technology, SWIPT has attracted significant research attention in recent years. Papers [22; 23; 24] analyzed the trade-off between the spectrum efficiency and energy efficiency of information transmission from the perspective of information theory. The authors in [25; 26; 27] studied the optimization of coding, modulation, and resource allocation. The problem of optimal power allocation in wireless propagation channels under transmit power constraints was investigated in [28; 29]. Paper [30] studied the issue of receiver operation and dynamic power allocation. Paper [31] exploited the non-orthogonal-multiple-access (NOMA) technique and proposed a transmit power allocation scheme to improve the spectrum efficiency of a SWIPT system. A comprehensive survey of recent advances in SWIPT systems was given in [32]. The nonlinear model of energy harvesting is also an important research topic for SWIPT systems [20]. Paper [33; 34; 35] studied the intelligent reflecting surface (IRS) system and investigated how to use IRS to improve the energy collection efficiency of a SWIPT system. In order to highlight the tradeoff between SNDR and PA efficiency, the above discussion focused on the transmitter side with the basic single-input-single-output (SISO) hybrid signal. When extended to the whole SWIPT system, there remain many important and need research areas, such as energy harvesting circuit, signal modulation, multiple-input-multiple-output (MIMO), energy waveform design, and multipath channel estimation [19; 20; 21].