# Performance Tradeoff Analysis of Hybrid Signaling SWIPT Systems with Nonlinear Power Amplifiers
## 1 Introduction
A research area that requires immediate attention is how to enhance the power efficiency of the total SWIPT system, which is a combination of the PA DC-to-RF conversion efficiency, RF-to-RF transmission efficiency, and the energy harvesting circuit RF-to-DC conversion efficiency. Especially when the signal power is low, the efficiencies are coupled with each other due to the nonlinearity of the energy harvester circuit [19]. Another promising area is the MIMO-OFDM modulation scheme; the use of multi-antenna beamforming can further increase the DC power of the energy harvesting circuit. However, it is more sensitive to the nonlinear distortion of PA, especially the phase distortion, and more efforts are needed to estimate the dynamic nature of a MIMO channel [21]. For the whole SWIPT system, more performance metrics at the  
Figure 1: Illustration of SWIPT systems in independent and coexistence modes. receiver side are needed to evaluate the efficiency of information transmission, such as signal-to-interference-plus-noise ratio (SINR) and bit-error-rate (BER). This also leads to interesting new challenges in terms of the information rate and delivered energy trade-off for the whole system. These works, along with the majority of theoretical SWIPT studies in the literature, imply an idealized assumption that energy receivers can effectively harvest radio signals of arbitrary power strength. However, in practice, due to the physical characteristics of electronic devices, RF energy harvesting is only feasible when the received signal strength is above a certain threshold. Compared with ID receivers that typically work at a signal strength of around \(-90\) dBm [36], EH receivers require a much higher signal strength of around \(-40\) dBm to function properly [37]. Such a drastic difference should be taken into account during the design of SWIPT signaling schemes. Two types of SWIPT signaling schemes have been proposed in the literature: identical signaling schemes and hybrid signaling schemes. The former uses a common transmit signal for EH and ID, as illustrated in Figure 1(a). The advantage of this scheme is that it encourages the maximum reuse of existing information signals transmitted by legacy wireless communication systems. However, as wireless communications systems are designed for high spectrum utilization, tight transmit power constraints are imposed on information-bearing wideband RF signals. As a result, the effective range of energy transfer in a SWIPT system with identical signaling would be very short; hence, this system has limited practical value. To overcome the drawback of identical signaling, a hybrid signaling scheme has been proposed [38], as shown in Figure 1(b). It consists of a low-power broadband information signal and a high-power, small-bandwidth energy signal [39]. The energy signal is typically an unmodulated multitone sinusoid signal, whose signal waveform can be optimized to improve the energy harvesting efficiency by increasing the peak-to-average power ratio (PAPR) [40; 41; 42; 43]. To date, reported research on hybrid signaling SWIPT has covered multiple topics, including the topics of beamforming and optimal power allocation [44; 45], the tradeoff between information transmission and energy transfer [46], and the impacts of baseband modulation on the efficiency of energy and data transmission [41]. In the existing SWIPT research literature, most studies have focused on the receiver side, while assuming that the transmitter performs ideal linear amplification on the signal. Receiver-side nonlinearity has also been studied in energy harvesting schemes for multiple applications such as IoT [47]. On the contrary, transmitter-side nonlinearity is still under-investigated. In practice, the high-power energy signal transmission may drive the transmit PA to work on the nonlinear region and cause serious interference with the information signal.