  Waveform optimization has shown its great potential to boost the performance
of far-field wireless power transfer (WPT). Current research has optimized
transmit waveform, adaptive to channel state information (CSI), to maximize the
harvested power in WPT while accounting for energy harvester (EH)'s
non-linearity. However, the existing transmit waveform design disregards the
non-linear high power amplifiers (HPA) at the transmitter. Driven by this, this
paper optimizes the multi-carrier waveform at the input of HPA to maximize the
harvested DC power considering both HPA's and EH's non-linearities. Two
optimization models are formulated based on whether the frequencies of the
multi-carrier waveform are concentrated within the transmit pass band or not.
Analysis and simulations show that, while EH's non-linearity boosts the power
harvesting performance, HPA's non-linearity degrades the harvested power.
Hence, the optimal waveform shifts from multi-carrier that exploits EH's
non-linearity to single-carrier that reduces HPA's detrimental non-linear
distortion as the operational regime of WPT becomes more sensitive to HPA's
non-linearity and less sensitive to EH's non-linearity (and inversely).
Simultaneously, operating towards HPA's non-linear regime by increasing the
input signal power benefits the harvested power since HPA's DC power supply is
better exploited, whereas the end-to-end power transfer efficiency (PTE) might
decrease because of the increasing non-linear degradation. Throughout the
simulations, the proposed waveforms show significant gain over those not
accounting for HPA's non-linearity, especially in frequency-flat channels. We
also compare the two proposed waveforms and show that the severity of HPA's
non-linearity dictates which of the two proposed waveforms is more beneficial.
