  Over the last decade, simultaneous wireless information and power transfer
(SWIPT) has become a practical and promising solution for connecting and
recharging battery-limited devices, thanks to significant advances in low-power
electronics technology and wireless communications techniques. To realize the
promised potentials, advanced resource allocation design plays a decisive role
in revealing, understanding, and exploiting the intrinsic rate-energy tradeoff
capitalizing on the dual use of radio frequency (RF) signals for wireless
charging and communication. In this paper, we provide a comprehensive tutorial
overview of SWIPT from the perspective of resource allocation design. The
fundamental concepts, system architectures, and RF energy harvesting (EH)
models are introduced. In particular, three commonly adopted EH models, namely
the linear EH model, the nonlinear saturation EH model, and the nonlinear
circuit-based EH model are characterized and discussed. Then, for a typical
wireless system setup, we establish a generalized resource allocation design
framework which subsumes conventional resource allocation design problems as
special cases. Subsequently, we elaborate on relevant tools from optimization
theory and exploit them for solving representative resource allocation design
problems for SWIPT systems with and without perfect channel state information
(CSI) available at the transmitter, respectively. The associated technical
challenges and insights are also highlighted. Furthermore, we discuss several
promising and exciting future research directions for resource allocation
design for SWIPT systems intertwined with cutting-edge communication
technologies, such as intelligent reflecting surfaces, unmanned aerial
vehicles, mobile edge computing, federated learning, and machine learning.
