  The efficiency of wireless information and power transfer (WIPT) systems
requires an essential reevaluation and rethinking of the entire transceiver
chain, which is characterized by a bottom-up multi-layer design approach. In
this paper, we introduce and describe the key design layers: i) "Mathematical
modeling", associated with the investigation of mathematical models for the
wireless power transfer process, ii) "Information-theoretic limits", which
refers to the fundamental limits of the WIPT channel, iii) "Link design",
corresponding to signal processing techniques that make WIPT feasible, iv)
"System-level perspective", which studies the developed WIPT techniques from a
macroscopic system-level point-of-view, and v) "Experimental studies", that
refers to real-world implementation of WIPT systems. These layers are
well-connected and their interplay is imperative for the effective design of
WIPT systems. Specific case studies are discussed, which demonstrates the
interdisciplinary nature of the aforementioned multi-layer design framework.
