IEEE Microwave Magazine - September/October 2018 - 74
dance with the channel status [commonly called
channel state information (CSI)], rendering state-ofthe-art multiple-input/multiple-output (MIMO)
processing an indispensable part of WPT. An
example of a closed-loop and adaptive WPT architecture is shown in Figure 2.
* A systematic approach to design and optimize the signal at the transmitter, as a function of the channel
(encompassing beamforming and waveform) to
maximize e 2 # e 3 subject to transmit power and
PAPR constraints. This requires that the rectifier's nonlinearity and the wireless channel be
captured as part of signal design and optimization. Such a systematic design methodology will
lead to the implementation of efficient strategies
as part of the "Transmission Optimization" module in Figure 2.
* A link and system design approach that takes wireless
power from a rectenna paradigm to a network paradigm, with multiple transmitters and/or receivers.
Instances of such network architectures may be a
deployment of colocated transmit antennas delivering power to multiple receivers or a dense and
distributed deployment of well-coordinated antennas/transmitters, as illustrated in Figure 3.
* WPT design has remained very much centered
around an open-loop approach, with the waveform being static and beamforming relying on tag
localization not on channel state.
* The design of the transmit signals is heuristic
(with conclusions based exclusively on observations obtained from measurements using
various predefined and standard waveforms),
but no systematic approach and performance
bounds exist to design and evaluate them. The
waveform and beamformer have been studied
independently, despite being part of the same
transmit signal.
Tackling the Challenges
Effectively tackling the challenges discussed so far
will require several developments:
* A closed-loop and adaptive WPT architecture with a
reverse communication link from the receiver to the
transmitter that supports various functions, such
as channel feedback/training, energy feedback,
and charging control. The transmitter should be
able to flexibly adjust the transmission strategy,
jointly optimized across space and frequency
(through beamforming and waveform) in accor-
Power Transmitter
RF
Source
Rx
Antenna
Tx
Antenna(s)
Transmission
Optimization
Rectenna
Propagation
Channel
Reverse
Communication Link
PMU
Matching
Network
dc-dc
Converter
Storage
Rectifier
Channel Estimation
and Feedback
Figure 2. A block diagram of a closed-loop and adaptive WPT architecture.
WPT with Colocated Antennas
WPT with Distributed Antennas
PR
PT
PR
PR
(a)
PR
PT
PR
PT
PT
(b)
Figure 3. An illustration of a WPT network with (a) colocated and (b) distributed transmit antennas and multiple receivers.
PT: power transmitter; PR: power receiver.
74
September/October 2018
Table of Contents for the Digital Edition of IEEE Microwave Magazine - September/October 2018
Contents
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