IEEE Microwave Magazine - September/October 2018 - 79

PXI Equipment

Tx Antenna
Transmitter

Signal
Optimization

Signal
Generation

External
Power Amplifier
Coaxial Cable

Power Amplifier

Rx Antenna
Pilot

RF Switch
or Power Splitter

IFFT

OFDM Modulator
for Channel Estimation

Receiver
Part A

Channel
Estimation

Local
Oscillator
2.4 GHz
Receiver
Part B

FFT

Energy Harvester

OFDM Demodulator
for Channel Estimation

downlink from one or more access points to one
or more receivers. The energy receiver(s) [ER(s)]
and information receiver(s) [IR(s)] can be colocated or separated. In SWIPT with separated receivers, the ER and IR are different devices: the ER
is a low-power device being charged; the IR is a
device receiving data. In SWIPT with colocated
receivers, each receiver is a single low-power
device that is simultaneously being charged and
receiving data.
* Wirelessly powered communication network: Energy
is transferred in the downlink, and information
is transferred in the uplink. The receiver is a lowpower device that harvests energy in the downlink and uses it to send data in the uplink.
* Wirelessly powered backscatter communication (WPBC):
Energy is transferred in the downlink, and information is transferred in the uplink; however, backscatter modulation at a tag is used to reflect and
modulate the incoming RF signal for communication with a reader. Because tags do not require
oscillators to generate carrier signals, backscatter
communications benefit from orders-of-magnitude
lower power consumption than conventional radio
communications. RFID is an example of WPBC.
A major challenge in WIPT is to characterize the
fundamental tradeoff between conveying information and energy (or harvested dc power) wirelessly
[80]-[82] and identify corresponding transmission and
reception strategies.
A tradeoff between rate and energy exists because
the transmit signal used to maximize rate is not

September/October 2018

Harvested dc Power (µW)

Figure 8. The prototype architecture with three key modules: signal optimization, channel acquisition, and energy harvester
[77], [78]. FFT: fast Fourier transform; IFFT: inverse FFT; OFDM: orthogonal frequency-division multiplexing.

3.5
3
2.5
2
1.5
1
0.5
0

1 Tone

2 Tone 4 Tone 8 Tone 16 Tone
Number of Sinewaves N
Nonadaptive Multisine Waveform
Adaptive Multisine Waveform

Figure 9. Harvested dc power using the architecture of
Figure 8 in an indoor NLoS deployment as a function of
N (N sinewaves whose frequencies are uniformly spread
within a 10-MHz bandwidth) [78].
necessarily the same as the one that maximizes energy.
A simple example is obtained when we want to transmit information and energy simultaneously across
multiple subbands on a frequency-selective channel
[81]. To maximize the amount of energy collected at
the receive antenna (and, therefore, maximize e 2), one
would transmit all power on the strongest subband,
i.e., the one corresponding to the strongest frequencydomain channel. Unfortunately, such a strategy does
not necessarily maximize the information rate. To
maximize this rate, it is more efficient to transmit
information across multiple subbands and allocate
power across subbands according to the well-known

Table of Contents for the Digital Edition of IEEE Microwave Magazine - September/October 2018