2020 IEEE Wireless Power Transfer Conference (WPTC)
November 15 - 19, 2020, Seoul, Korea
Low-Power Wireless Communication for Wireless
Power Transfer Device
Yong Hun Jang
Sang Hyun Lee
Illsoo Sohn
School of Electrical
Engineering
Korea University
Seoul, Korea
disclose@korea.ac.kr
School of Electrical
Engineering
Korea University
Seoul, Korea
sanghyunlee@korea.ac.kr
Dept. Of Computer
Science and Engineering
Seoul National University
of Science and Technology
Seoul, Korea
isohn@seoultech.ac.kr
Muhammad Fithratur
Rahman
Dept. Of Computer
Science and Engineering
Seoul National University
of Science and Technology
Seoul, Korea
fithraturrahman@seoultech
.ac.kr
Abstract—The Implantable Medical Devices (IMDs) are
playing an increasing role for its benefit in monitoring and
detecting a patient's disease symptoms. One of the challenges
faced by the IMD developer is the need for a long lifetime of the
devices. Along with the implementation of wireless power
transfer (WPT) technology, a low-power communication
between the implants and the receiver outside the patient's body
is needed to ensure a low medical cost and risk from the need of
battery replacements through surgery. Adopting optical
wireless communication, in this article, a novel approach to
solve the problem is introduced. Using an unsynchronized pulseinterval modulation (UPIM) technique, a system working
principle is addressed, and a prototype is also built within a
10mm x 10mm x 2.3mm form factor. Experiments are
conducted to test the system design, ensuring its ability to carry
out low-power wireless communication with µW-level power
consumption. The result shows that the proposed approach can
achieve as low as 392uW power consumption. Finally, the
technical challenges along with the possibility for further
development is discussed.
Keywords—optical
power transfer device.
communication,
low-power,
Fig 1. IMD biosensing scenario using the optical wireless communication
to transmit and receive data.
IMDs. IMDs are mostly a battery-powered device, which
battery needs to be replaced when the energy has drained. A
battery replacement procedure for IMDs is not a trivial task
and demands a costly invasive procedure that is inconvenient
to the patient and prone to infection [5]. The other limitations
are the size of the IMDs, which need to be kept minimal to
reduce the patient's discomfort feelings caused by a foreign
object in their body. The next issue is that transmit power
needed to overcome the high tissue channel path loss to meet
the stringent specific absorption rate (SAR) requirements.
Finally, energy dissipation from the IMDs should not exceed
wireless
I. INTRODUCTION
Implantable medical device (IMD) has been rapidly
developed and deployed over recent decades. The IMD
industries are forecasted to grow $116.3 billion during the
period 2016-2022, which corresponds to a 7.1% annual
growth rate [1]. The advancement in IMD development is
highly enforced by the advancement of emerging technologies
including nanomaterials and microelectronics. The IMD
history dates to 1958 [2],[3]. The first deployment of
commercial heart pacemakers is followed by various other
IMD applications, such as the oximeters, bionic vision
implants, and glucose sensors [4]. Medical applications
incorporate bio-signal data reading as a sign of the body's
physiological conditions and stored the data by passing it to
the external access point via the wireless communication
channels. This channel is subject to different characteristics
with the typical technologies that transmit data over the free
air medium.
what the body can tolerate as it can increase the risk of
developing cancer.
Studies on using wireless power transfer (WPT) for IMDs
to reduce the need of battery replacement surgeries have been
conducted to provides alternative for a one-time battery as the
energy source of an IMDs [6],[7]. Low-power communication
approaches, such as medical implants communication system
(MICS), working at 402MHz-495MHz frequency band [8],
ultrasonic communication [9], inductively coupled
communication [10], and optical infrared communication [11],
have been introduced recently to further increase the lifetime
of an IMD. The approaches have different characteristics,
including their data rate, maximum penetration depth and
distance, size, and use case. A new optical infrared wireless
technique is studied in this article. The illustration of the IMD
system using the concept is depicted in Fig. 1. The proposed
working principle of the optical wireless communication is
introduced. Afterwards, the system design and the structure
Living human tissue is a poor communication channel due
to its high absorption rate of the electromagnetic signals.
Higher power for transmission is needed to compensate for the
loss when the signals traveled through the human skin and
other tissues. This, in turn, imposes critical limitations for
This work was supported in part by the National Research Foundation
of Korea (NRF) grant funded by the Korea government (MSIT) (No.
2017R1A5A1015596, NRF-2019R1A2C1090650).
978-1-7281-4238-8/20/$31.00 ©2020 IEEE
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Fig 2. Proposed low-power optical communication system design and protocol
results are described. The experiments to measure the
implemented design is carried out to verify a µW-level lowpower communication is achieved.
III. DESIGN AND IMPLEMENTATION
A. System Structure
The system is designed as depicted in Fig 2. The
transmitter part includes a modulator part, and an optical
driving circuit with an IR LED for transmitting in 850nm
wavelength. This wavelength corresponds to the 750nm –
950nm spectrum which has low penetration loss in human
skin tissue. The receiver part is consisted of a PD, which will
receive the incoming light and turn it into electrical current
that corresponds to the light intensity received. An amplifier is
used to increase the power of the signal and then a comparator
decides if the signal is an intended pulse by comparing it with
a certain threshold. The last part is the UPIM demodulator that
will demodulate the reconstructed signal into a readable biosignal data.
II. LOW POWER OPTICAL COMMUNICATION
A. Skin Penetration Channel
Skin penetration loss is one of the major challenges when
designing a low-power wireless communication for IMDs.
The typical RF spectrum is easily absorbed by the skin due to
its high absorption rate. This leads to the need for a higher
transmission power to deliver the data through skin tissue.
However, a specific spectrum between 600nm to 950nm is
found to have a low absorption rate [12]. The use of visible
light around this spectrum is excluded due to high interference
from ambient light that can cause errors in data transmission.
Therefore, an infrared (IR) spectrum between 750nm to
950nm is the best candidate for this application. An IR lightemitting diode (LED) is carefully picked for the designed
system.
B. Optical Communication Protocol
The typical bio-monitoring applications use a relatively
low sampling frequency in comparison to the transceiver
clock speed, ranging from dozens to hundreds of Hz. On the
other hand, the biosensing data is usually more delay sensitive
and thus a real-time streaming data transmission is
recommended compared to the batch transmission method. A
new protocol is developed for a sparse but delay sensitive
nature transmission. An unsynchronized pulse interval
modulation is adopted instead of synchronized method to
avoid high power demand for synchronization between the
transmitter and the receiver.
B. Power-Efficient Transmission
The fast advancement of LED and Photodiode (PD)
technology makes it very attractive for implementing lowpower communication. The new LED technologies feature a
small-sized, fast transition time, and high-power efficiency
package. One simple modulation is the on-off keying (OOK),
which presents binary data as on/off state of an LED. The
commercially available short-range optical wireless system,
such as the Infrared Data Association (IrDA), fails to satisfy
the µW-level communication power.
The proposed PIM protocol is depicted in Fig 2. The
transmitter will be in standby state most of the time. Upon
receiving the data from the biosensing unit, it will wake up
and transmit the 1st pulse in 1us period. Then it will go into
chip counting state, which uses a timer to count how many
chips have passed, with other functionality is turned off. If the
chip for the symbol interval is passed, the chip will wake up
again and transmit the 2nd pulse. The size of the chip in this
design is 2us. A wider chip will increase robustness to jitter
but consumes more power. The system is designed for an
optimized value between the two.
For more power-efficient data transmission, a pulse
interval modulation (PIM) technique can be employed. This
technique features modulation with the interval time between
pulses. The input message symbol will correspond to the
duration of the interval, and at the receiver side, this duration
will be counted and interpreted as the symbol value.
Compared to the OOK modulation, the PIM employs a smaller
number of optical pulses which then will lead to lower energy
consumption per symbol.
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Fig 3. Experiment results: average current consumption of the transmitter versus data rate (left), average maximum distance achieved versus supply voltage
applied (right)
rate. The maximum distance achieved by the 20mA
transmitter module is 93cm with an average current
consumption of 133uA at 5V supply voltage.
C. Prototype Implementation
A testbed for implementing and testing the low-power
wireless communication system design is made in the form of
a transceiver prototype. For the transmitter, an 8-bit
microcontroller unit (MCU), ATMEGA328PB, is chosen as
the UPIM modulator part. The UPIM modulator implements
the unsynchronized pulse interval modulation protocol. For
implementing the optical transmitter part, an IR LED type
with 850nm peak wavelength, the SFD4240s, is used. A
considerable amount of effort is put into packing these
components and its supporting circuits into a small PCB with
10mmx10mmx2.6mm in size. The receiver part, on the other
hand, is not restricted too much because it will operate outside
of the body. A high-speed PIN PD, BPV22NF is used for
sensing the optical pulse and then turned it into electrical
pulses. The electrical pulses will then be reconstructed by a
built-in integrated amplifier and comparator, MAX3120.
ATMEGA328P is adopted in the last part as the demodulator,
retrieving the bio-signal.
C. Power Consumption vs Data Rate
The second experiment is conducted to measure the
variation of power consumption due to the variation of the
data rate. Three types of low-power wireless communication
modules are examined. The HM-10 BLE 4.0 module, the
MIKROE Click 3 IrDA module, and the proposed optical
wireless transmission module. The average current
consumption increases as the data rate used increases in all
three modules. The proposed design reduces a considerable
amount of power consumption in comparison to the two other
transmission modules. The proposed module design with
20mA forward current module uses only 119uA, including the
power for UART communication for reading the input. At
3.3V operating voltage, it corresponds to 392µW. This result
makes the proposed low-power optical wireless
communication approach attractive for low power IMDs.
IV. EXPERIMENTAL RESULTS
V. CONCLUSION
A. Experiment Setup
The proposed low-power optical wireless system design
performance is verified through experiments. The transmitter
part is placed in a line of sight with the receiver part facing
each other. A multimeter is connected between the transmitter
power line to measure the average current at 3.3V applied
voltage for the power consumption vs data rate measurement.
The biosensing signal is made by emulated ECG waveforms
and sent to the transmitter part using UART protocol in 16 bits
per sample and 250 Hz sampling rate. The output of the
demodulator at the receiver part is monitored using a laptop
PC that records the received signal and plots the monitored
ECG.
This article addresses major technical challenges faced in
designing a communication system for IMD. The approach
using optical spectrum that has high skin penetrability and the
efficient data transmission technique is one enabler for the
emerging IMDs. The developed approach achieves µW-level
power consumption for transmission, which hard to get using
typical RF communication. The optical spectrum also has high
safety for human skin in comparison to the typical RF
spectrum based on the SAR limitations. For IMDs that
requires low-power communication to enhance its lifetime
and relieves the patient with battery replacement surgery, the
proposed system design and implementation serve as a
guideline to fulfill the requirements.
B. Power Consumption vs Distance
The first experiment is conducted to see how the
maximum reachable distance is related to the current
consumed by the transmitter. For this experiment, two types
of transmitters are made. The first transmitter is set to give
20mA forward current to the LED, and the second transmitter
uses a 100mA forward current to the LED. In this experiment,
the data shows a longer distance achieved using the 100mA
transmitter module, as displayed in Fig 3. The farthest distance
of 209cm is achieved with the 100mA transmitter settings at
5V supply, with an average current of 208uA at 4kbps data
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