A Near-Threshold, Multi-Node, Wireless Body Area
Network Sensor Powered by RF Energy Harvesting
Jiao Cheng1, Lingli Xia1, Chao Ma1, Yong Lian2, Xiaoyuan Xu3, C. Patrick Yue4, Zhiliang Hong5, Patrick Y. Chiang1
1
Oregon State University, Corvallis, OR; 2NUS, Singapore; 3CVPL, Singapore;
4
HKUST, HK; 5Fudan University, Shanghai, China
Abstract - A wirelessly-powered, near-threshold, body area
network SoC supporting synchronized multi-node TDMA
operation is demonstrated in 65nm CMOS. A global clock source
sent from a base-station wirelessly broadcasts at 434.16MHz to
all sensor nodes, where each individual BAN sensor is phaselocked to the base-station clock using a super-harmonic injectionlocked frequency divider. Each near-threshold SoC harvests
energy from and phase locks to this broadcasted 434.16MHz
waveform, eliminating the need for a battery. A Near-VT MICSband OOK transmitter sends the synchronized local sensor data
back to the base-station in its pre-defined TDMA slot. For an
energy-harvested local VDD=0.56V, measurements demonstrate
full functionality over 1.4m between the base-station and four
worn sensors, including two that are NLOS. The sensitivity of the
RF energy harvesting and the wireless clock synchronization are
measured at -8dBm and -35dBm, respectively. ECG Lead-II /
Lead-III waveforms are experimentally captured, demonstrating
the end-to-end system application.
Sensor3
OOK data
(EEG)
Sensor2 Sensor1
(Temp.)
402-405MHz
Data Path
(ECG)
433MHz
Smart Phone
(base station)
30dBm
18dBm
SensorN
(EMG)
Energy & Clock Path
Fig. 1. Proposed multi-node synchronized body area network
powered by RF energy harvesting.
I. INTRODUCTION
VDD (1.1V)
The simultaneous acquisition of multiple vital signs from
the human body, such as ECG, EEG, EMG, pulse oximetry,
activity, heart-rate, and temperature, will be a key
differentiating feature for next generation wireless body area
network (WBAN) systems. Energy-efficient designs have
been previously demonstrated that optimize an individual
wireless sensor node for low power operation [1-2]. However,
the necessary scheme for operating multiple nodes coherently
has been largely overlooked. The challenge is to minimize the
network protocol complexity and system power consumption,
while providing precise timing synchronization to enable dutycycled wake-up simultaneously of each node. Finally, batteryfree operation is desirable, since the battery is a significant
limitation to cost, size, and sensor lifetime. In this work, we
demonstrate a battery-less, multi-node WBAN system that
features: (1) wireless energy harvesting using power broadcast
from a base station such as a smart-phone; (2) duty-cycling
and TDMA synchronization of multiple nodes by employing
injection-locked wireless clock distribution.
II. WBAN ARCHITECTURE
A. Overall Architecture
Fig. 1 illustrates the system architecture of the proposed
WBAN for multiple wearable sensors. The base station (for
example, smartphone) broadcasts power and clock within the
433MHz ISM band to all the sensors, and also receives each
node’s allotted TDMA-based wirelessly-transmitted data that
occupies the 402-405MHz MICS band. The core of each
433MHz
wireless power
and clock from
base station
Match.
Net+
Balun
100uF
Voltage
Reference
Generator
VBG
(1.1V)
Bandgap
Rectifier
VREF2
Analog
Comparator
TDMA
Slot
Start
code 12b
Digital
Comparator
Counter
12b
MICS Tx
Enable
En
End code
12b
VDD (0.56V)
Digital
Comparator
/9
/8
/2
144MHz
16MHz
Clock Synchronization
/2
402MHz
data to
base
station
Data Gen
& Sel
MICS TX
Match.
Net
Pre Amp
Edge
Combiner
BB CLK SEL
ILFD
DIN
PA
off
Dummy
This Work
NODE1
VREF2
VDD (0.56V)
on
Wakeup
BB_clk
VREF1
ECG
Electrodes
VDD (0.56V)
Energy Harvesting
LDO
VREF1
Bio[3]
Sensor
EnergyHarvested
VDD = 0.56V
/2
fREF
SHILRO
80MHz
Fig. 2. System block diagram for each sensor node.
sensor node (Fig. 2) is a near-threshold SoC consisting of a RF
energy-harvesting front-end, a micro-power bandgap reference
generator, a low-dropout (LDO) regulator, a super-harmonic
injection locked frequency divider for clock synchronization, a
digital TDMA slot generator, and a 402-405MHz MICS-band
OOK transmitter. A biomedical signal acquisition chip from [3]
provides the sensor data input from a captured ECG waveform.
B. TDMA Protocol
As shown in Fig. 3, the WBAN base station initiates
operation by broadcasting a 2-ASK, 434.16MHz waveform in
Harvesting
Synchronized TDMA Transmission
Base
station
Rectifier
Unit 6
…
Wakeup (Comp.)
…
…
BB Clock
0
1
2 … B1 …… E1 … X
Node1
on
Slot 1
Enable
……
X … …
off
…
TX Out
…
BB Clock
…
Counter
Node2
Enable
1
2 … X
……
X … B2 …… E2 … …
on
off
Slot 2
…
w/o limiter (simulated)
w/ limiter (simulated)
w/ limiter (measured)
4
Rectifier
Unit 5
VDD (0.56V)
LDO
M4
out
3
Rectifier
Unit 4
in
2
1
-12
M3
Rectifier
Unit 3
0
-2
-4
-6
-8
-10
out
Input Power (dBm)
-20dBm
M2
PIN
M1
Rectifier
Unit 1
VRF -
…
VRF-
10u
150n
10u
150n
in
3pF
Matching
Network
20u
150n
Rectifier
Unit 2
VRF +
-8dBm
20u
150n
VRF+
20u
65n
Analog
Comp.
3pF
…
TX Out
0
VTOP (V)
…
VBG (1.1V)
Bandgap
M5
5
Rectifier Output
Counter
VTOP
Fig. 4. Merged rectifier-limiter energy-harvesting circuit.
Fig. 3. The TDMA handshaking flow chart for multiple nodes.
the 433MHz ISM band. During the energy-harvesting phase,
when the incoming power received by each sensor exceeds the
on-chip rectifier sensitivity (-8dBm), two off-chip surfacemount capacitors (100uF) are charged to 1.1V and 562mV,
respectively. The higher supply is used for powering the
bandgap reference and the comparator (2uW of total power),
while the lower supply powers the rest of the SoC. After the
energy-harvesting phase, the base-station transitions into data
transmission phase, controlled by the base station reducing its
434.16MHz broadcast signal strength by 12dB. This signal
amplitude reduction is then detected by the sensor’s analog
comparator, which then generates a wake-up signal. Local
clock synchronization to the base-station clock is achieved by
utilizing a divide-by-3 injection-locked frequency divider
(ILFD) that produces a 144.72MHz signal from the incoming
434.16MHz base station signal. As a result, the local baseband
clocks of all the sensor nodes are phase-locked to the central
base station. Once the wake-up signal is detected, a digitally
programmable counter within each sensor node begins
counting. The nodes interleave transmission based on their
pre-programmed TDMA time slot, set by the begin and end
codes (Bx and Ex in Fig. 3). The guard band interval between
two adjacent TDMA slots can be set either extremely short
(one data period) to minimize dead time and power
dissipation, or relatively long in order to provide margin for
any differences in time-of-flight between physically separated
nodes on the body.
C. Merged Rectifier-Limiter
A rectifier with a cross-coupled bridge configuration is
adopted here for both low on-resistance and small reverse
leakage [4]. Six identical rectifier units are stacked to boost as
small as a -8dBm incoming energy up to over 1.2V for
powering the bandgap. The second highest output voltage of
the rectifier (~0.75V) is fed into the LDO that supplies the rest
of the system-on-chip.
In the proposed multi-node WBAN system, the received
signal power at the input of each sensor node’s rectifier may
exhibit an extremely large dynamic range, due to distance and
Subharmonic Injection-Locked
Ring Oscillator
VBP
VDD = 0.56V
fREF (16MHz)
In
A1
A2
A3
A3
A4
A5
VDD = 0.56V
A1
VBN
Constant
Gm Bias VBN
A2
A2
A3
A4
Out
A5
VBP
VBN
A1
A4
A5
VDD =
0.56V
A2
A3
A4
A5
A1
A1
A2
A3
A4
A5
A2
A3
A4
A5
A1
VDD = 0.56V
PreAmp
Match
Net
Power
Amplifier
3b
Edge Combiner
Fig. 5. Near-VT MICS-band OOK transmitter.
channel loss variations between the base-station and the
sensors. As a result, a voltage limiter is needed to prevent any
over-harvested charge stored on the capacitors from damaging
any subsequent circuit block that employs thin-oxide
transistors. Fig. 4 illustrates the proposed merged rectifierlimiter circuit along with its simulated and measured
characteristics. Since the gate voltages of M1~M5 are set to
VBG = 1.1V, their drain nodes can only be charged up to V BG,
independent of the transistors’ source voltages. As a result, for
all received input powers (Pin) up to 0dBm, the output voltage
(VTOP) of the rectifier-limiter is limited to below 2.5V, which
is below the tolerance limit of the thick-oxide I/O devices
implemented within the rectifier.
D. Transmitter Architecture
The near-threshold MICS-band transmitter operating with a
harvested supply of VDD=0.56V is shown in Fig. 5. To
enhance the transmitter global efficiency (defined as the ratio
of the transmitter output power divided by the entire
(a)
(b)
Fig. 8. a) Measured TX output spectrum and b) TX & ILFD phase
noise.
25
Global Efficiency (%)
20
15
10
5
Fig. 6. Measured time-domain waveforms for TDMA transmission
of four sensor nodes from the front/back of person.
1
10mV
TX Output
Wakeup
20
30
Output Power (uW)
40
50
From 0 to 0.56V
0
10
-1
10
-2
10
From 0.55V to 0.56V
-3
10
5.55ms
10
Fig. 9. Measured TX global efficiency vs. output power.
10
Time (s) for Charging
100uF LDO Load
Energy-Harvested VDD
0
0
-8
-6
-4
-2
0
PIN (dBm)
Fig. 7. Measurement results of the energy-harvesting front-end.
transmitter power consumption), a sub-harmonic injectionlocked ring oscillator (SHILRO), and edge combiner are
employed to generate the 402-MHz carrier [5]. Compared with
traditional phase-locked loops, this SHILRO structure has the
advantage of fast start-up time, which facilitates the precise
duty cycling requirements for the multi-node TDMA operation.
The 16.08-MHz local reference clock derived from the
434.16-MHz RF input is injected into the 80.4-MHz, 5-stage
SHILRO, eliminating the need for an off-chip crystal
oscillator for each sensor. An inverter-based pre-amplifier is
added between the edge combiner and the class-C power
amplifier to ensure sufficient driving capability.
Programmable output power for the power amplifier is
achieved by a 3-bit current DAC, which tunes the current
flowing through the bottom resistor, modifying the bias
voltage.
III. EXPERIMENTAL RESULTS
Fig. 6 shows the lab setup and the measured waveforms for
four sensor nodes operating simultaneously. A signal
generator (Agilent 8643A), used as the base station, transmits
a 2-ASK 434.16MHz signal with a +30dBm output power to
four sensor nodes placed on a user standing 1.4m away. The
measured sensitivity of the rectifier and the ILFD are -8dBm
and -35dBm, respectively, using a quarter-wavelength antenna
for the base station and 1.8inch 433MHz antennas for the
sensors. Sensor Node3 and Node4 are placed on the back side
of the user to demonstrate full functionality for non-line-ofsight operation.
As shown in Fig. 7, using two 100uF surface-mount
capacitors, the energy-harvested supply voltages (1.1V and
0.56V) can remain stable for 5.55ms before exhibiting a 10mV
drop at the LDO output, when the MICS transmitter is sending
data at a 1Mbps data rate with -16dBm output power. When
the minimum rectifiable input power at -8Bm is received,
5.8ms is required to charge up the 100uF capacitors by 10mV.
Hence, for a 25% duty-cycle duration between harvesting and
transmission modes for a network of four nodes, the overall
effective data rate per sensor is over 180kbps. Furthermore,
the proposed periodic harvesting scheme allows the trade-off
between the storage capacitance size and the duty-cycle ratio
between harvesting and transmitting, expanding the range of
applications to other cost and size constrained scenarios.
The phase noise of the 402MHz carrier shows only minor
degradation as the 434.16MHz received power decreases (Fig.
8), insuring robust radio operation even as the surrounding
environment and wireless channel conditions alter. The
carrier-to-spur ratio at the transmitter output is a measured
31.2dBc for a 16.08MHz spacing. The measured global
transmitter efficiency is over 16% when the output power is
25uW, as shown in Fig. 9.
TABLE I: PERFORMANCE SUMMARY
Technology
65nm-CMOS
Die Size
1mm x 1mm
Harvested VDD for TDMA Slot, Clock Sync. and TX
0.56V
Frequency Band (Harvesting)
433MHz
Frequency Band (TX Transmission)
402-405MHz
MICS TX Output Power (Pout)
-27dBm ~ -13dBm
MICS TX OOK Data Rate
250kbs-2Mbps
Transmission Time Before 10mV Drop in Harvested VDD
5.55ms
Number of Synchronized Nodes (Measured)
4
Sensitivity (Harvesting)
-8dBm
Sensitivity (Clock Synchronization)
-35dBm
Max Experimental Distance for a Fully Operational Sensor
1.4m
MICS TX Global Efficiency
16.7% (When Pout = -16dBm)
Power Break Down
Bandgap
< 1uW
LDO
< 1uW
Digital TDMA Slot
< 1uW
Clock Synchronization RX
8uW
MICS TX
150uW (When Pout = -16dBm)
Fig. 10. Die photo.
ECG Lead-III (mV)
ECG Lead-II (mV)
TABLE II: MULTI-NODE BAN PERFORMANCE COMPARISON
[6]
[7]
Technology
0.18um
0.18um
65nm
Supply
0.9V
1V
0.56V
RF Energy Harv.
2
1.5
Original
1
Reconstruction
This work
Power Source
Remote Battery
N/A
0.5
Channel
eTextiles
BCC
MICS/ISM
0
Frequency Band
10MHz (Clock)
40M - 120MHz
402MHz/433MHz
Multiple Access
TDMA/CSMA
FDMA
TDMA
Range
1m
N/A
1.4m
TX Energy/bit
0.7-18pJ
0.20nJ
0.15nJ
TX Data Rate
10Mbps
1k-10Mbps
250k-2Mbps
0
0.5
1
Time (s)
1.5
2
2
1.5
Original
1
Reconstruction
0.5
0
0
0.5
1
Time (s)
1.5
2
Fig. 11. Measured ECG Lead-II/Lead-III signals.
Fig. 10 shows a die photo of the 1mm x 1mm body-area
network prototype, fabricated in a 65nm CMOS technology.
Fig. 11 shows the ECG waveforms of both Lead-II and
Lead-III of the subject under test, when the BAN chip is
connected with a biomedical sensor chip interface [3] that is
powered by the bandgap from our chip. The RF data
transmitted by the MICS-band TX is sampled by an
oscilloscope (Tektronix TDS7404) and reconstructed in
MATLAB.
The performance summary and comparison with previous
body area network prototypes are summarized in TABLE-I
and TABLE-II, respectively.
IV. CONCLUSION
This work proposed a wirelessly-powered, body area
network SoC supporting synchronized multi-node operation.
Wireless clock synchronization based on an injection-locked
frequency divider enables low-overhead TDMA duty-cycled
transmission for multiple nodes. RF energy harvesting further
eliminates the requirement of the battery. Both techniques help
reduce the sensor node’s size, weight and cost, and enable the
future possibility for disposable wearable sensors.
ACKNOWLEDGEMENTS
This work was funded by grants from the Center for the
Design of Digital-Analog Integrated Circuits (NSF-CDADIC),
NSF-0901883, and the Catalyst Foundation. The authors thank
Yajie Qin for help with the chip fabrication.
REFERENCE
[1] F. Zhang, Y. Zhang, J. Silver, Y. Shakhsheef, M. Nagaraju, A. Klinefelter,
J. Pandey, J. Boley, E. Carlson, A. Shrivastava, B. Otis, B. Calhoun, “A
Batteryless 19uW MICS/ISM-Band Energy Harvesting Body Area Sensor
Node SoC,” ISSCC Dig. Tech. Papers, pp. 298-299, Feb. 2012.
[2] M. Vidojkovic, X. Huang, P. Harpe, S. Rampu, C. Zhou, L. Huang, K.
Imamura, B. Busze, F. Bouwens, M. Konijnenburg, J. Santana, A.
Breeschoten, J. Huisken, G. Dolmans, H. de Groot, “A 2.4GHz ULP OOK
Single-Chip Transceiver for Healthcare Applications,” ISSCC Dig. Tech.
Papers, pp. 458-459, Feb, 2011.
[3] X. Zou, X. Xu, L. Yao, Y. Lian, "A 1-V 450-nW Fully Integrated
Programmable Biomedical Sensor Interface Chip", IEEE J. Solid-State
Circuits, vol. 44, no. 4, pp. 1067-1077, Apr. 2009.
[4] K. Kotani, A. Sasaki, T. Ito, "High-Efficiency Differential-Drive CMOS
Rectifier for UHF RFIDs", IEEE J. Solid-State Circuits, vol. 44, no. 11,
pp. 3011-3018, Nov. 2009.
[5] J. Pandey, B. Otis, “A 90uW MICS/ISM Band Transmitter with 22%
Global Efficiency,” IEEE Radio Frequency Integrated Circuits, pp. 285288, 2010.
[6] P. Mercier, A. Chandrakasan, “A 110μW 10Mb/s eTextiles Transceiver
for Body Area Networks with Remote Battery Power,” ISSCC Dig. Tech.
Papers, pp. 496-497, Feb, 2010.
[7] J. Bae, K. Song, H. Lee, H. Cho, L. Yan, H. Yoo, “A 0.24nJ/b Wireless
Body-Area-Network Transceiver with Scalable Double-FSK Modulation,”
ISSCC Dig. Tech. Papers, pp. 34-35, Feb, 2011.