A Near-Threshold, Multi-Node, Wireless Body Area Network Sensor Powered by RF Energy Harvesting

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Nov 21, 2013 (3 years and 6 months ago)

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A
Near
-
Threshold,
Multi
-
Node
,

Wireless Body

Area
Network

Sensor
Powered by RF Energy Harvesting


Jiao Cheng
1
, Lingli Xia
1
, Chao Ma
1
, Yong Lian
2
, Xiaoyuan Xu
3
, C. Patrick Yue
4
, Zhiliang
Hong
5
, Patrick Y. Chiang
1


1
Oregon State University, Corvallis, OR;
2
NUS, Singapore;
3
CVPL, Singapore;

4
HKUST, HK;
5
Fudan 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 a
t

434
.16
MHz to
all
sensor
nodes
,
where

e
ach
individual
BAN
sensor

is
phase
-
locked

to

the base
-
station clock using a

super
-
harmonic injection
-
locked frequency divider.

E
ach near
-
threshold
SoC
harvests
energy from and phase locks to
this

broadcasted 434.16MHz
waveform
,
eliminating the need for a battery
.

A Near
-
V
T

MICS
-
band
OOK
transmitter
sends the
synchronized
local
sensor data
back to the base
-
station

in its pre
-
defined TDMA slot
.
For a
n

energy
-
harvested
local
V
DD
=0.56V,

m
easurements 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.

I.

I
NTRODUCTION

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 optimiz
e 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

provid
ing

precise timing
synchronization
to enable duty
-
cycled wake
-
up
simultaneously
of each node.
Finally
,
battery
-
free

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
-
lo
cked wireless clock distribution.

I
I
.

WBAN

A
RCHITECTURE

A. Overall Architecture

Fig. 1 illustrates the system architecture of the proposed
WBAN for multiple
wearable
sensors.
The

base station

(
for
example,
smart
phone)

broadcasts
power and clock

within

the
433MHz ISM band

to all the
sensors
, and also receives each
node’s allotted TDMA
-
based wireless
ly
-
transmitted

data

that

occupies

the 402
-
405MHz MICS band
. The core of each
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
su
per
-
harmonic
injection locked

frequency

divider for
clock synchronization, a
digital TDMA slot
generator, and a 402
-
405
MHz MICS
-
band
OOK transmitter.

A

biomedical signal acquisition chip
from
[3]
provides the sensor data input from a

captur
ed

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

Fig
.
2
.


System block
di
a
gram for
each sensor node.


Fig
.
1.


P
ropos
ed
multi
-
node

synchronized

body area network

powered by RF energy harvesting
.

Dummy
144
MHz
/
2
BB CLK SEL
Counter
12
b
Start
code
12
b
End code
12
b
Digital
Comparator
MICS Tx
Enable
Digital
Comparator
on
off
Analog
Comparator
TDMA
Slot
16
M
Hz
Wakeup
/
2
/
2
/
8
Clock Synchronization
SHILRO
80
MHz
Edge
Combiner
Pre Amp
PA
BB
_
clk
Data Gen
&
Sel
En
f
REF
MICS

T
X
V
DD

(
0
.
56
V
)
LDO
Band
g
ap
Rectifier
Voltage
Reference
Generator
V
REF
1
V
REF
2
V
REF
1
V
REF
2
Energy Harvesting
Match
.
Net
Match
.
Net
+
Balun
100
uF
V
BG
(
1
.
1
V
)
433
MHz
wireless power
and clock from
base station
402
MHz
data to
base
station
NODE
1


Energy
-
Harvested
V
DD
=

0
.
5
6
V
V
DD

(
0
.
56
V
)
V
DD

(
0
.
56
V
)
ILFD
/
9
Bio
[
3
]

Sensor
V
DD
(
1
.
1
V
)
ECG
Electrodes
This Work
D
IN
Sensor
3

(
EEG
)
Sensor
2

(
Temp
.
)
Sensor
1

(
ECG
)
Sensor
N

(
EMG
)
Smart Phone
(
base station
)
402
-
405
MHz
OOK data
Data Path
433
MHz
30
dBm
18
dBm
Energy
&
Clock Path
the 433MHz ISM band. During t
he energy
-
harvesting phase
,
when the incoming power received by each sensor exceeds the
on
-
chip rectifier sensitivity (
-
8dBm), two off
-
c
hip surface
-
mount 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
ene
r
gy
-
harv
esting

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
c
lock s
ynchronization
to the base
-
station clock
is achieved by
utilizing a divide
-
by
-
3 injection
-
locked frequency divider
(ILFD) t
hat

produce
s

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 e
ach sensor node begins
counting. The nodes interleave transmission based on their
pre
-
programmed TDMA time slot, set by the

begin and end
codes

(B
x

and E
x

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 physicall
y separated
nodes

on the body.

C.
Merged Rectifier
-
L
imiter

A
rectifier with
a
cross
-
coupled bridge configuration is
adopted here for both low on
-
resistance a
nd 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
in
to 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
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 empl
oys
thin
-
oxide
transistors. Fig.
4

illustrates the proposed merged rectifier
-
limiter circuit
along with
its simulated and measured
characteristics. Since the gate vo
ltages of M1~
M5
are

set to
V
BG

= 1.1V, their drain nodes can only be charged up to V
BG
,
independent of the transistors’ source voltages.

As a result, f
or
all
received
input powers (P
in
)
up to

0dBm, the output voltage
(V
TOP
) of the rectifier
-
limiter is
limited to

below 2.5V, which
is
below
the tolerance limit of the thick
-
oxide I/O devices
imp
lement
ed within

th
e rectifier
.

D. Transmitter Architecture

The
near
-
threshold
MICS
-
band transmitter operating
with
a

harvested
supply of
V
DD
=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

Fig
.
4
. Merged rectifier
-
limiter energy
-
harvesting circuit
.


Fig.
3
.

The TDMA handshaking flow chart

for multiple nodes.


Fig
.
5
.

Near
-
V
T

MICS
-
band OOK transmitter.

-12
-10
-8
-6
-4
-2
0
1
2
3
4
5
w/o limiter (simulated)
w/ limiter (simulated)
w/ limiter (measured)
Rectifier
Unit
1
M
1
M
2
M
3
M
4
M
5
P
IN
V
RF
+
V
RF
-
V
BG

(
1
.
1
V
)
V
TOP

V
DD

(
0
.
56
V
)
-
8
dBm
-
20
dBm
LDO
Bandgap
Matching
Network
Input Power

(
dBm
)
V
TOP

(
V
)
Analog
Comp
.
20
u
65
n
3
pF
3
pF
Rectifier
Unit
2
Rectifier
Unit
3
Rectifier
Unit
4
Rectifier
Unit
5
Rectifier
Unit
6
in
V
RF
+

V
RF
-
20
u
150
n
20
u
150
n
10
u
150
n
10
u
150
n
out
in
out
Base
station
Harvesting
Synchronized TDMA Transmission
Rectifier Output
Wakeup
(
Comp
.)
Counter

BB Clock
1
0
2

B
1
……
E
1

Enable
TX Out
Counter
0
1
2
……
B
2
……
E
2

Enable
TX Out

X
X

……
X
X

on
off
on
off
Slot
1
Slot
2










Node
1

Node
2



BB Clock

A
1
V
BN
f
REF

(
16
MHz
)
V
BN
In
V
BP
Out
Subharmonic Injection
-
Locked
Ring Oscillator
Edge Combiner
A
1
A
2
A
3
A
4
A
5
A
1
A
2
A
3
A
4
A
5
Pre
-
Amp
Power
Amplifier
Match
Net
A
2
A
3
A
4
A
5
A
1
A
2
A
3
A
4
A
5
A
1
A
2
A
3
A
4
A
5
A
1
A
2
A
3
A
4
A
5
Constant
G
m

Bias
V
BP
V
B
N
V
DD

=
0
.
56
V
V
DD

=
0
.
56
V
V
DD

=
0
.
56
V
3
b
V
DD

=
0
.
56
V
transmitter power consumption)
,

a sub
-
harmonic injection
-
locked ring oscillator (SHILRO)
,

and edge combiner are
empl
o
yed
to generate the 402
-
MHz carrier [
5
].

Compared with

traditi
onal phase
-
locked loops, this
SHILRO
structure has the
advantage of fast start
-
up time
, which

facilitate
s

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 i
n
verter
-
based pre
-
amplifier is
added between the edge combiner and the class
-
C power
amplifier to ensure
sufficient
driving capability.

P
rogrammable
output power
for the power amplifier is
achieved by
a
3
-
bit current DAC, which tunes the current
flowing through the bottom resistor
,
modify
ing

the
bias
voltage.

III.

E
XPERIMENTAL
R
ESULTS

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
+
30
dBm 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 Node
3
and Node
4

are

placed on the back side
of the user to demonstrate full functionality for non
-
line
-
of
-
sight 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 b
efore 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 10
mV.
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 402
MHz 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 channe
l conditions alter. The

carrier
-
to
-
spur ratio

at the transmitter output

i
s

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
.


Fig
.
6
.


Measured time
-
domain waveforms for TDMA transmission
of four sensor
nodes from the front/back of person
.


Fig
.
7
.

Measurement results of the energy
-
harvesting front
-
end
.



(a) (b)

Fig
.
8
.

a) Measured
TX output spectrum and
b) TX & ILFD phase
noise
.


Fig
.
9
.

Measured TX global efficiency vs. output power.

Energy
-
Harvested V
DD
TX Output
Wakeup
10
mV
5
.
55
ms
-8
-6
-4
-2
0
10
-3
10
-2
10
-1
10
0
10
1
P
IN

(
dBm
)
Time
(
s
)
for Charging
100
uF LDO Load
From
0
to
0
.
56
V
From
0
.
55
V to
0
.
56
V
0
10
20
30
40
50
0
5
10
15
20
25
Output Power (uW)
Global Efficiency (%)
Fig.
10

shows
a
die photo of the
1mm x 1mm
body
-
area
network prototype, fabricated

in a
65nm
CMOS technology.

Fig. 1
1

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 TDS740
4) 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.

C
ONCLUSION

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 sensor
s.


A
CKNOWLEDGEMENT
S

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.

R
EFERENCE

[1]

F. Zhang, Y. Zhang, J. Silver, Y. S
hakhsheef, 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 B
iomedical 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. 285
-
288, 2010.

[
6
]

P. Mercier, A
.
Chandrakasan
, “A 110μW 10Mb/s eTextiles Transceiver
for Body Ar
ea 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.



Fig
.
1
1
.


Measured ECG Lead
-
II/Lead
-
III signals.


Fig
.
10
.


Die photo
.

TABLE

II:

M
ULTI
-
N
ODE
BAN

P
ERFORMANCE
C
OMPARISON


TABLE

I:

P
ERFORMANCE
S
UMMARY


0
0.5
1
1.5
2
0
0.5
1
1.5
2
Time (s)
ECG Lead-II (mV)
0
0.5
1
1.5
2
0
0.5
1
1.5
2
Time (s)
ECG Lead-III (mV)
Original
Reconstruction
Original
Reconstruction
Technology
Supply
Channel
Power Source
Range
Multiple Access
TX Energy
/
bit
[
6
]
0
.
18
um
0
.
9
V
eTextiles
Remote Battery
1
m
TDMA
/
CSMA
0
.
7
-
18
pJ
[
7
]
0
.
18
um
1
V
BCC
N
/
A
N
/
A
FDMA
0
.
20
nJ
This work
65
nm
0
.
56
V
MICS
/
ISM
RF Energy Harv
.
1
.
4
m
TDMA
0
.
15
nJ
TX Data Rate
10
Mbps
1
k
-
10
Mbps
250
k
-
2
Mbps
Frequency Band
10
MHz
(
Clock
)
40
M
-

120
MHz
402
MHz
/
433
MHz
Technology
Die Size
Harvested V
DD

for TDMA Slot
,
Clock Sync
.
and TX
Frequency Band
(
Harvesting
)
Frequency Band
(
TX Transmission
)
MICS TX OOK Data Rate
Transmission Time Before
10
mV Drop in Harvested V
DD
Number of Synchronized Nodes
(
Measured
)
Sensitivity
(
Harvesting
)
Sensitivity
(
Clock Synchronization
)
Max Experimental Distance for a Fully Operational Sensor
Bandgap
LDO
Digital TDMA Slot
Clock Synchronization RX
MICS TX
Power Break Down
65
nm
-
CMOS
1
mm x
1
mm
0
.
56
V
433
MHz
402
-
405
MHz
250
kbs
-
2
Mbps
5
.
55
ms
4
-
8
dBm
-
35
dBm
1
.
4
m
<
1
uW
<
1
uW
<
1
uW
8
uW
150
uW
(
When Pout
=
-
16
dBm
)
MICS TX Output Power
(
Pout
)
-
27
dBm
~
-
13
dBm
MICS TX Global Efficiency
16
.
7
% (
When Pout
=
-
16
dBm
)