A 180 nm-CMOS Asymmetric UWB-RFID Tag with Real-time
Remote-monitored ECG-sensing
Jue Shen
1
, Jia Mao
1
, Geng Yang
1
, Li Xie
1
, Yi Feng
1
, Majid Baghaei Nejad
2
, Zhuo Zou
1
,
Fredrik Jonsson
1
, Hannu Tenhunen
1
and Lirong Zheng
1,3
1
iPack Center, KTH-Royal Institute of Technology, Electrum 229, Stockholm, Sweden
2
Electrical and Computer Engineering Department, Hakim Sabzevari University, Sabzevar, Iran
3
School of Information Science and Technology, Fudan University, 200433, Shanghai, China
Keywords: Real-Time Remote-Monitoring, Electrocardiogram (ECG), Asymmetric Ultra-Wideband - Radio Frequency
Identification (UWB-RFID), Inkjet-Printed Electrodes.
Abstract: This paper proposes an asymmetric ultra-wideband - radio frequency identification (UWB-RFID) tag with
electrocardiogram (ECG)-sensing capability for patients remote-monitoring in hospital environment. A
UWB-RFID communication protocol is suggested for real-time transmission of undistorted ECG by
interleaving ADC sampling and burst-mode UWB transmission. The proposed system shows a maximum
accessing capability of 400 tags/second at 1.5 KHz ECG sampling rate with 10 Mbps UWB pulse rate. The
tag consists of UHF-RFID receiver, UWB transmitter, ECG analog front-end, multi-input ADC and
baseband circuitry integrated on two silicon dies. It was implemented by 6 mm
2
-sized 180 nm CMOS
technology. Electrodes for ECG-sensing are manufactured by inkjet-printing on polyimide substrate.
Experiment results show that the tag transmits UWB pulses at 1 Mbps rate with 18 µW power. The printed
electrodes conduct ECG waveform comparable to commercial electrodes.
1 INTRODUCTION
Wireless electrocardiogram (ECG) tags, enabling
real-time remote-monitoring of ECG waveform for
in-hospital patients, greatly reduce hospital
manpower, increase response accuracy and improve
patient comfort. As an example, for patients
recovering from serious diseases, such tags facilitate
them to do rehabilitation exercises and shorten
recovering periods while ECG signals are
continuously monitored at backend central server.
However, typical solutions of ECG tags store data in
large-sized memories and do not activate
transmission unless certain pre-defined conditions
are detected; or transmit ECG signal that is sampled
below 1 KHz (Medtronic, 2014; Pantelakis et al.,
n.d.). As results, though tag complexity and power
consumption are greatly decreased, these conditions
significantly reduce detection accuracy and range of
cardiac symptoms, unsuitable for hospital-targeted
real-time remote-monitoring.
This paper proposes an ultra-wideband - radio
frequency identification (UWB-RFID) tag with
ECG-sensing for real-time remote-monitoring of in-
hospital patients. ECG data of multiple patients are
sampled at KHz rate and transmitted to backend
server in real-time. It reduces buffer size for data
storage and design complexity for data processing at
the tag side. Among various item-level short-ranged
wireless communications, ultra-high frequency
RFID (UHF-RFID) has been verified as one most
cost-power-effective method for multi-object data
collection (Reinisch et al. 2011). UWB Impulse
Radio (UWB-IR) which outputs ultra-short pulse
and ultra-wide bandwidth supports Mbps data rate
without significant increase in circuit power and
implementation cost (Zheng et al., 2010).
Following of the paper is organized as: section 2
introduces system architecture and communication
protocol of UWB-RFID tag. Section 3 discusses tag
circuit and inkjet-printed ECG electrodes
implementation. Section 4 demonstrates test results.
Section 5 concludes and outlooks the work.
2 SYSTEM DESCRIPTION
2.1 System Architecture
System architecture of the tag is illustrated in Fig. 1.
210
Shen J., Mao J., Yang G., Xie L., Feng Y., Nejad M., Zou Z., Tenhunen H. and Zheng L..
A 180 nm-CMOS Asymmetric UWB-RFID Tag with Real-time Remote-monitored ECG-sensing .
DOI: 10.5220/0005286302100215
In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2015), pages 210-215
ISBN: 978-989-758-071-0
Copyright
c
2015 SCITEPRESS (Science and Technology Publications, Lda.)
Figure 1: System architecture of UWB-RFID bio-tag.
ECG data sampled at several KHz rate are
transmitted to and processed at cloud server in real-
time. Low communication power and high
transmission data rate are hence the challenges of
tag design. Especially in multi-tag accessing
scenarios, uplink traffic from tags to reader is far
more crowded than downlink from reader to tags.
Therefore, the tag integrates UHF-RFID for
downlink reception because RFID network is star-
structured, suitable for multi-tag accessing; and
RFID receiver is non-coherent, suitable for low
power tag. To the contrast, it integrates UWB-IR for
uplink transmission because data rate is high and
UWB-IR transmitter is digital-intensive, suitable for
low-power real-time transmission. The asymmetric
transceiver is much less complicated than
transceiver for longer-ranged communications such
as WLAN (wireless local area network), greatly
reducing tag cost and increasing battery life time.
Such asymmetric radio links pay cost at the
reader side which is area and power hungry. UWB-
RFID reader works as gateway to relay the
communications between tag and server as shown in
Fig. 1. However for in-hospital applications, the
number and allocation of readers are basically fixed
and power consumption is not a fatal problem.
Communication range of tag-to-reader layer is above
10 meters (Zheng et al., 2010).
Targeting future wearable ECG-patch, chip part
of the tag is minimized by implementation in 180-
nm CMOS process. Moreover, inkjet-printing
technology is utilized to fabricate ECG electrodes on
polyimide substrate based on noise characterization
and impedance study of dry electrodes in (Xie et al.,
2012). Battery is used to improve communication
distance and performance stability.
2.2 UWB-RFID Communication
Protocol for ECG Transmission
Fig. 2 illustrates the UWB-RFID communication
protocol for ECG transmission in real-time. In
general, ECG sampling and UWB transmission (TX)
in burst mode are interleaved based on an improved
frame-slotted-ALOHA protocol.
t
acess
t
tag
Tag N
Reader
Frame
Synchronizing
Command
Set
Response (Time Slots)
TX window
Sample
#2
Format
UWB-IR TX data rate: 1 ~ 10 Mbps
ACK for Tag N-1
ACK for Tag N
ECG sampling
Sample
# K
TX window
ACK for Tag N-1
ACK for Tag N
ECG sampling
TX window TX window
TagN+1
ECG sampling
ECG sampling
TX of
sample #1
Sample
#2
ECG sampled rate: several KHz.
They are buffered in registers during TX intervals .
Sample
# K
ECG
sample #1
Figure 2: UWB-RFID protocol for real-time ECG TX.
Frame-slotted-ALOHA is a typical algorithm for
anti-collision in multi-tag accessing. A frame
represents an operation initiated by readers and is
composed by three phases: start of frame (SOF),
commands, and response. A frame time is divided
into discrete time intervals, called slots. A tag
randomly selects a slot number and responds to the
reader. An acknowledgement (ACK) is sent from
reader to a specific tag to ensure successful
reception and to resolve collision. Collided tags
retransmit in next frames. Such anti-collision
protocol can be improved by pipelining reader ACK
and tag TX to reduce slot length, optimizing frame
size, and skipping idle slot, achieving simulated
throughputs up to 2000 tags/s (Zhuo et al., 2007).
Real-time ECG TX makes use of the improved
protocol as follows: ECG data are sampled and
buffered during TX intervals of one tag, and the
buffered data are completely transmitted in next TX
window. To reduce complexity of tag register
management, ECG sampling is temporarily disabled
during UWB TX. Since UWB TX rate is much
higher than ECG sampling rate, missing percentage
of ECG data is very low. For an example of 3.75 ms
UWB TX window, data missing percentage for ECG
at 1.5 KHz sampling rate is 5.625%.
A180nm-CMOSAsymmetricUWB-RFIDTagwithReal-timeRemote-monitoredECG-sensing
211
Tag UWB TX window is designed no shorter than
reader ACK window to decrease TX gap and
increase system efficiency. In optimized case, there
is no time gap or no idle slot between consecutive
tag TX windows. Therefore, the number of accessed
tags per second is shown in expression 1, of which
the related parameters are shown in table I. For ECG
signals with 1.5 KHz sampling rate and 16-bit UWB
data per sample, it offers approximate 400 tags/s
accessing capability at 10 Mbps UWB-IR TX rate
and 0 dB processing gain. However, in real case, tag
selects TX slot in a frame randomly; moreover idle
slots and tag collision cannot be eliminated. Thus,
system throughput is dependent on trade-off of
frame size and collision probability in proportional
to N
opt
(Zhuo et al., 2007).
Because UWB TX is aggressively duty-cycled in
the protocol, average power consumption of UWB
TX is much lower than active power as shown in
expression 2.1 and 2.2 (Nejad et al., 2009), making
real-time ECG TX feasible within tag power budget.
N
opt

ƒ
tx
K N
b
PG ƒ
ECG
(1)
P
active
=E
pulse
·ƒ
tx
(2.1)
P
avg
=P
active
·t
tag
/t
access
(2.2)
Table 1: Parameters for UWB-RFID communication
protocol and power consumption.
N
opt
Optimized number of accessed ECG-tags per
second
K ECG samples per TX window
f
tx
UWB-IR TX data rate (bps)
f
ECG
ECG sampling frequency (Hz)
N
b
bits per sample
PG TX processing gain (pulse per bit)
E
p
ulse
circuit energy consumption per UWB pulse
t
ta
g
tag TX window length
t
access
averaged tag access period
P
active
active UWB TX power
P
av
g
averaged UWB TX power
3 TAG IMPLEMENTATION
According to the system architecture in Fig. 1, tag
diagram is illustrated in Fig. 3. Circuit blocks are
implemented in two silicon dies, and are composed
of UWB-IR transmitter, UHF-RFID receiver, power
management unit (PMU), successive-approximation-
register ADC (SAR-ADC), baseband (BB) circuitry
and analog front-end (AFE) sensing ECG signals.
Power Management Unit
Decoder
Dec En/
DisEN
Baseband
oscillator
UHF envelop detector
Bias
generator
PBW PGA
IA
SAR
ADC
ECG AFE
UHF
Ant.
UWB
Ant.
Analog Baseband
RX
check
Register
UWB
BB Gen
ADC
Ctrl
Die 2
Die 1
Printed
electrodes
1.2V 1.8V
Flexible
battery
UWB-IR Transmitter
Data CLK Recovery
Clk_rx
ADC
out
BB
ctrl
clkg
UWB
CLK Gen
UWB
CLK
BB data
Data_rx
RST_rx
renew
CMD
Modules
1.2V
FSM
matching
matching
Figure 3: Tag implementation diagram.
Both UWB TX distance and power consumption
are key consideration for bio-tag applications.
Therefore supply voltage for UWB transmitter is
kept 1.8 V while that for other blocks are reduced to
1.2 V. Thus, a power management unit (PMU)
integrates two voltage regulators to generate the two
supplies from a single battery voltage input. In the
voltage regulator, feedback comparator establishes
negative feedback to stabilize the output voltages.
Tag receives UHF-RFID commands that are
modulated by amplitude shift keying and encoded by
pulse interval (ASK-PIE) from reader. Modulation
index is 90%. Low interval for data 0 is 6.25 µs and
18.75 µs for data 1. Reception (RX) block is
composed of envelop detector, envelop recovery and
data decoder. To reduce reception power, analog
decoder is implemented by charging a capacitor
when envelop is high. Sequential baseband block of
RX check checks ID while shifting in the serially-
decoded data, and generates a renew signal once the
check is correct. The renew signal triggers the data
to the baseband registers in this clock period.
3.1 Operation Flow
According to the communication protocol in section
2, UWB TX and ADC sampling are both controlled
by reader. A renew signal marks the RX of a new
reader command and the start of assigning configure
registers. Data in configure registers enable/disable
ADC sampling, and together with (pseudo number)
PN counting, enable/disable UWB TX in random
slots. Corresponding to the protocol in Fig. 2, ADC
is also disabled when UWB TX is enabled; and
UWB TX is also disabled when data buffered during
TX interval are all read out and successfully
received. Such operation flow is shown in Fig. 4. TX
state and ADC sample state are interleaved in
applications of real-time ECG remote-monitoring.
BIODEVICES2015-InternationalConferenceonBiomedicalElectronicsandDevices
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Figure 4: Operation flow.
3.2 Uwb Tx
AFE output of ECG signal is sampled and converted
to 8-bit data by SAR-ADC at ADC clock rate. Data
are continuously buffered until UWB TX is
activated. When UWB TX is activated according to
operation flow in Fig. 4, ID and ADC data are read
out in serial by rising edge of UWB TX clock
(UWB_CLK), and each bit are repeated by PG
times. Rising edge of the output data triggers the
UWB-IR transmitter. Transmitter NOR the rising
edge and its delayed inverted result, and filters the
output to UWB-shaped pulse (Mao et al., 2014).
Two clock domains are used. ADC clock is
divided from global clock and the division ratio is
received from reader command for different bio-
applications. It is further gated by ADC enable
signal to reduce power consumption. UWB TX
clock are generated by down-scaling UHF carrier
wave (CW) with harmonic injection locked divider
(HILD) first and then with digital dividers according
to commanded data rate (Mao et al., 2014).
3.3 Inkjet-Printed Electrodes and AFE
Inkjet-printed electrodes are fabricated by direct
printing of NPS-JL (nano-particle silver inkjetable
low temperature ink from Harima Chemicals) with a
10 pL printhead on commercial polyimide (PI) foil
(Kapton 500HN, DuPont) substrate. The substrate
has advantages of smoother surface, less shrinking,
and more stable chemical and moisture conductance.
They are sintered at 145 °C for 1 hour (Xie et al.,
2012).
Amplitude of ECG signals varies due to size,
pair distance and contact impedance of the printed
electrodes. Therefore, a variable-gain-bandwidth
amplifier is implemented in AFE. The AFE consists
of three amplifier stages, of which the first stage is
differential instrument amplifier with common-mode
feedback and the other stages are single-end
amplifiers with programmable capacitor arrays at
input and output ends for gain and bandwidth tuning.
It outputs up to 49 dB gain from 0.35 Hz to 1.5 KHz
at 2.76 µW power consumption (Yang et al., 2012).
4 EXPERIMENTAL RESULTS
Fig. 5 illustrates the silicon die photo of UWB-RFID
tag with ECG-sensing. UWB-RFID tag die is shown
in Fig. 5 (a). AFE for ECG is currently implemented
in another die as shown in Fig. 5 (b). Most area-
consuming parts are inductors of UWB transmitter
and capacitors of SAR ADC in UWB-RFID die, as
well as programming capacitors in AFE die. Both
dies are implemented in UMC-180 nm CMOS
technology process. UWB-RFID die is tested
separately by inputting ADC with emulated
amplified cardiac signals from signal generator
(Agilent 33250A). In future design, both dies will be
integrated in a single die and signal generator will be
replaced with AFE output.
4.1 UHF-RFID Reception
Tags demodulate and decode UHF-RFID envelop to
generate decoded data (Dec_out). Analog decoder is
Figure 5: Silicon die photos of UWB-RFID tag (a) with ECG sensing (b).
A180nm-CMOSAsymmetricUWB-RFIDTagwithReal-timeRemote-monitoredECG-sensing
213
conditionally enabled to save power consumption.
As illustrated in Fig. 6, the decoder enable
(Dec_enb) outputs high to disable decoding when a
high envelop duration over 22 µs is detected and
outputs low to enable it when a falling edge of RF
envelop is detected.
Dec_out consists of four parts: starter, data
command, tag ID, and ID checking. Data command
includes configuration data such as UWB en/disable
and bio-sensing en/disable. ID check validates one
specific tag to be accessed. The time window of one
reader command package lasts around 2.2 ms when
RX data rate is 40 - 80 Kbps. According to the
protocol in section 2, tag TX window is selected
more than 2.2 ms to reduce time gaps between
transmission slots of multiple tags. Thus, 3.75 ms is
used as an example in the following UWB TX
experimental results.
Figure 6: UHF-RFID command RX.
4.2 Real-time UWB-IR TX
Fig. 7 shows multiple transmission slots of one tag,
demonstrating UWB TX interleaved with ECG
samplings of one tag. UWB BB data and UWB TX
pulse are illustrated in Fig. 7 (a) and (b).
Corresponding to the protocol in Fig. 2, UWB-IR
TX happens in the response phase. According to the
reader command window length of 2.2 ms as
demonstrated in Fig. 6, 3.75 ms of tag TX window is
selected as an example. For an assumed multi-
accessing scenario of 40 tags, the transmission
interval is thus 146.25 ms and is left for the UWB
TX of the other 39 ECG-tags in ideal case. ECG
signal sampled by ADC at 1.5 KHz rate are buffered
Figure 7: UWB TX interleaving with ADC sampling.
during the transmission intervals, and the number of
buffered samples is around 219. They are
transmitted together with tag ID in burst mode at 1
Mbps data rate in the next TX window.
Fig. 8 zooms in one tag TX slot of one ECG
sample. UWB pulse rate is 1 Mbps (Fig. 8 (a)) and
modulation is on-off-keying (OOK) (Fig. 8 (b)). As
described in section 3, BB data is input to UWB-IR
transmitter and triggers a pulse at its rising edge.
Fig. 8 (c) illustrates the further zoomed-in result of
one UWB pulse. Pulse shape is not optimized when
chip output impedance is not matched to antenna
impedance by matching network.
Figure 8: UWB TX of 1 ADC data @ 1 Mbps pulse rate.
Fig. 9 demonstrates the UWB pulses after
impedance matching. Pulse Amplitude is increased
to 1.02 V and duration decreased to 620 ps. UWB
transmitter is measured to consume 18 µW when
continuously transmitting pulses at 1 Mbps rate.
Figure 9: UWB pulses @ 1 Mbps pulse rate.
4.3 ECG by Inkjet-Printed Electrodes
Fig. 10 (a) shows ECG signals captured by inkjet-
printed electrodes in Fig. 10 (b). Although the
electrodes have different sizes and pair distances, the
effects on electric parameters are compensated by
variable-gain-bandwidth amplifier in AFE. The
result is measured at AFE output separately. In
future single-die solution, it will be input to ADC.
BIODEVICES2015-InternationalConferenceonBiomedicalElectronicsandDevices
214
Figure 10: Measured ECG by inkjet-printed electrodes.
5 CONCLUSIONS AND
OUTLOOKS
This paper proposes a UWB-RFID tag with ECG-
sensing ability for low power and real-time patients
remote-monitoring in hospital application scenarios.
For undistorted ECG TX of multiple patients, burst-
mode TX and ECG sampling are interleaved based
on an improved frame-slotted-ALOHA protocol. It
suggests a maximum accessing capability of 400
tags/s at 10 Mbps transmission data rate and 1.5
KHz sampling rate. UWB-IR transmitter is
implemented to achieve Mbps data rate with low
power consumption, whereas non-coherent UHF-
RFID receiver is used for low-power tag accessing.
The tag is implemented in two silicon dies
manufactured by UMC-180nm-CMOS technology
process. Total die area is 6 mm
2
. Targeting future
wearable ECG-patch, ECG electrodes are fabricated
by inkjet-printing on polyimide substrate.
Experimental results show that UWB-IR transmitter
consumes 18 µW at 1 Mbps rate, corresponding to
18 pJ/pulse. AFE output of ECG signals captured by
the printed electrodes is comparable to that by
commercial electrodes.
To outlook this work: in current prototype, AFE
is implemented in a separate die, yet it can be
integrated with UWB-RFID tag in one single die.
UWB and UHF communication are realized by
conventional antenna, however flexible antenna can
be used in the future with correspondent passive
matching networks (Khaleel et al., 2012).
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