A 180 nm-CMOS Asymmetric UWB-RFID Tag with Real-time

Remote-monitored ECG-sensing

Jue Shen

, Jia Mao

, Geng Yang

, Li Xie

, Yi Feng

, Majid Baghaei Nejad

, Zhuo Zou

Fredrik Jonsson

, Hannu Tenhunen

and Lirong Zheng

1,3

iPack Center, KTH-Royal Institute of Technology, Electrum 229, Stockholm, Sweden

Electrical and Computer Engineering Department, Hakim Sabzevari University, Sabzevar, Iran

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

-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

 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.

acess

tag

Tag N

Reader

Frame

Synchronizing

Command

Set

Response (Time Slots)

TX window

Sample

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

TX of

sample #1

Sample

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.

opt



K ∙ N

∙ PG ∙ ƒ

ECG

(1)

active

pulse

·ƒ

(2.1)

avg

active

·t

tag

access

(2.2)

Table 1: Parameters for UWB-RFID communication

protocol and power consumption.

opt

Optimized number of accessed ECG-tags per

second

K ECG samples per TX window

UWB-IR TX data rate (bps)

ECG

ECG sampling frequency (Hz)

bits per sample

PG TX processing gain (pulse per bit)

ulse

circuit energy consumption per UWB pulse

tag TX window length

access

averaged tag access period

active

active UWB TX power

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-

and analog front-end (AFE) sensing ECG signals.

Power Management Unit

Decoder

Dec En/

DisEN

Baseband

oscillator

UHF envelop detector

Bias

generator

PBW PGA

SAR

ADC

ECG AFE

UHF

Ant.

UWB

Ant.

Analog Baseband

check

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

ctrl

clkg

UWB

CLK Gen

UWB

CLK

BB data

Data_rx

RST_rx

renew

CMD

Modules

1.2V

FSM

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.

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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

. 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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