A 2.4 GHZ WIRELESS ELECTRONIC SHIRT

FOR VITAL SIGNALS MONITORING

J. P. Carmo

, P. M. Mendes

, C. Couto

and J. H. Correia

Polytechnic Institute of Braganca, Campus Santa Apolonia, 5300, Braganca, Portugal

University of Minho, Dept. Industrial Electronics, Campus Azurem, 4800-058 Guimaraes, Portugal

Keywords: Wireless electronic shirt, wireless sensors networks, RF CMOS transceiver.

Abstract: This paper presents a wireless sensor network for wireless electronic shirts. This allows the monitoring of

individual biomedical data, such as the cardio-respiratory function. The solution chosen to transmit the

body’s measured signals for further processing was the use of a wireless link, working at the 2.4 GHz ISM

band. A radio-frequency (RF) CMOS transceiver chip was designed in the UMC RF 0.18 μm CMOS

process. The power supply of the RF CMOS transceiver is of only 1.5 V, thus it can be supplied by a single

coin-sized battery. The receiver has a sensibility of -60 dBm and consumes 6.2 mW. The transmitter

delivers an output power of 0 dBm with a power consumption of 15.6 mW. Innovative topics concerning

efficient power management was taken into account during the design of the RF CMOS transceiver.

1 INTRODUCTION

Today, the link between textiles and electronics is

more realistic than ever. An emerging new field of

research that combines the strengths and capabilities

of electronics and textiles into one: electronic

textiles, or e-textiles is opening new opportunities.

E-textiles, also called smart fabrics, have not only

wearable capabilities like any other garment, but

also have local monitoring and computation, as well

as wireless communication capabilities. Sensors and

simple computational elements are embedded in

e-textiles, as well as built into yarns, with the goal of

gathering sensitive information, monitoring vital

statistics, and sending them remotely (possibly over

a wireless channel) for further processing

(Marculesco et al, 2003).

The e-shirt’s goal is the monitoring of the

cardio-respiratory function. This makes it able to

recognize qualitatively and quantitatively the

presence of respiratory disorders, both during wake

and sleep-time in free-living patients with chronic

heart failure, providing clinical and prognostic

significance data.

In wireless sensors networks, the continuous

working time of sensorial nodes are limited by its

average power consumption (Mackensen et al,2005).

To optimise the power consumption, it was designed

a RF CMOS transceiver for the operation in the

2.4 GHz ISM band. It was used the UMC RF

0.18 μm CMOS process in the design. This process

has one poly and six metal layers, allowing the use

of integrated spiral indutors (with a reasonable

quality factor), high resistor value (a special layer is

available) and the low-power supply of 1.5 V. The

use of this technology, allows the development of a

complete system-on-a-chip (SOC), with the

additional advantage to be supplied by a single 1.5 V

coin-sized battery. Moreover, in order to optimize

power consumption, the RF CMOS transceiver

design predicts the use of control signals. With these

control signals it is possible to enable and disable all

the subsystems of the transceiver. These signals

allows, e.g., to switch off the receiver when a RF

signal is being transmitted, to switch off the

transmitter when a RF signal is being received, and

allows the transceiver to enter to sleep when RF

signals are neither being transmitted, nor being

received.

2 TRANSCEIVER DESIGN

Figure 1 shows the architecture of the RF CMOS

transceiver, which consists of a receiver, a

162

P. Carmo J., M. Mendes P., Couto C. and H. Correia J. (2008).

A 2.4 GHZ WIRELESS ELECTRONIC SHIRT FOR VITAL SIGNALS MONITORING.

In Proceedings of the First International Conference on Biomedical Electronics and Devices, pages 162-165

DOI: 10.5220/0001048301620165

 SciTePress

transmitter, an antenna-switch and a frequency

synthesizer.

2.4/2.48 GHz

Switch

RXD

TXD

Filter

Envelope

detector

LNA

@ 80 MHz

2.48 GHz

2.4 GHz

Figure 1: The block diagram of the RF CMOS transceiver.

The receiver adopts a direct demodulation, by means

of heterodyne detection. It is enough to achieve a bit

error probability less that 10

-6

with a sensibility of

-60 dBm, in a transmitted power of 0 dBm using

Amplitude Shift Keying (ASK) modulation. These

specifications makes the RF CMOS transceiver

suitable for short range applications.

2.1 Receiver

Figure 2 shows the receiver’s front-end schematic.

The amplified RF signal at the output of the

low-noise amplifier (LNA) is directly converted to

an intermediate frequency (IF) with a

double-balanced downconversion mixer, followed

by a low-pass filtering. The final downconversion to

the baseband is made by means of envelope

detection.

The low-noise amplifier (LNA) is the first gain stage

in the receiver path. In a LNA, the signal must be

amplified as much as possible, with a small

signal-to-noise ratio (SNR) decrease. This is

achieved with the best noise figure (NF). The LNA

is an inductively degenerated common source

amplifier. This makes the input impedance at

2.4 GHz equal to 50 Ω, for matching with

antenna-switch (Yao, 2007). The LNA is putted in

the sleeping mode, by cutting the current in the

polarization stage. The same principle applies to the

all subsystems of the transceiver. The

downconversion to the IF uses a double-balanced

switched transconductor mixer (Klumperink, 2004).

The advantage is that the switching stage is directly

connected to the power rails, requiring a less voltage

headroom. This simplifies the switching process and

has a slightly bigger conversion gain. Moreover, this

topology also eliminates the presence of the strong

carrier at the output, producing only odd harmonics

of the local oscillation frequency. The signal at the

output of the low-pass filter enters an envelope

detector. The 80-MHz IF frequency is produced

from the 2.4 GHz RF and from the 2.48 GHz local

frequency.

Bias 2

Bias 3

Bias 5

Output

stream

Output buffer

Bias 1

Input

LNA

Envelope

detector

IF filter

Mixer circuit

Bias 4

Figure 2: The schematic of the receiver.

2.2 Transmitter

Bias 1

Bias 2

Mixer circuit

Bias 3

Input

stream

Antenna

Power amplifier

Filter

Figure 3: The schematic of the transmitter.

Figure 3 shows the schematic of the RF CMOS

transmitter. The mixer used in the frequency

upconversion is of the same type of the previous

mixer used in the frequency downconversion. The

main differences are on the MOSFET’s sizes. The

RF signal is further amplified by a power amplifier,

which provides a transmitted signal with an

appropriated output power. The upconversion mixer

generates the RF signal, from the input bitstream and

from a 2.4 GHz local generated frequency. The

external filter follows the power amplifier, and

removes the spectral components around the

2.4 GHz carrier frequency while adapts it to the

impedance of the antenna.

A 2.4 GHZ WIRELESS ELECTRONIC SHIRT FOR VITAL SIGNALS MONITORING

163

2.3 Frequency Synthesizer

As depicted in Figure 4, the PLL has a reference

generator circuit with a crystal based oscillator at

20 MHz, followed by a phase-frequency difference

circuit (PFD) without dead zone, a current steering

charge pump (CP) and a third order passive filter.

The passive section output is connected to the VCO,

which generates the desired frequencies of 2.4 GHz

or 2.48 GHz. These frequencies must be divided by

120 or by 124 and connected to the PFD again,

closing the loop. The frequency division ratio is

selected by the one bit control signal 120/124. This

selects the value of the frequency to be connected to

the mixer, in order to make the up or the

down-conversion operation.

The division by 120/124 in the feedback path is done

with a cascade constituted by one half divider

implemented with a true single phase clock (TSPC)

logic, one divider by 30/31 (digitally selected by the

signal 120/124), followed by a toggle flip-flop to

ensure a duty-cycle of 50% at the PFD input. The

TSPC logic was used to overcome the impossibility

to implement the first toggle flip-flop with static

logic in this technology. It is required a rail-to-rail

input to work properly. Moreover, at these

frequencies, the power consumptions are lowest

compared with the SCL logic (Pellerano et al, 2004).

The ratios of 30 and 31 were achieved with the use

of frequency dividers by 2/3 with modulus control.

3 WIRELESS ELECTRONIC

SHIRT

Like any other every-day garment piece, the wireless

electronic shirt (WES) will be lightweight, machine

washable, comfortable, easy-to-use shirt with

embedded sensors. To measure respiratory and

cardiac functions, sensors are plugged into the shirt

around patient’s chest and abdomen. The WES also

uses small sizes and compact modules, made with

microsystems, containing the 2.4 GHz RF CMOS

transceiver, the electronic of control and processing,

and the interfaces to make the connections to the

sensors. These modules have also an associated

antenna and are supplied by a coin-sized battery.

The size-reduction achieved with these modules,

make them suitable to be easily plugged in the WES,

according the interest of the medical doctors.

Conventional applications of electronic shirts uses

wires to connect the sensors to a central unit, and

then to make the RF communication to the external

base-station (Marculescu et al., 2003). An

interesting application and an easy way to

implement wireless buses, is using modules able to

communicate between them and between the

base-station.

Reference

generator

PFD

VCO

ref

div

Down

C1 C2

R3 V

control

Buffers

out

Static logic

30/31

TSPC

Figure 4: The structure of the PLL.

A single channel measures heart rate and an

inductive elastic band will be used for monitoring

the respiratory function.

It is used a bending sensor to monitor the shoulders

positions, that suffers changes in the electrical

resistance when it is bent. As this sensor is bent the

resistance gradually increases. When the sensor is

bent at 90 degrees, its resistance will range between

from the 30 kΩ up to the 40 kΩ. It was also used an

elastic band to measure the changes in thoracic

circumference due to respiration. The transducer

contains an variable inductance, used indirectly to

measure the changes in thoracic circumference. The

device should be placed around the body at the level

of maximum respiratory expansion. This level will

change between erect and supine positions. At

maximum inspiration the belt should be stretched

almost to maximum extension, making its

inductance minimum. As shown in the Figure 5, the

PLL reference oscillator is reused, to generate the

20 MHz sinusoidal signal, required for this circuit.

The variations in the inductance, changes the

attenuation for the 20 MHz signal, in the first RL

low-pass of first order filter. Thus, the attenuation

increases with the decreasing of the thorax

perimeter. This filtered signal is further amplified,

before a second low-pass filtering, to eliminate noise

and spurs generated in the 20 MHz oscillator. Then,

a peak detector gets the amplitude of the 20 MHz

processed signal. This amplitude is a low-frequency

signal, and it is converted to the digital domain,

using a ΣΔ analog-to-digital converter (ADC) of first

order, with an output with resolution of eight bits.

This allows the recording of respiratory changes

with maximum sensitivity and linearity.

BIODEVICES 2008 - International Conference on Biomedical Electronics and Devices

164

Peak detector

20 MHz

Oscillator

(PLL

reference)

(Indictive sensor)

Filter 2Filter 1

output

Figure 5: The block diagram of a signal conditioning used

in the measure of the respiratory function.

It is shown in Figure 6, a photo of the patient

wearing an WES ready to plug the RF modules. It

can be seen the three connections for heart-rate

respiratory function.

Figure 6: A photo of the patient wearing a electronic shirt

ready to plug the RF modules.

Interface 1 Interface 2

Analog MUX

Activation

signals

Selection

On-chip

sensor 1 sensor 2

ADC

RF CMOS

transceiver

Control

antenna

Memory

Figure 7: Block diagram of the microsystems used in the

plug-and-play modules of the WES.

In WES, the sensor interfaces, data processing, the

wireless interface and antenna are integrated in the

same microsystem (Figure 7) by Multi-Chip-Module

(MCM) techniques. The wireless communication is

between the base-station and the multiple processing

elements placed in the shirt. The main advantage is

to allow the positioning of the sensors where we

like. The sensors can also be removed from the shirt,

either when the sensors are no more need or when

the shirt is to be cleaned and washed. This wireless

bus introduces the concept of plug-and-play in

textiles.

4 CONCLUSIONS

The simulations of the RF CMOS transceiver,

shown a total power consumption of 6.2 mW for the

receiver (1.5 mW for the LNA 3.2 mW for the

downconversion mixer, and 1.5 mW for the

envelope detector). For the transmitter, it was

observed the power consumptions in the following

blocks: 5.3 mW in the upconversion mixer and

10.3 mW in the PA. The transmitter delivers a

maximum output power of 1.08 mW with a total

power consumption of 11.2 mW. These

characteristics fulfill the requirements for short-

range communications for using the 2.4 GHz ISM

band.

The main goal of the WES, is improving the

monitoring of the cardio-respiratory function, by

using devices which reduces healthcare costs and

facilitates the diagnostic while at the same time

preserving the mobility and lifestyle of patients.

REFERENCES

Mackensen, E., et al, 2005, Enhancing the lifetime of

autonomous microsystems in wireless sensor actuator

networks (WSANs), in Proceedings of the

XIX Eurosensors, TC4, pp.1-2, Barcelona, Spain.

Klumperink, E., et al, 2004, A CMOS switched

transconductor mixer, IEEE Journal of Solid-State

Circuits, Vol. 39, No. 8, pp. 1231-1240.

Marculescu, D., et al., 2003, Electronic Textiles:

A Platform for Pervasive Computing, Proceedings of

the IEEE, Vol. 91, No. 12, pp. 1995-2018.

Pellerano, S., et al, 2004, A 13.5 mW 5-GHz frequency

sinthesizer with dynamic logic frequency divider,

IEEE Journal of Solid-State Circuits, Vol. 39, No. 2,

pp. 378-383.

Yao, T., et al, 2007, Algorithmic design of CMOS LNAs

and PAs for 60-GHz radio, IEEE Journal of

Solid-State Circuits, Vol. 42, No. 5, pp. 1044-1057.

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