A 2.4 GHZ WIRELESS ELECTRONIC SHIRT
FOR VITAL SIGNALS MONITORING
J. P. Carmo
1
, P. M. Mendes
2
, C. Couto
2
and J. H. Correia
2
1
Polytechnic Institute of Braganca, Campus Santa Apolonia, 5300, Braganca, Portugal
2
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
Copyright
c
SciTePress
transmitter, an antenna-switch and a frequency
synthesizer.
~
2.4/2.48 GHz
IF
Switch
RXD
TXD
Filter
PA
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
LO
Bias 5
Output
stream
Output buffer
C
1
C
2
C
3
R
2
R
1
L
s
L
g
C
b
Bias 1
Input
M
1
M
2
LNA
Ω
=
50
in
Z
L
d
C
b
C
f
R
f
Envelope
detector
IF filter
Mixer circuit
Bias 4
M
3
M
4
M
5
Figure 2: The schematic of the receiver.
2.2 Transmitter
Bias 1
Bias 2
LO
Mixer circuit
Bias 3
Input
stream
M
pa
L
sk
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
F
ref
F
div
Up
Down
I
cp
I
cp
C1 C2
R2
C3
R3 V
control
Buffers
F
out
2
Static logic
30/31
2
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 k up to the 40 k. 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
A
Peak detector
20 MHz
Oscillator
(PLL
reference)
(Indictive sensor)
L
R
1
R
2
C
1
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
A
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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