Performance evaluation of zigbee transceiver for wireless body sensor system

International Journal of Computer Networks & Communications (IJCNC) Vol.6, No.3, May 2014
DOI : 10.5121/ijcnc.2014.6312 145
PERFORMANCE EVALUATION OF ZIGBEE
TRANSCEIVER FOR WIRELESS BODY
SENSOR SYSTEM
Ruchita Pandey1
,Mukesh kumar2
,A.K.Jaiswal3
,Rohini Saxena4
1,2,3,4
(Electronics and Communication Engineering,SHIATS,U.P.,India)
ABSTRACT
A cost effective impedance measurement system and a low cost transceiver device has been presented for
wireless body sensor systems.The proposed device has an analog front end to measure bioimpedance and
ZigBee device which provides reliable wireless communication.Bioelectric Impedance measurement
enables to characterize the state of tissues.Tetrapolar method is an advance method for measuring
impedance since it is a very easy and simple method for practical implementation.The principle of modified
tetrapolar method and its wireless transimission through zigbee has been investigated here.Different
modulation technique has been applied and it has been found that MSK based transceiver is an efficient
one since it has low bit error rate and it produce constant envelope carrier signals which have no
amplitude and phase varations,hence it will be a more power saving technique.
KEYWORDS
Bioimpedance,Imaginary value,Phase sensitive detector method,Real value,Tetrapolar,ZigBee.
1.INTRODUCTION
Many advantages of wireless transmission using ZigBee forced us to use it in various applications
.One such application is in the field of medical science. Millions of people in this world suffer
from healthcare issues.Hence to increase the level of medical supervision ,the concern for
designing medical devices as comfortable as possible and for easy use has grown. Electrical
bioimpedance measurement has been proved to be suitable for determining body composition and
also for monitoring physiological parameters[1,2,3].It is more fruitful in tissue ischemia
monitoring[4].Due to these facts ,Electrical bioimpedance measurement technique as wearable as
possible has been proposed to reduce complication in patient’s life.To observe the measurement
distantly,a wireless transmission technique ZigBee can be used as it offers low power
consumption ,high reliability and it is more secure technology[5,6,7,8].This technology is more
simpler and cost effective than other wireless personal area networks such as
Bluetooth[9].Performance of ZigBee transceiver has been analysed using different modulation
techniques that are Offset Quadrature Phase Shift Keying and Minimum Shift Keying in which
MSK is most promising technology[10].The characteristics ,performance and physical realization
of wireless sensor communication system can be varied with varying the choice of modulation
scheme.
Here for analog front end design, conventional methods and new tetrapolar method have been
discussed for bioimpedance calculation in which new tetrapolar method is more accurate and has
simpler design than previous old methods such as phase sensitive detection method.It excludes
the use of analog demodulator .In this method that is new tetrapolar method,three voltages

146
Vଵ, Vଶ, Vଷ are measured across tissue impedance,electrode impedance and across the resistor
R.The real part Z୰ୣୟ୪and imaginary part Z୧୫ୟ୥ of the complex impedance can be measured by
calculating modular ratios of the two input voltage to phase gain detector and phase differences
of those two inputs [11].
Thus the design of real time wireless sensor system must consider the energy consumption[12],it
must be reliable and cost effective.
The whole work has been organized as:section two deals with methodology which has been
further divided in two subsections,first deals with different methods for bioimpedance
measurement and second with the proposed zigbee technology transceiver for wireless
transmission of measurement.section three deals with the result obtained for bit error rate
comparision for two modulation techniques used and fourth with application of bioimpedance and
zigbee technology.and lastly conclusion in section fifth.
2.METHODOLOGY
2.1 ANALOG FRONT END TO MEASURE IMPEDANCE
Some conventional methods has been described below showing how new tetrapolar method is
more efficient.
2.1.1 PHASE SENSITIVE DETECTION METHOD
This is a fast computational method based on two phase reference coherent demodulation
technique.Voltage between voltage electrode is Vୣ ሺtሻ
Vୣሺtሻ ൌ ZI sinሺω଴t ൅ θሻ, (1)
where Z is the magnitude of the bioimpedance, Ѳ is the phase, and I is the amplitude of current
provided by current source.
Figure 1. PSD method for measuring bioimpedance
Due to the high input impedance of the instrument amplifier IA, voltage electrodes draw
practically no current, so the tissue impedance measurement is not affected by contact
impedance.Output of IA Vz(t) can be represented as
V୸ሺtሻ ൌ V sinሺω଴t ൅ Ѳሻ
Current
Source
IA
In phase signal
Multiplier
Multiplier
Quadrature signal
LPF ADC
LPF ADC
V୸ሺtሻ

147
= V.cosѲ.sinω଴t + V.sinѲ.cosω଴t (2)
The multiplier multiplies V୸ሺtሻ with inphase signal Usinω଴t and quadrature signal Ucosω଴t.The
output obtained are passed to low pass filter and then sampled with A to D converters.Two
output Z୰ୣୟ୪ value is proportional to VcosѲ/I and Z୧୫ୟ୥ value to VsinѲ/I.
But use of multipliers lead to phase errors if the component are not matched.In addition the total
volume of the measurement system will not be suitable for hand held design for its complexity of
realization.
2.1.2 NEW TETRAPOLAR METHOD
It is based on the use of phase gain detector which comprises phase detector and closely matched
pair of demodulating logarithmic amplifier.Resistor R is also added in series with two
impedance,electrode and tissue impedance.The circuit includes current source,amplifiers,phase
gain detector and analog to digital converter.Two electrodes are used to inject current to a
biological tissue and two electrodes are used to pick up the voltage drop across it.The voltage
across biological impedance,electrode impedance and across resistor R are amplified with the
help of three identical instrument amplifiers(IA).They are then given to phase gain detector.The
two DC output voltages V୮୦ୟୱ and V୫ୟ୥ are proportional to phase difference and logarithmic ratio
of the two inputs V୧ଵ and V୧ଶ respectively that is
V୮୦ୟୱ ൌ K୮ሺθ୧ଵ െ θ୧ଶሻ (3)
V୫ୟ୥ ൌ K୫log ቀ
୚౟భ
୚౟మ
ቁ (4)
Figure 2. Tetrapolar method for bioimpedance measurement
Where K୮and K୫ are proportionality constant,if one of the PGD input is switched to IA1 then
V୮୦ୟୱ ൌ K୮ሺθଵ െ θଶሻ (5)
Current
Source
R
Phase Gain
Detector
ADC
ADC
V1
V2
V୮
V୫
V3
Vi1
Vi2

148
V୫ୟ୥ ൌ K୫logቀ
୅భ୚భ
୅మ୚మ
ቁ (6)
If switched to IA2 then V୮୦ୟୱ is proportional to phase difference of θଶ and θଷwhile is
proportional to logarithmic ratio of AଶVଶ and AଷVଷ .By getting the modular ratio Vଵ Vଷ⁄
and Vଶ Vଷ⁄ ,phase difference ሺθଵ െ θଷሻ and ሺθଶ െ θଷሻ,we can calculate Z୰ୣୟ୪ and Z୫ୟ୥
Z୰ୣୟ୪ ൌ R
୚భ
୚య
cosሺθଵ െ θଷሻ െ R
୚మ
୚య
cosሺθଶ െ θଷሻ (7)
Z୫ୟ୥ ൌ R
୚భ
୚య
sinሺθଵ െ θଷሻ െ R
୚మ
୚య
sinሺθଶ െ θଷሻ (8)
Hence,the overall impedance can be measured accurately.
2.2 PROPOSED METHOD FOR WIRELESS TRANSMISSION USING ZIGBEE
ZigBee satisfies the market needs of wireless sensor system since it is more reliable,secure and
has long battery life.The block diagram for transmission of impedance data through it is shown
below
Figure 3. Wireless transmission of impedance measurement through ZigBee
2.2.1. ALGORITHM:
Step1: Measure the bioimpedance using tetrapolar method.
Step2: The output from phase gain detector used in tetrapolar method is fed into the
microcontroller unit for analog to digital conversion.
Step3: This impedance data is transmitted to human computer interaction module through
ZigBee.
Step4: The receiving module of human computer interaction module will then receives the
impedance data and transfer it to monitoring system.
2.2.2 NEED FOR ZIGBEE
Following points supports,why there is need for using ZigBee in present scenario.
1) A number of standards need low latency and very low energy consumption for long battery
lives.
2) Many wireless sensor system require low cost designing.
Analog Front End
for Impedance
measurement
MCU ZigBee PC

149
Thus it can be an efficient easy technique for wireless transmission purposes.
2.2.3 LAYERED ARCHITECTURE OF ZIGBEE
The layered architecture for ZigBee is shown in figure 3.The architecture is divided in many
layers.
Figure 4. ZigBee Architecture
The functions for each layer can be summarized as:
1.Phsical layer is used for transmitting and receiving data.
2. With the use of Advanced Encryption Standard (AES), The MAC layer provides reliable
communications.
3.The network layer is used for routing and broadcasting messages.
3)The application layer is ued for providing transport services to both ZDO and applications.
2.2.4 FREQUENCY AND DATA RATES
The frequency band and data rate within IEEE 802.15.4 Std or the ZigBee describes different data
rates for different frequency bands,according to which different band and coverage are also
mentioned below.
The following table shows the available frequency band and data rate.
PHYSICAL LAYER
MAC LAYER
NETWORK LAYER
APPLICATION SUPPORT
SUBLAYER
ZIGBEE DEVICE OBJECT
APPLICATION LAYER

150
Table 1. Available frequency band and data rate within IEEE 802.15.4 Std.
FREQUENCY BAND COVERAGE DATA RATE
2.4 GHz ISM World Wide 250 Kbps
868 MHz ISM Europe 20 Kbps
915 MHz ISM America 40 Kbps
2.2.5 BLOCK DIAGRAM OF ZIGBEE TRANSCEIVER
The design of ZigBee transmitter and receiver can be explained as shown below.
2.2.5.1 Transmitter End:
1.Following steps have been done for the transmission of data.
Firstly,Bit to symbol mapping is done for input signal.On the obtained signal then logical
operation is done with PN sequence.
After it,the signal is passed to serial to parallel converter that operates using a clock,two D
flipflops and one J flipflop.In phase and outphase data are thus obtained which when multiplied
by sine wave generator,provides half sine pulse shaping.
Further after modulation of both data signals with high frequency carrier and adding them,the
signal is finally transmitted.

151
Figure 5. Block diagram of transmitter end of ZigBee
2.2.5.2 Receiver End:
2.Following steps have been done for receiving the data.
Incoming signal is multiplied with inphase and quadrature phase high frequency carrier followed
by low pass filter.
The resultant is passed to sample and hold circuit and then comparator that has a fixed threshold
value for making a decision.If the value is greater than threshold,it takes the bit as 1 otherwise
0.The signal is then delayed at inphase side of incoming signal and further both signal processed
to parallel to serial conversion.
Now logical operation is done on this signal with PN sequence generator and the data is
received.The whole process can be shown with the diagram as in figure 6.
Input signal
Bit to symbol
PN sequence Logical operation
Serial to parallel conversion
Half sine pulse shaping Half sine pulse shaping
modulation modulation
add
Output signal

152
Figure 6. Block diagram of receiver end of ZigBee
2.2.6 MODULATION TECHNIQUES USED
2.2.6.1 Offset Quadrature Phase Shift Keying
Offset QPSK and QPSK are same.The similarity between the two is that their power spectral
density and error performance is the same.But the differences which makes them different are:
1.There is delay in Q-channel.
2.The pulse train of both the channels are staggered.
3.OQPSK is more power-efficient than QPSK technique since the maximum phase change in the
waveform is П/2.
Incoming signal
Carrier multiplication Carrier multiplication
LPF LPF
Sample
and hold
circuit
Sample
and hold
circuit
comparator comparator
delay
Parallel to serial converter
PN sequence
Logical operation Received data

153
2.2.6.2 Minimum Phase Shift Keying
In place of OQPSK,MSK modulation techniques has been applied next.
The benefits of using MSK are:
1.The baseband waveform of MSK is smoother,that multiplies the quadrature carrier than
rectangular waveform of QPSK.
2.The main centre lobe in MSK is 1.5 times wider than QPSK and the side lobes in MSK are
relatively smaller in comparision to the main lobe,hence filtering becomes easier.
3. MSK possess phase continuation that is no phase change,while QPSK not.Hence intersymbol
interference is reduced.
Minimum shift keying helps orthogonal detection since it allows minimum frequency separation
,called bandwidth.Over any symbol interval,MSK can have one of two possible frequencies:
sሺtሻ ൌ a୍ሺtሻ cos ቀ
П୲
ଶ୘
ቁ cosሺ2Пftሻ ൅ a୕ሺtሻ sin ቀ
П୲
ଶ୘
ቁsinሺ2Пftሻ (9)
2.2.7 ADVANTAGES OF ZIGBEE
Advantages of ZigBee can be summarized as:
1) Power consumption is low, simple implementation.
2) Long Battery life.
3) Device cost and installation cost is low and low maintenance is required.
4) It can handle many devices at a time,Since it provides high density of nodes per network.
5) Simple protocol and has global implementation.
2.2.8 SIMULATION
The implementation has been built on Matlab/Simulink using fundamental components in
Simulink to demonstrate how reliably complex modulation schemes can be built, cost effectively
and efficiently.Where the input bit stream was generated using Bernoulli random generator.And
then logical operation is performed with PN sequence.The resulted signal has the data rate of
2Mbps.
After serial to parallel conversion,the signal gets divided into two components,Inphase and
Quadrature phase.Then modulation is done with a high frequency 2.4 GHz carrier on the In Phase
and Quadrature Phase data stream.Both are then summed up to get the required transmitter output
signal.For modulation,two modulation technique have been applied to study the efficient scheme
that can be applied in ZigBee transceiver.Then the Bit error rate has been studied for the two.
3.RESULT AND DISCUSSION
Different modulation schemes has been applied at transmitter end and receiver end.Those are
OQPSK that is offset Quadrature Phase Shift Keying.Second is MSK that is Minimum Shift
Keying.Then the bit error rate for these techniques has been calculated and following graph in
figure 7 shows the BER comparision.

154
Figure 7. BER comparision between different modulation technique
ZigBee standard is a great solution for wireless sensor network.Using simulation,ZigBee
transceiver has been designed for different modulation technique and it has been found that MSK
based designing is most promising technology which provides low data rate with low BER &
large acquired area. So, this technology can be used.
4.APPLICATION
Bioimpedance measurement using tetrapolar method can be widely used in studying the state of
tissues.It can be implemented in skin hydration,cerebral monitoring,dental decay diagnosis and
many other health problems.
Due to limitations in other wireless technology such as frequent need of charging,more power
consumption,ZigBee is becoming a smart choice for transmitting data.ZigBee technology is
programmed in chip form and can be applied to many other fields such as home
automation,energy management,industrial plant management etc.
5.CONCLUSION
The proposed system integrating the impedance measurement technique and ZigBee enables the
remote monitoring of health issues.ZigBee has longer battery life due to low power
consumption.Secure and reliable transmission can be achieved through it.ZigBee transceiver
based on minimum shift keying modulation technique are better one since in minimum shift
keying,bit error rate is lower as compared to other modulation technique and it produce constant
envelope carrier signals which have no amplitude and phase varations,hence it will be a more
power saving technique.Bioimpedance measurement through traditional methods are time
consuming and not safe even.PSD method is also not popular as the use of demodulators causes
phase error and complex circuitary hinders its use.Hence tetrapolar method is best among all
methods.Due to use of integrated chips,it liberates the traditional methods from the complexity of
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
2 4 6 8 10 12
BER
Graph between BER and samples per sec
OQPSK
MSK

155
design.Its accuracy is goog and it has wide frequency range.Power consumption and design cost
is also low.
REFERENCES
[1] R.Pallas and J.Webster(1993),Bioelectric impedance measurements using synchronous
sampling,Biomedical Engineering IEEE,vol.40,issue 8.pp 824-829 .
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transplants,Instrumentation and Measurement Technology Conference proceedings,pp1-4.
[3] S.Kun,B.Ristic,R.Peura and R.Dun(1999),Real time extraction of tissue impedance model
parameters for electrical impedance spectrometer,Medical and Biological Engineering and
Computing,springer vol.37,issue4, pp 428-432.
[4] S. Kun and R. A. Peura(1994),Tissue ischemia detection using impedance
spectroscopy,Engineering in Medicine and Biology Society,Proceedings of the 16th Annual
International Conference of the IEEE,vol.2,pp 868-869.
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networking through ZigBee technology,International Journal Of Advance Research in Computer
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[7] H.Chen and C.Zhang(2012).The result and implement of ZigBee technology for building wireless
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wireless sensor networks,International Journal Of Computer Networks and
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Biomedical Engineering,Worcester Polytechnic Institute,MSc.
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AUTHORS
Ruchita Pandey received her B.Tech Degree from department of Electronics And
Communcation Engineering from Krishna Institute Of Engineering and Technology
Ghaziabad in 2011.She is persuing M.Tech from the Department of Electronics and
Communication SHIATS,Allahabad.Her main research interest includes Communication
System Engineering,Biomedical Engineering and Signal Processing.

156
Mukesh Kumar is working as an Asst. Prof. in Electronics & Communication
Engineering department,SHIATS.He has done M.Tech in Advanced Communication
System En gineering from SHIATS, Allahabad in 2010.Signal processing and Microwave
Engineering are his main area of research.
A.K. Jaiswal is working as a Proffesor and is Head of ECE Department at SHIATS. He
has done M.Sc. in Technology,Electronic & Radio Engineering. from Allahabad University
in 1967.Various projects & research at undergraduate & postgraduate level are guided by
him.His experience is more than 35years in Industrial, research & Teaching.Optical
Networks and satellite communication are his main area of research.
Rohini Saxena is working as an Asst. Prof. in the Department of Electronics &
Communication Engineering in SHIATS, Allahabad.She has done M.Tech. Degree in
Advanced Communication System Engineering from SHIATS, Allahabad in 2009.Digital
Communication,Microwave Engineering,Wireless Sensor Network,Computer Networks and
Mobile Communication are her research fields.

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