SAW sensor read range limitations and perspectives

SAW sensor read range limitations and perspectives

Faiza Nawaz • Varun Jeoti

� Springer Science+Business Media New York 2014

Abstract Wireless monitoring systems with passive sur-

face acoustic wave (SAW) sensors offer novel and prodi-

gious perspectives for remote monitoring and control in

harsh environments. This paper reviews the working prin-

ciple of such passive sensors and their state-of-the-art

behavior. All major fields of the SAW sensor require

operation over a longer reading range, which is the current

limitation, hindering the massive implementation of these

passive devices. This foremost limitation of the SAW

based sensor system occurs when the reply signal strength

at the reader becomes too weak for reliable recognition and

identification. SAW sensor network read range limitations

and advanced techniques for its enhancement are deliber-

ated. Deployment design to maintain the communication

reliability and connectivity of the sensor/interrogators

network is also discussed.

Keywords Surface acoustic wave � Wireless sensor

network � Passive sensor � Read range

1 Introduction

With the recent advancement in ubiquitous computing,

surface acoustic wave (SAW) devices find numerous

applications in everyday life, especially in sensor systems,

and in identification mechanisms or a combination of both

[1, 2]. Implementation of SAW device as a sensor is an

upcoming field of interest due to its valuable characteristics

of high portability, sensitivity and passive nature. The

passive nature of the SAW sensor makes it highly desirable

in information and communication technology (ICT) where

energy efficiency is always required [3]. Today they are

widely used for monitoring temperature [4, 5], pressure [6,

7], chemicals [8] and gas leakage [9, 10] for both indoor

and outdoor applications.

A radio SAW sensor system usually has an interrogation

unit and one or more SAW sensor distributed over an area

to be monitored. The request signal is transmitted from the

interrogator to the sensor (downlink) while the SAW sensor

responds with a back scattered signal (uplink) that is

received by the receiver section of the interrogation sys-

tem. Up and downlink, the interrogator’s request signal and

the sensor’s response, all have to be separated. These radio

sensors can be of three categories, based on their source of

energy. Active devices; powered by a built in battery, semi

active devices; energized by strong radio frequency signal

or inductive coupling and passive sensors which uses the

mechanism of back scatter without requiring any internal

energy storage.

Silicon-based Integrated Circuits (ICs) are another

popular type of sensors, however SAW based sensors are

distinguished from them by the unique piezoelectric crystal

effect produced by applying a proper electrical field. The

SAW sensor being passive, offers a variety of advantages

as compared to IC based sensors. SAW signal has a longer

range as it is 100 times smaller than IC’s based signal, and

thus, it has better signal penetration, and can be tagged on

metals and liquid. The extended reading range of the pas-

sive SAW sensors can replace high-cost battery-powered

active sensors. In addition, because the substrate materials

used in SAW sensor are piezoelectric crystal, they can

work at temperatures up to several hundred degrees, can

F. Nawaz (&) � V. Jeoti

Department of Electrical and Electronic Engineering,

Universiti Teknologi Petronas, 31750 Tronoh, Perak, Malaysia

e-mail: [email protected]

V. Jeoti

e-mail: [email protected]

123

Wireless Netw

DOI 10.1007/s11276-014-0765-3

withstand high energy X-rays, gamma rays and they have

survived shock levels exceeding 1,000 g. All these prop-

erties make them suitable for operation in harsh and ver-

satile environments.

For designing SAW sensor two basic principles are

used. The delay line sensor and the resonator based sensors

as shown in Fig. 1. The resonators are narrowband device,

characterized by a resonance frequency dependent on a

physical quantity under investigation [11] whereas the

delay lines are wideband devices, characterized by a

propagation delay dependent on a physical quantity under

investigation [12]. Delay lines often require fast electronics

and large data storage memories with typical time con-

straints in the hundreds of MHz bandwidth, and 40-ns long

pulses. The reader interrogates either in time domain e.g.

by directly measuring the impulse response or in frequency

domain i.e. by measuring the transfer function [13].

Although SAW sensors have a longer read range as

compare to other passive sensors. There are still a need and

future perspectives to increase the readout distance.

Authors in [14] proposed the operation of a SAW device at

2.45 GHz ISM band and demonstrated a reading distance

of up to 11 m for indoor measurements and 13 m in out-

door operation. Time-position coding was used with one

reflector per 16 possible positions to visibly reduce the

losses and increase the read range.

The rest of this paper is organized such that the second

part of this paper deals with the operation principle and

design of SAW sensor devices, an overview of SAW sys-

tem, its components connectivity and signal processing is

also explained. In the third part the read range and com-

munication coverage of the SAW sensor network is eval-

uated and the capabilities of such systems are presented.

The last section gives the conclusion and future directions

in this research area.

2 Principles of operation

Transduction mechanism is the primary means of detection

employed by a SAW device. This method involves the

conversion of signals from the electrical domain to the

physical domain and vice versa [15]. The inter digital

transducer (IDT) enables the conversion of these signals

and therefore is the fundamental term of SAW technology.

2.1 SAW device operation principle

Piezoelectricity, the production of electrical charges by the

imposition of mechanical stress was discovered by Pierre

and Paul-Jacques Curie in 1880 while SAW was discov-

ered in 1885 by Lord Rayleigh [16]. All SAW devices use a

piezoelectric material, such as quartz crystal, to generate

the acoustic wave by applying a proper electrical field. A

mechanical wave is created which propagates through the

substrate and is then converted back to an electric field for

measurement. Since 1970, SAW devices have started

gaining attention for pulse compression radar, radios, and

bandpass filters for TV sets. A piezoelectric crystal is

electrically neutral and the design of devices is based on

combined Gaussian and Hooke’s law [17] governed by the

following equations.

Si ¼ SEijTj þ dki ð1Þ

Dl ¼ dlmTm þ eTlnEn ð2Þ

where i, j, m = 1, .6 and k, l, n = 1, 2, 3. S, D, E and T are

the strain, dielectric displacement, electric field and stress,

respectively. sEij , dki and eT

ln are the elastic compliances, the

piezoelectric constants and the dielectric permittivity.

SAW sensors are purely passive and the underlying

principle of the completely passive operation is the pie-

zoelectric effect. SAW sensor designing requires many

factors to be taken into consideration. The relative size,

sensitivity and efficiency is dependent on intended appli-

cation. For the wireless interrogation of SAW based sensor

a signal frequency of 40 MHz to a few GHz can be used.

The available bandwidth in the ISM bands increases with

the frequency. Figure 2 shows the layout of a common

SAW based sensor.

The SAW based sensor is achieved by placing a specific

sensing film on the surface of the device. When the sensing

film gets exposed to a chemical, mechanical or electrical

change; the oscillation frequency of the acoustic wave is

modified. The sensing films can be metal, metal oxide,

metal nitride, polymer, or biological material depending on

the type of application. Early semi-conductive sensors were

made of metal oxides, i.e. ZnO, TiO2, Fe2O3, Al2O3, Y2O3

and SnO2. Eventually tin dioxide (SnO2) became the

dominating gas sensitive material. Tin dioxide sensors are

generally used as flammable gas sensors. The fundamental

advantage of this technology is its extremely low cost,

lightweight design, high sensitivity, and high temperature

range of operation (up to 1,000 �F). A SAW sensor

Fig. 1 SAW sensor classification

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sensitive layer alters the SAW velocity resulting in fre-

quency shifts. For an ideal thin film this shift is calculated

as in (3).

Df ¼ k1 þ k2ð Þf 20 Vf q� k2f 2

0 tF4l � xþ lð ÞV2

r ðxþ 2lÞ ð3Þ

where k1; k2 are material constants, f0 is fundamental fre-

quency, Vf is film volume, q is film density, l is the elastic

modulus, and Vr is the raleigh velocity of the film.

2.2 SAW system operation principle

A wireless SAW system can be segmented into several

components: the transmitter, the receiver, the SAW sensor

device, and the communication channel. Combination of

transmitter and receiver into a single block is referred to as

the transceiver or interrogator. The basic components of

SAW system and their interaction is shown in Fig. 3. The

interrogator delivers power to SAW device. The input

source of the transceiver can be continuous wave, pulse or

chirped. However, there is a limit to the amount of power

that can be originated and therefore be received by the

SAW tag. One such technique that can lower the peak

transmit power while maintaining the same average power

is known as chirping and is frequently used in radar sys-

tems [18].

The interrogator signals are transmitted by using an

antenna who’s effective radiated power, with respect to an

isotropic radiator (EIRP), is defined as in (4).

EIRP ¼ PtGt ð4Þ

where Gt is the gain of the transmitting antenna and Pt is

the transmission power. The directive gain can be defined

as in (5).

G ¼ maximum power intensity

average power intensity over 4p steradiansð5Þ

The radiated power from the antenna is usually uniform

over the bandwidth of interest. The generated signal from

the transceiver are then passed to the channel which is the

propagation medium connecting the SAW devices with the

interrogator. A Free space channel assumes that the chan-

nel is free of any interference to signal such as reflection,

refraction or absorption, and the energy arriving at the

receiver is assumed to be a function of the distance from

the transmitter only. The main equations for the channel

are established on the basis of range equations. P t is

related to the power density of a hypothetical sphere at a

distance r from the source as in (6).

P(r) ¼ Pt

4p r2

watts

m2ð6Þ

The power extracted at a receiving antenna for a dis-

tance much greater than propagation wavelength is defined

as in (7).

P(r) ¼ pðrÞAer ¼PtAer

4p r2ð7Þ

where Aer is the absorption effective cross section area of

the receiving antenna, defined as in (8).

Aer ¼Total power extracted

incident power flux densityð8Þ

Thus the antenna gain of the SAW sensor device is defined

as in (9).

Fig. 2 Passive SAW sensor

design parameters

Fig. 3 Basic components of SAW system and their interaction

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G ¼ 4p Ae

k2ðfor Aeiik2Þ ð9Þ

The power received at the sensor is then defined as in (10).

Pr ¼ EIRPGrk

2

ð4pr)2ð10Þ

The signal passing through the channel is often affected

by the path loss as defined in (11).

Ls ¼4 p r

k

� �2

ð11Þ

The path loss is dependent on the frequency of operation

which ultimately affect the antenna size, communication

range and complexity of device fabrication.

The remote sensor node without a transmitter, working

on the backscattered waveform principle is extremely

susceptible to the near-far problem. As there is no way to

perform power control on the sensors, spread spectrum

multiple-access gets affected by, the near-far problem in

DSSS/MA, as is discussed in [19]. In a network of SAW

sensors, wireless nodes are never located at exactly the

same distance from a reader, which results in a range of

different powers seen by the reader. For two-way line-of-

sight communication standard free-space link equation is

defined as in (12)

PR ¼PT GT GR

4pRcr=p

� �2ð12Þ

Here the path loss falls off by a factor of r2 However, in

case of a backscatter the link equation is given as in (13)

were received power falls off by a factor of r4 (with single

antenna to send and receive) [20].

PR ¼PT G2

TRGtk2X2M

4pRcrð Þ4�2B2F2

ð13Þ

where X is polarization mismatch, M is modulation factor,

� is an antenna gain penalty, B is path blockage and F2 is

fade margin. At this point, there is no method for con-

trolling the power reflected from the sensor. Thus, the near-

far problem affects the ability to perform direct sequence

spread spectrum.

In a wireless SAW network, voltage is induced in the

sensor antenna by the RF signal, and this induced voltage

should be rectified to direct current in order to be utilized.

The receiver antenna converts the RF power to DC as long

as 100 mV of voltage is induced. The relation between the

induced voltage and communication range is explained by

[21] as in (14).

Gk2

4p¼ Vtj j

8Wi

21

Rr þ RL

� �ð14Þ

where Vt is voltage induced, Wi is incident power density,

and Rr and RL are radiation and load resistances. k is the

wavelength of the signal.

The emitted power from the reader is another useful

parameter. There are constraints on the regulations for

effective isotropically radiated power (EIRP). According to

which a directional source antenna can have the 4 W

maximum EIRP (FCC rule on WLANs). Using Eqs. 12 and

14, the maximum distance to induce 100 mV on the node

for 4 W EIRP was calculated by [22]. At 2 GHz, 13 m

range was observed; this distance increased to 26 m at

1 GHz and 51 m at 500 MHz. However, the distance of

26 m at 1 GHz requires the 4 W EIRP source to be mobile

and must come close to the passive node to transmit the

power. These calculations also prove that multiple readers

in close proximity are needed for the practical coverage of

a wireless passive sensor network, deployed in a large

industrial environment.

3 SAW network range evaluation

3.1 Read range

SAW-based system can be modeled in numerous ways.

Fundamental and traditional techniques can be used to

predict the read range of SAW devices. Microcell envi-

ronment characterization has been used successfully for

SAW based broadcasting at varying distances [23–25],

however the SAW sensor range is currently limited to

centimeters or a few meters. Coherent integration of mul-

tiple responses was proposed in [26] to increase the range

of SAW sensor, however collision free simultaneous

interrogation of multiple SAW sensors over a wide area is

still a prevalent research topic.

Received signal strength (RSS) can be used to predict

the read range of passive devices [27]. The dual-slope

model derived from the two-ray model is a general

approach for predicting the RSS. Two-ray model is shown

in Fig. 4, where r1 is the direct ray and r2 is a multipath

ray and d is the distance. According to dual-slope model

the received signal power at the receiver can be expressed

using revised Friis formula as in (15)

Pr ¼ 10� log10

k4p

� �2

GtGrPtp1rN

1

1þ r=Ro

� �� �NL�2

0B@

1CA

ð15Þ

where Pr is the received power, Gr is the reader antenna

gain, Gt is the tag antenna gain, Pt is the transmitted power,

k is the wavelength, p is the polarization mismatch, N is the

variation of power before the breakpoint and NL is the

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increased signal loss beyond the breakpoint. The break-

point distance Ro is calculated using (16)

Ro ¼4hthr

kð16Þ

where hr is the height of reader antenna and ht is the height

of tag antenna above the surface.

According to the traditional radar equation the level of

signal strength received by the reader decreases with the

fourth power of the distance from the SAW sensor. Hence

the request distance r is given by (17).

r ¼ k:W4p

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPtG2

r G2t

A:kTB:SNR

4

rð17Þ

where W is the EM wavelength, A is the insertion attenu-

ation of the SAW sensor whose value lies between 20 and

50 dB (depending on frequency and substrate material),

kTB is the thermal noise power. The signal to noise ratio

(SNR) at the SAW tag reader output can be calculated from

the following equation.

SNR ¼ Pt � Gr � PGt � Gsaw � PGri

NF � kTBð18Þ

where P is path gain, GSAW is the gain of SAW chip (less

than unity), and NF is the noise factor of the reader.

In Europe, SAW devices are allotted two frequency

bands 433.07–434.77 MHz and 2.4–2.483 GHz and the

equivalent isotropic radiation power (EIRP) allowed in

these bands is 25 mW. Using these values a request dis-

tance of 36 cm in the 2.45 GHz band and 2.5 m in the

434 MHz band can be achieved [28]. In order to achieve

longer range a coherent integration of the signals from

multiple request cycles can be used. This approach con-

siders the entire system as linear, in which no frequency

mixing happens and several RF responses are summed up

coherently in order to improve the SNR. Pulse compression

techniques are another useful method to increase the range.

The passive SAW sensor can only be read with an inter-

rogator with power, low enough to be used in applications

normally restricted to the silicon tag readers. In a multi-

sensor environment the readable distance of a SAW Sensor

is dependent on the insertion loss of the device and the

processing gain of the system [5]. The readable distance can

be increased by improving insertion loss, and optimizing the

device fractional bandwidth and correlation properties. In

[5] the pseudo-orthogonal frequency coding (POFC) is

presented to minimize the spectral domain overlap and

measure temperature wirelessly at a distance of 7 ft (*2 m).

Hartmann et al. in [28] analyzed the range of a 2.4 GHz

SAW based RFID. The proposed tag has a 309 superior read

range compared to passive semiconductor IC-based RFID.

Authors in [29] provide a detailed overview of current

challenges and issues of RFID systems, mainly on RFID

algorithms and protocols. Early SAW tags had a limited data

capacity, but the recently developed SAW sensors have used

new modulation schemes integrated with EPCglobal stan-

dards resulting in an increased data capacity of 128 bits or

higher. The Global SAW tag (GST) [1] operates with spread-

spectrum signal at 2.44 GHz instead of UHF and offer a low

power reader alternative to the silicon tags by -40 dB.

3.2 Communication coverage

All wireless monitoring networks are systematically clas-

sified into two categories, network coverage and network

connectivity [30]. In a SAW system network connectivity

and coverage are dependent to interrogator design and

functionality. A wireless SAW system consists of several

interrogators and SAW devices working on the principle of

modulated backscattering. In order to maintain the com-

munication reliability, connectivity of sensors and cover-

age of interrogators, it is important to carefully design the

deployment, especially the number of interrogators acting

as the RF source. In a network of size D where the com-

munication range of each interrogation source is repre-

sented by a circle of radius Rrf, we can find the required

number of interrogators k by using (19).

K ¼ DkR2

rf

ð19Þ

The read range of interrogators is considered to be non-

overlapping to avoid two types of interference as described

in [31]. The interrogator-to-Interrogator interference in

which the signal generated by one interrogator interferes

with the reception system of other interrogator resulting in

a situation where the SAW device is unable to respond to

any interrogator at all. As illustrated in Fig. 5(a); when I1

attempts to read data from T1 and I2 is trying to read data

from tags T2 using the same channel, the week response

signal on Tag T1 will be degraded because of the strong

interference signal of I2. Similarly interrogator to SAW

node interference is shown in Fig. 5(b) in which two or

more interrogators try to read the same node simulta-

neously because of an overlap in their read ranges. Due to

the passive nature of node, it is not able to select a

hr

ht

r1

r2

θ θ

d

Fig. 4 The two ray model

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particular interrogator to transmit its data, even if both

interrogators are operating at different frequencies.

Using the Friis’ transmission equation k can be obtained

as in (20).

K ¼2kDf 2 V2

t

c2PtGtGrðRr þ RlÞ

ð20Þ

where f is the carrier frequency and c is the speed of light

(k = c/f). This equation can be used to obtain appropriate

parameters for effective communication coverage in SAW

network. Wireless SAW sensor networks do not have the

lifetime constraints like conventional wireless sensor net-

works, however, the communication performance of SAW

network is directly related to the interrogator coverage,

read range and energy efficiency over the field of opera-

tion. Joint routing and advanced aggregation scheme can be

employed to enhance these characteristics of the interro-

gator network [32, 33]. Authors in [34] employ neural

network and particle swarm optimization for an energy-

efficient multi-source temporal data aggregation model that

predicts the future according to historical data. The tech-

nique significantly reduced the total data transmissions and

achieved higher energy conservation. Forming clusters of

interrogators is another important energy-saving method

where the cluster head node is responsible for the necessary

data aggregation and route selecting decision [35]. The rest

of the network communication is done in accordance with

the default network protocol after the formation of cluster

structure.

4 Conclusion

SAW devices are vital in many practical disciplines of

mechanical and electrical engineering. The communication

performance of the SAW sensor network is associated with

the interrogator coverage provided over the field. In this

paper the radio frequency communication coverage of the

wireless SAW network is investigated also the required

number of interrogators to obtain interference-free com-

munication is analyzed in terms of power, transmission

frequency and size of the network. The unique transduction

mechanism and back scattered operation principles of

SAW sensor is also presented. The analysis presented here

can be used towards determination of design approaches of

passive and wireless SAW sensor networks.

Acknowledgments We are thankful to Universiti Teknologi PET-

RONAS for providing the research facilities under Yayasan-UTP

Grant No. 0153AA-A71.

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35. Liu, Y., Xiong, N., Zhao, Y., Vasilakos, A., Gao, J., & Jia, Y.

(2010). Multi-layer clustering routing algorithm for wireless

vehicular sensor networks. IET Communications, Special Issue on

Vehicular Ad Hoc and Sensor Networks, 4(7), 810–816.

Faiza Nawaz received her M.S.

degrees from GIK Institute of

Engineering and Technology,

Pakistan in 2010, She is cur-

rently a Ph.D. student in Elec-

trical and Electronics

Engineering Department at

Universiti Teknologi PETRO-

NAS, Malaysia. Her research

interests are in the area of

wireless networking with

emphasis on wireless SAW

based sensor networks.

Varun Jeoti received his Ph.D.

degree from Indian Institute of

Technology Delhi India in 1992.

He worked on several sponsored

R&D projects in IIT Delhi and

IIT Madras during 1980–1989.

He was a Visiting Faculty in

Electronics Department in

Madras Institute of Technology,

Anna University for about

1 year from 1989 to 1990 and

joined Delhi Institute of Tech-

nology for next 5 years till

1995. He moved to Electrical

and Electronic Engineering

School of Universiti Sains Malaysia in 1995 and joined Electrical and

Electronic Engineering Department of Universiti Teknologi PET-

RONAS in 2001. His research interests are in the area of signal

processing, surface acoustic wave (SAW) devices, wireless SAW

sensor network and wireless communication for maritime applications

besides others.

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