Development of Smart Antenna for RFID Reader

Nemai Chandra Karmakar, Sushim Mukul Roy and Muhammad Saqib Ikram Department of Electrical and Computer Systems Engineering, Monash University, Bldg. 72, PO Box 35, Clayton,

VIC Australia 3800, Phone: +61 3 9905 1252, Fax: +61 3 99053454 Email: nemai.karmakar@eng.monash.edu.au; sushim.roy@eng.monash.edu.au

Abstract-Radio Frequency Identification (RFID) systems have many important applications in the 900 MHz frequency band. A 3××××2-element planar smart antenna for RFID readers in a low cost compact package is developed. The smart antenna covers 860-960 MHz frequency band with more than 10 dB input return loss, 12 dBi broadside gain and up to 40° elevation beam scanning with a 4-bit reflection type phase shifter array. Once implemented in the mass market, RFID smart antennas will contribute tremendously in the areas of RFID tag reading rates, collision mitigation, location finding of items and capacity improvement of the RFID system. Keywords: Radio frequency identification, smart antenna, RFID tag, Barcode, anti-collision, RFID reader, Switched beam array, beamforming, phased array antenna, phase shifter.

1 INTRODUCTION The RFID system is a wireless data transmission and reception technique for automatic identification, asset tracking, security surveillance and many other emerging applications. Figure 1 illustrates a generic block diagram of the RFID system. An RFID system consists of three major components: (i) a reader or interrogator, which sends interrogation signals to an RFID transmitter responder (transponder) or tag, which is to be identified; (ii) an RFID tag, which contains the identification code; and (iii) a middleware, which maintains the interface and the software protocol to encode and decode the identification data from the reader into a mainframe or a personal computer. In a further embodiment, the computer can be connected to the global network for remote real-time asset tracking and security surveillance.

At the dawn of the new millennium, as barcodes and other means for identification and asset tracking are becoming inadequate for recent demands, RFID technology has been facilitating logistics, supply chain management,

asset tracking, security access control, intelligent transportation and many other areas at an accelerated pace. This large huge number of applications represents the significant activities and applications of RFID in various sectors in either commercial domains or government agencies.

Due to the flexibility and numerous advantages of RFID systems compared to barcodes and other identification systems available so far, RFIDs are now becoming a major player in retail and government organisations. Patronization of the RFID technology by organisations such as Wal-Mart, K-Mart, the USA Department of Defense, Coles Myer in Australia and similar consortia in Europe and Asia has accelerated the progress of RFID technology significantly in the new millennium. As a result, significant momentum in the research and development of RFID technology has developed within a short period of time. The RFID market has surpassed $5 billion dollar mark in 2007 [1], and this growth is exponential, with diverse emerging applications in sectors including medicine and health care, agriculture, livestock, logistics, postal deliveries, security and surveillance and retail chains. Today, RFID is being researched and investigated by both industry and academic scientists and engineers around the world. The synergies of implementing and promoting RFID technology in all sectors of business and day to day life have overcome the boundaries of country, organisation and discipline.

ReaderTag/

Transponder

Clock

Data/Energy

Global Network

HostComputer

ReaderTag/

Transponder

Clock

Data/Energy

Global Network

HostComputer

Fig. 1 Generic RFID system.

Antennas play significant role in RFID systems. Smart antennas for RFID readers are a new area of research [2]. RFID smart antennas will contribute tremendously in the areas of RFID tag reading rates, collision mitigation, location finding of items and capacity improvement of the RFID system. This paper concerns development of a low cost compact smart antenna for RFID readers at 900 MHz frequency band. The smart antenna enhances the performance of automatic identification systems, asset

2008 IEEE International Conference on RFIDThe Venetian, Las Vegas, Nevada, USAApril 16-17, 2008

1B1.1

978-1-4244-1712-4/08/$25.00 ©2008 IEEE 65

tracking in real time and inventory control in warehouses. The paper is organised as follows. An introduction to RFID is presented first. An in-house developed phased array antenna from scratch for 900 MHz band RFID readers is then presented. The phased array antenna comprises an L-shaped slot loaded half disk (semi-circular) patch antenna, a corporate feed network, a four-bit reflection type phase shifter array and control electronics. The design of each component from scratch is presented. The results of the prototype components are presented next followed by conclusion. It is important to note that smart antennas for RFID readers are currently in the developmental stage. To the best of the author’s knowledge, such smart antenna systems for RFID have not yet emerged as the mainstream enabling technology.

2 SMART ANTENNAS FOR RFID READERS

2.1 Smart Antennas Smart antennas for RFID readers are scanned beam antenna systems, which point beams to selective transponders within their main lobe radiation zones, thus reduce reading errors and collisions among tags. This technique exploits spatial diversity, and in many cases polarisation diversity of tags. The directed beam also reduces the effects of multipath fading [3]. This new approach to RFID antenna technology is being adopted by RFID manufacturers such as Omron Corporation, Japan [4], RFID Inc. [5] and RFSAW, USA. In March 2006 Omron Corporation developed a new electronically controlled antenna technology in their UHF band RFID reader systems [4]. Recently, RFID Inc. [5] has introduced a 32-element compact package smart antenna with associated switches at 125 KHz frequency band. This new product advances RFID applications in factory

automation, process controls and original equipment manufacturer (OEM) markets. However, the technical details of these smart antennas for RFID readers are not available in open literature due to commercial-in-confidence nature of the works. This paper presents technical information on smart antennas for RFID readers and this is provided in subsequent sections. Smart antennas are

electronically steerable antennas, which are fixed with supporting platforms such as gantries. Electronic beam steering can be achieved by (i) switching on certain antenna elements and turning off the rest as in switched beam array antennas; (ii) changing the phase excitations of individual elements as in phased array antennas; and (iii) adaptively controlling the weight vectors of individual elements using array signal processing chips. Adaptive antennas can also be sub-classified as multiple input multiple output (MIMO) or multiple input single output (MISO) antennas. Finally, by combining switching and phase shifting of the turned on subsets of the antenna elements, a switched beam phased array is formed. Following are the detailed accounts of the smart antenna developed for RFID readers at Monash University.

2.2 Development of Smart Antenna for 900 MHz RFID Reader

So far various smart antenna systems for RFID readers have been discussed. In this section, the technical development of a planar phase scanned antenna developed by the Monash RFID Research Group led by the author is presented. The 900 MHz frequency band has many applications in RFID systems. The application areas are: 868-870 MHz for short range devices in CEPT countries including Australia and most European countries that recognize the U.S. participation in the CEPT Radio Amateur License (http://www.arrl.org/FandES/ field/ regulations/io/#cept, accessed 11 Sep. 2007); 902 – 928 MHz ISM band RFID for North America; 918-926 MHz RFID for Australia; and 950-965 MHz for Japan (Mendolia et al, 2005; Clampitt, 2006). Therefore, there is a demand for a smart reader antenna which can cover all these bands of operations. To fulfill this requirement the author has developed a 3×2-element planar phased array antenna operating at 900 MHz frequency band covering 100 MHz bandwidth. The objective is to make a

low cost, low profile and light weight antenna for easy mounting and portable operation, aimed at the mass worldwide market. These research and development activities in RFID have also trained personnel for this potential RFID market. Table 1 shows the specification requirements of the smart antenna for RFID readers.

Table 1: Specification requirements of 900 MHz RFID reader phased scan array antenna.

Frequency 910 MHz 10 dB RL Bandwidth 100 MHz Beam scanning 3D azimuth and elevation beam scanning Beamwidth 42° (bore sight) Peak gain 12 dBi Weight Less than 1 kg Dimensions 3 cm×50 cm × 50 cm

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3 SMART ARRAY ANTENNA DESIGN

3.1.1 Antenna Element Design

To make the antenna compact and fully planar the most suitable candidate is the microstrip patch antenna, but the conventional microstrip patch antenna is a half-wavelength resonator. At 900 MHz, the antenna element is sizable compared to the wavelength and is not portable and easily mounted. The resonant frequency of a circular patch antenna at dominant TM110 mode is expressed [5] as:

1.8412|110 2

cfr a

e r� �

= (1)

where, c = velocity of light in free space (3×1010 cm/s); ae= effective radius of the circular patch antenna (fringing field effect included); and �r = dielectric constant of the microwave laminate on which the patch is etched. Using (1), the diameter of a circular patch antenna on a low dielectric constant microwave laminate for efficient radiation at 900 MHz is approximately 19 cm, which is quite large for array application. Besides this physical constraint, obtaining a

large bandwidth in the order of 100 MHz at 900 MHz resonant frequency is not a trivial task. This constitutes 11% bandwidth at 900 MHz. A comprehensive investigation for a suitable compact antenna configuration, which can meet the required bandwidth and antenna size without much compromising antenna radiation efficiency and gain, is made. Finally, an L-slot loaded crescent antenna [7] is culled as the most suitable candidate for portability and ease of mounting.

The antenna element is designed with full-wave electromagnetic solver CST Microwave Studio. Fig. 2a shows the L-shaped slot loaded half disk antenna. The slot perturbation on the patch radiator creates parasitic loading comprised of extra capacitance due to the slot and inductance due to the modified current path on the radiating half disk. Thus broad bandwidth of approximately 110 MHz is achieved. The single antenna element is designed on FR4 substrate with a dielectric constant of 4.4 and loss tangent of 0.002. The thickness of the material is 1.6 mm. The radius of the half disk antenna is 8 cm. The total height of the antenna element is 25 mm; this includes FR4 and an airgap height of 23.4 mm. The single antenna element produces a RL bandwidth of more than 100 MHz centered at 900 MHz with a maximum RL of 35 dB (Fig. 2b). The antenna element produces about 7 dBi gain across the frequency band (Fig. 2c).

Ground plane

Half-disk patch

Feed probe

Ground plane

Half-disk patch

Feed probe

(a)

700 1100900

Frequency (MHz)

-20

0

-10

Ret

urn

Loss

(dB

)

700 1100900

Frequency (MHz)

-20

0

-10

Ret

urn

Loss

(dB

)

(b)

(c)

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3.1.2 Feed Network

The feed network is designed using equal power dividers in two stages. The first stage is a two-way T-splitter power divider and the second stage is comprised of two 3-way power divider as shown in Fig. 3. The 3-way power divider is squiggled to get perfect phase matched outputs as well as a fine geometry to connect the phase shifter arrays in correct phase matching. As per the diagram in Fig. 3, a 50� line is split into two 50� lines carrying equal power using 70.7� quarter wave transformers. Further, the 50� line is divided into three 50� lines carrying equal power using 86.6 � quarter wave transformers in the final stage of the power division.

(a)

(b)

Fig. 3(a) CST generated layout of a 1-to-6-way feed network and (b) S-parameter vs frequency plots.

Figure 3(b) shows ADS simulated s-parameters vs frequency plot of the 6-way equal power divider. The input return loss of the power divider is about 30 dB and the transmission loss for each output port is about 8 dB. The

output ports are well phase matched at the center frequency of 910 MHz and fall linearly with the frequency.

3.1.3 Phase Shifters

Microwave phase shifters are widely used in telecommunications systems, and are core elements to perform tasks such as digital modulation or antenna beam forming. There is a particular interest in p-i-n diode phase shifters due to their high switching speed, small size, and ease of control by a digital computer or a microcontroller. Generally, there are two main types of phase shifters that employ a p-i-n diode as a switching element – the reflection type and the transmission type. The reflection-type phase shifter uses a 3 dB quadrature hybrid coupler as its essential component. The transmission-type phase shifter includes a loaded line, switched line, and three-element T- or �-networks [7-11].

Standard designs do not consider the issue of minimization of size and the development expense of phase shifters. This issue is of significant importance, for example, in the design of a phased-array antenna [12], which usually employs a large number of such devices. It is apparent that minimization of the size and cost of individual phase shifters can be of a great advantage from the commercial point of view.

In typical designs, cascades of binary phase shifters [10], including 180°, 90°, 45°, and 22.5° phase bits, are employed. Out of different bits, the 180° phase bit introduces the most significant challenge to the designer, as it occupies a large area or it uses a large number of diodes that increase production cost. This situation is evident with the two most popular designs, namely, the reflection type and the switched-line type [7, 8]. The reflection-type phase shifter minimizes the number of diodes, as only two diodes per 180° bit are required. However, it occupies a considerable space due to the use of a 3dB quadrature hybrid. The switched-line phase shifter minimizes the occupied area, as it uses no hybrid, but only two lines that relay the transmitted signal. However, in its standard form [7, 9], this device requires four diodes to accomplish the phase-shifting function. This paper presents the designs of hybrid coupled phase shifters, employing 180° phase shifters that minimize the occupying area and manufacturing cost. The size of the hybrid-coupled phase shifter is reduced by employing a compact 3 dB branch-line hybrid coupler. In order to minimize the manufacturing cost, the phase shifters employ low-cost UHF (500 MHz) p-i-n diodes in the switching networks. A special compensation technique is applied to these diodes to make them high-quality switches in the L-band. Using these compensation techniques, high-quality performances of the developed phase shifters over an approximately 10% bandwidth centered at 910 MHz are demonstrated.

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3.2 Design of Hybrid-Coupled Phase Shifter The basic configuration of the reflection-type phase shifter shown in Fig. 4, which uses a 3 dB branch line coupler, is shown in Fig. 5. As seen in Fig. 4, this device employs two identical loads at ports 3 and 4, which are varied in a binary manner using two band switching p-i-n diodes. Assuming that the two loads are reactive and that the coupler is ideal, the changes in the two identical loads results in a reflectionless phase shift ��, which takes place between ports 1 and 2. In the present case, �� =180° is the required phase shift. In many applications, the required operational bandwidth is in the order of 10%. In this case, the coupler’s behavior can be assumed to be close to the ideal one. Consequently, the operation of the phase shifter depends upon its reflective network. This, in turn, depends on the quality of the diodes that perform the switching function. Low-quality packaged diodes often feature significant parasitic elements, which adversely affect their switching function, and consequently the performance of the phase shifter. In order to improve the performance of the diode as a perfect switch, a suitable compensation circuit is applied. This technique, in relation to the design of hybrid-coupled and switched-branch-line phase shifters, is demonstrated in the following sections.

Zo

Zo

Zo------�2

Zo------�2� �

PinPout

1 2

3 4

Zo

Zo

Zo------�2

Zo------�2� �

PinPout

Zo

Zo

Zo------�2

Zo------�2� �

PinPout

1 2

3 4

Fig. 4 Configuration of a hybrid-coupled phase shifter including diodes arrangements

Design of a Compact Hybrid Coupler. The standard 3dB branch-line coupler includes two sets of parallel quarter-wave transmission lines which are arranged in a rectangle. As such, this device occupies a significant area, which is an undesired feature, especially in low microwave frequency applications, which is the case here. The area occupied by the coupler can be reduced using the configuration shown in Fig. 5. It can be seen that, using 90° bends, the 50 � lines can be sandwiched into the 35.5 � lines. Consequently, the area occupied by the hybrid is reduced to approximately 50%. This maneuver produces a compact form of a branch-line hybrid coupler.

Diode-Compensated Reflection Network Fig. 6 shows an equivalent circuit for the reflective network with compensated diodes. This network includes a section of transmission line with length l1 and characteristic impedance Z1, in front of the diode whose equivalent parameters in the

FB and RB states are given by lumped-element equivalents, as shown in Fig. 6(b).

Fig. 5 Agilent ADS generated layout of a compact hybrid coupler

The other elements of this network include capacitors Cs and Cd and a section of transmission line with length l2 and characteristic impedance Z2. In order to obtain the required reflection network parameters for the assumed phase shift �� =180° over a given frequency band, the microwave design package Agilent ADS is used. An operational bandwidth of approximately 10% centered at 910 MHz is considered.

Fig. 6 Equivalent circuits of (a) the reflective network and (b) the diode for forward-bias (FB) and reverse-bias (RB) conditions

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Four phase shifters were designed, giving phase shifts of 22.5°, 45°, 90° and 180° respectively. The generic layout of the reflection type phase shifters is shown in Fig. 7. As can be seen in the figure, the compact hybrid is connected to two symmetric switched reactive loading at ports 3 and 4 to generate required phase shift. For each bit phase shifter, the reactive loading is different.

Fig. 8 shows the phase shifter performances of the 22.5°, 45°, 90° and 180° phase shifters. As can be seen all phase shifters are well matched in both on and off states of the phase shifts with more than 50 dB RL at 910 MHz. The maximum phase offset at the band edge is less than ±15°. The phase shifters have an average insertion loss of 3.5 dB in both on and off states and the maximum phase deviation of about ±18° for the four bit phase shifter array. This quantization error in phased array antennas is quite common. The assembled array antenna is very well matched with more than 10 dB return loss in the prescribed bandwidth of 100 MHz centered at 910 MHz.

Compact hybrid coupler

Switc

hed

Rea

ctiv

e lo

adin

g to

hyb

rid

coup

ler

Port 2Port 1

Diode with LCcompensationnetwork

Compact hybrid coupler

Switc

hed

Rea

ctiv

e lo

adin

g to

hyb

rid

coup

ler

Port 2Port 1

Diode with LCcompensationnetwork

Fig. 7 Diode compensated reflection type phase shifter (Agilent ADS generated layout).

(a)

(b)

(c)

(d)

Fig. 8 phase shifter performances: (a) 22.5, (b) 45, (c) 90 and (d) 180°

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3.2.1 Beamforming Network Design

The next design is the integrated beamforming network for the 3×2-element array antenna. Fig. 9a below shows the layout of the beamforming network developed using Agilent’s ADS 2005A. The beamforming network comprises a 1-to-6-way power divider (Fig.3), 4-bit reflection type phase shifters and biasing control of the 4-bit phase shifter array. All components are designed on Taconic TLX0 of thickness of 0.787, dielectric constant of 2.45 and

loss tangent of 0.00015.The details of the reflection type phase shifter can be found in previous work by the author and a colleague [13]. The complete beamforming network is delicately phase matched to each branch with zig-zag transmission lines. The feed network is a corporate type feed network with the first stage of a 2-way T-junction power divider followed by a three way power divider. Thus 1-to-6-way equal power division is achieved. In this primary design amplitude tapering was not attempted. However, the readers are referred to [15].

Input port

4-bit phase shifter array Feed network

Output to antenna element

Bias control

Input port

4-bit phase shifter array Feed network

Output to antenna element

Bias control

(a)

Top antenna array layer

Ground plane

Inter-elementmetallic barrier

Top antenna array layer

Ground plane

Inter-elementmetallic barrier

(b)

(c)

Fig. 9 CST Microwave Studio generated (a) beam-forming network, (b) array layout and (c) CST simulated radiation patterns at 900 MHz. FR4 laminate �r1 = 4.2, thickness 1.6 mm and airgap height

23.4 mm.

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3.3 Integrated Array Antenna

3.3.1 Mitigating Mutual Coupling between Antenna Elements

Mutual coupling greatly affects the array antenna’s performance, especially when the antenna elements are brought too close together. The situation worsens when the heights of the antenna elements are considerable (in this case 25 mm). CST Simulation shown about 10 dB mutual coupling between antenna elements’ ports. This strong mutual coupling between antenna elements distorted the radiation patterns and generates considerable sidelobes and back lobes. Metallic barriers as shown in Fig. 9b have been used to reduce the mutual coupling effect between the antenna elements by more than 30 dB. Thus the antenna aperture area is maintained at 50 cm× 50 cm without any significant radiation pattern distortion and input impedance match. This area is about 70% of a conventional patch array antenna at that frequency. Fig. 8b shows the top view of the complete antenna array generated in CST Microwave Studio Layout platform. The antenna is housed in a microwave transparent plastic casing. The total weight of the antenna is about 1.8 kg, making it suitable for easy transportation and mounting. Fig. 9c shows the radiation pattern of the array antenna in the 30° elevation angle direction. As the figure shows, the antenna generates a beam of 42° beamwidth with a gain of 10.7 dBi. The results indicate that the antenna is capable of scanning main beams in 3D orientations. However, due to the compact design with only 6 antenna elements the degree of freedom is confined to about ±40° elevation scan angles from the broadside direction. Beam squints more than ±40° elevation scan angles result in larger grating lobes. However, with the number of antenna elements this scan range can be improved at the expense of more complex beamforming network, associated switching electronics and a larger antenna aperture.

4 CONCLUSION RFID is an enabling technology which is transforming the identification, asset tracking, security surveillance, medicine and many other industries at an accelerated pace. This paper has presented a 3×2-element low cost, light weight and compact planar phase scanned antenna array for RFID readers. A detailed design guideline for the component level physical layer development of the smart antenna has been presented. The antenna is designed at the 910 MHz frequency band with 100 MHz bandwidth to cover a plethora of applications in

this frequency band. The development certainly has a potential international market.

5 Acknowledgements The work is supported by Australian Research Council’s Discovery Project Grant # DP665523: Chipless RFID for Barcode Replacement. The software supports from Agilent and CST are also acknowledged.

6 References [1] R. Das. & P. Harrop, “RFID Forecasts, Players &

Opportunities 2006 – 2016,” IDTechEx, London, UK,. Retrieved from http://www.idtechex.com/products/en/ view.asp?productcategoryid=93 on 11 September 2007.

[2] M. A. Ingram, “Smart reflection antenna system and method”, US patent no. US 6,509,836, B1, Jan. 21, 2003

[3] T. Nakamura, & J. Seddon, “Omron Develops World’s First Antenna Technology That Boosts UHF RFID Tag Read Performance,” Omron Corporation press releases, 2006. Retrieved from http://www.omron.com/news /n_270306.html in June 2007.

[4] Profibus Device Net-Ethernet IP-Modbus-Remote I/O, Retrieved from http://www.rfidinc.com in June 2007.

[5] C.A., Balanis, Antenna Theory: Analysis and Design, 2nd Ed. Wiley, New York, 1982.

[6] A. A. Deshmukh, & G. Kumar, “Various slot loaded broadband and compact circular microstrip antennas” Microwave and Optical Technology Letters, vol. 48, no. 3, pp. 435-439, 2006.

[7] M.H. Kori and S Mahapatra, Integral Analysis of Hybrid Coupler Semiconductor phase shifters, Proc Inst Elect Eng 134 (1987), 156-162.

[8] S.K. Koul and B. Bhat, Microwave and millimetre wave phase shifters, Artech House, Norwood, MA, 1991, vol. II.

[9] R.V. Garver, Microwave diode control devices, Artech House, Dedham, MA, 1975, Chap. 10.

[10] R.W. Burns, R.L. Holden, and R. Tang, Low cost design techniques for semiconductor phase shifters, IEEE Trans Microwave Theory Tech MTT-22 (1974), 675-688.

[11] N.C. Karmakar and M.E. Bialkowski, Designing a low cost L-band phased array antenna for mobile communications, Proc 1997 Asia-Pacific Microwave Conf, Hong Kong, Dec. 1997

[12] N. C. Karmakar, and M. E. Bialkowski, “An L-Band 90° Hybrid-Coupled Phase Shifter Using UHF Band

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PIN Diodes”, Microwave and Optical Technology Letters, vol. 21 no. 1, pp. 51-54 , Jan., 1999.

[13] N. C. Karmakar, Antennas for Mobile Satellite Communications, Unpublished doctoral dissertation, The University of Queensland, Australia, 1999.

[14] N. C. Karmakar, “A Low Sidelobe Sub-Array of Portable VSATS” Proc. 2003 IEEE International Antennas and Propagation Symposium and URSI North American Radio Science Meeting, Columbus, Ohio, USA, June 22-27, 2003.

[15] J.F. White, Diode phase shifters for array antennas, IEEE Trans Microwave Theory Tech MTT-22 (1974), 658 – 674.

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